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

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 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

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

Examples of critical values for arable crops

Saxmundham:- response to fresh P

Effect of soil organic matter 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 44 45 61 17 72 89 S. beet sugar 6.6 6.6 32 18 61 87

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

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

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.

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