Why pH Matters

ALA Meeting Rothamsted January 2015

Why pH Matters

Johnny Johnston

Lawes Trust Senior Fellow

Rothamsted Research

ALA Meeting, Rothamsted, January 2015

Effect of low pH (< 5) on barley

and wheat on a sandy loam soil at

Woburn, UK

Visual effect of soil acidity on cereals

Treatment

None +Lime +N +N +Lime

Soil pH 5.4 6.7 5.5 6.8

Yield index 100 113 188 224

Nitrogen and lime interaction on the yield of cereals

Benefits from maintaining soil pH by liming

Effect of soil acidity on plant nutrient availability

Roman writers in 1st century knew that applying calcareous

materials to soils had beneficial effects on crop growth

Largely forgotten in the Middle Ages

Liming started again in the 1700-1800s

Calcareous materials

Marls, especially on light textured soils

Sources of materials and some interesting effects

1930s Recognition that some soils were becoming acid

1937 Introduction of Lime Subsidy

1976 Lime Subsidy removed

Where now? Mainly education

A very brief history of liming in the UK

Chalk and liming

Problems with uneven distribution of chalk

pH of the soil outside the plot at the top right hand

corner of the pH map had declined to 3.7 in the

1960s and spring barley failed to grow at this level

of acidity but correcting acidity in the plots on the

right of the picture to about pH 7 allowed barley to

grow

Effect of lack of uniformity of chalk application

Section from 1968 onwards

Section 0 Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8 Section 9

- - - -

1.48 2.00 2.65 2.64 2.66 1.50 1.14 1.28 1.50 1.48

2.72 2.62 2.86 3.06 2.98 1.96 1.14 1.08 1.15 1.46

2.74 2.82 2.92 2.85 4.07 2.47 2.04 1.56 1.27 1.36

1.75 2.14 2.04 2.26 3.34 1.93 1.24 0.83 0.84 1.12

1.44 0.84 1.36 1.29 1.86 1.07 0.82 0.50 0.45 0.44

1.14 0.88 1.20 0.93 2.12 1.02 0.62 0.66 0.09 0.12

0.36 0.04 0.64 0.99 1.46 0.38 0.09 0.03 0.04 0.04

1.53 2.21 2.16 2.80 2.26 2.26 1.07 1.07 1.22 1.22

1.12 1.42 1.50 1.46 1.38 0.40 0.48 0.22 0.10 0.37

0.99 1.11 1.32 1.19 1.38 0.32 0.31 0.02 0 0.20

1.24 1.80 1.57 1.10 1.38 0.95 0.42 0.02 0.20 0.21

1.50 1.80 1.73 1.42 0.88 0.62 0.40 0.07 0 0.11

1.23 1.03 1.11 1.58 0.80 0.68 0.54 0.16 0 0.10

0.78 0.26 0.50 1.28 1.10 0.94 0.50 0.10 0 0.14

1.71 1.71 1.54 1.54 1.60 1.60 1.43 1.43 0.89 0.89

1.74 1.40 2.10 2.10 1.73 1.73 1.75 1.75 1.26 1.26

1.68 1.59 2.24 1.76 2.44 2.52 2.58 1.17 1.03 0.80

1.52 1.38 1.92 1.88 1.34 0.56 0.21 0.02 0.39 0.32

2.14 2.74 - - - - - - - -

>2.01%

1.01 - 2.00%

0.21 - 1.00%

0.01 - 0.20%

0%

Broadbalk 1944 %CaCO3 in topsoil


- - - -

7.44 7.68 7.69 7.66 7.66 7.62 7.48 7.68 7.68 7.72

7.74 7.80 7.82 7.80 7.74 7.72 7.53 7.52 7.70 7.73

8.02 8.08 8.13 8.14 8.14 8.10 8.04 8.08 8.12 8.16

8.10 8.14 8.18 8.17 8.20 8.11 8.09 8.09 8.09 8.08

8.00 8.04 8.07 8.06 8.09 8.06 7.94 7.74 7.88 8.04

8.10 7.87 8.10 8.13 8.13 7.96 7.72 7.77 7.48 7.56

7.74 7.14 7.98 8.01 8.06 7.70 7.13 6.16 6.06 6.63

8.11 8.14 8.18 8.18 8.24 8.20 8.10 8.10 8.18 8.16

7.50 7.44 7.42 7.56 7.60 7.44 7.24 7.06 6.94 7.40

7.46 7.50 7.46 7.37 7.60 7.34 7.42 7.34 7.12 7.40

7.25 7.43 7.74 7.51 7.70 7.68 7.50 7.12 6.86 7.42

7.60 7.70 7.66 7.62 7.46 7.48 7.40 7.04 6.24 7.26

7.54 7.55 7.48 7.64 7.64 7.66 7.55 7.28 5.30 6.99

7.50 7.60 7.26 7.52 7.60 7.70 7.44 7.40 6.40 7.34

8.12 8.14 8.18 8.15 8.18 8.19 8.08 8.07 8.12 8.10

7.96 7.98 8.02 8.05 8.02 8.05 7.72 7.70 7.71 7.70

7.94 7.92 7.96 7.98 8.00 8.00 7.99 7.96 7.93 7.91

7.86 7.84 7.92 7.96 7.94 6.96 6.36 6.25 7.24 7.72

7.86 7.99 - - - - - - - -

>8.01

7.71 - 8.00

7.41 - 7.70

7.11 - 7.40

<7.10

Broadbalk 1944 pH water in topsoil


7.65 7.76 7.94 7.76 7.54 7.60

7.72 7.70 7.94 7.80 7.90 7.74 7.60 7.78 7.62 7.64

7.90 7.82 7.79 7.76 7.91 7.80 7.58 7.66 7.65 7.59

8.26 8.14 8.23 8.26 8.24 8.19 8.10 8.12 7.98 7.98

8.16 8.10 8.15 8.22 8.17 8.12 7.99 7.85 7.52 7.62

7.88 7.89 8.04 8.06 8.10 7.96 7.63 7.40 7.20 7.52

7.60 7.50 7.83 7.89 8.08 7.64 7.28 7.28 7.08 7.24

7.28 7.30 7.43 7.74 7.96 7.00 7.13 7.12 7.04 7.05

7.92 7.98 8.10 8.04 8.12 7.72 7.80 7.57 7.50 7.66

7.92 7.93 8.21 8.15 8.16 7.41 7.51 7.59 7.49 7.45

7.86 7.90 7.98 7.93 8.04 7.53 7.24 7.42 7.24 7.34

7.98 8.00 8.10 7.92 7.98 7.48 7.44 7.36 7.22 7.26

7.95 8.00 8.10 7.96 7.74 7.42 7.55 7.24 7.06 7.14

7.89 7.95 7.92 7.91 7.72 7.52 7.62 7.30 6.92 7.06

7.25 7.07 7.02 7.50 7.60 7.39 7.26 6.99 6.82 6.80

7.91 7.80 7.76 7.82 7.93 7.96 7.85 7.44 7.26 7.40

8.08 8.04 8.10 8.10 8.08 8.06 8.03 7.87 7.68 7.61

8.09 8.14 8.20 8.10 8.12 8.08 7.99 8.02 7.58 7.60

7.96 8.13 8.21 8.15 8.10 7.53 7.66 7.48 7.36 7.26

8.09 8.17

7.87 7.86 7.95 7.96 8.00 7.70 7.63 7.53 7.35 7.40

>8.01

7.71 - 8.00

7.41 - 7.70

7.11 - 7.40

<7.10

Broadbalk 2000 pH water in topsoil

‘Pure’ rain has a pH of 5.0-5.6 Consequently the pH of non-calcareous soils in unpolluted areas is also about 5.0-5.6 The unmanured, permanent grassland soils on Park Grass, which never received large dressings of chalk started at about this pH But man-made pollution increases the acidity of rain, and also of mist, fog and snow Consequently in the 1960s and 1970s some rain was at pH 4 or less, and some unlimed soils became very acid often with a pH 4

Soil acidification is a natural process

Acid precipitation, i.e. H+ ions Acidifying gases or particles, e.g. SO2, NH3

Acidifying fertilisers, e.g. ammonium-N, elemental S Nutrient offtake by crops (Ca, Mg, K) Microbial processes in soil e.g. mineralisation Root exudates

Soil acidification is increased by many other factors

Broadbalk Topsoil 0-23cm %CaCO3

0

1

2

3

4

5

6

1840 1860 1880 1900 1920 1940 1960 1980 2000 2020

Year

% C

aC

O3

FYM1843 Nil PKMg N3PKMg

Effect of inputs on calcium carbonate level in topsoil

2

3

4

5

6

7

8

1840 1860 1880 1900 1920 1940 1960 1980

pH

(in

wa

ter)

Year

pH (in water) 1856-1959

14 N*2PKMgNa Unlimed 14 N*2PKMgNa Limed 3 Nil Unlimed

3 Nil Limed 9 N2PKMgNa Unlimed 9 N2PKMgNa Limed

Liming stops increasing acidification

Permanent grassland soils, Park Grass, Rothamsted

Since about 1980 emissions of sulphur have decreased by over 80% and sulphur fertilisers are now being used more widely (For example, at Woburn Farm atmospheric sulphur inputs have declined from 85 to 10 kg S/ha/year) Attention to soil pH becomes ever more important where large amounts of nitrogen are used to achieve larger yields and sulphur is applied where needed Regular monitoring of soil pH is very worthwhile because increasing acidity can be corrected before soil acidity leads to loss of yield and the risk of adverse effects on soil physical and biological properties

Some aspects of soil acidification

Improve soil structure, stability and earthworm activity.

Suppress some diseases (Club Root) but exacerbates

others (Take-all).

Limit the benefits of surface-acting herbicides because

they are strongly adsorbed on mineral particles in acid

soils.

Other benefits of maintaining soil pH by liming

Soil structure is important

Soil acidification - 1

Compounds are electrically neutral, e.g. salt (NaCl) and potassium sulphate

(K2SO4), but when dissolved in water they breakdown (dissociate) into

charged elements or molecules which have a positive or negative charge, e.g.

Na+ & Cl-, and 2K+ & SO42- The positively charged ions are called cations

those with a negative charge are anions

A important feature of soils is that the mineral particles and organic matter that

constitute soil have negative charges especially on the edges of clay particles

To maintain electrical neutrality positive ions (cations) “sit on” negative

charges, and this is what holds cations like potassium (K+), calcium (Ca2+),

magnesium (Mg2+) in soil.

The total amount of negative charges is called the “cation exchange capacity”

(CEC) because the cations held by these negative charges can be readily

exchanged with other cations depending on the relative concentrations in the

soil solution

Soil acidification - 2

As the concentration of hydrogen ions (H+) in the soil solution

increases, they displace other cations on the CEC

In neutral/calcareous soils where the dominant cations on the CEC are

calcium (Ca2+) and, in some cases magnesium (Mg2+), these are

displaced by H+ and thus the soil becomes increasingly acid as H+

occupies more and more sites on the CEC

Whenever there is excess rainfall the Ca2+ and the Mg2+ displaced

from the negative sites can be lost from the soil in the drainage water.

BUT for this to happen there has to be an anion (i.e. with a negative

charge) to accompany the Ca2+ and Mg2+ to maintain the electrical

neutrality of the drainage water

Anions which are frequently available are nitrate (NO3-), chloride (Cl-),

sulphate (SO42-) and bicarbonate (HCO3

-)

-- -- -- -- -- -- -- -- -- -- -- -- -- --

K+ K+ H+ Ca2+ K+ Na+ H+ Mg2+ Ca2+ Ca2+

soil solution

K+

K+ Mg2+ Ca2+ Na+

Ca2+ H+ K+

H+ H+ H+ H+ H+ H+ H+ H+

---------------------------------------------------------------------------------------------------------------------------------------------------------

-- -- -- -- -- -- -- -- -- -- -- -- -- --

H+ H+ K+ H+ H+ Ca2+ H+ H+ Na+ Mg2+ H+ K+

K+

Mg2+

Ca2+

Soil mineral surface


SO42-

NO3-

HCO3-

Soil solution

Acidifying inputs

Visual depiction of soil acidification

Drainage

Causes:

Causes:

Nitrification of ammonium-N and nitrate leaching:

NH4+ + 2O2 NO3

- + 2H+ + H2O NH4

+ + 2O2 NO3- + 2H+ + H2O Sulphur/sulphur dioxide oxidation:

NH4+ + 2O2 NO3

- + 2H+ + H2O

2SO2 + O2 + 2H2O 2H2SO4 4H+ + SO42-

Examples of acidification

There is no mechanism in UK soils that can hold

nitrate and sulphate so these will always be lost

from soil together with a balancing cation

Reversing acidification - liming

Ameliorating acid soils involves reversing the acidification process,

i.e. calcium (Ca2+) and/or magnesium (Mg2+) ions have to be “put

back” on the negative sites on the CEC and H+ removed

This is done by supplying an excess of calcium and/or magnesium

together with an anion which can associate with the H+ and be lost

in drainage water

Calcium and magnesium are usually applied in the cheapest

available forms and when using the carbonate an excess is added

because both calcium and magnesium carbonates are only

sparingly soluble in water and a large quantity well mixed

throughout the topsoil ensures that there is sufficient to saturate the

soil solution with Ca2+ and Mg2+. The CO3 will dissolve in the soil

solution forming the bicarbonate ion, HCO3- a balancing anion for

the H+

-- -- -- -- -- -- -- -- -- -- -- -- -- --

H+ H+ K+ H+ H+ Ca2+ H+ H+ Na+ Mg2+ H+ K+

Soil solution

Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------

-----


Base cation input

Ca and Mg

-- -- -- -- -- -- -- -- -- -- -- -- -- --

K+ Ca2+ Ca2+ K+ Na+ H+ Mg2+ Ca2+ Ca2+


Drainage

H+ NO3-

H+ SO42-

H+ Cl-

H+ HCO3-

Ca2+ K+ Mg2+ H+ Ca2+ K+ Ca2+ Ca2+

Visual depiction of liming process

Soil solution

1st edition 1973 8th edition 2010

Liming recommendations in RB209

1st edition 1973

• Identified that soil pH is NOT lime requirement

• Gave a table of crops and pH below which yield is

restricted

• Gave a table of recommended pH for different farming

systems

A considerable “decrease in emphasis” on liming in the

second and subsequent editions until the 7th and 8th

Major changes introduced in the 7th edition

• Optimum soil pH for arable and grassland

• “look up tables” for amounts of lime (t/ha of

ground limestone or chalk NV 50-55) needed to

raise the soil pH of different soil types to achieve

the optimum pH level

• introduction of NV (Neutral Value) of liming

materials relative to pure calcium oxide

Changes in liming recommendations in RB209

Determination of soil pH on a representative soil sample

Use the look tables in RB209 to determine the amount of lime

to apply

Determine the buffer capacity of the soil in a detailed

laboratory analysis

Use the Rothamsted liming model “RothLime”

Downloadable from:

www.rothamsted.bbsrc.ac.uk/aen/rothlime

Lime requirement methods

Base Cation Saturation Ratios, BCSR

The Albrecht system

Neal Kinsey’s “Hands on Agronomy” by Neal Kinsey and Charles

Walters 1st published 1993 by Acres USA, Metairie. Louisiana

Kinsey attributes the concepts to Professor Wm A Albrecht

Professor Albrecht, was a respected scientist at the University of

Missouri from 1916 to the mid 1950s when he retired

In that period I can find no papers by Professor Albrecht in scientific

journals that discuss the BCSR system for managing the nutrient

status of soils

Recent alternative approaches to the base cation

nutrition of crops

This is that there is an ideal ratio for the readily extractable base

cations, i.e. calcium, Ca; magnesium, Mg; potassium, K;

sodium, Na and further that they should be a given proportion of

the total cation exchange capacity, CEC

Work in the US reported near maximum yields of lucerne on

soils with Ca: Mg ratios varying from 100: 1 to 10: 1 and there is

much other published research there that shows that cation

ratios in soil do not relate to yield

A student working in Professor Albrecht’s lab about the time he

retired, concluded that there was no relation between lucerne

yield and the Ca: Mg ratio. In fact the relationship was better

between yield and the quantity of available Mg in soil – as in the

Index system we know

The underlying concept of the BCSR system

I believe this is a serious misuse of Professor Albrecht’s name

As frequently “sold”, soils are analysed for a range of parameters

other than just the base cations. These include pH, organic

matter, active humus, P and trace elements. Advice given about P

is based on an index system. If this system is suitable for P then

why not for K and Mg?

The question to ask is whether the additional data represents

“value for money” and can it be used to help improve nutrient use

efficiency on the farm

The “Albrecht” system

The base cations are extracted as usual in the current system in

England and Wales but Ca is rarely determined because for soils

with pH above 7, ammonium nitrate extracts some Ca from free

CaCO3. Thus exchangeable Ca is overestimated leading to

serious errors

The analytical data for the base cations are converted to

milliequivalents per 100 g soil before being expressed as a

percentage of the total CEC

This makes it very difficult for farmers to relate the data to any

previous data they may have and independent advisors can only

help if they are conversant with this system

The percentages of each cation are shown as a ratio. The extent

to which a cation is above or below its “ideal” ratio leads to a

recommendation for action

Soil analysis and data interpretation in the BCSR system

Cation Early Neal Two UK proponents

American Kinsey*

Literature (USA)

Ca 65-85 60-70 68 65

Mg 6-12 10-20 12 15

K 2-5 3-5 5 5

Na 1 1 1

H 8 10-1 8

Other 6 2-4 6

* Neal Kinsey states that the sum of Ca plus Mg should be 80

Some base cation percentages used in BCSR systems

Site and Soil Crop

yield mg/kg % in dry matter

Ca K Ca: K Ca K Ca: K

ratio ratio

Rothamsted 3290 244 13: 1 0.6 3.5 0.2: 1

10.0 t/ha

Woburn 1300 158 8: 1 0.5 3.3 0.15: 1

10.8 t/ha

Levels of calcium and potassium in soil and grass

Soil Cations as % of total Grain Cations as mg/kg and index

pH Ca Mg K Na t/ha Ca Mg K Na

7.2 76 4 4 0.2 9.6 2380 66 (2) 247 (3) 10

7.0 69 19 8 1 9.8 1460 243 (4) 347 (3) 32

(Ideally Ca plus Mg should equal 80)

Yields of wheat, BCSR ratios and K and Mg indices

Soil No. Ratio of Grain yield Cations as mg/kg and index

Ca: Mg t/ha K Mg

1 37: 1 8.5 305 (3) 33 (1)

2 9: 1 8.0 189 (2) 60 (3)

(Ideally Ca 60-70 and Mg 10-20

i.e. Ca: Mg ranges from 3: 1 to 7: 1)

Yields of winter wheat on soils with contrasted Ca: Mg ratios

Documents

Why pH Matters