Measurement of soil mineral nitrogen to predict the response of winter wheat to fertilizer nitrogen after applications of organic manures or after ploughed-out grass

Abstract

SUMMARY: In 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 (N_min i.e. NH₄-N and N0₃-N). Amounts of N_min varied widely between sites, indicating the difficulty in estimating soil supply following ploughing grass or manure application although type of manure and N_min 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, N_min was greater in the second year after application than the first. The relationship between N_min 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 N_min 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 N_min. N_min 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.