| RESEARCH AND DEMONSTRATION FARMS - THE ALLERTON PROJECT COVER CROPS
Can cover crops recover legacy phosphorus? Cover crops such as buckwheat have been shown to reduce nitrogen loss to water and to reduce soil erosion. © Chris Stoate/GWCT
BACKGROUND As one of five research and demonstration farms in Defra’s national Sustainable Intensification research Platform (SIP), we set up an experiment to explore the potential benefits of cover crops to soil and commercial crops. This article describes a PhD project, in partnership with Nottingham University and Rothamsted Research, which investigated one biological aspect of phosphorus cycling in more detail.
24 | GAME & WILDLIFE REVIEW 2019
Phosphate fertiliser is essential for crop growth but is a finite resource. It is available only from a limited number of sources worldwide, many of them at risk of political instability. The price to farmers has increased substantially in recent years. Like other farms across the country, we try to maintain soil phosphate at an index of 2 for optimum use of this resource, although even with regular soil nutrient testing, this is not always easy to manage. Once in the soil, phosphate becomes bound to soil particles and most of the phosphate applied as fertiliser over the years is estimated to have become unavailable to crops in this way. This soil-bound fraction is known as legacy phosphate. Loss of soil to water is associated with transport of phosphate into watercourses where it is a major cause of eutrophication and deterioration of aquatic ecosystems. From both an environmental and an economic perspective, there is an increasingly recognised need to improve the efficiency of phosphate use on farmland. One area of interest is the store of phosphate currently locked up in soil. This can be remobilised by phosphatase enzymes produced by soil microorganisms and some plant roots. We were interested in exploring the potential of cover crops grown over the autumn and winter between the harvest of one crop and the drilling of a following spring-sown crop. Cover crops have been shown to reduce nitrogen loss to water and to reduce soil erosion, with implied benefits in terms of phosphate conservation, but we were interested in potential additional benefits associated with the biological activity in the soil. In 2017, we set up a replicated experiment involving plots of oats, radish, phacelia, vetch and buckwheat, with bare stubble plots lacking cover crops as controls, with three replicates of each. We collected soil samples for laboratory analysis for phosphatase in March, and in the following June when the spring-sown oats crop was actively growing. Through laboratory analysis at Rothamsted Research, this enabled us to assess the presence of phosphatase enzymes at the end of the cover crop growing period, and during the period of peak growth for the following cash crop. In March, cover crop plots of oats showed significantly greater phosphatase activity than any of the other plots, with phacelia showing intermediate activity. The same relationship was found in the following oats crop, with the plots that had been oats cover crop showing the highest activity, and phacelia intermediate levels (see Figure 1), so cover crop effects were following through potentially to benefit the spring-sown cash crop.
www.gwct.org.uk
