Keywords: Nitrate leaching, Potatoes, Spring barley.
Nitrate can leach from soil when drainage follows shortly after nitrogen fertiliser application. In addition to the environmental consequences, crop yield and quality may be poorer than expected. Experiments between 1989 and 1992 studied the effects of drainage on soil mineral nitrogen (Nmin) distribution with depth in the soil profile, crop nitrogen (N) uptake and yields of spring barley and potatoes. Sufficient water was applied through an irrigation system to cause approximately 50mm or 100mm drainage after nitrogen fertiliser application.
Drainage after the first N dressing (40 kg/ha N) to spring barley, at the three leaf stage, caused over 20 kg/ha Nmin to be lost from the soil profile but there was no measurable yield reduction. Drainage after N application at stem extension reduced neither Nmin or crop yield, presumably because rapid crop uptake was competing for nitrate against the leaching process, and consequently, there was no yield reduction. Thus, splitting the nitrogen application offered some protection against leaching and even after 100mm drainage there was no need to apply extra N. Yields of potatoes, however, were reduced after drainage. The reductions were slightly larger from drainage after planting (the crop received half of its N in the seedbed) than when drainage occurred after N application at tuber initiation (t.i.). Placing the nitrogen on the ridge top at t.i., rather than broadcasting, provided some protection against leaching. There was also a small, but consistent, yield advantage from this technique, even in the absence of drainage.
There is a greater risk of substantial nitrogen loss from potatoes than from spring barley, first because more fertiliser N is applied to potatoes and, second, potatoes are often irrigated to keep the soil close to field capacity after t.i., so that additional rain could cause leaching. Although irrigation increases this risk, a larger Nmin residue will remain at harvest when un-irrigated crops are restricted by drought. Therefore to reduce the risk of nitrate loss from potatoes on light soils during and after the growing season, irrigation should be correctly scheduled and applied evenly, and nitrogen fertiliser applications should be timed to meet crop needs.
M A Shepherd, ADAS Gleadthorpe, Meden Vale, Mansfield, Nottinghamshire, UK.
32 pages, 9 figures, 12 tables, 18 references.