Sulphur‐coated urea as a fertiliser for potatoes

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Abstract
In a field experiment comparing sulphur‐coated urea (SCU) and “Nitro‐Chalk” at rates up to 400 kg N ha−1, potato tuber yields were greater with “Nitro‐Chalk” at rates up to 200 kg N ha−1, but there was no consistent difference between forms of N at larger rates. The crop recovered more N from “Nitro‐Chalk” at all rates except 350 kg N ha−1. The largest measured yield was at 200 kg N ha−1 with “Nitro‐Chalk” and 250 kg N ha−1 with SCU, and the optimum N rate inferred from split‐line regressions of yield on N rate was also at a lower N rate with “Nitro‐Chalk”. Yields of winter wheat following the potatoes increased significantly with the N rates given to the potatoes and, at rates above 200 kg N ha−1, were larger where SCU had been used. SCU residues also lessened the response of the wheat to a spring top‐dressing of new N slightly more than those of “Nitro‐Chalk”. The sum of the apparent recoveries by the two crops of N given to the potatoes was greater from “Nitro‐Chalk” at rates up to 200 kg N ha−1. In the glasshouse, potato plants grown in 3 kg soil took 71 days to initiate tubers when given 2 g N as ammonium nitrate but this time was much decreased as increasing proportions of SCU were used. When 1 g N was given, the time was only 47 days with ammonium nitrate and the proportion of SCU had little effect. In the field, the number of tubers per plant by mid‐June decreased with increasing N rate with “Nitro‐Chalk” but not with SCU.

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