The ability of soil to ensure P nutrition for the crop – using data from the

long-term trials at Rothamsted.

Paul Poulton & Johnny Johnston

Rothamsted Research

Phosphorus – the issues

• Phosphorus is an essential, irreplaceable nutrient in crop and animal nutrition

• Phosphorus lost from agriculturally managed soils to surface water bodies, e.g. lakes, causes eutrophication

• Global reserves of phosphorus are limited

Exhaustion Land

Treatments 1856-1901

No P or K fertilizer

NPK fertilizers

Farmyard manure

300m

Soil is a silty clay loam (Chromic Luvisol)

Microplots testing available soil P and fresh fertilizer P

Current concepts of the behaviour of P in soil

Fertilizers

and

manures

Soil solutionReadilyavailablepool

P in crop

P offtake

Lessreadilyavailable pool

Slowly available pool

Soil analysis measures P in the soil solution and the readily available pool

Losses

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

Examples of critical values for arable crops

Saxmundham:- response to fresh P

0

10

20

30

40

50

60

0 10 20 30 40 50

Olsen P, mg/kg

Po

tato

yie

ld,

t/h

a

No fresh P With fresh P

0

2

4

6

8

10

0 10 20 30 40 50

Olsen P, mg/kg

Su

ga

r fr

om

su

ga

r b

ee

t,

t/h

a

No fresh P With fresh P

SOM %

Yield t/ha

Olsen P mg/ha

% variance acc’ted for

Barley grain

1.5 2.4

4.4 5.0

45 16

46 83

Potato tubers

1.5 2.4

44 45

61 17

72 89

S. beet sugar

1.5 2.4

6.6 6.6

32 18

61 87

Effect of soil organic matter

0

1

2

3

4

5

6

7

0 48 96 144

N applied, kg/ha

Bar

ley

gra

in, t

/ha

at 8

5% d

ry m

atte

r

Effect of plant available soil P on the response to N

140 mg/kg

13 mg/kg

3 mg/kg

Olsen P

Transport of P from soil to water

In eroded soil

By movement throughsoil into drainage ditches and rivers

Incidental losses from surface applied slurries and fertilizers when rainfall causes surface run-off

Relationship between yield, Olsen P and total P losses

Conclusions

There is a critical level of plant available P in the soil below which yield will be limited and N used less efficiently. Thus, soils should be maintained at slightly above this value.

BUT, there is also a threshold value for available P, above which there is an increasing risk of P being lost to ditches or streams. Unnecessarily high levels of available P should therefore be avoided.

Both of the above values will vary; depending on eg soil type and soil organic matter.

Acknowledgements

Thanks to the many scientific

and farm staff at

Rothamsted Research