Keywords: Soil, nitrogen, additionally available nitrogen, organic matter, winter wheat, crop N uptake.
The accurate determination of soil nitrogen supply (SNS) in spring would increase profits and decrease unnecessary loss of nitrogen (N) to the environment. One of the uncertain factors that can be improved is the determination of N mineralised from organic matter between time of sampling and harvest. In this paper, we describe the development of the determination of additionally available N (AAN), which is the amount of N that will realistically become available throughout the growth season from mineralisation under UK field conditions. It was found that the inclusion of AAN improved the prediction of SNS or fertiliser N requirement when measured against N offtake at harvest such that for all soil types SNS explained about 62% of the variation in crop N uptake. AAN was closely related to soil organic matter (SOM) and its C:N ratio. Soils containing greater than 5% SOM (C:N ratio 10-15) showed the most consistent increases in AAN resulting in fertiliser savings >20 kg N/ha. For soils containing about 10% SOM, AAN was on average 60 kg N/ha. Whilst winter rainfall greatly influences soil mineral N (SMN) in spring, the inclusion of both AAN and spring crop N uptake indicated that temperature had a greater influence on N availability throughout the growing season. The unexplained variation in measured SNS and crop N uptake appears to be driven by the influence of soil textures on soil temperature and moisture retention. The paper describes the development of the GrowHow N-Min ® test which is commercially available and most effective for soils where SNS is estimated to be higher than 90 kg/ha, particularly those receiving organic input such as manures and leafy residues.
M. M. A. Blake-Kalff and L. Blake, Hill Court Farm Research Ltd., Frogsmarsh, Corse Lawn, Gloucester GL19 4PW, UK.
20 pages, 10 figures, 2 tables, 11 references.