Response of cereals to soil and fertilizer phosphorus

Data from 1969 to 2008 on the response of 102 cereal crops to plant-available soil phosphorus (Olsen P) in three contrasting soils, each with a wide range of Olsen P, have been summarised. For each crop, the response curve, relating grain yield to Olsen P, was fitted statistically, and from the curve was determined i) the maximum yield and ii) the critical Olsen P associated with 98% of the maximum yield.

On a well structured silty clay loam at Rothamsted (Herts), maximum yield of 16 crops of winter wheat and 7 of spring barley was achieved on soil with:
- 6 to 15 mg/kg Olsen P (top P Index 0 to Index 1) in 20 of the 23 crop years.
- 16 to 25 mg/kg Olsen P (P Index 2) in 2 years and P Index 3 in only 1 year.

On a poorly structured sandy clay loam at Saxmundham (Suffolk), maximum yield of 44 winter wheat crops and 23 of spring barley was achieved on soil with:
- 8 to 15 mg/kg Olsen P (top P Index 0 to 1) in 29 (43%) of the 67 crop years.
- 16 to 25 mg/kg Olsen P (P Index 2) in 24 (36%) of the years.
- 26 to 36 mg/kg Olsen P (P Index 3) in 14 (21%) of the years.
- larger concentrations of Olsen P were needed where little nitrogen was given.

On a poorly structured, heavy silty clay loam at Rothamsted on which it was difficult to get a good seedbed for early drilling, maximum yield of 8 spring barley crops was achieved on soil with:
- 10 to 25 mg/kg Olsen P (P Index 1 to 2) in 6 of the 8 crop years.
- 26 to 35 mg/kg Olsen P (P Index 3) in 2 of the 8 crop years.
- on the same soil, but with less SOM and a very poorly structure, 40-52 mg/kg Olsen P were needed to achieve maximum yield.

Year to year variation in maximum yield was due to weather, mainly rainfall, and the length of the grain filling period. Year to year variation in critical Olsen P on each soil type reflected differences in soil and seedbed conditions and the way they interacted with weather factors. These results highlight:
- the importance of maintaining a good soil structure and using appropriate, timely, cultivations such that roots can readily find nutrients within the soil to achieve maximum yield.
- until more data are available, most fields should be maintained at P Index 2 for cereals (i) to ensure that maximum yield is achieved in most years, (ii) to allow for in-field variation in Olsen P.

Changes in Olsen P reflect changes in the P balance.
- Where large crops were grown and no phosphate was applied, Olsen P declined rapidly; from the mid-point of P Index 2 to the bottom of Index 1 in six years.
- Large amounts of phosphate were required to build up Olsen P. To increase Olsen P from the mid-point of P Index 1 to the mid-point of Index 2 required 300-330 kg/ha P2O5 (670-750 kg/ha triple superphosphate).