Assessing the need of maincrop potatoes for late nitrogen by using isobutylidene di-urea, by injecting trification inhibitors with aqueous N fertilizers and by dividing dressings of ‘Nitro-Chalk’

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Abstract
Summary: In experiments with maincrop potatoes at Rothamsted in 1976, 1977 and 1978 ‘Nitro-Chalk’ was broadcast, either as a single dressing on the seed bed, or half on the seed bed and half at tuber initiation. Isobutylidene di-urea was broadcast on the seed bed and aqueous ammonia was injected into the seed bed, without a nitrification inhibitor or with either ‘N-Serve’ (nitrapyrin) or sodium trithiocarbonate; all were compared with ‘Nitro-Chalk’ at rates of 200 or 300 kg N/ha. Additionally, single dressings of ‘Nitro-Chalk’ supplied 150, 250, 350 and 400 kg N/ha. In similar experiments at Woburn in 1979 and 1980 aqueous urea replaced aqueous ammonia and the additional amounts of ‘Nitro-Chalk’ (100 and 400 kg N/ha) were tested as both single and divided dressings. Winter wheat was grown in each of the following years to measure residual effects.Retaining half of the total N dressing of 200 or 300 kg N/ha as ‘Nitro-Chalk’ until tuber initiation improved yields at Rothamsted in 1976 and 1978 when irrigation was applied but not in 1977 when it was not. Dividing the N at Woburn improved yields at 100 and 400 kg N/ha but not at 200 or 300 kg N/ha. At Rothamsted injected aqueous ammonia was on average similar in effect to the undivided ‘Nitro-Chalk’ dressing at 300 kg N/ha, but less effective at 200 kg N/ha. In 1977, the wettest summer, it was, however, the most effective of the N-sources, provided that an inhibitor had been applied with it. At Woburn, injected aqueous urea did not differ consistently in effectiveness from ‘Nitro-Chalk’ and inhibitors did not improve its performance. Isobutylidene di-urea (IBDU) gave the smallest yields at both farms, apparently because it was too slowly available. Tuber size distributions were affected little by the amount or the form of N.IBDU residues increased wheat yields the most following the potatoes at Rothamsted. This apart, the N-sources had similar residual values.