Amounts of NO 3 -N and NH 4 -N in soil, from autumn to spring, under winter wheat and their relationship to soil type, sowing date, previous crop and N uptake at Rothamsted, Woburn and Saxmundham, 1979–85

• Published in: The Journal of agricultural science 1987-02, Vol.108 (1), p.73-95
• Main Authors: Widdowson, F. V., Penny, A., Darby, R. J., Bird, E., Hewitt, M. V.
• Format: Article
• Language: English
• ISSN: 0021-8596; 1469-5146
• DOI: 10.1017/S0021859600064145

Soil NO 3 -N and NH 4 -N were measured to 90 cm depth in autumn and again in spring, under several sets of winter wheat experiments, on contrasting sites. Crop samples were taken throughout the growing season, both before and after the fertilizer N was applied, to measure N uptake. The amount of NO 3 -N in soil at the outset of growth in autumn was related to the uptake of N by wheat not given any fertilizer N until April. The effect of sowing date (September v . October) on both crop and soil N was compared, as also was the effect of soil type (retentive of NO 3 -N v . readily leached) and previous crop (potatoes v . oats and wheat v . beans). The amounts of NO 3 -N in the soils in autumn related well with previous crop and declined gradually during winter on the heavier soils, but rapidly on the sandy soil, in the latter case as a consequence of leaching. On the heavier soils, where little leaching occurred, the decline in soil NO 3 -N related well with the amount of N taken up by September-sown wheat during autumn and winter, but not with that taken up by October-sown wheat, where NO 3 -N accumulated in the soil during winter, because uptake was so small. Hence delayed sowing enhanced the likelihood of losses of NO 3 -N by leaching or by denitrification. On the sandy soil at Woburn, whilst the September-sown wheat removed more N than the October-sown, losses of NO 3 -N by leaching were severe, so that late winter growth was restricted by shortage of N in soil, and the amount of N taken up was far smaller than at Rothamsted. The soil measurements distinguished between the NO 3 -N residues remaining after beans or wheat in the same field and between residues after oats or potatoes on soils of the same soil series, but in different fields on the same farm. The amount of NO 3 -N in soil and the N taken up by wheat in February-March were together used to adjust the amount of fertilizer N applied in April, using a balance sheet approach to meet a specific yield objective. Some of the N uptake data from these experiments are presented. This should aid the calculation of N requirement during specific growth periods and thus help improve the prediction of fertilizer N dressings in spring.