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‘Nitrogen fertilising materials’ Report for Defra project NT2601

Report for Defra Project NT2601

Nitrogen fertilising materials

Peter Dampney, ADAS Boxworth

Ian Richards, Ecopt

Anne Bhogal, ADAS Gleadthorpe

June 2003

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Contents

1. ABBREVIATIONS........................................................................................................................................................4

2. GLOSSARY OF TERMS..............................................................................................................................................5

3. EXECUTIVE SUMMARY............................................................................................................................................7

4. INTRODUCTION AND ACKNOWLEDGEMENTS..............................................................................................10

5. REQUIREMENTS OF A GOOD NITROGEN FERTILISER PRODUCT..........................................................12

6. MATERIALS FOR MANUFACTURING NITROGEN FERTILISERS..............................................................14

6.1 AMMONIUM CARBONATE (AC)..................................................................................................................................146.2 AMMONIUM CHLORIDE (ACL)....................................................................................................................................156.3 AMMONIUM NITRATE (AN)........................................................................................................................................166.4 AMMONIUM POLYPHOSPHATE (APP).........................................................................................................................186.5 AMMONIUM SULPHATE (AS)......................................................................................................................................196.6 AMMONIUM SULPHATE NITRATE (ASN)....................................................................................................................216.7 AMMONIUM THIOSULPHATE (ATS)............................................................................................................................226.8 ANHYDROUS AMMONIA (ANA)..................................................................................................................................236.9 AQUEOUS AMMONIA (AQA).......................................................................................................................................246.10 CALCIUM AMMONIUM NITRATE (CAN).................................................................................................................256.11 CALCIUM CYANAMIDE (CC)..................................................................................................................................266.12 CALCIUM NITRATE (CN)........................................................................................................................................276.13 CHILEAN POTASSIC NITRATE (CHILEAN KN).........................................................................................................286.14 CROTONYLIDENEDIUREA (CDU)...........................................................................................................................296.15 DI-AMMONIUM PHOSPHATE (DAP)........................................................................................................................306.16 ISO-BUTYLIDENEDIUREA (IDBU)..........................................................................................................................316.17 MAGNESIUM AMMONIUM PHOSPHATE (STRUVITE) (MGAP).................................................................................326.18 MAGNESIUM NITRATE (MGN)...............................................................................................................................336.19 METHYLENE UREA (MU).......................................................................................................................................346.20 MONO-AMMONIUM PHOSPHATE (MAP)................................................................................................................356.21 OXAMIDE (OX)......................................................................................................................................................366.22 POTASSIUM NITRATE (KN)....................................................................................................................................376.23 SODIUM NITRATE (NITRATE OF SODA) (NAN).......................................................................................................386.24 SULPHUR COATED UREA (SCU).............................................................................................................................396.25 UREA (U)...............................................................................................................................................................406.26 UREA AMMONIUM NITRATE (UAN SOLID)............................................................................................................426.27 UREA AMMONIUM NITRATE (UAN FLUID)............................................................................................................436.28 UREA AMMONIUM SULPHATE (UAS).....................................................................................................................446.29 UREA CALCIUM NITRATE (UCN)...........................................................................................................................456.30 UREA PHOSPHATE (UP).........................................................................................................................................466.31 NITRIFICATION INHIBITORS....................................................................................................................................476.32 UREASE INHIBITORS...............................................................................................................................................486.33 FERTILISER FILLERS, ADDITIVES AND COATING AGENTS.......................................................................................49

7. THE PRODUCTION OF COMPOUND FERTILISERS........................................................................................50

8. NITROGEN FROM ORGANIC MANURES...........................................................................................................54

8.1 FORMS OF NITROGEN..................................................................................................................................................548.2 PRODUCTION OF ORGANIC MANURES AND THEIR POTENTIAL N VALUE....................................................................548.3 REGULATORY CONTROLS...........................................................................................................................................558.4 FARM MANURES.........................................................................................................................................................568.5 SEWAGE SLUDGES (BIOSOLIDS)..................................................................................................................................598.6 INDUSTRIAL ‘WASTES’ AND COMPOSTS......................................................................................................................60

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9. NITROGEN FROM LEGUMES................................................................................................................................61

10. DISCUSSION AND CONCLUSIONS...................................................................................................................62

10.1 NITROGEN FERTILISERS.........................................................................................................................................6210.2 NITROGEN FROM ORGANIC MANURES....................................................................................................................6610.3 NITROGEN FROM LEGUMES....................................................................................................................................66

11. REFERENCES........................................................................................................................................................68

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1. Abbreviations

AC Ammonium carbonateACl Ammonium chlorideAN Ammonium nitrateAnA Anhydrous ammoniaAPP Ammonium polyphosphateAqA Aqueous ammoniaAS Ammonium sulphateASN Ammonium sulphate nitrateATS Ammonium thiosulphateCAN Calcium ammonium nitrateCC Calcium cyanamideCDU CrotonylidenediureaChilean CN Chilean potassic nitrateCN Calcium nitrateDAP Di-ammonium phosphateFMA Fertiliser Manufacturers AssociationHSE Health and Safety ExecutiveIBDU Isobutylidene ureaK PotassiumKN Potassium nitrateMAP Mono-ammonium phosphateMg MagnesiumMgAP Magnesium ammonium phosphateMgN Magnesium nitrate MOP Muriate of potashMU Methylene ureaN NitrogenNaN Sodium nitrate (nitrate of soda)Ox OxamideP PhosphorusP2O5 PhosphatePSDA Product Safety Data SheetS SulphurSCU Sulphur coated ureaSNS Soil nitrogen supplySSP Single superphosphateTSP Triple superphosphateU UreaUAN Urea ammonium nitrateUAS Urea ammonium sulphateUCN Urea calcium nitrateUP Urea phosphate

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2. Glossary of terms

Blended fertiliser Compound fertiliser produced by dry mixing of two or more different particulate or powder materials.

Bulk density Density of a mass of material, often expressed as kg/litre. The mass comprises the particles and the air spaces between them so bulk density is determined by the shape and size of particles as well as by the true density of the material from which the particles are formed. Particulate materials show differences in bulk density between loose and tamped or shaken states, in some materials as great as 15%. The bulk densities shown are intended to describe those of material in a spreader hopper. A value of 1.00kg/l means that a 1000 litre hopper should hold 1tonne of material.

Caking Formation of large hard agglomerations of fertiliser particles due to chemical properties of the materials or to absorption of water. This phenomenon occurs when fertiliser granules adhere to one another through crystal bridges or plastic deformation.

Complex fertiliser Compound fertiliser where all particles have the same composition.

Compound fertiliser Product containing more than one of the major nutrients

Deliquesce Absorption of atmospheric water vapour resulting in the loss of physical structure of particles.

Fluid fertiliser Products supplied in liquid form, either as solution or suspension.

Granulation Methods of forming fertiliser particles, mainly in the range 2 to 4mm. There are two main classes of granulation: slurry and non-slurry processes. In slurry processes, solid particles of the fertiliser (obtained through recycling of undersize particles) are coated with a slurry of the fertiliser in successive layers. In non-slurry processes, a liquid component is added to finely divided particles causing them to agglomerate. Most granular products are slightly irregular in shape but some, those made by fluidised bed processes for example, are nearly spherical.

Granular fertiliser Solid fertiliser where particles are all produced by granulation. May be complex or blended though the term is sometimes erroneously used as an alternative to complex.

Hygroscopic Material absorbs moisture from the air.

IBC Intermediate bulk container or big bag, usually containing 500, 600 or 1000kg of fertiliser. IBC also can refer to 1m3 containers of solution fertiliser.

Median size The particle size at which 50% of the material by weight is smaller and 50% larger. The median size can vary in some materials and the values shown should be treated as guides. The particle size for most manufactured granular and prilled fertilisers is in the range 2 to 4mm range.

Particle crushing strength

Force that must be applied to cause a particle to shatter or break. Measured in newtons (N).

Particle or true density

Density of the solid material from which the particles are formed. Particle density therefore is independent of particle size and shape. The weight of a particle is determined by it’s size and density and is an important factor in spreading properties.

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Prilling Method of particle formation in which the molten fertiliser is forced through holes in a metal disc or spinning bucket and allowed to fall as droplets in a tower. The particles solidify as they fall. Prills tend to be more spherical and slightly smaller than granules

Solution fertiliser Products where the nutrients are present in true solution.

Straight fertiliser Product containing only one of the major nutrients (nitrogen, phosphate or potash)

Suspension fertiliser

Products where the nutrients are present partly in solution and partly as finely divided particles in suspension.

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3. Executive summary

1. This report forms part of the NT2601 project for Defra. It describes and discusses the potential of different nitrogen (N) fertilising materials for use in UK agriculture and horticulture. The report considers manufactured N fertilisers, N supplied from organic manures, and N ‘fixed’ by leguminous crops. The information sources are published international literature and information provided by representatives of the UK and international fertiliser industries. Other reports from the NT2601 and 2602 projects cover ‘The production and use of nitrogen fertilisers’ and ‘Evaluation of urea-based nitrogen fertilisers’.

2. A good quality and commercially viable manufactured nitrogen fertiliser must satisfy a wide range of different requirements as listed below. A weakness in any characteristic may mean that the material is not acceptable or usable by the (farmer) end-user or the national agri-environment industry in general.

i) Acceptable security of national/international supply of raw materials.ii) Acceptable reliability of supply to end-users.iii) Economic cost of manufacture.iv) Satisfactory health and safety characteristics.v) Ability to produce compound (multi-nutrient) fertilisers.vi) Good medium/long term storage characteristics.vii) Able to be transported safely and economically.viii) Able to be spread accurately in fields.ix) Achieves good crop response with no significant adverse effects on crops, livestock or human health.x) Has minimum or acceptable impact(s) on the water, soil or air environments.

3. Ammonium nitrate (AN) is currently the dominant source of fertiliser N used in British agriculture. AN is not used in Northern Ireland. Consideration of an alternative must take account of the main characteristics of AN which are as follows:-

i) A UK production capacity currently exists. Approximately 90% of the UK’s demand for carbon dioxide is produced from these production plants.

ii) AN can be used flexibly in the production of compound fertilisers (both complexes and blends), and solution fertilisers.

iii) AN has a high nitrogen concentration (33.5-34.5% N) for ease of transport, handling and storage both nationally and on farms.

iv) AN stores well and is not easily crushed.v) AN prills have good spreading characteristics through farm fertiliser spreaders.vi) The nitrogen from AN is rapidly available for crop uptake once applied to soils and has proven

agronomic characteristics. vii) Although nitrogen from AN can leach through soils and cause nitrate pollution of water, gaseous losses

(e.g. ammonia) to the atmosphere are very small.

4. There is a wide range of manufactured nitrogen fertilisers used in the world. The characteristics of each material are summarised in the report. Although a wide range of N materials have been identified and considered, only three ‘Good Prospect’ alternatives to AN have been identified for use as straight N fertilisers – urea, calcium ammonium nitrate (CAN) and urea ammonium nitrate (UAN) solution. Many materials have been identified as ‘No Prospect’ due to reasons such as environmental impact (e.g. ammonia emissions from ammonium carbonate); health and safety (e.g. anhydrous ammonia); slow release of N (e.g. IBDU) or low concentration of N (e.g. calcium nitrate, 15.5% N). Other materials have been identified as ‘Possible Prospect’ pending further investigation.

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5. The main difference between AN (33.5-34.5% N) and CAN (26-28% N) is that CAN contains a limestone filler and thus has a lower concentration of N. There is thus more bulk to handle which can add to the logistical problems of transport and handling for both manufacturers and farmers. This can add to the overall costs. Otherwise, little difference is expected between the agronomic efficacy or environmental impact of CAN as compared to AN.

6. Although urea is already used as a straight N fertiliser in the UK, there are several potential problems associated with its use as a complete replacement for AN.

i) There is no existing UK manufacturing plant for urea; all current supplies are imported with supplies and costs influenced by world market forces. Based on free-market economics, it is doubtful if manufacturers would build new urea production plant in the UK.

ii) Urea is strongly hygroscopic (absorbs moisture) and is thus much less suitable than AN for the production of high N compound fertilisers. Urea-based compounds tend to have a short shelf life and potentially serious caking problems.

iii) Urea has a lower bulk density than AN and can be more difficult than AN to spread evenly especially over typical wide bout widths (24 metres) used on many arable farms.

iv) The way that urea behaves in soil can result in large losses of ammonia gas following application to land. Ammonia pollution of the atmosphere is a major environmental concern and the UK is a signature to the Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (Gothenburg Protocol) of the UNECE Convention on Long-Range Transboundary Air Pollution.

7. Methods may be available to mitigate ammonia emissions following use of urea. For instance, addition of a urease inhibitor as part of the urea production process is suggested as an effective mitigation method. A commercial urease inhibitor called ‘Agrotain’ is available but not currently used in the UK. Current knowledge on mitigation methods is reported in the NT2601 project report ‘Evaluation of urea-based nitrogen fertilisers’. The effectiveness of Agrotain is being investigated in the Defra NT2603 research project being carried out in 2003 cropping season.

8. Some N-containing fertilisers also contain other nutrients (e.g. di-ammonium phosphate, DAP) but these materials could still replace some of the AN currently used as a straight N fertiliser. However, use of materials such as DAP (18% N, 45% phosphate) would only be a partial solution because the maximum application rate would be severely limited by its phosphate content. Many fields have a zero phosphate requirement and DAP would not be suitable for use on these fields else the risk of phosphorus pollution of water would increase.

9. Several commercially available N-containing materials also contain sulphur (S) - e.g. ammonium sulphate (AS), ammonium sulphate nitrate (ASN), urea ammonium sulphate (UAS). Because of the increase in sulphur deficiency, the use of NS fertilisers is increasing and these materials may have wider scope for use as a primary source of N.

10. Organic manures (farm manures, sewage sludges, composts and other industrial ‘wastes’) contain significant quantities of nitrogen. Individual applications to land can provide useful quantities of crop available N which is equally available for crop uptake as fertiliser N. However, the national annual supply of manure crop available N (equivalent to fertiliser N) is only around 10% of the consumption of manufactured fertiliser N. Although improved manure management practices (e.g. spring application of manures) could increase this to around 15%, this source of N will not be able to replace the current national use of fertiliser N. Additionally:-

i) Farm manures are mostly produced in areas of livestock production. Areas without livestock (e.g. Eastern England) have little scope to acquire supplies of organic manures without lengthy road transport, which would be prohibitively costly, add to problems of road congestion and potentially conflict with the national management of bio-security risks.

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ii) Regulatory controls (e.g. the Defra Water Code, Nitrate Vulnerable Zones) limit the amounts of organic manures that may be applied to individual field areas. The current limit of 250kg/ha of total N (up to c.125kg/ha crop available N depending on manure type and application circumstances) will not usually allow the crop N requirement to be fully met from manure N.

iii) Since the fertiliser N value of organic manure applications cannot be estimated as precisely as the nitrogen supplied from manufactured fertiliser, current Defra recommendations state that no more than 50-60% of the crop N requirement should be met using manure N.

11. Legumes can ‘fix’ atmospheric N and are another potential source of N. However in arable rotations, peas and beans are the only significant leguminous crops that require no inputs of nitrogen fertilisers; these crops cover less than 5% of the cropped area. Without specific breeding developments in the major arable crops such as cereals, there is no prospect for an increased contribution of ‘fixed’ nitrogen by leguminous crops in arable rotations. In grassland systems, clover is used in some swards and provides useful quantities of fixed nitrogen that reduces the need for manufactured nitrogen fertilisers. However, grass/clover swards will not generally support intensive production systems, notably intensive dairy production. Less than 10% of dairy swards have a significant clover content. It is recognised though that there is considerable potential for grassland farmers to increase reliance on clover as a source of nitrogen.

12. In conclusion, there is no candidate replacement for AN that is problem-free. No fundamentally new materials under development have been identified. Urea-based fertiliser materials offer the best prospective alternative to AN-based materials, but there are significant problems that would need resolution before the UK supply of nitrogen fertilisers could be based on urea as a substitute for AN.

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4. Introduction and Acknowledgements

Nitrogen (N) is an essential nutrient for economic agricultural production and correct use can result in large increases in crop yields. It is one of the most cost-effective inputs used by farmers. However, nitrogen can be lost to the water and air environments where it can cause pollution. Voluntary adoption of good agricultural practices for using nitrogen will minimise the risk of causing environmental pollution though various regulatory controls are in place which farmers must comply with (e.g. Nitrate Vulnerable Zone Action Programme rules).

There are 4 main sources of available nitrogen for use by agricultural crops:-

1. Natural sourcesThese include:- small amounts of nitrogen deposited to land from the atmosphere nitrogen ‘mineralised’ due to decomposition of soil organic matter atmospheric nitrogen ‘fixed’ by leguminous crops such as peas, beans and clover

2. Residues from agricultural crop managementResidues will depend on the nature of the previous cropping, the use of nitrogen fertilisers and organic manures, and losses of nitrogen by leaching and other loss processes. The soil nitrogen supply (SNS) will usually be high if grass has been ploughed out and/or where lots of organic manures have been used; it will be low under continuous arable cropping especially on sandy or shallow soils.

3. Manufactured nitrogen fertilisersFertiliser will normally be used to make up the balance between the contribution of nitrogen from all other sources of nitrogen and the crop's nitrogen requirement. Defra’s recommendations for the use of fertiliser N for agricultural and horticultural crops are given in ‘Fertiliser Recommendations (RB209)’ (Defra, 2000).

4. Organic manuresFarm manures (e.g. slurries, farmyard manure), sewage sludges (biosolids), composts and other industrial ‘wastes’ are applied to agricultural land. These materials contain a range of nutrients, including nitrogen, which can provide some or all of the crop’s nutrient requirement.

In this report, Section 5 outlines the range of requirements needed for a good nitrogen fertiliser product. Section 6 gives a complete list of all manufactured nitrogen fertiliser materials that are currently used in temperate agriculture or have been considered for use at some time. The list is exhaustive and includes many materials which are not expected to be suitable for use under UK conditions; a summary is given at the start of the section. The basic chemical and physical properties, manufacturing process, worldwide use, actual and potential uses in UK agriculture, and constraints are summarised for each manufactured nitrogen fertiliser material.

Each material is classified as a ‘No Prospect’, ‘Possible Prospect’ or ‘Good Prospect’ for their potential i) as a straight nitrogen fertiliser ii) increased use in the production of compound fertilisers. A few materials can immediately be classified as potential ‘Good Prospects’ if a replacement for ammonium nitrate (AN) is required. Many materials are classified as ‘No Prospect’ due to the reasons stated. The majority of materials are classified as ‘Possible Prospects’ though many if these have severe constraints. During the course of the NT26 programme, the classifications of these materials will change as information is obtained and evaluated.

Details of the current supply and use of different N fertiliser materials in UK agriculture, and international practice and experience on the use of N fertiliser materials are given in other NT2601 reports.

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Section 7 describes the potential and constraints of different materials for the production of N-containing compound fertilisers.

Section 8 summarises the nitrogen value of organic manures that are either produced on UK farms from farm livestock, or are available to farmers from other sources such as sewage sludges (biosolids), composts and other industrial ‘waste’ materials. Section 9 considers the supply of nitrogen from leguminous crops that fix atmospheric N.

Section 9 discusses the ‘fixing’ of atmospheric N by leguminous crops.

Acknowledgements

The information in this report has been obtained from published and unpublished information, including discussions with:

Fertiliser Manufacturers Association (FMA) Hydro Agri (UK) Ltd Kemira Growhow (UK) Ltd Terra Nitrogen (UK) Ltd The Fertiliser Institute (TFI), America International Fertiliser Development Centre (IFDC), Muscle Shoals, America

The factual content of the report and appraisal of the information has been verified by the Fertiliser Manufacturers Association (FMA).

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5. Requirements of a good nitrogen fertiliser product

There are a range of requirements that need to be met for any fertiliser product in order for it to be technically, practically and commercially viable.

1. Security of supply of raw materials

All fertiliser products are manufactured from raw materials which are either supplied from within the UK or imported. Secure and reliable supplies of raw materials are preferred. Imported raw materials will be more subject to the vagaries of world trade, fluctuations in world prices and reliability of supply.

2. Reliability of supply to end-users

Fertiliser supply must match demand – most nitrogen fertilisers are used in the spring. Farmers need to equip themselves to handle and spread one or more fertiliser types that they plan to use. Minor changes (e.g. changes from one form of solid fertiliser to another) can usually be managed, but large, unexpected or last minute changes in the availability of expected fertiliser products could cause serious difficulties.

3. Cost of manufacture

Cost of manufacture is critical for the producer whose fertiliser must compete with other supplies, particularly imported materials. Fertiliser N production costs are dominated by feedstock and energy prices. Manufacturing cost is also important for the farmer as it is an important determinant of the price of N. Around 10–35% of the variable costs of crop production are accounted for by fertilisers. If the manufacturing process (per kg of nutrient) for a particular product is much higher than comparable alternatives, it is likely to preclude widespread use of the product though use in speciality markets may still be economic.

4. Health and safety Health and safety are important considerations during manufacture, transport and on-farm use and for potential misuse of products.

5. Ability to produce compound fertilisers (complex and/or blended fertilisers)

A large proportion of the solid compound fertilisers used in the UK are produced by blending and a suitable straight N material is necessary for this. An N fertiliser that is suitable for use both as a straight fertiliser and for the production of compound fertilisers is a significant advantage to the manufacturer. The production of high N compounds (e.g. 20–10–10 or 25–5–5) requires use of an N fertiliser with a high N concentration. Some N materials are not suitable for the production of either complex and/or blended products due to their chemical or physical characteristics.

6. Storage characteristics

Solid fertilisers must be capable of being stored for up to 10 months before use. This storage period may be spent largely on farm and through the UK winter under conditions of high humidity. Fertiliser properties that influence storage characteristics are hygroscopicity (propensity to absorb moisture), chemical nature, chemical compatibility with any other components of a blend and granule hardness. Susceptibility to caking will adversely affect storage characteristics. Suspension fertilisers have a very limited storage life since the suspended material will solidify in store.

7. Transport characteristics

Transport represents the major component of national distribution costs. Virtually all fertiliser in the UK is transported by lorry and payload is important; the N concentration and bulk density of the fertiliser determine the amount of nitrogen that a lorry can carry. Similarly, the amount of bulk to store, handle and

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spread has a large effect on farm costs. Bulk density and N concentration determine the amount of N held in a spreader hopper and thus the downtime needed for re-filling. More time and machinery capacity will be needed to handle and spread N fertilisers that have a low bulk density and/or a low N concentration. Fertilisers with a high N concentration and high bulk density are desirable to minimise both transport and on-farm handling costs.

8. In-field spreading characteristics

All fertiliser products, especially those high in nitrogen, must be able to be spread accurately and evenly over the crop under a range of weather conditions (e.g. wind) and with a range of fertiliser spreader types. Accurate spreading is needed to maximise the economic return and minimise the environmental impact from the use of fertilisers. Fertiliser must have consistent physical properties to allow spreader calibration. The true density of the fertiliser material and its particle size determine the particle weight which in turn affects the spreading properties. Light particles are more affected by cross-winds during spreading. Dustiness or caking affect flow of fertiliser through the spreader and the accuracy of spread.

9. Crop response In UK agriculture, crop production is critically dependent on the use of manufactured fertilisers, especially nitrogen. Typically, the yield of a crop (with the exception of legumes) will halve if fertiliser nitrogen is not applied. Fertiliser products should be suitable for use under a range of crop, soil and climatic conditions so that required amounts of nitrogen can be supplied when needed to meet the N requirements of different crops. The efficiency of use of the fertiliser N should be as high as possible.

10. Crop quality and livestock health

Fertiliser products must be able to produce agricultural foods that are acceptable to the consumer and have no adverse effects on livestock or human health.

11. Environmental impact

Some fertiliser materials are prone to the loss of nitrogen to the water and/or air environments by processes such as volatilisation of ammonia-N and leaching of nitrate-N. Use of the fertiliser should have no significant adverse effects on the long-term sustainability of the soil resource.

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6. Materials for manufacturing nitrogen fertilisers

6.1 Ammonium carbonate (AC)

6.1.1 IntroductionAmmonium carbonate is used as a fertiliser in the Far East, especially in China. It is cheap to manufacture.

6.1.2 ManufactureAmmonia is reacted with carbon dioxide to form ammonium carbonate. This part of the process is the same as that for urea. However, for urea, there is a dehydration phase which is omitted for ammonium carbonate.

6.1.3 UsesAs a straight N fertiliser for top-dressing.

6.1.4 Nutrient concentrationAround 25% N.

6.1.5 Main chemical form of nutrientsAmmonium carbonate (NH4)2 CO3. Readily soluble in water.

6.1.6 Physical characteristicsWhite crystalline particles. It has a short shelf life because it loses free-flowing properties within a few days and tends to cake.

6.1.7 Behaviour in soilAC easily breaks down into ammonium-N and carbon dioxide on contact with soil. The N is in an immediately available form but subject to significant losses as ammonia gas.

6.1.8 Other factorsUse of AC is prohibited under the Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (Gothenburg Protocol) of the UNECE Convention on Long-Range Transboundary Air Pollution. Full details can be found at www.unece.org/env/lrtap/protocol/99multi_a9.htm.

6.1.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. Use of AC is banned under the UNECE Gothenburg Protocol.

6.1.10 Potential for use in the production of compound fertilisers in the UKNo prospect. Use of AC is banned under the UNECE Gothenburg Protocol.

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6.2 Ammonium chloride (ACl)

6.2.1 IntroductionACl is used in India and the Far East as a nitrogen fertiliser, mainly for rice, coconut, kiwifruit and oil palm. It has not been used as a fertiliser in Europe. About two thirds of production capacity is in Japan, the remainder in China and India.

6.2.2 ManufactureACl is a by-product of soda ash production. Alternatively, ammonia can be reacted with hydrochloric acid.

6.2.3 UsesAs a solid source of nitrogen and chloride for soil application.

6.2.4 Nutrient concentrationUsually 25% N but can be 26% N.

6.2.5 Main chemical form of nutrientsAmmonium chloride (NH4Cl). The nitrogen is water-soluble. Solubility in water at 200C is 2,838 g/l.

6.2.6 Physical characteristicsWhite crystalline solid; granular formulations are produced. ACl is prone to caking.

6.2.7 Behaviour in soilThe nitrogen is in the ammonium form but converts rapidly in the soil to the nitrate form which is at risk of leaching.

6.2.8 Other factorsContact with oxidising agents should be avoided. The chloride input could be a problem as high concentrations can cause damage to germinating seeds and young plants.

6.2.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. Unlikely to be adequate current capacity or supplies to be a major contributor to the UK fertiliser N market. The chloride content could be a problem for certain crops (the amount of chloride applied would be 2.5 times the amount of nitrogen).

6.2.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. ACl is present in some commercial complex fertilisers due to chemical reactions with MOP or AN. However, it is not currently used as a raw material for the production of compound fertilisers. Due to its crystalline nature, it would not be suitable for blending.

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6.3 Ammonium nitrate (AN)

6.3.1 IntroductionAN has been the main form of straight N fertiliser used in Great Britain since the 1960s, but UK use is only 0.7% of total world straight N use which is dominated by urea. Total UK use of AN represents about 7% of total world AN use. AN is a major source of straight N in some other European countries (e.g. France) though in other countries, the main source is CAN or urea.

6.3.2 ManufactureAmmonia is reacted with nitric acid to give a concentrated AN solution. Water content is then reduced and the molten AN is solidified by granulation or prilling. Caking is minimised by the use of additives or coating powders. In the UK, there is AN manufacturing plant at Teeside and Severnside, and at Ince, Chester. In 2000, the plant at Immingham, Humberside was closed. Significant quantities of AN are imported annually into Britain mainly from Eastern Europe, Russia and the EU.

Carbon dioxide (as liquid gas and dry ice) and nitric acid (used in manufacturing textiles, explosives, industrial chemicals and the metals industry) are by-products of the UK production of AN. Around 90% of the UK market for carbon dioxide (total demand is over 570,000t per year) and 100% of the nitric acid market is provided from the 3 existing AN manufacturing plants (source: industry sources).

6.3.3 UsesThe main use of AN is as a solid straight N source for soil application. It is also used as a component of high N complex, blended and fluid fertilisers. In solid fertilisers, AN can be successfully mixed with AS, DAP, MAP and MOP to produce compound fertilisers. Solid AN/urea complexes are not practical because they rapidly turn sticky due to hygroscopicity; solid AN/urea blends have been made experimentally but not commercially because of chemical reactions which occur from intimate contact which leads to loss of physical structure, caking and a very short shelf life. However, mixtures of AN and urea are dissolved in water to manufacture high N fluid fertilisers such as UAN. AN is also used in horticultural liquid feeds, fertigation and growing media; it can be used in hydroponics but restricted by the proportion of nitrogen in the ammonium form.

6.3.4 Nutrient concentrationDomestic prilled AN contains 34.5% N, granular materials may contain 33.5-34.5% N. Imported prills often contain 33.5% N.

6.3.5 Main chemical form of nutrientsAmmonium nitrate (NH4NO3). Water soluble (1,920g/l at 20oC).

6.3.6 Physical characteristicsAN prills are shiny white spheres with a median diameter of around 2.5mm. Some non-EU material has a median size of 1.6-2.0mm. Granular products are larger, with a median diameter of around 3.5mm when made by the fluidised bed granulation method. The colour of 33.5% N products tends to be slightly off-white. The bulk density of prilled AN is around 0.99kg/l, around 0.94kg/l for granular AN and can be as low as 0.85kg/l for some other imported prilled AN. Particle crushing strength (1.5-1.9kg) is high relative to some other N fertilisers.

6.3.7 Transport, health and safety, handling and storageAN is well known to be potentially explosive (e.g. Shah, 1996) and is classified as an oxidising substance Class 5, Division 5.1 in the UN classification system. It is classified as a Group 1 product in the scheme of FMA Product Safety Data Sheets. In spite of considerable study, no modifications to the method or formulation of fertiliser grade AN has been successful in mitigating the risk of detonation. So called ‘technical grade’ AN is manufactured for the explosives market (e.g. as used by quarry operators) – this grade is of lower density than fertiliser grade AN.

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The storage of large quantities is also controlled throughout the EU. In the UK, this is covered by the Control of Major Accident Hazards (COMAH) Regulations 1999. Under the Notification of Installations Handling Hazardous Substances (Amendment) Regulations 2002 (SI 2979), the HSE must be informed where 150t or more of ‘ammonium nitrate or mixtures containing ammonium nitrate where the nitrogen content exceeds 15.75% of the mixture by weight’ is to be stored. In the principal 1982 Regulations, effective until 31 December 2002, notification was required for quantities exceeding 500t. Guidance on storage and handling is given by the Health and Safety Executive (HSE, 2001).

There are special requirements for transport throughout the EU and in the UK these are given in the Carriage of Dangerous Goods by Road Regulations (1996). Transport of AN is covered under The Control of Major Accident Hazards Regulations 1999.

The Ammonium Nitrate Materials (High Nitrogen Content) Safety Regulations 2003 (SI 1082, effective from May 2003) require both home-produced AN and AN proposed for import into the UK, to undergo a standard detonation test before use is permitted.

6.3.8 Behaviour in soilNitrate-N is immediately available for plant uptake but may leach to water. Ammonium-N can also be taken up by plants but most is rapidly nitrified to nitrate-N. AN is the standard for agronomic effectiveness against which other products are judged.

6.3.9 Other factorsTo be accepted as an EU grade fertiliser, AN must have additional characteristics covering, for example, additives, oil retention, pH, and size and it may only be supplied to the end user in packaged form.

6.3.10 Potential for use as a straight nitrogen fertiliser in the UKUnless regulatory controls are introduced, AN is likely to continue as the main form of straight N fertiliser used in Britain and as a major component in the manufacture of complex, blended and fluid fertilisers. It is a concentrated form of N with good storage and spreading characteristics, and has proven agronomic characteristics. The main potential environmental impact is from leaching of nitrate to watercourses. The current British fertiliser manufacturing industry and supply chain is based on AN.

6.3.11 Potential for use in the production of compound fertilisers in the UKUnless regulatory controls are introduced, AN is likely to continue as a major component in the manufacture of complex, blended and fluid fertilisers. Use of AN allows the production of compound fertilisers with a high N concentration.

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6.4 Ammonium polyphosphate (APP)

6.4.1 IntroductionAPP is a common source of nitrogen and phosphate in the UK.

6.4.2 ManufactureWhen heated, phosphoric acid condenses, losing water to form chain molecules of super-phosphoric acid which contains around 69% P2O5. The length of the chain can vary with process condition. The super-phosphoric acid is then reacted with ammonia to form APP. The material sometimes contains a significant proportion of orthophosphate.

6.4.3 UsesAPP is used as a source of nitrogen and phosphate in fluid fertilisers having advantages of solubility and nutrient concentration.

6.4.4 Nutrient concentrationThe crystalline solid usually contains 13% N and 69% P2O5 and is readily soluble. APP is usually supplied to solution fertiliser manufacturers as an aqueous solution containing 10-11% N and 34-37% P2O5.

6.4.5 Main chemical form of nutrientsTri-ammonium pyrophosphate ((NH4)3 HP2O7).

6.4.6 Physical characteristicsProduced as a crystalline solid but invariably supplied as a clear green solution containing 10% N and 34% P2O5 with a density of 1.4kg/l.

6.4.7 Behaviour in soilAmmonium-N can be taken up by plants but most is rapidly nitrified to nitrate-N before uptake. Polyphosphate is rapidly converted to orthophosphate after application. This usually happens in a few days. The effectiveness of AP as a phosphate and nitrogen source is similar to that of mono-ammonium phosphate.

6.4.8 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. APP contains over 3 times as much phosphate as nitrogen, which would preclude its widespread use without adding to the phosphate pollution problem. Additionally, there is no known technology for manufacturing APP as prills or granules.

6.4.9 Potential for use in the production of compound fertilisers in the UKNo prospect for solid compound fertilisers. APP is likely to continue to be a source of nitrogen and phosphate in the manufacture of fluid fertilisers.

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6.5 Ammonium sulphate (AS)

6.5.1 IntroductionAS was at one time a major straight N agricultural fertiliser in Europe. Use in agriculture declined with the advent of more concentrated AN and CAN and until recently, AS was a minor product. The increasing need for sulphur in fertilisers has renewed interest in AS both as a component of compound fertilisers and as a straight N in its own right.

6.5.2 ManufactureAS is available as a by-product of non-fertiliser related industrial processes (principally manufacture of caprolactam and recovery from coke oven gas) or as specially manufactured fertiliser. AS fertiliser is manufactured by reacting ammonia with sulphuric acid and then removing water from the resulting solution. AS is difficult to granulate but methods have been developed; granular AS is manufactured in the US and Europe. Crystalline AS cannot be blended due to it’s small particle size and could not be added to the AN melt in current domestic prilling. Granular mixes of urea/AS are produced in the Netherlands and used in S Ireland.

6.5.3 UsesAs a solid source of nitrogen and sulphur for soil application, in liquid feeds, fertigation or growing media and for turf. The ammonium form of the nitrogen restricts use in hydroponics. AS can be mixed with urea to produce a blend or complex (UAS) fertiliser, or blended with CAN. Only granular, not crystalline, AS can be blended successfully.

6.5.4 Nutrient concentrationUsually 21% N and 60% SO3 (24% S).

6.5.5 Main chemical form of nutrientsAmmonium sulphate ((NH4)2 SO4). Both nitrogen and sulphur are water-soluble. Solubility in water at 200C is 750g/l.

6.5.6 Physical characteristicsBy-product AS is crystalline, brownish in colour, with a relatively small particle size (96-100% in the range 0.2 to 2.0mm), similar to that of granulated sugar. When manufactured as a fertiliser, AS is in the form of off-white granules with a median size of around 2.6mm. Bulk density of by-product around 1.76kg/l, granular products around 0.90kg/l.

6.5.7 Behaviour in soilBoth nitrogen and sulphur are immediately available to crops. The nitrogen is as effective as that in AN. The sulphur is immediately available to the plant. The nitrogen converts rapidly in the soil to the nitrate form when it is at risk of leaching as is the sulphate-S. AS has a greater acidifying effect of soils than some other fertilisers.

6.5.8 Other factorsWhen used for soil application as the sole source of nitrogen, the high sulphur to nitrogen ratio should be borne in mind. Field crops rarely require more than 70 kg SO3/ha.

6.5.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. AS has been used in the past as a major straight N fertiliser but it’s low N concentration (21%) is a negative factor for storage and transport. The inclusion of sulphur in AS should be a minor problem since sulphur is a required nutrient in many cropping situations and has no known negative impact on the environment. However, the sulphur content could be a problem if AS were used as the only N source for grassland prone to copper deficiency in livestock.

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6.5.10 Potential for use in the production of compound fertilisers in the UKGood prospect. AS is commercially complexed with AN or urea (to produce UAS); granular AS is commercially blended with DAP, MOP and/or SOP; mixtures with CAN, TSP, SSP and/or Kieserite are feasible.

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6.6 Ammonium sulphate nitrate (ASN)

6.6.1 IntroductionASN is a mixture of AN and AS produced as a solid complex fertiliser. Once common as a blend component, use in Europe is now more limited.

6.6.2 ManufacturePowdered AS is mixed with molten AN before granulation or, occasionally, prilling.

6.6.3 Uses

6.6.4 Nutrient concentrationUsually 25-26%N and 38-42% SO3 (15 to 17% S); should contain at least 5% nitric N.

6.6.5 Main chemical form of nutrientsAmmonium nitrate (NH4NO3) and ammonium sulphate ((NH4)2SO4). Both nitrogen and sulphur are water-soluble.

6.6.6 Physical characteristicsUsually in the form of off-white granules with a median size of 2.7-3.2mm. Granular products have a bulk density of around 0.98kg/l.

6.6.7 Behaviour in soilThe AN converts rapidly in the soil to the nitrate form when it is at risk of leaching as is the sulphate-S. The sulphur is immediately available to the crop. ASN, like AN, AS and urea, is suitable for application in spring to arable crops and in spring and summer to grassland. However, the material is usually used as a blend component. No special requirements for transport or storage.

6.6.8 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. ASN requires AN in the manufacturing process but has a reasonably high N concentration (25-26%).

6.6.9 Potential for use in the production of compound fertilisers in the UKGood prospect. ASN is commercially blended with DAP; it is feasible to blend with CAN, urea, MOP, SOP and Kieserite, but not TSP or SSP.

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6.7 Ammonium thiosulphate (ATS)

6.7.1 IntroductionATS is a common source of sulphur in fluid fertilisers, offering advantages of concentration and compatibility over AS.

6.7.2 ManufactureSulphur dioxide and ammonia are reacted and the product dissolved in water. Both sulphur dioxide and ammonia are by-products of crude oil processing.

6.7.3 UsesAs a sulphur source in solution fertilisers.

6.7.4 Nutrient content19% N and 108% SO3 (43% S) in the solid material, around 12% N and 65% SO3 (26% S) in solution.

6.7.5 Main chemical form of nutrientsAmmonium thiosulphate ((NH4)2 S2O3). The nitrogen and sulphur are soluble in water.

6.7.6 Physical characteristicsWhite crystalline solid that is dissolved in water when included in fluid fertilisers or clear green solution. Typical bulk density is 1.32kg/l for the 12% N: 65% SO3 solution.

6.7.7 Behaviour in soilOn application the sulphur forms colloidal S and sulphate ions. Colloidal S oxidises to sulphate almost immediately. The nitrogen and sulphur in ATS therefore are as effective as the same nutrients in AS. The material is mainly when incorporated in fluid fertilisers but a 11% N, 65% SO3 solution is available for direct application. ATS is sometimes added to foliar urea.

6.7.8 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. ATS is of low N concentration and is mainly a source of sulphur.

6.7.9 Potential for use in the production of compound fertilisers in the UKNo prospect for solid compound fertilisers. ATS is likely to continue to be a source of nitrogen and sulphur in the manufacture of fluid fertilisers.

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6.8 Anhydrous ammonia (AnA)

6.8.1 IntroductionAnA was last used in the UK in the 1970s when Calor was the supplier. Use stopped when Calor ceased supply. Although cheaper to produce than AN, there were increased costs for local storage, handling and application.

6.8.2 ManufactureA gaseous hydrogen source (now invariably natural gas) is reacted with gaseous nitrogen under high pressure and in the presence of a catalyst. The ammonia produced is compressed before sale as anhydrous ammonia.

6.8.3 UsesAs a straight nitrogen fertiliser.

6.8.4 Nutrient concentration82% N.

6.8.5 Main chemical form of nutrientsAmmonia gas (NH3) under pressure.

6.8.6 Physical characteristicsGas in pressurised containers up to 950 kilopascals.

6.8.7 Behaviour in soilAmmonia is trapped when injected into moist soils, though escape of ammonia gas is difficult to prevent completely. Ammonium-N is then nitrified to nitrate-N for plant uptake.

6.8.8 Other factors AnA must be injected into the soil to minimise losses of ammonia gas using specialised equipment usually operated by contractors. AnA is a hazardous material to transport and handle on farms requiring specialised equipment. Such equipment is not currently available within UK agriculture. When used in the UK, operators had to follow a strict safety code.

6.8.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. The hazardous nature of AnA, the need for specialised handling and application equipment, direct injection into soil and the high risk of ammonia emission preclude this material as a realistic prospect as a major straight N fertiliser for use in the UK.

6.8.10 Potential for use in the production of compound fertilisers in the UKNo prospect. Compound fertilisers in gaseous form cannot be produced.

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6.9 Aqueous ammonia (AqA)

6.9.1 IntroductionAqA has been used in the UK in the past but is no longer used; it’s place being taken by the more concentrated UAN solution.

6.9.2 ManufactureAqA is manufactured by dissolving ammonia gas in water to produce a clear solution.

6.9.3 UsesAs a straight nitrogen fertiliser injected into the soil.

6.9.4 Nutrient concentration22-29% N, typically 27% N.

6.9.5 Main chemical form of nutrientsAmmonia (NH3) dissolved in water.

6.9.6 Physical characteristicsClear solution under slight pressure up to 100 kilopascals.

6.9.7 Behaviour in soilAmmonium-N is nitrified to nitrate-N for plant uptake. There is a risk of loss of ammonia gas though this is less than for AnA.

6.9.8 Other factorsBecause it is a pressurised solution, AqA must be injected into the soil, usually by specialist contractors. This is a slow operation. Although less hazardous than AnA, strict safety precautions are needed.

6.9.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. Although less hazardous than AnA, the need for specialised handling and application equipment, direct injection into soil preclude this material as a realistic prospect as a major straight N fertiliser for use in the UK.

6.9.10 Potential for use in the production of compound fertilisers in the UKNo prospect. Other nutrients cannot be mixed with pressurised AqA.

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6.10 Calcium ammonium nitrate (CAN)

6.10.1 IntroductionCAN is a major straight N product in continental Europe but is used mainly in compound fertilisers in the UK. Worldwide CAN use is 7% of total N use; European CAN use is 24% of total N use.

6.10.2 ManufactureMolten AN is produced by reacting ammonia with nitric acid. Finely divided calcium carbonate (limestone) is introduced into the melt which is then solidified by granulation or prilling. The final product is coated with an anti-caking agent.

6.10.3 UsesAs a straight N source for soil application and occasionally as an additive to growing media. CAN can be blended with a range of other fertiliser materials including AN, AS, DAP, MAP, TSP and MOP. All of these blends are currently produced commercially in the UK and have good storage and handling characteristics. However, such blended products will have only medium or low concentrations of N - usually no more than 22% N (22.5.5 compounds would be possible using CAN, but not 25.5.5 or 27.5.5 which are possible using AN).

6.10.4 Nutrient concentrationUsually 26-28%N.

6.10.5 Main chemical form of nutrients.Ammonium nitrate (NH4NO3). Water soluble.

6.10.6 Physical characteristicsOff-white granules or prill. Median size around 2.8mm (granules) and 2.6mm (prills). Bulk density around 1.05kg/l for both granular and prilled materials.

6.10.7 Behaviour in soilAs the nitrogen source is AN, effectiveness is the same. Inclusion of the calcium carbonate in CAN does not affect availability of the nitrogen but can compensate partially for the soil acidifying effect of AN.

6.10.8 Other factorsWhere less than 28% N, CAN is not classed as an oxidising agent so there are no special transport or storage requirements. Because of its lower concentration of N, a change from AN to CAN as the main source of straight N fertiliser would incur significant extra costs of transport.

CAN (27% N) contains c.200-220kg of limestone per tonne of CAN product (0.8kg per kg N). Complete substitution of CAN for the current 500,000t annual use of N as AN in the UK would require the supply of an extra 400,000t of limestone. This is a small proportion of the current annual use of limestone for all purposes, including construction (c.95 million tonnes; pers.comm DTI), or c.15% of the current annual use of limestone or chalk for agricultural liming purposes (c.3 million tonnes). In part, the liming value of CAN should reduce the need for agricultural lime but only where CAN is used on soils which are potentially acid.

6.10.9 Potential for use as a straight nitrogen fertiliser in the UKGood prospect. CAN is a widely used source of straight N in Europe.

6.10.10 Potential for use in the production of compound fertilisers in the UKGood prospect. CAN is a widely used source of straight N used in the production of compound fertilisers.

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6.11 Calcium cyanamide (CC)

6.11.1 IntroductionCC is one of the oldest manufactured N fertilisers which is still used in limited amounts, mainly in Germany.

6.11.2 ManufactureLime is heated with coke to produce calcium carbide. On heating and exposure to atmospheric nitrogen, this converts to CC. CC is manufactured mainly in Germany.

6.11.3 UsesA source of straight N fertiliser mainly to high value vegetables. Not used in compound fertiliser manufacture.

6.11.4 Nutrient concentration21-22% N. Must contain at least 18% N of which a minimum of 75% must be in the form of CC.

6.11.5 Main chemical form of nutrientsCalcium cyanamide (Ca (CN)2). May contain small amounts of calcium oxide and ammonium salts. Soluble in water but decomposes.

6.11.6 Physical characteristicsBlack granules of 1-6mm size range.

6.11.7 Behaviour in soilEnzyme activity in the soil converts the cyanic-N to ammonium-N which is nitrified to nitrate-N which is available to plants. This conversion occurs rapidly at soil temperatures when plants are growing.

6.11.8 Other factorsIn powdered form can be used as a herbicide. Intermediate reaction products are toxic.

6.11.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. Production costs are high and storage properties are poor.

6.11.10 Potential for use in the production of compound fertilisers in the UKNo prospect. CC is very hygroscopic and cannot be used in the production of compound fertilisers.

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6.12 Calcium nitrate (CN)

6.12.1 IntroductionCN is usually regarded as a specialist fertiliser for protected horticulture use, though a solid field grade is available for use as a source of straight N for field crops.

6.12.2 ManufactureCN is a co-product in the manufacture of nitro-phosphate fertilisers. Phosphate rock is reacted with nitric acid to produce NP fertilisers and CN. There is usually a small AN content in the product. The molten CN is solidified by prilling. Where intended for field application, the final product is coated with an anti-caking agent. CN is currently only manufactured in significant quantities in Norway and Germany.

6.12.3 UsesAs a source of nitrogen and soluble calcium for soil application, in hydroponics, liquid feeds or fertigation, for foliar application or as an additive to growing media. CN can be mixed with AN or AS to produce complex fertilisers, but CN is not suitable for blending due to its crystalline nature. Blends of CN and urea are produced commercially and used in Ireland and Spain.

6.12.4 Nutrient concentrationField grade CN is 15.5% N (usually 14.5% nitric-N and 1.0% ammonium-N) and 19% soluble calcium (Ca). CN for protected horticulture is usually 12%N, 17%Ca.

6.12.5 Main chemical form of nutrientsCalcium nitrate (CaNO3) and ammonium nitrate (NH4NO3) in field grade CN. This is 5Ca(NO3)2. NH4NO3.10H2O. For protected horticulture, CN is as Ca(NO3)2.4H2O; both the nitrogen and the calcium are water-soluble. Solubility in water at 200C is 1,290g/l.

6.12.6 Physical characteristicsThe greyish prills are smaller than those of AN with a median size of around 2.3mm. Field grade material for soil application is coated with anti-caking agents. Material for use in hydroponics is not coated. Bulk density is around 0.9kg/l. Both materials are highly hygroscopic.

6.12.7 Behaviour in soilBoth nitrate-N and calcium are immediately available for plant uptake.

6.12.8 Other factorsThe low nitrogen concentration is an issue when CN is used for outdoor crops. However, the product has a role where there is a need for soluble calcium, for example on vegetable crops. Unlike AN, urea, AS and CAN, CN does not acidify the soil.

6.12.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. CN is used for field scale applications of straight N but it is currently an expensive source of N. The low N concentration (15.5%) is a problem. Production volume is limited since it is a by-product of nitrophosphate production. Dedicated production might be possible but this is unlikely to be economic.

6.12.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. The low N concentration would preclude production of high N compounds.

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6.13 Chilean potassic nitrate (Chilean KN)See also Potassium nitrate and Sodium nitrate

6.13.1 IntroductionChilean KN is a naturally occurring 2:1 mixture (approx.) of sodium nitrate and potassium nitrate.

6.13.2 Manufacturing process Naturally occurring deposits in South America are mined, crushed and screened. Restricted volumes are available.

6.13.3 UsesAs a nitrogen and potassium source for some higher value crops such as sugarbeet.

6.13.4 Nutrient concentration15% N, 14% K2O and 24% Na2O.

6.13.5 Main chemical form of nutrientsSodium nitrate (NaNO3) and potassium nitrate (KNO3). The nitrogen, sodium and potash are water-soluble.

6.13.6 Physical characteristicsCrystalline or granular solid material. Granules have a median size of around 2.3mm. Typical bulk density is around 1.05kg/l for crystalline and 1.15kg/l for granular material.

6.13.7 Behaviour in soilChilean KN is a fully soluble and readily available source of nitrogen, sodium and potash. It should not be used on sodium sensitive crops such as potatoes. There are no special requirements for transport or storage.

6.13.8 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. The low N concentration together with the content of K and Na mean that Chilean KN would not be a suitable as a major source of N.

6.13.9 Potential for use in the production of compound fertilisers in the UKNo prospect. Chilean KN is commercially blended with DAP but other complexes/blends are not feasible. The low N concentration would preclude production of high N compounds. The sodium content would be unsuitable for many uses.

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6.14 Crotonylidenediurea (CDU)

6.14.1 IntroductionCDU is a slow release source of nitrogen.

6.14.2 ManufactureUrea and crotonaldehyde are chemically condensed.

6.14.3 UsesSource of nitrogen for prolonged release of N mainly for specialist turf.

6.14.4 Nutrient concentration32% N.

6.14.5 Main chemical form of nutrientsCrotonyldenediurea (C6H12O2N4). Slowly dissolves in water.

6.14.6 Physical characteristicsWhite crystalline powder that can be formulated as granules.

6.14.7 Behaviour in soilNitrogen is released as the particles dissolve. Rate of release therefore depends on particle size and so surface area exposed to water. The rate is also influenced by temperature.

6.14.8 Other factorsExpensive to produce.

6.14.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. CDU is a slow release fertiliser that would not meet the rapid N supply need of mainstream agriculture. It is expensive to manufacture.

6.14.10 Potential for use in the production of compound fertilisers in the UKNo prospect. CDU is a slow release fertiliser that would not meet the rapid N supply need of mainstream agriculture. It is expensive to manufacture.

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6.15 Di-ammonium phosphate (DAP)

6.15.1 IntroductionFertiliser grade DAP is imported to Europe from North Africa and the USA sometimes as a component of blended fertilisers. Technical grade DAP is a component of liquid feeds and fertigation solutions.

6.15.2 ManufactureFor the fertiliser grade, phosphate rock is reacted with excess sulphuric acid to produce phosphoric acid. This is then reacted with ammonia to give DAP. The molten DAP is then solidified by granulation. The technical grade is produced by reaction between thermal acid (a purer form of phosphoric acid) and ammonia, followed by separation of crystalline DAP.

6.15.3 UsesSource of phosphate and nitrogen in hydroponics liquid feeds or fertigation or for soil application, either alone or more usually as a component of compound fertilisers. DAP can be mixed with urea to form blended or complex fertilisers, or with CAN to form blended fertilisers. Ammonium-N content restricts the use of DAP in hydroponics.

6.15.4 Nutrient concentrationThis can vary slightly but is usually around 18% N and 45% P2O5.

6.15.5 Main chemical form of nutrientsDi-ammonium phosphate ((NH4)2 HPO4). The nitrogen is water-soluble. The phosphate can vary in water-solubility between 91-94%, the remainder being soluble in neutral ammonium citrate. Solubility in water at 200C is 569g/l.

6.15.6 Physical characteristicsThe grey, brown or dark green granules have a median size of around 2.8mm. Bulk density of granular material is around 0.91kg/l.

6.15.7 Behaviour in soilThe nitrogen is as effective as that in AN and the phosphate as effective as that in any other phosphate source.

6.15.8 Other factors

6.15.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. DAP contains over 2 times as much phosphate as nitrogen, which would preclude its widespread use without adding to the phosphate pollution problem. DAP is likely to continue to be a source of nitrogen and phosphate in the manufacture of compound fertilisers or used as an NP fertiliser where both nutrients are needed.

6.15.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. DAP is already widely used in the production of compound fertilisers containing N and P. However use of DAP as the only source of N would not allow production of compounds with a high N concentration.

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6.16 Iso-butylidenediurea (IDBU)

6.16.1 IntroductionIBDU is a commonly used slow release nitrogen source.

6.16.2 ManufactureUrea and iso-butyraldehyde are chemically reacted and condensed.

6.16.3 UsesSolid slow release nitrogen source for soil application. Frequently used as a turf fertiliser. May be soil applied alone or as a component of compound fertilisers. Rarely used for container plants as some toxicity has been found under protected conditions.

6.16.4 Nutrient concentrationCommercial materials contain 31-32%N. Pure material contains 32.2% N.

6.16.5 Main chemical form of nutrientsIsobutylidenediurea (C6H14N4O2). Sparingly soluble in water.

6.16.6 Physical characteristicsPowder with particle size ranging from fine dust to 0.3mm.

6.16.7 Behaviour in soilNitrogen release is by slow dissolution of the material. There is some immediate and then continued release of nitrogen over 2 to 5 months depending on temperature and soil condition with residual release for a further month. Rate of release is influenced by particle size, being greater with smaller size. Release is also affected by temperature (2 to 3 times faster at 250C than at 100C) and by soil moisture.


6.16.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. IBDU is a slow release fertiliser that would not meet the rapid N supply need of mainstream agriculture. It is expensive to manufacture.

6.16.10 Potential for use in the production of compound fertilisers in the UKNo prospect. IBDU is a slow release fertiliser that would not meet the rapid N supply need of mainstream agriculture. It is expensive to manufacture.

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6.17 Magnesium ammonium phosphate (struvite) (MgAP)

6.17.1 IntroductionMgAP is an extended release source of nitrogen, phosphate and magnesium that, in one route, is derived from treated waste water. It is produced also by chemical reaction between inorganic salts and acids in solution.

6.17.2 ManufactureIn one method, a solution of a soluble phosphate is mixed with ammonia, ammonium chloride and either magnesium sulphate or chloride. The MgAP is obtained as a crystalline precipitate. In another process, magnesium salts are added to industrial or sewage plant waste water at a controlled pH. The magnesium reacts with ammonium and phosphate ions in the water to form fine white crystals of MgAP. These are removed from the waste water by filtration.

6.17.3 UsesSource of nitrogen, phosphate and magnesium in growing media and for soil application during tree planting.

6.17.4 Nutrient content10% N, 51.5% P2O5 and 29% MgO.

6.17.5 Main chemical form of nutrientsMagnesium ammonium phosphate hexahydrate (MgNH4PO4). 6H20. Low solubility in water.

6.17.6 Physical characteristicsWhite crystalline solid.

6.17.7 Behaviour in soilAlthough water-solubility is low, nutrients will become available over a month or two following application to soil or during the growth of a plant in growing media.


6.17.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. MgAP has a low N concentration (15.7%) and over 5 times as much phosphate as nitrogen. It could not be formulated into solid granules.

6.17.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. MgAP could only be used to produce complex fertilisers with a low N concentration.

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6.18 Magnesium nitrate (MgN)

6.18.1 IntroductionMgN is a readily soluble form of magnesium that is used mainly for high value crops.

6.18.2 ManufactureIn one process, magnesium hydroxide derived from brine is reacted with nitric acid. In an alternative, magnesium oxide may be reacted with nitric acid.

6.18.3 UsesSource of magnesium for foliar application, in hydroponics or in fertigation.

6.18.4 Nutrient contentTypically 15.7% MgO and 10.8% N.

6.18.5 Main chemical form of nutrientsMagnesium nitrate hexahydrate (Mg (NO3)2.6H2O). The magnesium and nitrogen are water-soluble.

6.18.6 Physical characteristicsSupplied as highly hygroscopic off-white flakes for making up into aqueous solution or as made up solution.

6.18.7 Behaviour in soilMgN is an effective source of magnesium when foliar applied. It is claimed to cause less risk of scorch of foliage than other foliar applied materials. The nitrogen is unlikely to be of any special nutrient benefit.

6.18.8 Other factorsSupplies both magnesium and nitrogen but high cost relative to other sources.

6.18.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. MgN has a low N concentration (15.7%) and could not be formulated into solid granules.

6.18.10 Potential for use in the production of compound fertilisers in the UKNo prospect for solid compound fertilisers. MgN is likely to continue to be a source of nitrogen and magnesium in the manufacture of fluid fertilisers.

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6.19 Methylene urea (MU)

6.19.1 IntroductionMeU is a commonly used slow release nitrogen source. It can also be known as urea formaldehyde.

6.19.2 ManufactureIn one process, urea, formalin and a solution of triethanolamine are reacted to form methylol urea. Sulphuric acid is then added under controlled conditions to promote the formation of methylene ureas of various chain lengths. The reaction mixture solidifies rapidly.

6.19.3 UsesSolid slow release nitrogen source for soil application or addition to growing media. Frequently used as a turf fertiliser. May be soil applied alone or as a component of compound fertilisers.

6.19.4 Nutrient concentrationCommercial materials contain 38-39%N.

6.19.5 Main chemical form of nutrientsPolymethylene ureas of varying chain length, for example monomethylolurea (NH2.CO.NH.CH2OH) and methylene diurea (NH2.CO.NH.CH2.NH.CO.NH2). Usually, some urea (CO (NH2)2) is present. Sparingly soluble in water; decomposes if heated.

6.19.6 Physical characteristicsWhite powder or chips. Granules can be produced by a cold compaction process but can be unstable.

6.19.7 Behaviour in soilNitrogen release is through microbial action on the material. There is usually some immediate release, then continued release of nitrogen over 2 to 5 months. Rate of release is influenced by chain length, being greater with shorter chains. Typically around 30% of the material is soluble in cold (250C) water. Of the insoluble portion, at least 40% (usually 50 to 55%) should be soluble in hot (1000C) water. Release is also affected by temperature (up to 10 times faster at 250C than at 100C) and by soil moisture. Release characteristic of different forms of the material is indicated by the Activity Index defined by ((%CWIN - %HWIN) / %CWIN) x 100 where CWIN is cold water insoluble nitrogen and HWIN is hot water insoluble nitrogen. The activity index is usually in the range 38 to 80. The extractant used in determining solubility is buffered phosphate, not water.


6.19.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. ME has a high N concentration (38-39%). The high cost of production and slow release characteristic is likely to make it unsuitable for widespread use.

6.19.10 Potential for use in the production of compound fertilisers in the UKPossible prospect.

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6.20 Mono-ammonium phosphate (MAP)

6.20.1 IntroductionMAP is used mainly in blended and compound, solid and fluid NPK fertilisers but is sometimes applied alone. Like DAP, MAP is sometimes referred to as a straight fertiliser despite containing two nutrients. It is imported to Europe, mainly from North Africa and the USA.

6.20.2 ManufactureFor fertiliser grade, phosphate rock is reacted with excess sulphuric acid to produce phosphoric acid. This is then reacted with ammonia to give MAP. The liquid MAP is solidified by granulation. The technical grade used in liquid and soluble fertilisers is made from phosphoric acid derived by the thermal process.

6.20.3 UsesSource of nitrogen and phosphate for soil application or in hydroponics, liquid feeds, growing media or fertigation. Often a component of blended fertilisers for soil application. MAP can be mixed with urea to form blended or complex fertilisers, or with CAN to form blended fertilisers. The ammonium-N content restricts use in hydroponics.

6.20.4 Nutrient concentrationFor the fertiliser grade this can vary slightly but is usually around 11% N and 52% P2O5. The technical grade is usually 12% N and 61% P2O5.

6.20.5 Main chemical form of nutrientsMono-ammonium phosphate (NH4 H2PO4). The nitrogen is water-soluble in both grades. The phosphate in the fertiliser grade can vary in water-solubility between 91-94%, the remainder being soluble in neutral ammonium citrate, that in the technical grade is 100% water-soluble. Solubility in water at 200C is 276g/l.

6.20.6 Physical characteristicsGreyish or pale green granules with a median size of 2.7mm. Bulk density of granular material around 0.93kg/l.

6.20.7 Behaviour in soilThe nitrogen in MAP is as effective as that in AN and the phosphate as effective as that in any other phosphate source. There are no special transport or storage requirements.


6.20.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. MAP contains over 4 times as much phosphate as nitrogen, which would preclude its widespread use without adding to the phosphate pollution problem. MAP is likely to continue to be a source of nitrogen and phosphate in the manufacture of low N compound fertilisers.

6.20.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. MAP is widely used in the production of compound fertilisers containing N and P. However use of MAP as the only source of N would only allow production of compounds with a low N concentration.

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6.21 Oxamide (Ox)

6.21.1 IntroductionOxamide is a stable, non-hygroscopic slow release nitrogen source, limited in use but technically effective.

6.21.2 ManufactureAlkyl oxalate is reacted with ammonia, or oxalic acid is heated with urea at 150-1600C. Alternatively, hydrogen cyanide is oxidised to cyanogens using hydrogen peroxide. The cyanogen is then partially hydrolysed to form oxamide.

6.21.3 UsesSolid slow release nitrogen source for soil application or for incorporation in growing media.

6.21.4 Nutrient concentration31.8%N.

6.21.5 Main chemical form of nutrientsDiamide of oxalic acid ((CONH2)2). Sparingly soluble in water (0.4g/l at 200C).

6.21.6 Physical characteristicsCrystalline solid usually supplied as granules with particle size ranging from 0.4mm to 4.5mm. Auxiliary materials such as AN, gypsum, sulphite waste liquor formaldehyde may be used to enable granulation.

6.21.7 Behaviour in soilRate of release depends on dissolution from the particle surface so is affected by particle size and temperature. Usually provides consistent release of nitrogen for around 5 months followed by prolonged release at a lower rate.

6.21.8 Other factorsHigh melting point (2000C) and decomposes at 2900C. Good shelf life.

6.21.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. The high N concentration (31.8%) and non-hygroscopic property are positive characteristics but the slow release nature of oxamide may not meet the rapid N supply need of mainstream agriculture. It is costly to manufacture which may not be practically feasible on a large scale.

6.21.10 Potential for use in the production of compound fertilisers in the UKNo prospect.

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6.22 Potassium nitrate (KN)

6.22.1 IntroductionKN is a soluble source of potash and nitrogen with good agronomic effectiveness but relatively high cost. It is available as a manufactured fertiliser and is a component of some Chilean Nitrate grades and of guano. KN is found in some compound fertilisers where it is formed in the manufacturing process.

6.22.2 ManufactureNatural salt deposits are quarried, crushed and screened. KN is separated from the mixture of salts by re-crystallisation. Alternatively, KN can be produced by reacting muriate of potash with nitric acid. KN is also formed in one process for NPK fertiliser manufacture involving reaction of rock phosphate with nitric acid and subsequent ion exchange.

6.22.3 UsesSource of nitrogen and potash for foliar application, soil application, in hydroponics, liquid feeds, fertigation and in growing media.

6.22.4 Nutrient concentration13% N and 45% K2O.

6.22.5 Main chemical form of nutrientsPotassium nitrate (KNO3). Both the potash and the nitrogen are water-soluble. Solubility in water at 200C is 316g/l.

6.22.6 Physical characteristicsWhite crystalline solid or aqueous solution. Some prilled material is produced with a median diameter around 2.1mm. Bulk density around 1.15kg/l for crystalline and 1.20kg/l for prilled material.

6.22.7 Behaviour in soilPotash and nitrogen are immediately available.

6.22.8 Other factors Use tends to be restricted by cost which is significantly higher than for the same nutrients in materials such as AN and muriate of potash. KN is an oxidising agent and should be stored away from combustible materials.

6.22.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. KN contains over 3 times as much potash as nitrogen, which would cause problems of excessive potash application in some agricultural situations (e.g. grassland).

6.22.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. KN is likely to continue to be a source of nitrogen and potash in the manufacture of compound fertilisers.

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6.23 Sodium nitrate (nitrate of soda) (NaN)

6.23.1 IntroductionNaN is found mixed with KN in natural deposits in Chile and elsewhere. It is sometimes used where there is a need for both nitrogen and sodium. The material may be incorporated in blended fertilisers or applied alone. Some sodium nitrate is marketed, alone or in combination with potassium nitrate, under the Chilean Nitrate name.

6.23.2 ManufactureNaturally occurring deposits in South America are mined, crushed and screened. NaN is separated from other salts by re-crystallisation. NaN can also be produced by reacting sodium carbonate with nitric acid.

6.23.3 UsesSource of sodium and nitrogen for soil application.

6.23.4 Nutrient concentration26% N and 35% Na2O. There are also small contents of boron and other trace nutrients.

6.23.5 Main chemical form of nutrientsSodium nitrate (NaNO3). The nitrogen and the sodium are water-soluble.

6.23.6 Physical characteristicsOff-white granules with a median size of around 2.7mm. Bulk density around 1.05kg/l.

6.23.7 Behaviour in soilNaN is a fully soluble and readily available source of both nitrogen and sodium. It should not be used on sodium sensitive plants such as potatoes. There are no special requirements for transport or storage.

6.23.8 Other factorsShould not be used on sodium sensitive plants such as potatoes.

6.23.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. Although containing a reasonable concentration of N (26%), NaN contains more sodium oxide than nitrogen. Although sodium additions are needed by some crops (e.g. sugar beet), unnecessary use can cause serious deterioration of the structural stability of soils.

6.23.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. NaN is likley to continue to be used in the production of blended fertilisers with a low/medium N concentration where there is a need for both N and Na.

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6.24 Sulphur coated urea (SCU)

6.24.1 IntroductionSulphur coating has been used for many years to delay the release of nitrogen from urea particles. The sulphur coating is impregnated with a waxy material intended to improve nitrogen release characteristics.

6.24.2 ManufactureUrea particles, either granules or prills, are coated with elemental sulphur. The particles are then further coated with a polymer, wax or petroleum product. An anti-caking agent is then applied.

6.24.3 UsesA controlled release source of nitrogen mainly for turf. Sulphur used as the coating may also have a useful nutrient effect. The material is also used as a component of compound fertilisers.

6.24.4 Nutrient concentrationTypically 35-42% N and 21-55% SO3 (8.5-22% S).

6.24.5 Main chemical form of nutrientsUrea (CO (NH2)2) and sulphur (S). The urea is water-soluble. The elemental sulphur becomes soluble in the soil following microbial conversion to the sulphate form.

6.24.6 Physical characteristicsYellow particles usually in the 1 to 4mm size range with a median size of around 2.4mm for prills and 2.9mm for granules. A small prill material is available for blending with fine turf fertilisers. Bulk density around 0.75kg/l.

6.24.7 Behaviour in soilNitrogen is available after release from the particles and microbial conversion to ammonium and nitrate forms. Sulphur becomes available following microbial conversion to sulphate form. All these processes are influenced by soil temperature and moisture (faster in warm moist soil) as well as by the thickness of the sulphur coating and the nature of the hydrocarbon coating. Typically, 20 to 30% of the nitrogen is released within seven days of application and subsequently around 1% per day.


6.24.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. SCU is a slow release fertiliser that would not meet the rapid N supply need of mainstream agriculture. It is expensive to manufacture. There may be scope to develop more suitable coatings but manufacture would remain costly.

6.24.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. SCU could be mixed with AS, ASN, MOP, TSP or Kieserite.

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6.25 Urea (U)

6.25.1 IntroductionFertiliser grade urea is the predominant source of nitrogen used in agriculture throughout the world (c.50% of total N use). It can be manufactured by prilling or granulation (c.20% of total production capacity). In Europe, urea is manufactured in Germany, France, Netherlands, Italy, Spain and Ireland but not in the UK. However, some urea is currently used in the UK (8% of total N use) but all supplies are imported. (See NT2601 report on ‘Production and use of nitrogen fertilisers’ for more information).

6.25.2 ManufactureAmmonia is reacted with carbon dioxide which itself is a secondary product of the ammonia fixation process. Formaldehyde is used as a hardener. The molten urea is solidified by granulation or, more commonly, by prilling. The final material may be coated with an anti-caking agent.

6.25.3 UsesAs well as being used as a straight N fertiliser, urea can be a component of both solid and liquid compound fertilisers but not in combination with TSP (Anon, 1970). Urea can be blended with AS, MAP or MOP – one commercial blender in England produces the following granular urea based NPK blends 36:7:7, 26:14:14, 19:19:19, 15:18:25, 35:17:0, 30:0:21, 30:6:16. Urea can also be complexed with AS, MAP or MOP (e.g. UAS), but such complexing using urea is not practised in UK production facilities. However, urea-based compound fertilisers may have serious caking problems and a short shelf life.

Blends of urea with AN have been made experimentally but not commercially because of chemical reactions which occur from intimate contact which leads to loss of physical structure, caking and a very short shelf life. The critical relative humidities at which urea and AN absorb moisture from the air are:- urea 75.2%; AN 59.4% and urea/AN mixture 18.1% (Overdahl, 1991).

Mixtures of AN and urea are dissolved in water to manufacture high N fluid fertilisers such as UAN which is the main form of straight N solution fertiliser. Urea can be dissolved in water for foliar application (e.g. late N applications to increase wheat protein content) or in fertigation systems.

6.25.4 Nutrient concentrationUsually 46% N (solid urea).18% maximum (urea alone in solution)

6.25.5 Main chemical form of nutrientsUrea (CO (NH2)2). The nitrogen is water-soluble. Solubility in water at 200C is 1080 g/l.

6.25.6 Physical characteristicsUrea melts at 1350C. Urea prills are white or off-white spheres, smaller than those of ammonium nitrate with a median size of around 2.1mm. Granular urea also is white or off-white with a median size of around 2.8mm. Bulk density of prilled urea around 0.73kg/l, granular urea around 0.77kg/l. Because of this low bulk density, accurate spreading of urea usually needs narrower bout widths than other commercially available fertilisers. Experimental work in N Ireland has produced an 80 urea: 20 dolomite granule; because dolomite has a higher bulk density than urea, this mixture could have marginally better spreading characteristics than straight urea.

6.25.7 Behaviour in soilUrea is converted in the soil to ammonium-N and then to nitrate-N. This conversion is normally quite rapid, taking a few days. Many studies have shown a significant loss of ammonia to the atmosphere following application of urea (higher than from AN). This can lead to a loss of agronomic effectiveness. Plants can take up small amounts of urea through the roots but the fertiliser becomes effective after conversion of urea to ammonium-N and nitrate-N.

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6.25.9 Potential for use as a straight nitrogen fertiliser in the UKGood prospect. Urea is the most widely used form of straight N fertiliser worldwide and is currently used in Britain. It has a very high concentration of N (46%) though low bulk density. The main on-farm problems are the high potential loss of ammonia as a pollutant, and the poor field spreading characteristics.

6.25.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. Urea can be mixed with AS to produce urea ammonium sulphate (UAS) which is available commercially as a complex granular fertiliser. Some commercial urea-based blended and complex NPKS fertilisers are produced but these can have serious problems of caking and poor storage.

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6.26 Urea ammonium nitrate (UAN solid)

6.26.1 IntroductionMixtures of urea and ammonium nitrate in solid form are not produced commercially because of problems of caking and a short storage life due to the hygroscopic nature of the mix; the critical relative humidities at which urea and AN absorb moisture from the air are:- urea 75.2%; AN 59.4% and urea/AN mixture 18.1% (Overdahl, 1991). However, research (Garrett, 1987) has suggested that blended mixes of a urea/limestone granule with an AN/AS granule may be more attractive.

6.26.2 ManufactureThe research has produced a blend based on a 37.5 : 62.5 mix of the following 2 components:-i) uncoated urea/dolomite granules containing 80% urea and 20% dolomiteii) coated AN/AS/dolomite granules containing 48% AN, 16% AS and 36% dolomite.

6.26.3 UsesAs a blended N fertiliser for top-dressing with some sulphur content. Use as a component for NPK compound fertilisers has been suggested.

6.26.4 Nutrient concentration26% N (based on the above blend ingredients).

6.26.5 Main chemical form of nutrientsUrea (CO (NH2)2) and ammonium nitrate (NH4NO3).

6.26.6 Physical characteristicsThe research indicated potentially serious problems of caking and deterioration:-- the urea/dolomite granule was relatively hard and stored well- the AN/AS/dolomite granule was more subject to physical deterioration- addition of AS in the blend improved the hardness of the granule.

6.26.7 Behaviour in soilThe mix would be expected to behave in a similar way to AN and urea applied separately.

6.26.8 Other factors The research suggested that acceptable spreading characteristics were possible. Storage properties required further investigation.

6.26.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. A UAN blend (solid) could have a reasonable N concentration but questions remain concerning the potential for the physical characteristics of a commercial product and its potential for acceptable handling, storage and spreading characteristics.

6.26.10 Potential for use in the production of compound fertilisers in the UKNo prospect. Unlikely to be suitable for complexing or blending.

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6.27 Urea ammonium nitrate (UAN fluid)

6.27.1 IntroductionUAN is a fluid fertiliser widely used as a straight N in Europe. In the UK, UAN accounts for around 8% of the straight N market. It is a true solution with no requirement for agitation or mixing. UAN can be mixed with other solutions or with suspensions to give fluid NPK fertilisers.

6.27.2 ManufactureUAN is a solution in which 50% of the N derives from AN and 50% from urea. By using both AN and urea, the final N content of the solution is greater than could be achieved if either form were used alone. Most UAN used in the UK is imported as UAN solution or manufactured from UK-produced AN liquor with added solid urea. Some UK-produced UAN is made from solid urea and solid AN dissolved in water.

6.27.3 UsesAs a straight N fertiliser for top-dressing mainly arable crops but sometimes grassland.

6.27.4 Nutrient concentrationCan vary between 28-30% N (w/w) depending on grade.

6.27.5 Main chemical form of nutrientsAmmonium nitrate (NH4NO3) and urea (CO (NH2)2). All nitrogen is in solution.

6.27.6 Physical characteristicsClear solution with a light greenish colour. Typical bulk density is in the range 1.20-1.40kg/l depending on N concentration.

6.27.7 Behaviour in soilAs the nitrogen sources are AN and urea, agronomic effectiveness is intermediate between those of these materials. Fluid form can have advantages in storage and spreading. However, special storage requirements involving siting and bunding of tanks are needed and, in the UK, are covered in Defra and FMA Codes of Practice.

6.27.8 Other factorsUAN solutions will often scorch green plant tissue if there is direct contact. In practice, field application makes use of streamer nozzles (large droplet size) or dribble bars, both of which reduce crop scorch to acceptable levels though this could be a problem if used on e.g. fresh vegetables.

6.27.9 Potential for use as a straight nitrogen fertiliser in the UKGood prospect. UAN (solution) is already used as a source of straight N in Britain.

6.27.10 Potential for use in the production of compound fertilisers in the UKNo prospect for solid compound fertilisers. UAN is likely to continue to be a source of nitrogen in the manufacture of fluid fertilisers.

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6.28 Urea ammonium sulphate (UAS)

6.28.1 IntroductionUAS is a mixture of urea and AS, usually in 8:1 ratio, produced as a complex fertiliser. It is used where both nitrogen and sulphur are needed by the crop. It is used in S Ireland and outside Europe but not currently to any significant extent in the UK.

6.28.2 ManufacturePowdered AS is mixed with molten urea before granulation. It is manufactured by Hydro in the Netherlands.

6.28.3 Nutrient concentration38-40% N and 14-19% SO3.

6.28.4 Main chemical form of nutrientsUrea (CO (NH2)2) and ammonium sulphate ((NH4)2 SO4). The nitrogen and sulphur are water-soluble.

6.28.5 Physical characteristicsOff-white or yellow granules with a median size around 3.0mm. Typical bulk density is around 0.8kg/l which is marginally higher than straight urea (prills 0.73kg/l; granules 0.77kg/l).

6.28.6 Behaviour in soilThe urea and AS components probably behave in the same way as if these are applied individually. In Europe, the material is most suitable for sulphur responsive crops such as oilseed rape or grassland cut for silage. There are no special requirements for transport or storage.

6.28.7 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. UAS offers advantages of high N concentration (41%) and a useful sulphur content. The inclusion of sulphur in UAS should be a minor problem since sulphur is a required nutrient in many situations and has no negative impact on the environment. However, the sulphur content could be a problem if UAS were used as the only N source for grassland prone to copper deficiency in livestock.

6.28.8 Potential for use in the production of compound fertilisers in the UK.Possible prospect. Could be used for compounding but there are no apparent advantages compared to the use of straight urea and AS.

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6.29 Urea calcium nitrate (UCN)

6.29.1 IntroductionMixtures of urea and calcium nitrate have been produced for experimental purposes. One objective was to test the possible urease inhibiting effect of the calcium nitrate.

6.29.2 ManufactureNo commercial production capacity currently exists.

6.29.3 UsesPotentially as a straight N source for agricultural crops.

6.29.4 Nutrient concentrationVaries with ratio of components but typically 40% N.

6.29.5 Main chemical form of nutrientsUrea (CO(NH2)2) and calcium nitrate (CaNO3).

6.29.6 Physical characteristicsNo information.

6.29.7 Behaviour in soilExperiments revealed little urease inhibiting effect of the calcium nitrate so behaviour would be determined by the components and ratio of the components.


6.29.9 Potential for use as a straight nitrogen fertiliser in the UKPossible prospect. Only if CN is shown to have activity as a urease inhibitor.

6.29.10 Potential for use in the production of compound fertilisers in the UKPossible prospect. Only if CN is shown to have activity as a urease inhibitor.

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6.30 Urea phosphate (UP)

6.30.1 IntroductionUP is a concentrated source of nitrogen and phosphate, mainly for use in liquid feeds and soluble concentrates.

6.30.2 ManufactureUrea is reacted with phosphoric acid and the UP is precipitated as crystals.

6.30.3 UsesMainly in hydroponic and fertigation systems although sometimes used in solid form for soil application. Being acidic, the material is useful where the water supply is high in calcium.

6.30.4 Nutrient concentration18% N and 44% P2O5.

6.30.5 Main chemical form of nutrientsUrea phosphate (CO (NH2)2 H3PO4). Readily soluble in water.

6.30.6 Physical characteristicsWhite crystals.

6.30.7 Behaviour in soilBoth the nitrogen and the phosphate are immediately available. As with all phosphate sources, availability of the phosphate will decline with time after contact with soil.

6.30.8 Other factorsUP has an acidifying effect on the soil.

6.30.9 Potential for use as a straight nitrogen fertiliser in the UKNo prospect. UP contains over 2 times as much phosphate as nitrogen, which would preclude its widespread use without adding to the phosphate pollution problem.

6.30.10 Potential for use in the production of compound fertilisers in the UKNo prospect for solid compound fertilisers. UP is likely to continue to be a source of nitrogen and phosphate in the manufacture of fluid fertilisers.

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6.31 Nitrification inhibitors

6.31.1 IntroductionNitrification inhibitors have been available as commercial products for several years and are used in mainland Europe notably Germany, though only to a very limited extent in the UK.

6.31.2 ManufactureInhibitors are organic chemicals; manufacturing methods are specific to the inhibitor.

6.31.3 UsesFor inclusion in solid or liquid straight N or NPK compound fertilisers.

6.31.4 Nutrient concentrationUsually small nitrogen content.

6.31.5 Main chemical form of nutrientsThere are many different chemicals that have been identified as nitrification inhibitors including the following (Wozniak et al., 1999). Some mixtures are possible.

2-chloro-6-trichloromethyl-pyridene (Nitrapyrin, trade name N-serve; manufactured by Dow Agro-Science)

dicyanamide (DCD, trade names Alzon, manufactured by SKW Trostberg AG; Didin and Ensan manufactured by BASF)

3,4 dimethylpyrazole-phosphate (DMPP, trade name Entec, manufactured by BASF) (Zerulla et al. (2000) 2-ethinylpyridene 5-ethoxy-3-trichloromethyl-1,2,4-thiadiazole 4-amino-1,2,4-triazole 1-carbamoyl-3-methylpyrazole N-(2.5 dichlorophenyl)-succinic acid monoamide calcium carbide polyolefin coated urea

6.31.6 Physical characteristicsNitrapyrin has a high vapour pressure and cannot be used with solid fertilisers unless special packaging is used; it can be used with liquid fertilisers. A granular formulation of Nitrapyrin has been produced experimentally, and liquid Nitrapyrin has been incorporated in solid NPK fertilisers though double bagging was required to prevent loss by volatilisation. DCD can be added to liquid fertilisers or incorporated in solid fertilisers.

6.31.7 Behaviour in soilNitrification inhibitors provide temporary inhibition of the nitrification of ammonium-N to nitrate-N. Since ammonium-N is not at risk to loss by leaching, this can reduce the risk of N loss. However, ammonium-N can be less available for crop uptake. The degree and duration of control on the nitrification process is an important issue.

6.31.8 Other factorsSome nitrification inhibitors may have adverse agronomic, environmental and/or toxicological effects.

6.31.9 Potential for use in nitrogen fertiliser production in the UKPossible prospect. The use of nitrification inhibitors may help improve the overall suitability of some alternative forms of straight N fertiliser.

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6.32 Urease inhibitors

6.32.1 IntroductionUrease inhibitors are commercially available, used in the US and some European countries.

6.32.2 ManufactureThe manufacturing plant for the only commercially available urease inhibitor (trade name Agrotain) is based in the US. The cost of adding an inhibitor will vary with the type of inhibitor but typically, addition of Agrotain increases the cost of urea by £5-7 per tonne of product.

6.32.3 UsesMany inhibitors are organic and will not withstand the high temperatures of molten fertilisers. Incorporation of such organic materials must therefore be by coating or spraying the solid fertiliser. Metal salts that are active as inhibitors form metal complexes with urea and can be added at melt stage for subsequent product granulation.However, Agrotain (nBTPT or N-(n-Butyl) Thiophosphoric Triamide) can be used as a spray impregnation of urea granules, added to the urea melt or to UAN solutions.

6.32.4 Nutrient concentrationNil N concentration.

6.32.5 Main chemical form of nutrientsMany compounds have been evaluated as urease inhibitors (Mulvaney and Bremner, 1981). Watson (2000) reports 4 groups of compounds as follows:-

Reagents which interact with the sulphydryl groups (sulphydryl reagents). e.g. Hydroquinone; Catechol; p-Benzoquinone; 1,3,4-Thiadiazole-2,5-dithoiol; 5-Amino-1,3,4-thiadiazole-2-thiol; inorganic halide, CO3 and SO4 salts.

Hyroxamates. e.g. Acetohydroamic acid; Hyroxyurea; Caprylohydroxamic acid. Agricultural crop protection chemicals, e.g. herbicides. Structural analogues of urea and related compounds. e.g. Thiourea; Phenylurea; Phenylphosphorodiamidate

(PPD); N-(n-Butyl) Thiophosphoric Triamide (nBTPT)

nBTPT is the most widely tested inhibitor. It has been used in the US since 1996. Small quantities of urea+Agrotain are available commercially in Ireland (blended with bentonite/sulphur granule) and in the UK from spring 2003. In the US, IMC-Agro manufacture Super U which contains nBTPT and the nitrification inhibitor DCD.

6.32.6 Physical characteristicsAgrotain is a clear green liquid with a flashpoint of 980C.

6.32.7 Behaviour in soilUrease inhibitors reduce the speed of hydrolyis of urea to ammonium carbonate which can reduce the potential loss of N by volatilisation of ammonia.

6.32.8 Other factorsA 3 month half life for Agrotain impregnated urea is quoted. Agrotain has passed Irish environmental and toxicological tests.

6.32.9 Potential for use in nitrogen fertiliser production in the UKGood prospect. The addition of a suitable urease inhibitor(s) to urea could help mitigate the serious problem of ammonia volatilisation following use of urea fertiliser.

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6.33 Fertiliser fillers, additives and coating agents

6.33.1 Fillers and additivesInert fillers (for example powdered or chipped limestone) may be added to fertilisers to achieve a particular nutrient concentration or NPK ratio – e.g. the calcium carbonate in CAN is a filler, reducing nitrogen concentration from 34.5% N in pure ammonium nitrate to typically 27.5% N in CAN. The filler gives the product other desirable properties such as reduced moisture absorption and makes CAN more suitable for blending.

Various additives and coatings are also used to improve fertiliser properties. Generally, additives are used to reduce the rate of moisture absorption (e.g. small amounts of magnesium nitrate are used in prilled AN or formaldehyde to urea), to reduce the risk of caking or to change agronomic properties after application (e.g. urease or nitrification inhibitors).

6.33.2 Coating agentsMost fertilisers are treated with coating agents to improve storage properties (Ohlsson 2000). These coatings may be organic (for example mineral oils or amines) or inorganic (for example limestone). Coatings may be in the form of inert powders, liquids or solids. Examples of inert powders are dolomite, kaolin and talc. Typically, inert powder coatings comprise 2–3% of the fertiliser which can be a disadvantage. Liquid coating agents are organic, for example mineral oils, sulphonates, amines and polyoxyethylene condensates. Solid coating agents, for example amine acetate, are applied as melts to hot fertiliser. The coating agent used depends on the fertiliser being coated and on cost relative to other agents.

Fertilisers also may be coated to change their nutrient release characteristics, usually to slow release. Examples are the resin coatings used for some horticultural NPK fertilisers and the sulphur used to coat some urea products.

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7. The production of compound fertilisers

The production and availability of a range of compound fertilisers containing different proportions of 2 or more of the nutrients N, P, K, Mg and S is an important need of farmers. Approximately 40% of UK consumption of fertiliser N is as N in compound fertilisers. The current use of compound nitrogen fertilisers is described in the NT2601 report ‘Production and use of nitrogen fertilisers’.

Although in theory the nutrients contained in compound fertilisers could be applied as separate applications of straight fertilisers containing single nutrients, this approach could create severe logistical pressures on some farms. One pass across a field applying several nutrients as a compound fertiliser would need to be replaced by multiple passes each applying a straight fertiliser containing an individual nutrient. This would need more time, staff and equipment than currently budgeted for on farms.

On some farms, ‘rotational manuring’ of P and/or K (2 or 3 years worth of nutrient usually applied as a straight fertiliser in one application) is practised, and is a recommended option to annual applications in some situations. If compound fertilisers became less available, this practice could increase. In many situations, there are unlikely to be any penalties from rotational manuring though there may be an increased risk of P pollution of waters following rotational manuring if surface runoff occurs following a large application of P fertiliser. Rotational manuring is specifically not recommended in the following situations (Anon, 2000). where soil analysis shows that the soil is deficient in the nutrient for K on light sand soils (due to the risk of leaching of K out of the soil)

There are 2 types of compound fertiliser:-

1. Complex fertiliser. This is a compound fertiliser where all particles have the same composition.2. Blended fertiliser. This is a compound fertiliser produced by dry mixing of two or more different

particulate or powder materials.

The scope for producing compounds (complexes and blends) using different fertiliser materials is summarised in Table 1. This information has been derived from experience within the UK fertiliser industry and should be regarded as indicative.

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Table 1. Potential for producing fertiliser complexes and blends from nitrogen containing raw materials (Kemira Growhow, pers. comm.).

Acl AN AS ASN CAN CN ChileanKN

DAP MAP MU Ox KN NaN SCU U UP

AclAN S4 AS S4 S1,2ASN S4 S2 3CAN 4 3 3 3CN Q4 Q4 Q4 Q4 Q4Chilean KN SE4 2 Q4 4 2 3DAP 4 2 2 2 2 Q4 2MAP 4 2 2 2 2 Q4 2MU E4 ES3 4 E4 QS4 Q4 E3 E3Ox E4 ES3 4 E4 QS4 Q4 E3 E3 E3 E4KN SE4 SE4 4 4 E4 Q3 4 E3 E3 ES4 ES4NaN E4 S4 4 4 4? Q3 4 E3 E3 E4 ES4 4SCU E4 S4 3 S4 S4 S4 S4 E4 E3 E3 E4 ES4 E4U Q4 SQ4 1,2 QS3 QS4 4 4 2,3 2,3 E4 E4 E4 E4 E3UP QE4 QE4 Q4 Q4 QS4 4 EQ4 E4 E4 E4 E4 E4 E4 E4 E3

MOP 4 S1,2 2 3 2,3 4 E4 2,3 2,3 E4 E4 E4 E4 E3 2 E4TSP 4 S4 Q3 Q4 Q4 4 4 2,3 2,3 E4 E4 4 E4 E3 Q2,3 EQ4SSP 4 S4 Q3 Q4 Q4 4 4 Q3 Q3 E4 E4 4 E4 E4 Q4 EQ4Kieserite 4 1,2 3 3 2,3 4 4 Q3 Q3 E4 E4 4 E4 E3 3 EQ4

SOP E4 1,2 2 3 2,3 4 4 2,3 2,3 E4 E4 4 E4 E3 2 4

1 = Commercial complexes available2 = Commercial blends available3 = Mixtures not produced commercially but feasible4 = Mixture not practically possible

Q= Quality problems encounteredS= Safety concerns relate to some concentrationsE= Economics are not feasible for agricultural use

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7.1.1 Complex fertilisersComplex fertilisers contain the declared concentration of nutrients for the fertiliser product in each particle of fertiliser. Compared to blended fertilisers which are composed of separate particles of different raw materials, segregation of particles during handling, and unequal distribution of particles during spreading are not sigificant problems of complex fertilisers.

Urea is less suited as a raw material for complexing than AN due to its hygroscopicity. Kemira (pers. comm.). have tested the physical quality of a range of nitrate-based and urea-based complex fertilisers using a ranking scheme (0 to 200) based on fines, particle size distribution (Va), caking, dust, rolling tendency and crushing strength. Conventional AN-based complexes generally scored over 100 points whereas urea-based complexes below 100. The main problem with urea-based complexes was caking (due to moisture absorption) which means that such products have a short shelf-life compared to the 12 months or more of AN-based complexes. In Malaysia and Lithuania, urea-based complexes with a maximum of 15% N are produced but storage and spreading characteristics are reported to be poor.

7.1.2 Blended fertilisersSome products are more suitable than others for blending due to their physical or chemical properties. For N-containing blends where accurate spreading is especially important, the various blend components should have closely matched particle sizes so as to prevent segregation of components during handling, transport and spreading. Although prilled AN and granular MOP are used for some grassland fertilisers, it is not usual practice to mix prills and granules in a blend. Other physical properties important for production or blending include bulk density (affects size of bags, capacity of storage bins) and angle of repose (affects areas of storage bins, design of roof for bulk stores and a safety factor when transporting by ship) (Leonard, 1996).

A method developed for assessing the physical compatibility of blend components is the Size Grade Number (SGN). This has been extended to include a second measure, the Uniformity Index (UI). The SGN is the calculated mean particle diameter expressed in millimetres and multiplied by 100. The UI is the ratio of the sizes of small and large particles expressed as a percentage. Values of SGN and UI for several materials were reported by Lance (1996) (Table 2).

Table 2. Values of Size Grade Number (SGN) and Uniformity Index (UI) for some typical raw materials (Lance, 1996).

SGN UIAN 33.5%N granular 350 68AN prills 210 – 220 63AN prills (UK) 250 64CAN (Europe) 350 – 430 53 – 63DAP (Europe) 320 55DAP (USA) 281 58MAP 257 53KCl granular 275 – 280 42 – 44KCl coarse 345 42KCl screened 251 45TSP 288 52Urea prills 210 55Urea granular (Europe) 360 67

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For a good quality blended fertilisers, it has been suggested that the SGN and UI of components should be within ±10% of the mean values for the blend. The small particle size of prilled urea presents a problem for blending and this material is rarely used for this purpose.

Some fertilisers, when mixed together, become particularly hygroscopic giving the blend a short shelf life. Urea and AN, or urea and AN-based products, when mixed, quickly become wet and sticky. Similarly, mixtures of urea and superphosphate, or diammonium phosphate and superphosphate, deteriorate at a rate dependent on the moisture content of the superphosphate.

Particle segregation during transfer of bulk fertiliser, or when filling a fertiliser hopper, can also be a problem with blended fertilisers. Segregation will result in an uneven distribution following spreading. Work by Chaney (1990, Hydro Agri UK Ltd.) compared segregation of an AN-based blend with a urea-based blend (both blends containing TSP and MOP). Segregation of the urea-based blend was worse than for the AN-based blend; following segregation, the range of N content (%) in different sub-samples of the fertiliser product ranged between 17.8-23.1% N (AN-based) and 13.5-34.0% N (urea-based). Segregation is not a problem with complex fertilisers. Spreading tests comparing blended and complex products have also shown that uneven distribution of fertiliser particles following spreading of blended products can occur (Kemira, pers.comm.).

Formulating blended products from ‘building blocks’ of a selected range of complex fertilisers which have particles of similar physical characteristics, can help overcome the spreading problems of conventional blends (Kemira, pers.comm). Blends of this type (c.220 alternative nutrient analyses) are currently available on mainland Europe (Kemistar range) but not in the UK.

For these reasons, urea is not commonly used as a blend component. AN and CAN are the main straight N sources used for blending in the UK.

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8. Nitrogen from organic manures

The nitrogen content of organic manures is effectively nitrogen recycled from natural (e.g. mineralisation of soil organic N, fixed atmospheric N) and manufactured sources (e.g. fertilisers, feedstuffs). There are 3 main categories of organic manures applied to agricultural land:- 1. Farm manures (e.g. cattle, pig, sheep and poultry manures)2. Treated sewage sludges (biosolids)3. Industrial ‘wastes’ (e.g. paper waste, food processing waste, green waste compost)

These materials are valuable sources of organic matter and major plant nutrients, including nitrogen, which can result in substantial savings in the use of manufactured fertilisers. However, some of the nutrients in organic manures only become available for crop uptake gradually over a period of several years and/or may be lost to the environment. Thus, only a proportion of the total nutrient content of an organic manure application can be regarded as having a potential value for reducing the amount of fertiliser N needed. Additionally, there are regulatory controls that limit the rate and timing of manure applications. These factors together mean that organic manures cannot usually completely substitute for fertiliser N.

8.1 Forms of nitrogenOrganic manures contain N in different chemical forms which have different characteristics of availability for uptake by crops.

Total NThe total N content is the total amount of N contained per tonne or m3 of the manure. Each type of organic manure will contain a typical quantity of total N. The total N is present in readily available N (principally ammonium-N and uric acid-N for poultry manures) and organic N forms.

Readily available NReadily available N is the nitrogen that is potentially available for rapid crop uptake (i.e. is equivalent to fertiliser N) assuming that there are no losses of nitrogen following application. It is principally the ammonium-N content of the manure (plus uric acid-N for poultry manures). Typically, 10-25% of the total N content of straw-based farmyard manures (FYM) is present in readily crop available forms, 40-50% for poultry manures and 50-60% for cattle/pig slurries.

Organic NOrganic N is present as complex organic nitrogen compounds which will gradually decompose to simpler forms of N that are available for crop uptake. Most of the manure organic N is not available for crop uptake in the first season after application and will be slowly released over a period of years. It is difficult to allow for this source of N when planning nitrogen fertiliser applications.

Crop available NCrop available N is the readily available N less any N lost as a result of N losses by ammonia volatilisation, denitrification and nitrate leaching, including N released from organic N forms. This is the actual fertiliser N value of a manure application.

8.2 Production of organic manures and their potential N valueTable 3 shows the amounts of organic manures applied annually to agricultural land in the UK and their associated total and crop available N contents, based on current application practices – i.e. autumn or winter applied, predominantly surface applied without incorporation into the soil (Chambers et al., 2000). The supply of this crop available N from all sources of organic manures is estimated to be c.100kt of N, which is c.10% of the current consumption of manufactured fertiliser N in the UK. This could increase to 150kt of N (c.15% of

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fertiliser N consumption) if all manures were applied in the spring since there would reduced leaching of manure N to the environment (see 8.4 and 8.5). In practice, probably no more than 30kt of this crop available N is actually allowed for by farmers when planning the use of N fertilisers.

Table 3. Amounts of organic manures produced and applied to agricultural land in the UK; production as dry solids (ds) or fresh weight (fw).

Manure type Production (kt/yr)

Nitrogen content (kt N/yr)

Total Recycled to agricultural land

Total N Crop available N

Farm manures 90,000 fw 90,000 fwa 450 100Sewage sludgesb 1,120 ds 520 ds 18 5Industrial ‘wastes’ 7,200 fw 12c <1c

Total 480 105N as manufactured fertiliser d

- 1,115

a. c. 0.1-0.5 kt/yr of poultry litter is incinerated for energy productionb. Data for 1996/97c. Best estimate based on the analysis of a range of waste products (Gendebien et al., 2001)d. 2000/01 (FMA, 2002)

8.3 Regulatory controlsThere are several items of UK legislation that control the application of organic manures to land:-

8.3.1 The Defra Water Code and Nitrate Vulnerable Zones (NVZ).The Defra Water Code states that the amount of total N in organic manures applied to agricultural land should not exceed 250kg/ha/yr (Anon., 1998). In NVZs, which now occupy 55% of the land area in England (Figure 1), 3% in Wales, 4% in Scotland and cover four designated areas in Northern Ireland, adherence to the field based limit of 250kg/ha N is a statutory requirement of the Action Programme rules (Anon., 2002a). This limit controls the potential quantity of crop available N that may be supplied by organic manures applied to land within NVZs.

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Figure 1. Map of NVZs in England

8.3.2 Controls on the application of sewage sludge to land Where sludges are applied to agricultural land, the conditions of The Safe Sludge Matrix (Anon., 2001), The Sludge (Use in Agriculture) Regulations (SI, 1989 & 1990) and The Code of Practice for Agricultural Use of Sewage Sludge (Anon., 1996a) must be followed. The Safe Sludge Matrix provides the minimum standard for sustainable sludge recycling to agricultural land. These operating requirements ensure that sludge applications to farmland are strictly controlled, that elevated concentrations of heavy metals do not accumulate in soils and crops, and that disease risks to humans and livestock are minimised.

8.3.3 Controls on the spreading of industrial wastes to landThe Waste Framework Directive (91/156/EEC amending 75/442/EEC) provides the basis on which all waste is managed. This has been implemented in the UK by Part II of the Environmental Protection Act 1990 and the Waste Management Licensing Regulations (WMLR) 1994 (SI, 1994). The WMLR provide a system for the licensing of waste recovery, disposal and treatment operations. These regulations allow the spreading of some industrial ‘wastes’ onto agricultural land without licensing controls, providing certain conditions are met. In particular, applications should ‘confer agricultural benefit or provide ecological improvement’ and should not exceed 250kg/ha total N. However, the application of these ‘wastes’ must be registered with the Environment Agency and their chemical composition should be analysed.

8.4 Farm manuresApproximately 90,000kt of farm manures (fresh weight) are applied annually to agricultural land in the UK (Williams et al., 2000). Of these, c.52% are handled as slurries (42,000kt of cattle slurry and 5,000kt of pig slurry), and the remainder as solid manures (39,000kt of straw-based farmyard manure and 4,000kt of poultry manure). It is likely that the quantities of farm manures recycled to land will decline (c.10%) over the next decade due to decreases in livestock numbers, particularly pigs and dairy cattle. Also, that the trend back to solid manure systems will continue, particularly for pigs.

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Organic manures are applied annually to around 19% of tilled land (0.9 million hectares) and 44% of grassland (2.4 million hectares) in Britain. These manures supply in the region of 450kt of total nitrogen (N), 275t of total phosphate (P2O5) and 400t of total potash (K2O).

Organic manures present a risk of environmental pollution if not handled carefully. Applications need to be managed to limit N losses by ammonia volatilisation, denitrification (nitrous oxide and nitrogen gas) and nitrate leaching. Ammonia-N losses following the land spreading of farm manures have been estimated at 79kt NH3-N/yr (Anon., 2002b). Typically, 65% of the ammonium-N content of FYM and 35% of the ammonium plus uric acid N content of poultry manures can be lost through volatilisation following land spreading. In the case of slurries, ammonia losses are typically in the range 10-60% of the ammonium-N applied (Chambers et al., 1997; Chambers et al., 2001). N losses by denitrification following manure applications are typically in the range 1-5% of the total N applied (Ryden, 1987). Nitrate leaching losses following autumn-winter manure applications in the UK have been estimated at 58kt N annually (Chambers & Smith, 1995).

It is therefore recommended that manure applications are made during periods of active crop growth (spring-summer) to minimise leaching losses, and are rapidly incorporated into the soil (where practically possible) to minimise ammonia volatilisation losses (Anon., 2002b; Chambers et al., 2001).

8.4.1 Impact of regulatory controlsThe amount of crop available N supplied by a range of farm manures at an application rate of 250kg/ha of total N has been estimated in Tables 4 to 6, using data on the percentage of total N available to the following crop as given in Defra’s ‘Fertiliser Recommendations (RB209) publication (Anon., 2000). The amount of crop available N varies with manure type, soil type, application timing, how quickly the manures are incorporated into the soil and rainfall following application. The values given in Tables 4 to 6 assume that the manures are applied to the soil surface (the most common practice) at a rate of 250kg/ha total N, in either the autumn (to sandy soils) or spring; the values represent the potential extremes of crop available N. Due to decreased nitrate leaching, autumn (Aug-Oct) or winter (Nov-Jan) manure applications to more nitrate retentive medium or heavy textured soils will increase the amounts of crop available N.

Table 4. Estimated crop available N (kg/ha) from FYM applied to the soil surface at 250kg/ha total N, compared with typical N requirements of winter wheat and first cut grass (kg/ha).

Manure Application ratea Crop available Nb Typical N requirementc

(t/ha) Autumn(Aug-Oct)

Spring(Feb-Apr)

Winter wheat 1st cut grass

Cattle 42 13 50 200 150Pig 36 13 50 200 150Sheep 42 13 50 200 150

a. Assuming typical manure total N concentrations (Anon., 2000)b. Data refers to manures applied to sandy or shallow soils (autumn) or any soil type (spring)c. From RB209 (Anon., 2000)The values assume that FYM is fresh and not stored prior to use. Stored manures would have lower crop N availability (due to N immobilisation and losses during storage).

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Table 5. Estimated crop available N (kg/ha) from poultry manures applied to the soil surface at 250kg/ha total N, compared with typical N requirements of winter wheat and first cut grass (kg/ha).

Manure Application Crop available N Typical N requirementc

ratea (t/ha) Autumn(Aug-Oct)b

Spring(Feb-Apr)

Winter wheat

1st cut grass

Layer manure 16 25 88 200 150Broiler/turkey litter 8 25 75 200 150

a. Assuming typical manure total N concentrations (Anon., 2000)b. Applied to sandy or shallow soilsc. From RB209 (Anon., 2000)

Table 6. Estimated crop available N (kg/ha) from cattle and pig slurries applied to the soil surface at 250kg/ha total N, compared with typical N requirements of winter wheat and first cut grass (kg/ha).

Manure Application ratea Crop available N Typical N requirementc

(m3/ha) Autumn(Aug-Oct)b

Spring(Feb-Apr)

Winter wheat

1st cut grass

Dairy (6% DM) 83 13 88 200 150Beef (6% DM) 109 13 88 200 150Pig (4% DM) 63 13 125 200 150

DM = Dry mattera. Assuming typical manure total N concentrations (Anon., 2000)b. Applied to sandy or shallow soilsc. From RB209 (Anon., 2000)N availability increases with decreasing dry matter (DM) content, due to lower ammonia volatilisation losses.

8.4.2 Current practice and potential for improved utilisation of manure NAs can be seen from Tables 4 to 6, manure crop available N typically ranges between 13kg/ha from an autumn application to 125kg/ha following a spring application. In current practice, manure applications are made throughout the year depending on crop type, soil conditions and manure storage capacity. Around 50% of pig and poultry manures are applied in the autumn (Aug-Oct) to cereal stubbles in the predominantly arable areas where pig and poultry units are located. About 40% of cattle slurry is applied in spring (Feb-April), with 35% of cattle FYM applied in both autumn and spring (Williams et al., 2000). In terms of the potential crop available N supply from farm manures, autumn applications are likely to contribute as little as 5-15% of winter wheat and first cut silage N requirements (assuming a requirement of c.200kg/ha N for cereals and c.150kg/ha N for first cut silage), and spring applications between 25-80% of crop N requirement.

Thus, manures are a valuable source of crop N supply, but can not completely replace the need for manufactured N fertilisers (within the N application limits that apply in NVZs and are recommended in the Defra Water Code). Also, it is recommended that manure applications should supply no more than 50-60% of the total crop N requirement, with fertiliser N used to make up the difference; this is in order to minimise the potential impact of variations in manure N supply on crop yields and quality (Anon., 2000; Chambers et al., 2001). With livestock farming concentrated in the west of the UK and arable farming in the east, transport costs, logistics, economic and bio-security constraints restrict the opportunities for use of farm manures on many farms.

Based on the quantities and timings of farm manure applications to agricultural land in the UK, it is estimated that of the 450kt total N applied per year (Table 3), c.100kt is crop available (c.30 kg/ha crop available N). This would increase to c.150kt crop available N, if all manure applications were made in the spring. Annual statistics on fertiliser use (British Survey of Fertiliser Practice, 1994 et seq.) indicate that the typical allowances

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made by farmers for the N supply from manures are c.15kg/ha before cereals and sugar beet, and c.40kg/ha before oilseed rape, but no consistent allowances are made on potatoes or grassland (mean of 5 years data 1994-96 and 1998-99). Overall, it is evident that farmers do not make full allowance for the crop available N supplied from farm manures; there is considerable scope for farmers to increase the allowances made for manure crop available N when planning their use of fertiliser N (Smith & Chambers, 1993; Chambers et al., 2000). However, even these improvements would overall only have a small effect on the need for manufactured fertiliser N.

8.5 Sewage sludges (biosolids)Sewage sludges (biosolids) are a valuable source of crop available nutrients and organic matter (Anon., 1986). Recycling to agricultural land is generally recognised as the Best Practicable Environmental Option (Anon, 1996b). In 1996/97, around 520kt of sewage sludge dry solids (ds) were recycled to 80,000ha of agricultural land (60% of arable and 40% of grassland), at an average application rate of 6.5t/ha of dry solids (Gendebien et al., 1999). Sludge applications supply c.18kt N/annum (c.f. 450kt N from farm manures, Table 3) at an average application rate of 230kg/ha of N. It is likely that the quantities recycled to land will remain relatively static over the next decade, although there is considerable uncertainty associated with this prediction as market place forces have a strong influence on the security of this route and the EU Directive on ‘Sludge Use in Agriculture’ (CEC, 1986) is presently being revised. Digested cake and digested liquid sludge are the forms most commonly applied to farmland. As with farm manures, nitrogen is present in sewage sludges as both readily available (ammonium) N and organic N forms. The same factors that affect N supply and losses from farm manures also apply to sewage sludges (volatilisation, denitrification, leaching etc).

8.5.1 Impact of regulatory controlsAdopting the same procedure as for farm manures, Table 7 gives the crop available N from various sludges applied at a rate of 250kg/ha of total N, compared with ‘typical’ winter wheat and first cut grass silage crop N requirements.

Table 7. Estimated crop available N (kg/ha) from sewage sludges applied to the soil surface at 250kg/ha total N, compared with typical N requirements of winter wheat and first cut grass (kg/ha).

Sewage sludge Application ratea Crop available N Typical N requirementc

(m3/ha or t/ha) Autumn(Aug-Oct)b

Spring(Feb-Apr)

Winter wheat 1st cut grass

Digested liquid 125 13 125 200 150Digested cake 33 13 38 200 150Thermally dried 8 13 38 200 150Lime stabilised 42 13 38 200 150

a. Assuming typical total N concentrations (Anon., 2000)b. Applied to sandy or shallow soilsc. From RB209 (Anon., 2000)

Again these estimate represent the extremes of crop available N supply. The values will be higher on retentive soils (e.g. deep silt or clay soils), the closer that applications are made to spring, and where the biosolids have been incorporated into the soil.

A sewage sludge application will therefore typically supply between 13-125kg/ha of crop available N (Table 7). This represents between 5-10% of a typical winter wheat and first cut silage crop N requirement following an autumn application of sludge, and 20-80% following a spring application.

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8.6 Industrial ‘wastes’ and compostsApproximately 7,200kt (fresh weight) of industrial ‘wastes’ are recycled annually to agricultural land, including paper sludge, gypsum, waste lime, green waste compost, dairy, soft drink, brewing and vegetable processing ‘wastes’ (Gendebien et al., 2001). The current best estimate is that c.12kt N is applied to agricultural land annually via industrial waste materials, predominantly organic ‘wastes’ from the food processing industries. Crop available N from applications of these materials will normally contribute to the crop N requirement, though nitrogen will be subject to the same potential losses as described above. For paper sludge, however, a typical application of 100t/ha (fresh weight) will immobilise c.40kg/ha N (Aitken et al., 1998); a compensatory application of fertiliser N is justified to ensure that subsequent crop yields and quality are not compromised by this N immobilisation process.

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9. Nitrogen from legumes

Leguminous plants have rhizobia bacteria, acting in synergy with the crop roots, that have the ability to ‘fix’ atmospheric nitrogen and make this nitrogen available for uptake by the host crop. The main leguminous plants used in UK agriculture are red and white clover clover, peas and beans. Some lucerne and sainfoin are also grown.

In some grassland systems, white clover is deliberately grown as part of the sward in order to provide a natural source of nitrogen. The use of clover has probably increased in recent years, but there is no up to date information on the extent of these pastures in the UK. However, surveys conducted during the 1970s and 1980s by IGER and reported by Hopkins et al. (1995) have shown that reliance on white clover by farmers in Britain is vastly below its potential. Dairy farms occupy about 2.2 million ha of UK grassland and white clover is a significant sward constituent on less than 0.2 million ha, and a minor sward component on a further 0.2 million ha. This relatively low contribution reflects the reliance on high inputs of fertiliser-N and slurry on fields used for silage and aftermath grazing, and the favourable price of fertiliser-N relative to bought-in feed which led to farmers being encouraged to make maximum use of grass. Herbage production in April-May (for silage and spring grazing) is more easily met by applying fertiliser-N to ryegrass swards, than by white clover-based swards. Grass-white clover swards on dairy farms are mainly grazed by dairy young stock or other livestock, rather than by the dairy herd.

Of the 4.5 million ha of grassland on beef and sheep farms, white clover is a significant component on between 0.5 and 0.8 million ha and a minor component on a further 1.0 million, but swards with high levels of white clover (i.e. over 25% of herbage in summer) are relatively few. Low stocking rates and relatively low mean fertiliser-N use suit the management of grass-white clover swards. Furthermore, the seasonal growth of grass-white clover swards may coincide with seasonal variation in forage requirements, particularly in providing grazing for weaned lambs in early summer.

The use of nitrogen fertilisers on these grass/clover swards will be low or nil since excessive inputs of nitrogen to leguminous crops will inhibit activity of the rhizobia. On average, a good grass/clover sward will give an annual dry matter yield equivalent to that produced from about 200kg/ha of manufactured N fertiliser applied to a pure grass sward (Anon, 2000). This will be satisfactory for many moderately intensive grassland systems but will not achieve the potential dry matter yields sought by intensive grassland farmers who can be recommended to use up to 420kg/ha N per year for intensive silage cutting or up to 380kg/ha N per year for intensive dairy grazing (Anon, 2000). A further problem is that the amount of N fixed by clover is well known to be very variable and unpredictable and it is difficult to achieve the early dry matter production patterns that can be achieved with early applications of N to grass swards. Once incorporated into the cycle N derived from fixation behaves in an exactly comparable way to N entering the cycle via fertiliser addition.

Peas and beans are the only significant leguminous crops grown in arable systems but cover only c.200,000ha per year (less than 5% of the cropped area). No manufactured N fertiliser is applied to these crops. The nitrogen residues left in the soil after harvest can be significant, and these can leach causing nitrate pollution of waters. Nitrogen residues which do not leach will make a small contribution to the N requirement of the following crop. Mixed cropping of a legume with non-leguminous arable crops (e.g. cereals) is technically feasible but is likely to have problems of low yield (due to competition), poor quality (due to an uncertain N supply), increased pest and disease problems and harvesting difficulties (due to wet clover).

Another possible approach is to develop crop varieties that have associations with N-fixing bacteria. However, there are no prospects of this happening and any such development would probably have significant adverse consequences on the potential for crop yields and quality.

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10. Discussion and Conclusions

10.1 Nitrogen fertilisersWhen considering options for replacing AN as a source of manufactured nitrogen fertiliser, it is important to recognise the positive characteristics of AN. These fundamental characteristics have been important in allowing development of the current nitrogen fertiliser industry which is based on AN, supplied to farmers in straight or compound fertilisers. The nitrogen fertiliser industry has developed over many years so that the range of current N-containing fertiliser products generally meets the range of crop, handling and storage requirements that exist in practice. Most UK farmers have developed confidence in the efficacy of AN as a source of nitrogen and have little wish to find an alternative except for a few specific applications (e.g. foliar application).

The main characteristics of AN that must be considered for a complete or partial alternative are as follows:-

1. A UK production capacity exists. Approximately 90% of the UK’s demand for carbon dioxide is produced from these production plants.

2. AN can be used flexibly in the production of compound fertilisers, both complexes and blends, and solution fertilisers.

3. AN has a high nitrogen concentration (33.5-34.5% N) for ease of transport, handling and storage both nationally and on farms.

4. AN stores well and is not easily crushed.5. AN prills have good spreading characteristics through farm fertiliser spreaders.6. The nitrogen from AN is rapidly available for crop uptake once applied to soils and has proven agronomic

characteristics. 7. Although nitrogen from AN can leach through soils and cause nitrate pollution of water, gaseous losses

(e.g. ammonia) to the atmosphere are very small.

As with any nitrogen fertiliser, excessive or ill-timed use of AN will lead to an increased risk of pollution of the environment, mainly through the process of leaching causing pollution of water with nitrate.

10.1.1 Alternatives to ammonium nitrateTable 8 lists all nitrogen materials (including their typical N content) and inhibitors that are currently, or might be used as a source of manufactured fertiliser nitrogen. For each material, consideration is given to the prospects for replacing AN as a straight nitrogen fertiliser and to the prospects for use in the production of N-containing compound fertilisers.

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Table 8. Nitrogen materials and their potential to replace ammonium nitrate.

Nitrogen material N content (%N)

Potential to replace AN used as a straight

N fertiliser

Potential to replace AN in the production of

solid compound fertilisers

Solid N-only materialsAmmonium carbonate (AC) 25 No prospect No prospectAmmonium chloride (ACl) 25-26 Possible prospect Possible prospectAmmonium nitrate (AN) 33.5-34.5 n/a n/aCalcium ammonium nitrate (CAN) 26-28 Good prospect Good prospectCalcium cyanamide (CC) 21-22 No prospect No prospectCalcium nitrate (CN) 15.5 Possible prospect Possible prospectCrotonylidenediurea (CDU) 32 No prospect No prospectIsobutylidene urea (IBDU) 31-32 No prospect No prospectMethylene urea (MU) 38-39 Possible prospect Possible prospectOxamide (Ox) 32 Possible prospect No prospectUrea (U) 46 Good prospect Possible prospect

Solid N-only mixtures Urea ammonium nitrate (UAN solid) 26 Possible prospect No prospectUrea calcium nitrate (UCN) 40 Possible prospect Possible prospect

Solid materials containing N and other nutrientsAmmonium sulphate (AS) 21 Possible prospect Good prospectAmmonium sulphate nitrate (ASN) 25-26 Possible prospect Good prospectChilean potassic nitrate (Chilean KN) 15 No prospect No prospectDi-ammonium phosphate (DAP) 18 Possible prospect Possible prospectMagnesium ammonium phosphate (MgAP) 10 No prospect Possible prospectMono-ammonium phosphate (MAP) 11-12 No prospect Possible prospectPotassium nitrate (KN) 13 No prospect Possible prospectSodium nitrate (nitrate of soda) (NaN) 26 No prospect Possible prospectSulphur coated urea (SCU) 35-42 Possible prospect Possible prospectUrea ammonium sulphate (UAS) 38-40 Possible prospect Possible prospect

Crystalline solid materials only used in the manufacture of fluid fertilisersAmmonium polyphosphate (APP) 13 No prospect n/aAmmonium thiosulphate (ATS) 19 No prospect n/aMagnesium nitrate (MgN) 16 No prospect n/aUrea phosphate (UP) 18 No prospect n/a

Fluid and gaseous N fertiliser materialsAnhydrous ammonia (AnA) 82 No prospect n/aAqueous ammonia (AqA) 22-29 No prospect n/aUrea ammonium nitrate (UAN solution) 28-30 Good prospect n/a

InhibitorsNitrification inhibitors 0 Possible prospect Possible prospectUrease inhibitors 0 Good prospect Good prospect

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‘Nitrogen fertilising materials’ Report for Defra project NT2601 Table 9. Comparison of ammonium nitrate (AN), urea, calcium ammonium nitrate (CAN) and urea ammonium nitrate (UAN).

Ammonium nitrate Urea Calcium ammonium nitrate Urea ammonium nitrate

Physical form(s)

Prills Prills or granules Granules Clear solution

Chemistry NH4NO3; 33.5-34.5% N. CO (NH2)2; 46% N. CAN is ammonium nitrate (NH4NO3) mixed with a limestone filler; 26-

28% N.

UAN is a mixture of ammonium nitrate and urea; 37% N (w/v).

Manufacture Significant UK production capacity using natural gas;

some imports.

No UK production plant; all urea is imported. The urea manufacturing process is cheaper than AN though has a higher energy comsumption.

No UK production capacity; all CAN is imported.

Manufactured from AN liquor or prills which is mixed with solid urea.

Some UAN is imported.

Production of compound fertilisers

Allows production of a wide range of compounds

with a high N concentration.

Hygroscopic nature restricts range of compounds possible which can have

uncertain storage characteristics.

Flexible, similar to AN. A wide range of clear solution compounds can be produced.

Health and safety

AN is potentially explosive and is classified as an

oxidising agent.

Few restrictions. The AN raw material has similar risks as the production of solid AN. CAN is not classed as an oxidising agent; there are no special transport

or storage regulations.

Where manufactured from solid AN, this raw material will have the same risks as for the production of solid

AN products. UAN can be manufactured from AN liquor.

Transport and handling

There are stringent transport and storage

regulations.

High N concentration (46% N) but low bulk density so similar bulk to

AN.

Lower N concentration (26-28% N) than AN, so more bulk to handle and

transport.

High N concentration but requires road tanker transport and specialised

on-farm storage facilities.On-farm spreading

UK manufactured AN has been developed so that it

has good spreading characteristics.

Due to its low bulk density, urea has generally poor spreading

characteristics especially over wide bout widths.

Similar to AN. Requires tanker and boom equipment which can also be used for other crop spraying; potentially more accurate

application of N.Agronomic efficiency

Rapidly available for crop uptake.

Can be poorer efficiency than AN due to ammonia emissions to the

atmosphere.

Likely to be similar to AN. Probably similar to AN but risk of N lost by ammonia emission.

Environmental impact

Potential for significant losses due to leaching especially if overused.

Potential for high ammonia emissions and leaching losses if

overused.

Probably similar to AN. Potential for significant losses due to leaching especially if overused; uncertain potential for ammonia

emissions.

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Although a wide range of N materials have been identified and considered, only three ‘Good Prospect’ alternatives to AN have been identified for use as straight N fertilisers – urea, calcium ammonium nitrate (CAN) and urea ammonium nitrate (UAN) solution. Many materials have been identified as ‘No Prospect’ due to reasons such as environmental impact (e.g. ammonia emissions from ammonium carbonate); health and safety (anhydrous ammonia); slow release of N (e.g. IBDU) or low concentration of N (e.g. calcium nitrate, 15.5% N). Apart from generally higher costs of production, a potential problem of slow release forms of N is that some N may become available for crop uptake too late in the season; this N could then be at greater risk of being lost to the environment through processes such as nitrate leaching. Other materials have been identified as ‘Possible Prospect’ pending further investigation.

The main characteristics of AN, urea, CAN and UAN are compared in Table 9. A full appraisal of the potential of urea as a nitrogen fertiliser is given in the NT2601 project report on ‘Evaluation of urea-based nitrogen fertilisers’.

The main difference between AN and CAN is that CAN contains a limestone filler and thus has a lower concentration of N. There is thus more bulk to handle which will cause logistical problems for manufacturers and farmers, and add to the overall cost. Otherwise, little difference is expected between the use and efficacy of CAN as an N fertiliser material.

Although urea is already widely used as a straight N fertiliser in the UK, there are several potential problems associated with its use as a complete replacement for AN.

i) There is no existing UK manufacturing plant; all current supplies are imported with supplies and costs strongly influenced by world market forces. Based on free-market economics, it is doubtful if manufacturers would build new urea production plant in the UK.

ii) Urea is much less suitable than AN for the production of high N compound fertilisers. Urea-based compounds tend to have a short shelf life and potentially serious caking problems.

iii) Urea has a lower bulk density than AN and is thus difficult to spread evenly over typical wide bout widths (24 metres) used on many arable farms.

iv) The way that urea behaves in soil can result in much larger losses of ammonia gas following application to land. Ammonia pollution of the atmosphere is a major environmental concern and the UK is a signature to the Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (Gothenburg Protocol) of the UNECE Convention on Long-Range Transboundary Air Pollution.

It is possible that modifications to urea formulation and/or farm fertiliser spreaders may help mitigate the expected problem of poor spreading characteristics. Existing knowledge is being reviewed but new research may be needed. Similarly, methods may be available to mitigate ammonia emissions, for instance by addition of a urease inhibitor as part of the urea production process (see 6.32) and/or through adoption of agronomic measures during or following urea application. Current knowledge on these approaches are being investigated and will be reported fully in the NT2601 project report on ‘Evaluation of urea-based nitrogen fertilisers’. The effectiveness of urease inhibitors is being investigated in the NT2603 research project being carried out in 2003 cropping season.

Some N-containing materials contain other nutrients (e.g. di-ammonium phosphate, DAP) but could still replace some of the AN currently used as a straight N fertiliser. However, use of DAP would only be a partial solution because the maximum N application rate as DAP (18% N, 45% P2O5) would be severely limited by its phosphate content. Many crops only require around 50kg/ha P2O5, so a maximum of only around 20kg/ha N could be applied using DAP. Nevertheless, a fertiliser to meet all of the crop P requirement but only part of the crop N requirement is useful and is adopted in some current practice. Many fields have a zero phosphate requirement; DAP would not be suitable for use on these fields else the risk of phosphate pollution of water would increase. Several commercially available N-containing materials also contain sulphur (S) - e.g. ammonium sulphate (AS), ammonium sulphate nitrate (ASN), urea ammonium sulphate (UAS). An application of 200kg/ha N as

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UAS would also apply 95kg/ha SO3; this rate of sulphur would exceed any crop sulphur requirement. Because of the increase in sulphur deficiency, the use of NS fertilisers is increasing. These materials may have wider scope for use as N fertilisers, even if no sulphur is needed or if the application rate is above the crop S requirement. This is because sulphur is not known to cause or aggravate known agronomic or environmental problems, except possibly for aggravating copper deficiency in livestock; this effect may need further investigation. However, it is likely to be preferably to avoid use of any fertiliser material which provides a nutrient excess which may be perceived as undesirable or have unknown long term effects. ASN is based on AN and may have the same safety constraints as AN.

Table 8 shows other ‘Possible prospects’ to replace AN. Although none of these are expected to be realistic candidates for large scale production, the NT26 programme work should continue to consider their potential.

10.2 Nitrogen from organic manuresOrganic manures (farm manures, sewage sludges, composts and other industrial ‘wastes’) contain significant quantities of nitrogen. Individual applications to land can provide useful quantities of crop available N which is equally available for crop uptake as fertiliser N. Actual amounts of crop available N depend on the manure type, application rate, timing and method of application, soil type and rainfall. Provided farmers allow for this manure crop available N, application rates of manufactured fertiliser N can be reduced.

The total amount of manure crop available N supplied by organic manures currently applied to agricultural land in the UK is estimated to be c.100kt; this is c.10% of the consumption of manufactured fertiliser N. Improved manure management practices (e.g. spring application of manures) could increase this to c.150kt. Thus, although manure N can make a useful contribution to meeting the national annual crop N requirement, it cannot come anywhere near to fully meeting this demand.

Additionally:-1. Farm manures are mostly produced in areas of livestock production. Areas without livestock have little

scope to acquire supplies of organic manures, without lengthy road transport. Excessive road transport of manures would be prohibitively costly, add to problems of road congestion and potentially conflict with the national management of bio-security risks. Amounts of sewage sludge and industrial ‘wastes’ recycled to land make only a small contribution to the overall crop N requirement.

2. Regulatory controls (e.g. the Defra Water Code, Nitrate Vulnerable Zones) limit the amounts of organic manures that may be applied to individual field areas. The current limit of 250kg/ha of total N (up to c.125kg/ha crop available N depending on manure type and application circumstances) will not usually allow the crop N requirement to be fully met from manure N.

3. Since the fertiliser N value of organic manure applications cannot be estimated as precisely as the nitrogen supplied as manufactured fertiliser, current recommendations state that no more than 50-60% of the crop N requirement should be met using manure N.

10.3 Nitrogen from legumesIn arable rotations, peas and beans are the only significant leguminous crops that require no inputs of nitrogen fertilisers, but these crops cover less than 5% of the cropped area. Without specific breeding developments in the major arable crops such as cereals, there is no prospect for an increased contribution of ‘fixed’ nitrogen by leguminous crops in arable rotations.

In grassland systems, clover is used in some swards and provides useful quantities of fixed nitrogen that reduces the need for manufactured nitrogen fertilisers. However, grass/clover swards will not generally support intensive production systems, notably intensive dairy production. Less than 10% of dairy swards have a

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significant clover content. It is recognised though that there is considerable potential for grassland farmers to increase reliance on clover as a source of nitrogen.

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11. References

Anon (1970). Urea in compound fertilisers. A review of urea’s role as the principal source of nitrogen in granulated compound fertilisers. Nitrogen, The British Sulphur Corporation Ltd, London.

Anon (1986). The Agricultural Value of Sewage Sludge. A Farmers Guide. Water Research Centre, Medmenham, Marlow, Bucks.

Anon (1996a). Code of Practice for Agricultural Use of Sewage Sludge. Available from DETR Publications Sales Unit.

Anon (1996b). Sustainable Use of Soils (Nineteenth Report of the Royal Commission on Environmental Pollution) HMSO, London.

Anon (1998). The Water Code. Code of Good Agricultural Practice for the Protection of Water. MAFF Publications. London (PB0587).

Anon (2000). Fertiliser Recommendations for Agricultural and Horticultural Crops (RB209), Seventh Edition. The Stationery Office, Norwich.

Anon (2001). The Safe Sludge Matrix – Guidelines for the Application of Sewage Sludge to Agricultural Land (3rd edition). Available from ADAS Gleadthorpe Research Centre, Meden Vale, Mansfield, Notts. NG20 9PF.

Anon (2002a). Ammonia in the UK. Department for Environment, Food and Rural Affairs. Defra Publications, London (PB6865).

Anon (2002b). Guidelines for Farmers in NVZs – England. Defra Publications, London (PB5505).

Aitkin, M.N., Evans, B. & Lewis, J.G. (1998). Effect of applying paper mill sludge to arable land on soil fertility and crop yields. Soil Use and Management 14, 215-222.

British Survey of Fertiliser Practice (1994 et seq.). Fertiliser use on farm crops, England and Wales 1994; 1995; 1995; 1998; 1999. Survey of Fertiliser Practice. MAFF Publications, London.

CEC (1986). Commission of the European Communities Council Directive 12 June 1986 on the protection of the environment, and in particular the soil, when sewage sludge is used in agriculture. Official Journal of the European Communities No. 2.181 (86/278/EEC), pp. 6-17.

Chambers, B.J. & Smith, K.A. (1995). Management of farm manures: economic and environmental considerations. Soil Use and Management 11, 150-151.

Chambers, B.J., Smith, K.A. & van der Weerden, T.J. (1997). Ammonia emissions following land spreading of solid manures. In: Nitrogen Emissions from Grasslands (Eds. S.C. Jarvis & B.F. Pain), CAB International, pp. 275-280.

Chambers, B.J., Smith, K.A. & Pain, B.F. (2000). Strategies to encourage better use of nitrogen in animal manures. Soil Use and Management 16, 157-161.

Chambers, B., Nicholson, N., Smith, K., Pain, B., Cumby, T. & Scotford, I. (2001). Managing Livestock Manures – booklet 1; Making better use of livestock manures on arable land. Available from ADAS Gleadthorpe Research Centre, Meden Vale, Mansfield, Notts. NG20 9PF.

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Chaney, K. (1990). Evaluating blended fertilisers in comparison a complex fertiliser. Report 20, Levington Agriculture Ltd (on behalf of Hydro Agri UK Ltd.)

Defra (2002). Ammonium nitrate materials (high nitrogen content) safety regulations 2003. Consultation document issued 17 December 2003, www.Defra.gov.uk/corporate/consult/ammonium-nitrate.

FMA (2002). The Fertiliser Review 2002. Available from FMA, Great North Road, Peterborough, PE8 6HJ.Gendebien, A., Carlton-Smith, C., Izzo, M. & Hall, J.E. (1999) UK Sewage Sludge Survey: National Presentation (R&D Technical Report, P.165). Environment Agency.

Garrett K (1987). The rationale for mixed ammonium nitrate – urea fertilisers and assessment of granular products. Proceedings No 257. The International Fertiliser Society, York.

Gendebien, A., Ferguson, R., Horth, H., Sullivan, M., Davis, R., Brunet, H., Dalimier, F., Landrea, B., Krack, D., Perot, J. & Orsi, C. (2001). Survey of Wastes Spread on Land. Final Report of DG Environment Study Contract B4-3040/99/110194/MAR/03. Website http: //europa.eu.int /comm/environment /waste/landspreading.pdf.

HSE (2001). Storing and handling ammonium nitrate. Publication INDG230, Health and Safety Executive.

Hopkins, A., Davies, D.A. and Doyle, C.J. (1995). White clover - its present role and future prospects in British grassland farming. Journal of the Royal Agricultural Society of England, 156, 11-23.

MAFF (1980). Ammonia as a fertiliser. Leaflet 751.

Mulvaney R L and Bremner J M (1987). Control of urea transformations in soils. In: Paul E A and Ladd J N (Eds). Soil Biochemistry, 5, 153-196. Marcel Dekker, New York.

Ohlsson A (2000). Fertiliser coatings. Proceedings No. 453, The International Fertiliser Society, York.

Overdahl C J, Rehm G W and Meredith H L (1991). Fertiliser urea. University of Minnesota Extension Service, www.extension.umn.edu/distribution/cropsystems/DC0636.html

Richards I R (2001). The Fertiliser Directory. Context Design, Ashby de la Zouche, Leicestershire, UK. 108 pp.

Ryden, J., Skinner, J. & Nixon, J. (1987). Soil core incubation system for the field measurement of denitrification using acetylene inhibition. Soil Biology and Biochemistry 19, 753-757.

Shah K D (1996). Safety of ammonium nitrate fertilisers. Proceedings No 384. The International Fertiliser Society, York.

SI (1994). Waste Management Licencing Regulations 1994. SI 1056, The Stationery Office, London.

SI (1989). The Sludge (Use in Agriculture) Regulations 1989. SI 1263. The Stationery Office, London.

SI (1990). The Sludge (Use in Agriculture) (Amendment) Regulations 1990. SI 880. The Stationery Office, London.

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Smith, K.A. & Chambers, B.J. (1993). Utilising the nitrogen content of organic manures on farms – problems and practical solutions. Soil Use and Management 9, 105-112.

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Williams, J.R., Chambers, B.J., Smith, K.A. & Ellis, S. (2000). Farm manure land application strategies to conserve nitrogen within farming systems. In: Agriculture and Waste Management for a Sustainable Future (Eds. T. Petchey, B. D’Arcy & A. Frost) The Scottish Agricultural College, pp. 167-179.

Wozniak H, Michel H J and Fucjs M (1999). Nitrification inhibitors for economically efficient and environmentally friendly nitrogen fertilization. In proceedings of Managing Plant Nutrition conference, Barcelona June/July 1999, The International Fertiliser Society, York.

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