SUMMARYIn the autumns of 1985, 1986 and 1987, a total of 84 fields in England and Wales, which had received a recent dressing of organic manure or had recently been ploughed out of grass, was sampled to 90 cm depth to measure soil mineral nitrogen (Nmin i.e. NH4-N and N03-N). Amounts of Nmin varied widely between sites, indicating the difficulty in estimating soil supply following ploughing grass or manure application although type of manure and Nmin were related. Soils contained much inorganic nitrogen where manures containing a large proportion of readily available N (pig slurry or poultry manure) had been applied. Where manures containing straw were applied, Nmin was greater in the second year after application than the first. The relationship between Nmin and response of winter wheat to fertilizer was measured on 62 of the 84 sites sampled; there was no simple relationship between N min and response to N. For fields just ploughed out of grass, the relationship was particularly poor. However, for fields with a history of arable cropping, there was no response to N fertilizer when Nmin was > 300 kg/ha N in either autumn or spring. Below this amount, response to fertilizer N varied considerably even for sites with similar amounts of Nmin. Nmin measurements are of value in identifying soils containing sufficiently large N residues to allow fertilizer applications to be reduced, or even omitted, with confidence. Where residues are smaller, models need to be developed which take into account other factors, such as mineralization of organic nitrogen reserves, which modify nitrogen supply.
Winter Wheat Straw Decomposition under Different Nitrogen Fertilizers
