1. Paul Poulton & Johnny Johnston Rothamsted Research
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

2. 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

3. Exhaustion Land Treatments 1856-1901 No P or K fertilizer
NPK fertilizers
Farmyard manure
300m
Soil is a silty clay loam (Chromic Luvisol)

4. Microplots testing available soil P and fresh fertilizer P

5. Current concepts of the behaviour of P in soil
P offtake
Fertilizers
and
manures
P in crop
Less
readily
available
pool
Readily
available
pool
Slowly available pool
Soil solution
Losses
Soil analysis measures P in the soil solution and the readily available pool

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

7. Examples of critical values for arable crops

8. Saxmundham:- response to fresh P

9. Effect of soil organic matter
SOM %
Yield t/ha
Olsen P mg/ha
% variance acc’ted for
Barley grain
Potato tubers
S. beet sugar

10. Effect of plant available soil P on the response to N
Olsen P
140 mg/kg
13 mg/kg
3 mg/kg

11. Transport of P from soil to water
In eroded soil
By movement through
soil into drainage ditches and rivers
Incidental losses from surface applied slurries and fertilizers when rainfall causes surface run-off

12. Relationship between yield, Olsen P and total P losses

13. 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.

14. Thanks to the many scientific and farm staff at Rothamsted Research
Acknowledgements
Thanks to the many scientific
and farm staff at
Rothamsted Research