Meetings for Natural England - January/February 2014

Fertile soil, crop production and the

environment

Johnny Johnston

Lawes Trust Senior Fellow

Rothamsted Research

Defining soil fertility

A fertile soil is one that

produces optimum yields in an

economically sustainable way for

the grower

has least adverse environmental

impact

among other attributes must contain

adequate plant nutrients

Plant-available nutrients in soil

Plants take up nutrients like nitrogen, phosphorus,

potassium, magnesium, sodium through the roots from

the soil solution when present in plant-available forms

and these can be supplied by fertilizers

BUT fertilizers are not an end in themselves but a

means to an end in achieving optimum yields by

supplementing the indigenous supply in the soil

So knowledge of the soil supply is critically important

Hence soil analysis is an important management tool

in decisions as to how much fertilizer/manure to apply

Need to think about fertilizers in two groups

Group 1 - nitrogen and sulphur fertilizers

Irrespective of the fertilizer type most of the nitrogen is

converted to nitrate and sulphur to sulphate, the forms in

which these two nutrients are taken up by the plant

any residue of nitrate and sulphate is not retained in soil

- nitrate may be converted to nitrous oxide, a greenhouse

gas or be leached out when water drains through the soil

- sulphate is leached out and takes calcium with it leading

to soil acidification

Both nitrogen and sulphur need to be applied annually

Need to think about fertilizers in two groups

Group 2 - phosphate, potash and magnesium fertilizers

Negligible amounts of these three fertilizers are likely to

be leached from soil

Part of any residue remains in soil – usually the topsoil –

in plant-available forms so an increasing and valuable

reserve accumulates and soil analysis can be used to

measure the amount

Deciding on the amount of each of these three fertilizers

to apply depends on the amount in the soil which can be

taken up by plant roots

Nitrogen use increased from the 1960s

Availability of nitrogen fertilizers –

ammonium nitrate was no longer

required for munitions

Introduction of cultivars able to

respond to extra nitrogen

Availability of agrochemicals to

control weeds, pests and diseases

Winter wheat yields on Broadbalk

Inefficient use of nitrogen has both

a cost to the farmer

and

can have an adverse environmental

impact

Crop % SOM Fertiliser N applied

N0 N1 N2 N3

Potatoes 4.32 24.2 38.4 44.0 44.0

tubers, t/ha 1.73 11.6 21.5 29.9 36.2

Sugar beet 4.32 27.4 43.5 48.6 49.6

roots, t/ha 1.73 15.8 27.0 39.0 45.6

Spring barley 4.32 4.18 5.40 5.16 5.08

grain, t/ha 1.73 1.85 3.74 4.83 4.92

N0, N1, N2, N3: 0, 72, 144, 218 kg N/ha for root crops

0, 48, 96, 144 kg N/ha for barley

Silty clay loam soil, Rothamsted

Effect of soil organic matter on N-use efficiency - 1

Crop % SOM Fertiliser N applied

N0 N1 N2 N3

Potatoes 3.51 27.1 40.6 50.7 59.0

tubers, t/ha 1.31 25.7 35.6 41.7 43.2

Spring barley 3.37 2.58 5.12 6.85 7.81

roots, t/ha 1.31 2.19 5.00 6.73 7.05

Winter wheat 3.37 4.81 7.21 8.09 8.08

grain, t/ha 1.31 3.54 7.32 8.05 7.82

Winter barley 3.37 3.57 5.92 7.00 7.98

grain , t/ha 1.31 3.05 6.01 7.32 7.83

N0, N1, N2, N3: 0, 100, 200, 300 kg N/ha for potatoes

0, 50, 100, 150 kg N/ha for cereals

Sandy loam soil, Woburn

Effect of soil organic matter on N-use efficiency - 2

Effect of soil organic matter on N-use efficiency - 3

Yields, t/ha, of barley. Hoosfield, Rothamsted. Annual treatment since 1852; PK fertilisers ♦, 35 t/ha FYM ■. (A) cv Julia, 1976-79 (B) cv Triumph, 1988-91 (C) cv Cooper, 1996-99.

Efficient use of N requires sufficient plant-available

potassium in soil

Importance of both nitrogen and potassium in crop nutrition

Achieving large yields of most arable crops requires the rapid expansion of

the leaf canopy in spring so that the plant can capture sunlight energy to

convert carbon dioxide to sugars and then to dry matter

Nitrogen is a major driver of leaf canopy expansion which it does by

increasing both the number of individual cells and the size of cells

Some 80-90% of the total cell volume is water and to maintain cell turgor

(rigidity) there must be osmotic solutes within the water and plants prefer K.

So more and bigger cells, more water and thus more K

Compared to crops poorly–supplied with N cereals with adequate N can contain 10-15

t/ha more water and sugar beet 30-35 t/ha more water

30

35

40

45

50

55

0 50 100 150 200

Saxmundham - Sugar beet –roots – t/ha

192 mg

Kex/kg

114 mg

Kex/kg

0

2

4

6

0 50 100 150

Hoosfield - Spring barley - grain – t/ha

329 mg

Kex/kg

55 mg

Kex/kg

0

10

20

30

40

0 72 144 216

485 mg

Kex/kg

130 mg

Kex/kg

Potato tubers –

t/ha

Exchangeable K in soil and applied N interactions

N fertiliser, kg/ha

Immediately Readily available Less readily Much less readily

available P and extractable P available and available and

in soil solution extractable P extractable P

Pool 1 Pool 2 Pool 3 Pool 4

Olsen P as determined in most UK labs

Current concepts about plant-available, inorganic

phosphorus in soil

Grain yield:

with adequate P, 6.9 t/ha; with too little P, 2.9 t/ha

Importance of adequate plant-available soil P Phosphate ions taken up from soil solution, i.e. from pool 1 600,000litres water in topsoil with 0.31 mg P/L = 0.18kg P/ha Yet maximum P required about 0.6 kg P/ha per day - so P in pool 1 replenished 3-4 times per day Total P in annual crops 20-40 kg P/ha so there must be enough plant-available P in pools 2 and 3 Current recommendation to maintain soil at P Index 2

How much P should there be in the readily-available pool?

As nutrient supply increases, yield increases rapidly then more slowly to reach a maximum. The soil supply that gives near maximum yield is the critical level

Examples of critical Olsen P for arable crops. Although yield varied according to weather and nitrogen supply, the critical value changed little

Efficient use of nitrogen depends on adequate plant-

available soil P

Annual variation in winter wheat yield and critical P

Fitted plateau yields of winter wheat and the critical Olsen P associated with 98% of that yield. Saxmundham: 1st wheats,

squares; 2nd wheats, triangles; 3rd/4th wheats, diamonds. Exhaustion Land: continuous wheat, circles. Filled symbols denote

crops receiving sufficient N to achieve maximum yield; open symbols denote crops receiving insufficient N.

Red symbols,

recent data from

HGCA experiments

A risk assessment approach to plant-available P levels

in soil

Number (and percent) of soils in different critical Olsen P groups in relation to soil type

Soil type and crop Range of Olsen P, mg/kg

6-15 16-25 26-35

Well-structured silty clay loam

Winter wheat 14 (88) 2 (12)

Spring barley 5 (83) 1 (17)

Poorly-structured sandy clay loam

Winter wheat 20 (46) 17 (40) 6 (14)

Spring barley 9 (50) 5 (28) 4 (22)

Poorly-structured heavy silty clay loama

Spring barley 2 (25) 4 (50) 2 25)

a excluding the results from soils with 1.5% SOM

Yield at 95% Olsen P Variance

Soil maximum associated accounted for

organic C with 95% yield

% t/ha mg/kg %

Field experiment

Spring barley 1.40 5.00 16 83

grain, t/ha 0.87 4.45 45 46

Potatoes 1.40 44.7 17 89

tubers, t/ha 0.87 44.1 61 72

Sugar beet 1.40 6.58 18 87

sugar, t/ha 0.87 6.56 32 61

Pot experiment

Grass dry matter 1.40 6.46 23 96

g/pot 0.87 6.51 25 82

Effect of soil organic matter on plant-available P

Explaining annual variation in maximum yield and associated Olsen P

• Annual weather rainfall and sunshine • Soil conditions seedbed, soil structure

Soil structure

Compaction effects on soil structural conditions

Effect of a plough pan on

the growth and distribution of

winter wheat roots in

(a) December,

(b) March and

(c) June.

Total root length per unit ground area for

panned (solid bars) unpanned soil (open bars).

Effect of soil pan on root growth

WoburnRothamsted

Without deep loosening

Wye Double Digger

Depth - cm

Cone resistance - bars

Plough depth

Winged subsoiler

Effect of using the Wye Double Digger in 1977 and a winged

subsoiler in 1977 and 1979 on cone resistance in 1981

Importance of soil structure

Root tips cannot enter very small pores, for cereals they must exceed 0.05 mm Even with good structure, roots of annual crops rarely explore more than 25% of the top soil to take up nutrients Roots need to grow freely and quickly (especially for spring sown-crops) to access the nutrients they require to achieve optimum yields. This is especially so for P because the phosphate ion, H2PO4

-, only moves about 0.13 mm per day, the root must get to the P not P to the root! Using nitrogen efficiently, possibly using less and ensuring more is in the crop, requires adequate plant-available P and K in the soil. A cost benefit to the farmer and less nitrate to be lost to the environment Most P is transferred from soil to water in eroded soil. A good soil structure aids water infiltration and minimises surface runoff Developing and maintaining a good soil structure is not easy but has benefits to the farmer and the environment

Soil sampling and minimum cultivation

Illustration of the % of extractable P found at different depths of a soil which had

received only surface cultivations for 10 years

0 10 20 30 40 50 60

7.5-10.0 cm

5.0-7.5 cm

2.5-5.0 cm

Sa

mp

lin

g d

ep

ths

% of P in top 15 cm (6 in) of soil

Soil sampling after starting minimum cultivation and

following changes in soil P status over time

An illustration of the potential for a soil sample to show an over-high value when

taken to the standard depth in a field where minimum cultivation has been practiced

Plo

ugh d

epth

Normal

representative

soil sample

Ploughed soil

Plo

ugh d

epth

Normal

representative

soil sample

Ploughed soil Min-till soil

Soil surface

Normal soil sample depth

Skewed

soil sample in

min-till

Min-till soil

Soil surface

Normal soil sample depth

Skewed

soil sample in

min-till

Min-till soil sample depth

Representative

soil sample in

min-till

Min-till soil sample depth

Representative

soil sample in

min-till