Westermann, D.T. 1993. FertilityManagement. p. 77-86. In R.C. Rowe (ed.).Potato Health Management, Ch. 9. APS Press,ISBN 0-89054-144-2.

illfiSTER COPY 3°°

Potato Health Management

edited by

Randall C. RoweDepartment of Plant PathologyOhio State University, Wooster

Steering Committee

David CurwenDepartment of Horticulture

University of Wisconsin, Madison

David N. FerroDepartment of Entomology

University of Massachusetts, Amherst

Rosemary LoriaDepartment of Plant Pathology

Cornell University, Ithaca, New York

Gary A. SecorDepartment of Plant Pathology

North Dakota State University, Fargo

in cooperation with

The Potato Association of America

APS PRESSThe American Phytopathological Society

9000 >

Potato Health ManagementPotato Health Management is an interdisciplinary guideto complete potato health from seed to storage. EditorRandall C. Rowe and 26 other experts from such disciplines

as soil science, weed sci-ence, crop science, nema-tology, entomology, andplant pathology presentstep-by-step advice onraising a healthy potatocrop as well as easilyunderstood discussionsabout the interactions ofproduction practices and

pest- and disease-management strategies. Soil management;seed handling and planting; plant nutrition; weed, pest,and disease control; water management; and harvest andstorage techniques are all covered in this one holistic guide.Useful in all production regions of North America, PotatoHealth Management contains many color photos to assistin identifying important problems. The book is the secondtitle in the interdisciplinary Plant Health ManagementSeries.

This guide was written especially for:O Commercial potato growersO Field consultants and farm advisorsO Extension specialistsO ResearchersO StudentsO Professionals in all aspects of the potato industry

Potato GrowersLearn how to produce a health-ier crop using economicallyviable and environmentallysound methods beginningbefore planting and continuingafter harvest.

Potato Production AdvisorsGuide growers toward greater efficiency, profitability, andenvironmental stewardship through state-of-the-art potatoproduction and crop protection methods. This hard-workingreference integrates all the information growers need tomanage the whole health of the potato crop and the completecropping system.

ABOUT THE EDITORRandall C. Rowe is profes-sor of plant pathology atthe Ohio AgriculturalResearch and Develop-ment Center, The OhioState University, Wooster.For nearly 20 years, he hasbeen involved in researchand extension in diseasemanagement of potatoesand other crops, withprimary emphasis on root

diseases. He has extensive experience in diseasediagnosis and advising growers and has con-ducted numerous on-farm trials. He has workedwith researchers across North America and inseveral foreign countries to improve the under-standing and practice of potato health manage-ment. Originally from Michigan, Dr. Roweearned a B.S. degree from Michigan State Uni-versity and a Ph.D. in plant pathology from Ore-gon State University.

CONTRIBUTORSErnest E. Banttari, University of Minnesota;Robin R. Bellinder, Cornell University; GillesBoiteau, Agriculture Canada; Robert H. Callihan,University of Idaho; Richard W. Chase, MichiganState University; David Curwen, University ofWisconsin; Peter J. Ellis, Agriculture Canada;David N. Ferro, University of Massachusetts;K. L. Flanders, University of Minnesota; Neil C.Gudmestad, North Dakota State University; MaxW. Hammond, Cenex/Land ()lakes AgronomyCompany; Larry K. Hiller, Washington StateUniversity; Kenneth B. Johnson, Oregon StateUniversity; S. M. Paul Khurana, Central PotatoResearch Institute; Ann E. MacGuldwin, Univer-sity of Wisconsin; Alvin R. Mosley, Oregon StateUniversity; Edwin S. Plissey, University of Maine;Mary L. Powelson, Oregon State University;E. B. Radcliffe, University of Minnesota; D. W.Ragsdale, University of Minnesota; Randall C.Rowe, Ohio State University; Gary A. Secor,North Dakota State University; Steven A. Slack,Cornell University; Robert G. Stevens, Washing-ton State University; Walter R. Stevenson,University of Wisconsin; Robert E. Thornton,Washington State University; D. T. Westermann,U.S. Department of Agriculture

ISBN 0 - 89054- 144- 2

111 11 1119 780890 541449

D. T. WestermannU.S. Department of AgricultureAgricultural Research Service, Kimberly, Idaho

Fertility Management

A comprehensive strategy for potato health managementrequires that all essential plant nutrients be available inamounts needed for optimal potato growth and development.Essential nutrients are those required for a normal life cycleand for which no other nutrient can be substituted. Sixteennutrients are known to be essential for higher plants. Thesefunction as major constituents of plant cells or are used inmetabolic processes within the plant. Carbon, hydrogen, andoxygen are supplied in water and in the atmosphere. Theremaining essential nutrients must be taken up by the rootsof the plant from the soil solution or absorbed by the leavesfrom foliar sprays.

Nutritional Needsof the Potato Plant

The nutritional requirements of the potato plant change atvarious stages of crop development. Appropriate fertility man-agement strategies for potato production are based on thedifferent nutritional needs of the crop at each growth stage.

Changes in Nutritional NeedsAs the Crop Develops

The distribution of dry matter within a potato plant changesat each stage of growth and development (Fig. 9.1). Duringgrowth stage I (sprout development), the seed piece is the solesource of energy for growth, because photosynthesis has notyet begun. Soil nutrients are not available to the plant untilroots develop.

Photosynthesis begins during growth stage II (vegetativegrowth) and provides energy for the growth and developmentof vegetative parts of the plant. Roots actively absorb plantnutrients from the soil during this stage.

The onset of growth stage III (tuber initiation) is controlledby growth-regulating hormones produced in the plant. Beforetuber initiation can begin, photosynthesis must supply morecarbohydrate (in the form of sucrose) than is needed for thegrowth of leaves, stems, and roots. Soil moisture and temper-ature, nitrogen nutrition, and plant hormones all affect tuberinitiation.

During growth stage IV (tuber bulking), development pro-ceeds in a nearly linear fashion if no growth factor becomeslimiting. Tubers become the dominant sink for carbohydrates

and mobile inorganic nutrients. Carbohydrates in the formof sucrose are transported into tubers and converted to starch,which increases the ratio of dry matter to water in the tubers.Environmental and other factors may reduce the plant's abilityto supply carbohydrates or inorganic nutrients at sufficientrates for tuber growth. If this occurs, these materials aresolubilized in the vegetative parts of the plant and transportedto developing tubers, which eventually causes premature senes-cence of the vines. Plants in this condition may be moresusceptible to certain diseases, such as early blight and earlydying. Late-maturing cultivars may continue some vegetativegrowth during tuber bulking, but at a slower rate than ingrowth stage III. Most of the nutrients used by the plant aretaken up during growth stage IV, and uptake is nearly completeby the end of this stage.

During growth stage V (maturation), the dry matter contentof the tubers reaches a maximum. Nutrients in the tops androots are solubilized and moved into the tubers, which mayaccumulate an additional 10-15% of their dry weight duringthis stage. The portion of nutrients absorbed by the plant thatends up in the tubers at harvest depends upon the mobilityof each nutrient within the plant. In the case of highly mobile

Most of the nutrients used bythe plant are taken up duringgrowth stage IV.

nutrients, such as nitrogen, 90% or more of the total amounttaken up by the plant may end up in the tubers at maturity.In the case of relatively immobile nutrients, such as calcium,however, the portion is only 10-20% of the total uptake.

Nutritional DisordersNutritional disorders can result from deficiencies, excesses

(toxicities), and antagonisms between nutrients in the soil oras they are taken up by the plant. Soil pH plays a majorrole in nutrient availability (Fig. 9.2). At pH 5 and below,problems may be encountered with calcium, magnesium, andmolybdenum deficiencies; phosphorus fixation; ammonium,manganese, and aluminum toxicities; and increased leachingof some elements, such as magnesium. At pH 7.5 and above,deficiencies of boron, copper, iron, manganese, and zinc mayoccur, and phosphorus may be less available, because of the

77

Growth StageJI[ DT

40

20

78 • CHAPTER NINE

100

80

20 40 60 80 100 120

140

160

Days After PlantingFig. 9.1. Dry matter distribution in Russet Burbank, a late-maturing cultivar. Early-maturing cultivars have similar distributions,with their growth stages beginning earlier. Local growing conditions affect the actual timing of growth stages. The growth stagesare defined in Figure 1.3.

)..F Q9_ ie-co.

E.2.yV122 84

=. =a, 2 2 a)

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mt

..2 co

zzl,

sm...................

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3 NI.. ....-....

4.5 5 55 6 6.5 7 7.5

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85

9

pH

Fig. 9.2. Availability of nutrients to plants in relation to soil pH.The widest parts of the bars indicate maximum availability.(Reprinted, by permission, from R. J. Cook and R. J. Veseth,1991, Wheat Health Management, American PhytopathologicalSociety, St. Paul, MN, as redrawn from C. J. Pratt, 1965, Chemicalfertilizers, Scientific American 212 [June]: 62-72)

precipitation of calcium phosphates.Nutrient deficiencies in potato plants can sometimes be

identified from visible symptoms (Box 9.1), but visualdiagnoses can be misleading, and diagnosis can be complicatedby nonnutritional factors or multiple deficiencies. Symptomsmay not be readily apparent as nutritional problems develop,and by the time they are visible it is often too late to makecorrections to avoid a loss of tuber yield or quality. Another

problem with visual diagnosis is that symptoms of nutrientdisorders are sometimes masked by those due to infection bypathogenic microorganisms, and some disease symptomsresemble those of certain nutrient disorders. More precise andtimely identification of nutrient disorders can be obtained froma chemical analysis of plant tissues (see Nutritional Manage-ment Tools, below).

Factors AffectingNutritional Needs

Potential Tuber YieldAs long as pests and diseases are not limiting factors,

adequate nutrition will maximize tuber yield potential withinthe genetic limitations of the cultivar and the climate in whichit is grown. Rates of nitrogen, phosphorus, and potassiumuptake for tuber bulking (growth stage IV) in selected cultivarsare shown in Table 9.1. These may be adequate for early-maturing cultivars if the plants are large enough to meet thedemand for products of photosynthesis. In late-maturingcultivars, such as Russet Burbank, nutrient uptake rates duringgrowth stage IV must be slightly greater than that neededfor tuber growth. This is necessary to allow for some newvegetative growth to offset the effects of leaf aging and maintainmaximum production of dry matter. Low nutrient concen-trations may reduce the rate of photosynthesis and ultimatelyresult in lower tuber yield if insufficient quantities of theproducts of photosynthesis are generated during growthstage IV.

Potential tuber yield and appropriate fertilization strategiesare affected by the maturity class of the cultivar grown andthe length of the growing season. Early-maturing cultivarsgenerally have a shorter growing cycle from planting untilthe start of maturation. They generally have higher rates of

Nitrogen

Phosphorus

Potassium

Calcium

Magnesium

Sulfur

Iron

Manganese

Boron

Copper and Zinc

Molybdenum

FERTILITY MANAGEMENT • 79

nutrient uptake during growth stages II and III and henceneed adequate preplant fertilization. Late-maturing cultivarsmay have a greater maximum leaf area and may produce moretotal dry matter, and their potential tuber yield may thus begreater if healthy leaves remain active long enough.

The potential tuber yield of a late-maturing cultivar alsoincreases as the length of the growing season increases, pro-vided other factors are not limiting. In a 130-day frost-freegrowing season, growth stage IV may last about 70 days; ina 180-day growing season, it may last 100 days or longer.If the average daily tuber growth rate is 700 pounds per acre,the potential tuber yields in these two growing seasons wouldbe 490 and 700 cwt per acre, respectively. Approximately 70more pounds of nitrogen per acre would be required for tubergrowth in the longer growing season. Proportionally more ofthe other nutrients would also be needed.

Nutrient InteractionsThe plant's response to the application of a specific nutrient

depends on the availability of other nutrients. The responseto one nutrient is limited if any other nutrient is insufficientlyavailable. This situation can be corrected by identifying thenutrient or nutrients that are limiting and applying the amountsneeded to attain the potential yield of the cultivar in the pro-duction area where it is grown.

For example, chloride applications suppress the concentra-tion of nitrate in the petiole. Potassium suppresses magnesiumuptake, and vice versa. High amounts of ammonium suppresspotassium uptake. Excessive iron induces manganese defi-ciency. Heavy applications of phosphorus induce zinc defi-ciency in soils low in available zinc. Nitrogen and sulfur arerelated within the plant, because they are essential compo-nents of protein, in a ratio of about 15:1.

These problems can be offset somewhat by the capacity ofthe plant to make use of nutrients over a wide range of avail-ability in the soil if individual elements are not at deficientor toxic concentrations. In addition, fertile soils have somebuffering capacity to supply needed nutrients regardless ofwhat is applied as fertilizer.

Environmental FactorsSoil and air temperatures have a major effect on the early-

season growth of potatoes. Low soil temperatures duringgrowth stages I and II reduce rates of root growth and nutrientuptake, especially phosphorus uptake. It is possible to partiallycompensate for low soil temperatures by using starter fer-

Table 9.1. Tuber growth rates and nutrient use by selected potatocultivars during growth stage IV

Cultivar'

Average dailygrowth rate

of tubers(lb/ acre)

Daily nutrient use(lb/ acre)

Nitrogen Phosphorus PotassiumRusset Burbank 850 2.5-3.6 0.37-0.54 2.8-3.6Lemhi Russet 890 2.2-3.9 0.39-0.56 2.9-3.7Centennial Russet 800 2.5-3.6 0.35-0.50 2.6-3.4Norgold Russet 1,070 2.7-4.0 0.47-0.67 3.5-4.5Pioneer 1,200 3.1-4.9 0.53-0.76 3.9-5.0Norchip 700 1.8-2.8 0.31-0.44 2.3-2.9Kennebec 1,300 3.3-5.2 0.57-0.82 4.2-5.5Red McClure 1,000 3.4 0.50 3.3Oromonte 1,000 3.0 0.40 3.4White Rose 860 2.9 0.35 4.5Four unspecified

cultivars (avg.) 800 2.8 0.28 4.2'Russet Burbank, Lemhi Russet, Centennial Russet, Norgold Russet,

Pioneer, Norchip, and Kennebec from Kimberly, Idaho; Red McClureand Oromonte from Colorado; White Rose from California; andfour unspecified cultivars from Maine.

Box 9.1

Nutrient Deficiency Symptomsof Potato Foliage

Nitrogen Entire plants may turn light green. Youngleaves remain green; older leaves turn yel-low to light brown and become senescent.

Phosphorus Plants are stunted. Leaves are dark green,and their margins roll upward. Somepurpling occurs in pigmented leaves. Theseverity of leafroll increases as the defi-ciency increases.

Potassium Plants may be stunted. Young leavesdevelop a crinkly surface, and their mar-gins roll downward. Leaves have slightlyblack pigmentation. Marginal scorchingwith necrotic spots may occur on olderleaves.

Calcium The youngest mature leaves roll upwardand become chlorotic with brown spot-ting. Growing buds may die. In tubers, abrown discoloration develops within thevascular ring before symptoms appearon vegetation.

Magnesium Young mature leaves are affected withinterveinal chlorosis and brown spotting,which develop into interveinal scorchingand necrosis. Leaves near growing budsremain green.

Sulfur The symptoms are similar to those ofnitrogen deficiency, except that chlorosisdevelops first in young leaves. Affectedleaves turn uniformly yellow.

Boron Growing buds die. Plants appear bushy,having shorter internodes. Leaves thickenand roll upward. Leaf tissue darkens andcollapses.

Zinc Young leaves are chlorotic, narrow, andupward-cupped and develop tipburn.Other leaf symptoms are green veining,necrotic spotting, blotching, and erectappearance.

Iron Young leaves are yellow to nearly whitebut not necrotic. Leaf tips and edgesremain green the longest. Green veiningoccurs in leaves.

Manganese Young leaves are affected with interveinalchlorosis and then gray and black fleck-ing and leaf cupping. The flecks eventu-ally develop into small dead patches.

Copper Young leaves develop a pronouncedrolling, and then leaf tips wilt and die.Leaves remain green and are of normalsize.

Molybdenum Leaves turn yellow or greenish yellow.The symptoms are similar to those ofnitrogen deficiency.

Chloride Young leaves are light green, turnpurplish bronze, and may curl upwardor appear pebbled.

80 • CHAPTER NINE

tilizers, by banding fertilizers at planting, or by using higherfertilizer application rates. The objective is to provide nutrientssufficient to promote early development of leaf area for optimallight interception and photosynthesis, but not enough to stimu-late excessive vegetative growth. This is important for bothmaturity types, but particularly for late-maturing cultivars,as relatively high concentrations of available nitrogen duringgrowth stages II and III'tend to delay the onset of linear tubergrowth rates in these types.

High soil temperatures can accelerate early plant develop-ment and thus hasten senescence, particularly in early-maturingcultivars. Premature senescence can also result from physio-logical stresses on the plant, caused by high ambient temper-atures, air pollutants, or low soil moisture. Abnormally hightemperatures coinciding with high nitrogen status in the plantduring growth stage IV can stimulate excessive vegetativegrowth in late-maturing cultivars. This may limit the growthof tubers or cause problems with tuber quality.

The specific effects of adverse temperatures are difficult topredict, because of the many relationships between variousfactors affecting plant growth. In general, high nutrient concen-trations in the plant do not relieve plant stresses caused byclimatic abnormalities and may intensify their effects.

Soil moisture also has important effects on plant growthand affects some fertility relationships (Chapter 8).

Diseases and Tuber DisordersThe severity of many potato diseases often increases if the

plants are also stressed by heat, insufficient amounts of nutri-ents or water, or other adverse environmental factors. Goodmanagement of soil fertility and plant nutrition can help tosuppress many diseases and minimize their effects on yield.Appropriate applications of nitrogen, phosphorus, and potas-sium, based on analyses of soil and leaf tissue, will help suppresssymptoms of potato early dying (Chapter 17) and early blight(Chapter 16). Heavy applications of nitrogen can stimulateexcessive vegetative growth, resulting in an extensive plantcanopy. The moist microclimate maintained underneath sucha canopy promotes the development of aerial stem rot (Chapter15), Sclerotinia stalk rot (Chapter 17), and tuber diseasesassociated with wet soils, such as pink rot (Chapter 17).

The severity of many potatodiseases often increases if theplants are also stressed by heat,insufficient amounts of nutri-ents or water, or other adverseenvironmental factors.

A nutrition program that allows tubers to reach full maturitybefore harvest generally helps to minimize losses resulting fromtuber disorders and diseases. The specific gravity of tubertissues is lower in relatively immature tubers harvested becauseof premature senescence or because normal maturation wasprevented. Nutrient deficiencies can cause premature senes-cence, whereas high rates of nitrogen fertilization tend to delaynormal maturation, particularly in late-maturing cultivars.Tubers that attain high specific gravity with good fertilitymanagement practices also tend to have lower concentrationsof reducing sugars, good chip and fry colors, and fewer prob-lems with blackspot bruise and decay. Adequate calciumconcentrations in tubers may also help reduce bacterial softrot in storage. Additional calcium applied to soils in whichcalcium is limiting may also reduce internal tuber disorders,such as internal brown spot, brown center, and hollow heart.

Good fertilization and nutrient management practices willmaximize tuber quality (Chapter 10).

Nutritional Management Tools

Fertility management is an important part of any holisticpotato health management program. The goal in managingcrop nutrition is to promote uniform and continuous growthof plants and tubers throughout all growth stages. Soil testingand plant tissue testing for nutrient analysis should be usedroutinely by managers to guide nutritional programs.

Soil AnalysisThe relative availability of nutrients in the soil before

planting can be measured by soil testing. Soil test data canbe used in selecting fertilizers and application rates that areappropriate for the specific needs at a particular planting site.Soil testing can also identify production areas where additional

The goal in managing cropnutrition is to promote uniformand continuous growth of plantsand tubers throughout allgrowth stages.

fertilizers are not needed, enabling growers to cut expensesand avoid overfertilization and potential environmental pol-lution. Different soil-testing laboratories may use differentchemical procedures and may vary in their interpretations ofthe results and their subsequent recommendations. Labora-tories using procedures developed and calibrated for a specificgrowing region generally give the best results in that area.

Soil samples submitted for analysis must be representativeof the sampled area. It is essential that each field be sampledin such a way that variations within the field are accuratelyrepresented. In general, fields should be divided into areasof uniform soil color or texture, cropping history, and fertili-zation or manuring history. One sample should represent nomore than 20 acres, even if the soil is uniform. Usually, 10-20soil cores (0.75 inch in diameter) are randomly taken in azigzag pattern across the sampled area and then combinedto make up a single sample. A sampling depth of 12 inchesis adequate for most nutrients, but for analysis of residualnitrogen a second sample should be taken from the 12- to24-inch profile. Extensive precipitation that causes consider-able leaching following soil sampling can nullify the analysisof residual nitrogen. Managers are encouraged to seekprofessional help in determining correct sampling proceduresfor their particular fields. It cannot be overemphasized thatan appropriate preplant fertilization program depends onthorough and accurate soil sampling for nutrient analysis ineach field.

Fertilizer recommendations are based on soil test results,the potential yield of the chosen cultivar at a particular site,the intended market for the crop, the efficiency of variousapplication methods, the soil texture, and the amount of plantnutrients available from other sources, such as manure orirrigation water. All these variables must be integrated intoa single recommendation to fit a particular situation. Individualgrowers, assisted by professional advice, must ultimately deter-mine their own fertilizer programs.

Plant Tissue AnalysisChemical analysis of plant tissues is widely used as a

diagnostic tool to determine nutrient status during crop growth.

FERTILITY MANAGEMENT • 81

This method is based on known relationships between nutrientconcentrations in plant tissues and the growth rate or yieldof the plant at different growth stages. The growth rate islow at low nutrient concentrations but increases considerablyat slightly higher concentrations and continues to increase withnutrient concentration until nutrition no longer limits growth(Fig. 9.3). Further increases in nutrient concentrations do notincrease the growth rate and may actually reduce it if theyreach toxic levels. An appropriate nutrient managementprogram is one that maintains nutrient concentrations in the"sufficient" range during all stages of plant growth, but notin the deficient or the toxic range.

Particular tissues are usually selected for chemical analysisbecause their nutrient concentrations relate well to the nutri-tional status of the whole plant and are sensitive to changesin the availability of nutrients. In potatoes, the petiole of thefourth leaf from the top of the plant is generally used forchemical analysis (Fig. 9.4). The leaflets are stripped off thepetiole and discarded immediately after sampling. Sometimesthe entire leaf is used, rather than just the petiole.

Plant samples for tissue analysis must be representative ofthe sampled field. Approximately 40-50 petioles should becollected from the sampled area, in a pattern similar to thatused in collecting soil samples. Areas with different soil typesor different cropping and fertilization histories should besampled separately. Tissue samples should be dried imme-diately at 150° F or kept cool until submitted, because nutrientconcentrations may change in moist samples that are storedwarm. They should be placed in clean bags or containers inwhich they cannot be contaminated with any nutrient element.Details on sampling procedures and guidelines for handlingtissue samples should be obtained from the plant analysislaboratory where they will be submitted.

Problems encountered in using tissue analysis to monitornutritional status include selection of the correct leaf forsampling, changes in nutrient concentrations with plant age,recent fertilizer applications, and differences between cultivars.

It is very important that tissue samples be taken only fromthe fourth leaf from the top of the plant. Analysis of samplesconsistently taken from younger or older leaves gives signifi-cantly different results, which do not represent the actualnutritional status of the plant.

Nutrient concentrations in plant tissues change with the ageof the plant. Most nutrients are at their peak concentrationin vegetative tissues during tuber initiation (growth stage III),and the concentration usually declines until late maturation(growth stage V) unless additional amounts are applied during

Deficient Sufficient Toxic

Tissue nutrient concentration —+Fig. 9.3. Growth or yield of potato plants in relation to the nutrientconcentration in plant tissues.

the growth of the crop. Thus, a nutrient concentration thatis adequate during growth stage III is generally more thansufficient for growth during growth stage IV. Nitrogen,phosphorus, potassium, copper, zinc, and sulfur decrease inconcentration in foliage with increasing plant age, if no addi-tional nutrients are applied, while calcium, magnesium, boron,iron, chloride, and manganese increase in concentration.

A

4thleaf

6

Fig. 9.4. Vegetative shoot (A) and shoot with a floral spike (B).The fourth leaf from the top of the plant is used in tissue analysesto determine the nutrient status of the plant during growth.

Table 9.2. Suggested ranges of nutrient concentrations in the fourthleaf from the top of the potato plant during growth stage IV

Low Marginal Sufficient

Petiole without leaflet?Nitrate nitrogen, b ppm < 10,000 10,000-15,000 > 15,000Phosphate

phosphorus, b ppm < 700 700-1,000 > 1,000Phosphorus, % < 0.17 0.17-0.22 > 0.22Potassium, % < 7.0 7.0-8.0 > 8.0Calcium, % < 0.4 0.4-0.6 > 0.6Magnesium. % < 0.15 0.15-0.3 > 0.3Sulfur, % <0.15 0.15-0.2 > 0.2Sulfate sulfur," ppm < 200 200-500 > 500Zinc, ppm < 10 10-20 > 20Boron, ppm < 10 10-20 > 20Manganese, ppm <20 20-30 > 30Iron, ppm <20 20-50 > 50Copper, ppm < 2 2-4 > 4Molybdenum, ppm `...

Entire leaf (petiole plusleaflets)'

Nitrogen, % < 2.5 2.5-3.5 > 3.5Phosphorus, % < 0.15 0.15-0.25 > 0.25Potassium, % < 2.25 2.25-3.50 > 3.50Calcium, % < 0.30 0.30-0.60 > 0.60Magnesium, % < 0.15 0.15-0.25 > 0.25Sulfur, % < 0.12 0.12-0.20 > 0.20Zinc, ppm < 15 15-20 > 20Boron, ppm < 10 10-20 > 20Manganese, ppm < 10 10-20 > 20Iron, ppm < 11 11-30 > 30Copper, ppm < 2.0 2.0-5.0 > 5.0Molybdenum, ppm ... ` > 1.0

'Values for petiole concentrations are for Russet Burbank; those forthe entire leaf are suitable for many cultivars.

b Concentration of soluble nutrient (ppm = parts per million).`Concentration unknown.

82 • CHAPTER NINE

Differences in nutrient concentrations in various tissues andchanges in these concentrations with the age of the plant reflectdifferences in the mobility of nutrients within the plant andthe balance between the rate of supply and the use of nutrientsin various tissues. Thus the nutrient concentrations that areconsidered sufficient vary from one growth stage to another.

Nutrient concentrations in plant samples taken soon aftera fertilizer application are probably not indicative of the truenutritional status of the plant. Within the first few days aftera nitrogen application, the petiole nitrate concentration usuallyreflects a buildup of nitrate within the plant, because theconversion to protein has not yet occurred. The petiole nitrateconcentration may also not respond quickly to an applicationof ammonium if soil nitrification is inhibited by cool, wetsoil or a nitrification inhibitor. A high chloride concentrationor a heavy application of chloride suppresses the petiole nitrateconcentration but not the leaf protein nitrogen concentration.

Table 9.3. Nutrient concentrations in petioles of three potatocultivars relative to the concentration in Russet Burbank(the latter set at 1.00 for each nutrient)'

NutrientNorgoldRusset Norchip Kennebec

Nitrate nitrogen 0.88 0.99 0.96Phosphorus 1.25 0.98 1.32Potassium 0.98 0.94 1.05Calcium 0.89 1.19 0.77Magnesium 0.90 1.43 0.78Zinc 1.04 0.98 1.19Manganese 0.85 1.47 1.01Copper 1.26 1.24 1.25

'Suggested ranges of petiole nutrient concentrations in Russet Burbankare given in Table 9.2.

Table 9.4. Fertilizer materials suitable for application as foliar spraysto correct nutrient deficiencies in potato plants

Source Comment?

Monoammonium May cause leaf damage atphosphate high soluble rates; benefits

Orthophosphate are very short-livedmaterials

Sodium borates Apply at growth stage II orSolubor Ill; repeated applicationsBoric acid may be necessary; soil

applications are preferable

Copper sulfates Apply at growth stage II orCopper chelates III; one application may be

sufficient

Iron sulfate Repeated applications areIron chelates necessary to correct most

deficiencies

Magnesium sulfate One application may besufficient

Manganese sulfate Effective method for cor-Manganese chelates recting deficiencies; two or

three applications may benecessary

Sodium molybdate Effective method for cor-Ammonium recting deficiencies; very

molybdate low rates are sufficient

Zinc sulfates Apply at growth stage II orZinc chelates III; one application may be

sufficient

'Foliar sprays are generally not recommended for treatment of nitro-gen, phosphorus, potassium, calcium, or sulfur deficiencies. Consulta local fertilizer or crop advisor for correct application rates beforeapplying any of these materials.

Nutrient concentrations determined soon after foliar sprayshave been applied (including some pesticides that containnutrients) are likely to include external nutrients not yetabsorbed by the plant tissues.

Nutrient concentrations in the petiole without leaflets andin the entire leaf (petiole and leaflets) are divided into low,marginal, and sufficient ranges (Table 9.2). These ranges varyfor different cultivars (Table 9.3). Deficiency symptoms (Box9.1) are generally visible or soon develop when concentrationsare in the low range. Symptoms are probably not visible whenconcentrations are in the marginal range, but additionalnutrient applications may be needed if a significant portionof the growing season remains. Concentrations in the sufficientrange are generally adequate for plant and tuber growth atthe time of sampling. Nutritional problems may still developbefore the end of the growing season, but these can be detectedby later tissue samplings.

Fertilizer Nutrientsand Application Strategies

Crop managers should adopt fertilization practices thateffectively utilize their available resources, allow flexibility,and satisfy the nutritional requirements of the crop. Theavailability of equipment and materials, characteristics of thesite, relative costs, personal preference, and convenience arealso important factors.

Application MethodsFertilizer application methods include 1) preplant broad-

casting followed by incorporation, 2) banding before or atplanting, 3) side-dressing after planting, 4) spraying the foliageduring crop growth, and 5) injection into irrigation water.Each method has inherent advantages and disadvantages.

Broadcasting before planting is satisfactory if soil fertilityis generally high and no appreciable soil fixation is expected.Soil fixation occurs when nutrients become chemically boundin the soil and unavailable to the plants. Surface-broadcastfertilizers need to be incorporated into the moist layers ofthe seedbed where roots will be active, but not deeper than12-18 inches. Preplant application can largely eliminate theneed for additional fertilizer during planting, freeing more timefor management of the planting operation.

Banding during planting is a common fertilization practice.An efficient and safe band placement is about 2 inches tothe side and 2 inches below the seed piece. Sometimes fertilizersare banded preplant during marking-out operations. The bandsshould be placed close enough to the seed pieces to providebenefits for early growth, but they must not be in direct contactwith the seed pieces, or else injury may result.

Side-dressing is used mostly for applying nitrogen duringgrowth stage II, up to 60 days after planting. The later theapplication, the greater the risk of root pruning during theoperation. Side-dressing is generally not recommended forother fertilizer materials, particularly nutrients that stimulateearly growth, such as phosphorus.

Spraying liquid nutrients directly onto foliage often bringsforth a quicker response from plants than soil applications,and foliar sprays are effective for treating some existing nutrientdeficiencies (Table 9.4). This method is also advantageous forelements that are less effective in soil applications becauseof soil fixation. A surfactant tank-mixed with the nutrientsolution usually improves absorption through the leaf surface.The amount of any nutrient that can be applied in a singlespray is limited, because concentrated sprays can cause leafdamage.

Water-soluble fertilizers (Table 9.5) are commonly appliedby injection into irrigation water in some production areas.

Nutrient

Phosphorus

Boron

Copper

Iron

Magnesium

Manganese

Molybdenum

Zinc

FERTILITY MANAGEMENT • 83

This method has the advantage of providing nutrients accord-ing to the needs of the crop and moving them partially intothe root zone. Careful irrigation management is essential inthis method. The uniformity of the fertilizer application isno better than that of the water while the fertilizer is beinginjected. In addition, irrigation that supplies more water thanis lost to evapotranspiration may cause leaching or runoff ofapplied nutrients, leading to environmental pollution. Thistechnique should only be used on fields where the potentialfor runoff is low. The irrigation system must be equipped withcheck valves to prevent contamination of water sources dueto back-draining. The compatibility of the fertilizer materialswith the irrigation water should be checked, as materials thatremain soluble are the most effective. Many liquid fertilizermaterials are compatible with each other, but furtherinformation about compatibility should be obtained fromsuppliers or local advisors before any fertilizers are mixed.A strong acid, such as phosphoric or sulfuric acid, shouldnot be combined with a strong base, such as potassiumcarbonate. When application rates are calculated for specificnutrients, the portion of the crop's nutritional requirementssupplied by soluble nutrients already present in the irrigationwater itself must be taken into account.

Response to Applied FertilizerMany factors can alter the expected response of a potato

crop to fertilizer applications. Soil fumigants greatly changethe populations of microorganisms in the soil, many of whichplay roles in nutrient cycling. Some fumigants may retard thebiological conversion of ammonium to nitrate or the mineral-ization of nutrients from organic forms in the soil. Tillagepractices affect soil compaction and rooting depth, which mayinfluence the response to applied fertilizers. Overirrigation maycause nitrate leaching in sandy soils (Plate IC), whereas theavailability of nutrients is reduced if low soil moisture limitsgrowth. A large weed population reduces tuber yield by com-peting for nutrients and water, and plants with significant insect

Table 9.5. Liquid fertilizer materials suitable for application inirrigation water

Nutrient Analysis Source' Nitrogen 20-0-0

Aqua ammonia'82-0-0

Anhydrous ammonia'32-0-0

Urea and ammonium nitrate28-0-0

Urea and ammonium nitrate20-0-0

Ammonium nitrate17-0-0

Calcium ammonium nitrate

Phosphorus 8-24-0

Ammonium orthophosphate9-30-0

Ammonium orthophosphate10-34-0

Ammonium orthophosphateand polyphosphates

Phosphoric acid

Phosphoric and polyphosphoricacids

11-44-0

Urea phosphoric acidPotassium 0-0-30

Potassium carbonate8-8-8

Blends of ammonia, nitrogen8-16-8 solutions, phosphoric acids,4-8-12 urea ortho- and poly-7-21-7 phosphates, and

potassium chloride orpotassium hydroxide

Sulfur 0-0-0-32

Sulfuric acid12-0-0-26

Ammonium thiosulfate20-0-0-(40-45)

Ammonium polysulfides8-0-0-17

Ammonium bisulfite

'Check the compatibility of the fertilizer with the irrigation waterbefore any application is made.

"For surface irrigation only.

damage or disease are generally not able to use applied fer-tilizers as efficiently as healthy plants.

Nitrogen. Most soils need nitrogen applications to producea profitable yield of potatoes. The common forms of fertilizernitrogen are nitrate, urea, and ammonium. Plants can use eithernitrate or ammonium nitrogen.

Soil fumigants greatly changethe populations of microorgan-isms in the soil, many of whichplay roles in nutrient cycling.

Nitrogen fertilizers are most efficiently used in split appli-cations, with one-third to two-thirds of the total requirementside-dressed after plant emergence or applied by irrigation inseveral smaller applications. Dry nitrogen fertilizers may alsobe successfully top-dressed by aircraft during plant growthif the application is followed by either rainfall or sprinklerirrigation. Ammonia may volatilize from urea under someconditions if the material is not watered in soon afterapplication.

Potato crops intended for early harvest require less nitrogen.The entire nitrogen requirement for early-maturing cultivarscan be applied preplant if losses due to leaching are expectedto remain low during growth stages I and II. For late-maturingcultivars, application of the total nitrogen requirement atplanting is not recommended, because it tends to delay earlytuber development.

Several precautions should be taken in planning nitrogenapplications. High rates of banded urea or diammonium phos-phate can cause ammonia toxicity, particularly in calcareousalkaline soils. If these materials are placed with the seed piecesat planting, not more than 150 pounds per acre should beused. A much higher rate can be used if these fertilizers arebanded at least 2 inches from the seed pieces. The total saltindex of the fertilizer mix should also be considered. Nitrogenapplications 1.5-2 times higher than recommended rates canstimulate excessive foliar growth, leading to delayed tubermaturity, lower specific gravity, and increased problems withtuber quality (Chapter 10). Nitrate forms of nitrogen are subjectto leaching before plant uptake, particularly in coarse-texturedsoils (Chapter 8 and Box 9.2).

Nitrogen fertilizers aremost efficiently used in splitapplications, with one-third totwo-thirds of the total require-ment applied after plantemergence.

Nitrogen is particularly suitable for application by sprinklerirrigation. Several liquid nitrogen fertilizers can be appliedby this method (Table 9.5), and nitrogen is also a componentof most other liquid fertilizers containing phosphorus, potas-sium, and sulfur that are applied by sprinklers. This methodincreases the total efficiency of the fertilizer while maintainingor even increasing tuber yield and quality. Sprinkler applicationof nitrogen should not begin before the end of growth stageIII. Until then the root system is generally not sufficientlydeveloped to capture most nitrogen applied by this method,and thus the nutrients may be leached below the root zone.

In sprinkler irrigation, 20-40 pounds of nitrogen per acreis commonly applied every 10-14 days during tuber bulking

Box 9.2

A Checklist of Practicesfor Avoiding Nitrate Leaching

Nitrate nitrogen is readily leached by rainfall orirrigation, especially in coarse-textured soils. Leachingdecreases the efficiency of nitrogen fertilizer applica-tions, increases fertilizer expenses, and may lead tocontamination of surface waters and groundwater. Toavoid applying nitrogen in excess of the plant's needsand to avoid the loss of nitrates due to leaching, thefollowing practices should be part of any nitrogenmanagement program:

• Conduct soil tests to determine preplant fertiliza-tion rates.

• Make split applications of nitrogen.

• Make fertilizer applications as close as possible tothe actual time of plant demand for nitrogen.

• Conduct petiole tissue analysis for nitrate nitrogento evaluate the need for applications during cropgrowth.

• Allow the petiole nitrate nitrogen concentration todrop below 10,000 parts per million by the end ofgrowth stage IV.

• Schedule irrigations according to crop water use andsoil characteristics. Do not overirrigate.

• Plant a winter cover crop after the potato harvestto capture residual nitrogen in the soil.

1' it t40 40 40

Nitrogenapplications

84 • CHAPTER NINE

(growth stage IV). This maintains the petiole nitrate concen-tration in the desired range (Fig. 9.5). Repeated applicationof more than 40 pounds per acre is not recommended, becauseit stimulates excessive vegetative growth. An average RussetBurbank crop requires about 3 pounds of nitrogen per acreevery day to maintain a daily growth rate of 700 pounds oftubers per acre. An application of 40 pounds per acre issufficient for only about 10 days if the nitrogen uptakeefficiency of the plant is 75%. The number of applicationsneeded depends upon the length of growth stage IV.

It may be necessary to apply nitrogen with every irrigationin areas where the crop must be irrigated quite often, suchas fields with coarse-textured soils or environments with highevapotranspiration. A drawback of this practice is that it tendsto stimulate more vegetative growth than needed, which mayaggravate some diseases, particularly if the petiole nitrateconcentration is kept above 20,000 parts per million. To reducethis problem, only enough nitrogen should be applied to replacethat taken up by the crop since the last application.

To enhance tuber maturity at harvest, the petiole nitrateconcentration should be allowed to drop below 10,000 partsper million by the end of growth stage IV. Nitrogen shouldnot be applied within 4-6 weeks of vine killing. Applicationslate in growth stage IV and in growth stage V do not increasetuber yield and may reduce tuber quality, skin maturity, andthe storability of the crop (Chapter 6).

Anhydrous ammonia and aqua ammonia should not beapplied by sprinkler irrigation because of potentially high lossesdue to volatilization. Volatilization is also a problem in apply-

ing these materials by surface irrigation if the pH of the wateris high. These materials may also cause calcium to precipitateand increase the sodium hazard in some irrigation water. Theaddition of acidic materials, such as sulfuric acid, generallycorrects this problem.

Most other liquid nitrogen materials may be successfullyapplied by surface irrigation if precautions are taken to avoidrunoff. In general, injection of the fertilizer into the irrigationsystem should be started after the water is partway acrossthe field and should be completed by the time it begins toleave the field.

Ammonium forms of nitrogen may not be immediatelyavailable for plant uptake, because they are held by the soilin the irrigation furrow until they are converted to nitrate.As an alternative, growers using surface irrigation couldconsider banding the entire nitrogen requirement at plantingor side-dressing it later.

Phosphorus. Phosphorus fertilization is needed in many soilsfor a profitable yield of potatoes. This nutrient contributesto early crop development and tuberization and enhances tubermaturation. It is not readily leached, but phosphorus fertilizershould always be incorporated into the soil to prevent lossin runoff water, especially if soil erosion is likely to occur.

The total phosphorus fertilizer requirement may be broad-cast in the fall or in the spring before planting. It may alsobe banded at planting, to the side of and slightly below theseed piece. Banding gives the highest efficiency in phosphorus-fixing soils. The fertilization rate may generally be reducedby as much as 30% by banding, compared with broadcasting,if soil tests show low phosphorus concentrations. The avail-ability and uptake of phosphorus may be increased by bandingwith fertilizers containing ammonium.

Liquid and dry sources of phosphorus are equally effectivefor potato production. A phosphorus starter fertilizer low inammonium (such as monoammonium phosphate, 11-48-0)may be beneficial, particularly if it is placed about 1 inchabove the seed pieces at planting and applied at a rate ofup to 100 pounds per acre.

Potatoes respond to phosphorus applied through sprinklerirrigation if the fertilizer materials (Table 9.5) are compatiblewith the irrigation water and the plants have enough rootsnear the soil surface. Phosphorus applied by sprinklers pene-trates only about 2 inches into the soil. Maximum penetration

30000

25000Recommended

petiole nitrate nitrogenA concentration

9/ \_.......

July Aug. Sept.

Fig. 9.5. Petiole nitrate concentration in Russet Burbank. Arrowsrepresent points during the growing season when nitrogen wasapplied (at the rate of 40 pounds per acre) through a sprinklerirrigation system.

E

0

a)000a>co0

0a)

20000

1500 0

1000

5000

June

Box 9.3

Compatibilityof Phosphorus Fertilizerwith Irrigation Water

The compatibility of phosphorus fertilizer withirrigation water should always be determined beforeapplication by sprinkler irrigation is attempted. Theeffectiveness of this method is reduced if a precipitateforms before the water reaches the soil surface.Deposits may also form around and on sprinkler headsand nozzles, reducing their effectiveness or pluggingthem.

Compatibility can be tested by adding the appro-priate amount of fertilizer solution to a gallon of freshirrigation water. For hand lines, wheel lines, or solid-set irrigation, the amount of fertilizer solution (inteaspoons) to add to 1 gallon of water is calculatedas follows:

0.0283 X Fx=

WXtXP

where

x= amount of fertilizer solution (tsp) per gallon ofirrigation water

F= fertilizer application rate (P2O5 , lb/ acre)W= water application rate (in./hr)t= period of time during which the fertilizer is

injected (hr)P= fertilizer concentration (P 2O5 , lb/ gal)

For center-pivot irrigation systems, the calculation is

0.0283 X Fx=

DXP

where D= depth of water applied (in./ acre).If a fine white precipitate forms after the solution

is thoroughly mixed, a smaller amount of fertilizershould be applied, or the injection time should belengthened. In either case, the compatibility should beretested.

Irrigation water having a high pH or high calciumand magnesium content may be partially acidified toincrease its compatibility with phosphorus fertilizer.The final pH should be kept above 5.0 to preventcorrosion damage to the irrigation system. The pHmay be lowered by the use of an acidic liquid fertilizer,such as urea phosphoric acid or phosphoric acid, asa phosphorus source.

FERTILITY MANAGEMENT • 85

occurs when the soil surface is wet before the application.The concentration of phosphorus in petioles usually increaseswithin 10-14 days after application.

Conditions necessary for successful application of phos-phorus with irrigation water include 1) an active root systemin the upper 2 inches of soil under a full plant canopy, identifiedby small white roots visible on or immediately below the soilsurface under the canopy, 2) a fertilizer material that is com-patible with the irrigation water (Box 9.3), and 3) applyingthe fertilizer before a phosphorus deficiency develops. Nor-mally this practice is not recommended unless plant analysisshows that phosphorus will become limiting before the endof growth stage IV. If the irrigation water is not compatibleor if surface irrigation is being used, supplemental phosphorusmust be applied in foliar sprays (Table 9.4).

Potassium. Potassium fertilizer requirements for potatoesvary considerably in different soils. This nutrient influencesboth yield and tuber quality, including specific gravity,susceptibility to blackspot bruise, after-cooking darkening,reducing sugar content, chip fry color, and storage quality.Potassium chloride, particularly at high rates of application,usually results in lower tuber specific gravity than potassiumsulfate. This problem is reduced as the interval betweenapplication and planting is increased.

Banding increases the effectiveness of potassium fertilizersin soils where significant fixation occurs. Because of potentialproblems with salt toxicity, however, potassium fertilizershould not be banded at planting at rates exceeding 300 poundsof K205 per acre. High rates should be split between broadcastand banded applications. Fall broadcast applications can beeffective but are not recommended on sandy soils in areaswith winter rainfall, because of potential leaching. In irrigatedproduction, the potassium content of the irrigation watershould be considered when fertilization requirements arecalculated.

Several liquid potassium fertilizers are available for appli-cation by sprinkler irrigation (Table 9.5). In general, however,potassium should only be applied during crop growth if adeficiency is likely to occur before the end of growth stageIV. It is immobile in soil and can only be absorbed by active,healthy roots near the soil surface. It is also required inrelatively large amounts, with uptake rates similar to thosefor nitrogen (Table 9.1). In addition, potassium fertilizers haverelatively low analyses. All these factors tend to reduce theeffectiveness of irrigation systems in supplying the potassiumrequired during tuber growth.

Calcium and Magnesium. Supplemental applications ofcalcium and magnesium are needed for potato production insome acid soils, to provide needed calcium and raise the soilpH (Chapter 2). The increase in soil pH may also improvephosphorus uptake by the plant. If the magnesium contentof the soil is low, dolomitic lime should be used. Soils inwhich common scab is a problem, however, should be heldbelow pH 5.5 (Chapter 17). Calcium sulfate or magnesiumsulfate can be used without raising the soil pH. Calcium nitratecan be applied if both nitrogen and calcium are needed.

Most calcium and magnesium fertilizers should be appliedbefore or at planting. Magnesium sulfate may be applied asa foliar spray or added to fertilizer mixes. Calcium appliedto foliage is not translocated to developing tubers. The immo-bility of calcium in soil and within the plant and potentialproblems with its compatibility with irrigation water limit theeffectiveness of sprinkler-applied calcium.

Sulfur. Application of sulfur is usually needed for potatoeswhere the soil or irrigation water is naturally low in this nutrientor where it has not accumulated from previous applications.Many pesticides and some fertilizers (ammonium sulfate andpotassium sulfate) contain significant amounts of sulfur.

Sulfur is applied as a sulfate or as elemental sulfur. The

sulfate form is readily available for plant uptake, whileelemental sulfur must be oxidized to sulfate before being takenup by the plant. The soil pH may be lowered by the oxidationof elemental sulfur. Sulfates are susceptible to leaching fromsoil.

Supplemental sulfur can be applied with irrigation water(Table 9.5). Sulfate sources induce a quick response in theplant, whereas the response to elemental sulfur is slower.

86 • CHAPTER NINE

Several problems are associated with injecting sulfuric acidinto irrigation water because of the difficulty in handling thismaterial and the resulting excessively low pH of the water.

Micronutrients. Certain micronutrients must be supplied forpotato production in some soils. Zinc and manganese maybe needed in calcareous alkaline soils. Banding fertilizers con-taining ammonium may help correct some micronutrientdeficiencies in calcareous soils. Some fungicides containsignificant amounts of certain micronutrients and can be sig-nificant sources of these if the micronutrients are absorbedby the plant. For example, mancozeb is a source of zinc, andcopper fungicides supply that element.

Copper may be needed in peat and muck soils but is usuallysufficient in mineral soils. Boron may be needed where soilsor irrigation waters are naturally low in that element. Mostsoils do not require an application of iron for potatoes.Chloride and molybdenum are generally not deficient in soilsused for potato production.

The most effective application method depends upon themicronutrient needed, local soil conditions, and the point inthe growing season at which a deficiency is recognized.Generally, zinc, copper, manganese, and boron can be broad-cast and incorporated into the seedbed. On calcareous alkalinesoils, however, manganese should be banded or applied asa foliar spray. Foliar applications are effective for zinc, copper,and manganese. Boron can be applied as a foliar spray, butit is not translocated from the foliage to the tubers. Repeatedfoliar applications are necessary to correct an iron deficiency.Sources of micronutrients suitable for foliar application areoutlined in Table 9.4. Application of any micronutrient atrates higher than required by the plant may cause toxicityor deleterious interactions affecting the uptake of othernutrients. For this reason, micronutrients should only beapplied in response to recommendations based on results fromsoil tests or foliar analyses.