Identification of critical soil phosphate (P) levels for cereal and oilseed rape crops on a range of soil types

Abstract

Current advice for arable crop rotations is to maintain soils at P Index 2 (16–25 mg/l Olsen P) (Defra, 2010). This is considered the level of plant-available soil P needed to achieve optimum yields of arable crops in most years and to ensure that other agronomic inputs, especially nitrogen (N), are used effectively. This recommendation is based on results of many field experiments conducted by many organisations (see for example HGCA Research Review 18). Where soils are maintained at below the critical level, yield potential will be reduced with a risk of lower profitability and nitrogen fertiliser use efficiency. Previous research has indicated that even a large amount of fresh P fertiliser added to a P-deficient soil will not give yields equal to those on a P-sufficient soil. Where soils are maintained above the critical level, there will be little yield benefit and there is a potential environmental cost. Rising phosphate fertiliser prices and concerns about scarcity of supply have led some growers to question whether or not current recommendations are appropriate for all soil types, arable rotations and crop conditions, and many have asked if arable soils can be maintained at a P Index of less than 2 without risk of yield loss.

This project was in two parts. 1. A review of field experiments from 1969 to 2008 on specific silty clay loam and sandy clay loam soils (HGCA Research Review 74). 2. A series of new experiments on six sites with low Olsen P levels, representing soil types on which cereals and oilseed rape are widely grown, but for which critical P levels had not been determined specifically. The soils chosen were deep clays, loams and shallow soils over limestone or chalk. Field experiments were established at each site in autumn 2009 and continued in the same place for four cropping years. Winter wheat, oilseed rape or spring barley were grown following the host farmer’s rotation. Large plots were created and varying rates of phosphate fertiliser applied to create a range of Olsen P levels from P Index 0 to P Index 3. No further P fertiliser was applied to any plot in the first two years. Measured grain or seed yields were related to Olsen P levels. For the third and fourth years, each large plot was split into three sub plots, two of which continued to receive no P fertiliser. The third sub plot received fresh P fertiliser prior to cultivation and sowing in the autumns of 2011 and 2012 to test the response of the crop grown to fresh P at each level of Olsen P.

Results over four cropping years generally supported current advice, namely to maintain soils growing cereals and oilseed rape at P Index 2. In the few cases where 98% of maximum yield was only achieved on soils with more than 25 mg/kg Olsen P, it would not be justified economically to increase Olsen P above 25 mg/kg to achieve these yields. There were differences between sites and crops or years in the responsiveness of yield to Olsen P, but these could not obviously be related to soil conditions or other factors. Extremes of weather experienced during the project mean that further cropping years are required before more robust advice can be given about maintaining a certain level of Olsen P on a specific soil type. Three of the sites are now continuing within a new HGCA project that will provide more information on year to year variation. At Index 0, even a large application of fresh P fertiliser did not raise wheat yields to those achieved at Index 2. However, at P Index 1 a large application of P did increase yields to those achieved at P Index 2. This suggests that there may be the potential to manage annual P applications to achieve optimum yield where soil is at P Index 1 but further work is needed to determine the amount and method of applying the P that is needed. The amount of P fertiliser required to increase Olsen P by a given amount varied between sites, however, on average, the proportion (17%) of applied P that remained as Olsen P was similar to that (13-15%) found in other experiments. The proportion was highest on the heavy clay at Peldon and lowest on the shallow limestone soil at Cirencester. The Cirencester site required three times as much P fertiliser as Peldon in order to raise the soil P level by one Index, and at this site it was not possible to consistently maintain the soil at P Index 2 as recommended for arable rotations. An alternative approach could be to use an organic P source, although the likely effectiveness would require further investigation.