Plant-available soil phosphorus. Part I: the response of winter wheat and spring barley to Olsen P on a silty clay loam

Increasing fertilizer costs have prompted farmers to ask whether soils could be maintained at lower levels of plant-available phosphorus (Olsen P) than currently recommended without losing yield. To help answer this question, we assessed the response to Olsen P by spring barley grown from 1986 to 1991, followed by winter wheat from 1992 to 2008, on a silty clay loam soil. Each year the curve relating grain yield to Olsen P was fitted statistically to determine the asymptotic yield and the Olsen P associated with 98% of that yield, that is, the critical level of Olsen P. The variance accounted for by the relationship ranged between 83 and 97% in all but two years, suggesting that the availability of soil P was the major soil factor affecting yield and that Olsen P was a reliable measure of plant-available P in soil. Asymptotic annual yield of spring barley ranged from 2.34 to 7.12 t/ha and of winter wheat from 3.87 to 10.36 t/ha. In part, this range in yields was because of changes in the cultivar grown while the range of yields for any one cultivar was probably due to differences in weather, principally rainfall, between years. Critical Olsen P ranged from 7 to 18 mg/kg for both cereal crops (with one outlier at 26 mg/kg for winter wheat) most probably due to seedbed and soil structure conditions affecting root growth, and thus acquisition of available soil P, and the way these soil factors were affected by weather. Thus, a general recommendation for cereals grown on this silty clay loam, which is comparatively easy to cultivate, would be to maintain Olsen P at about 20 mg/kg in the plough layer to minimize the risk of losing yield in some years. This value, 20 mg/kg, equivalent to 20 mg/L, is the midpoint of P Index 2, the recommended P Index given in the Fertiliser Manual (RB209) (Defra 2010) for soils growing arable crops and grass in England, Wales and Northern Ireland.