Nitrogen contribution of various vegetable residues to succeeding barley and potato crops

Two 2-year field trials were carried out on loam soil, in order to measure the residual effect of four commonly-grown vegetable crops (cauliflower, Brassica oleracea L. var. botrytis; swede, Brassica oleracea L. var. rapifera; bulb onion, Allium cepa L.; peas, Pisum sativum L. var. medullare) compared with oats (Avena sativa L.). Residual effects of the vegetable crops were assessed in both spring barley and potatoes grown the following year at four levels of N-fertilizer (0, 40, 80 and 120 kg N ha(-1)). The amount of total nitrogen found in crop residues was greatest in cauliflower (I I I kg ha(-1)), and was for the other crops, in descending order, 86 kg ha(-1) in swedes, 54 kg ha(-1) in vining peas, 42 kg ha(-1) in mature peas and 21 kg ha(-1) in onions. The amounts of mineral nitrogen (N-min) found after harvest at 0-60 cm soil depth, were greatest after onions (> 60 kg ha(-1)), followed by peas (ca. 50 kg ha(-1)). The other vegetable crops left about 30-40 kg ha(-1) in the soil, whilst the amount after oats was < 30 kg ha(-1). The amounts found next spring, before fertilization of the following crops, were in the range 40-70 kg ha(-1), with about 50-60% present in the topsoil. Levels had risen, relative to the previous autumn, after mature peas, cauliflower and swedes. There was no change in level after oats, whilst a slight decline had occurred after vining peas and onions. Fertilizer trials in spring barley (Hordeum vulgare L and potatoes (Solanum tuberosum L.) showed in both years a significant residual benefit of vegetable crops on the yield of the former, but not of the latter crop. It was not possible to detect any difference between the vegetable crops with respect to their fertilizer value. This value was equivalent to the use of ca. 40 kg N ha(-1) in barley, irrespective of the fertilizer level at which it was compared. More than 70 % of the extra N-min found in the soil in spring after vegetable crops grown the previous year, was recovered in barley plants at harvest. The corresponding figure for potatoes was less than 25 %. This difference between crops was probably due to their contrasting uptake patterns, with later uptake in potatoes than in cereals. This means that leaching early in the growing season is more likely to occur in potatoes than in cereals. Both trials were characterised by higher than normal rainfall in May and June of the succeeding crop years. An analysis of the effect of excess rainfall at this time suggested that one can, under normal conditions for the region, expect a similar after- effect of vegetable crops on potato yields as that found in the case of spring barley.