Effects of restrictions and prices of N, P and K on fertiliser inputs and yield deficits of sugar beet

Abstract

Although excessive application of fertiliser has been commonplace, optimisation, as well as more limited use, is increasingly topical because of environmental and economic considerations that provide an impetus for either direct or price-induced restriction of fertiliser use. In this context, optimum N, P and K inputs, yields of sugar and yield deficits were calculated for sugar beet for current nutrient prices, multiples of 1.5, 2, 3 and 5 times current prices and for a range from 0% to 150% of the current optimum for each of four soil-fertility index categories. In terms of the optimum input, N was more impervious to price increase than P or K. For example, on low-fertility index 1 soils, reduction of inputs to circa 75% of current optimum required increases of 200% for N, 50% for P and 100% for K. Increases of 20–30%, representative of fertiliser taxes, had little impact. Variation in product price had a greater effect than nutrient price, as indicated by the exponential relationship between their proportional changes for similar effects on the price ratio, and the combined effect of small changes in each was appreciable.

All nutrient-price increases induced yield deficits in the order P > K > N. The aggregate financial effect on yield deficit and net nutrient cost, i.e. on gross margin, was in the order N > K > P for very large price increases, with little difference between nutrients for increases of 100% or less. Mean effects of 100% price increases were yield deficits (%) of 0.3 to 0.4 for N, 0.7 to 2.3 for P and 0.4 to 1.3 for K, depending on index category, and reductions in gross margins (%) of 5.1 to 5.4 at index 1 and 0.7 to 1.6 at index 4 depending on the nutrient. Direct input restriction to 75% or 50% of optimum had negligible financial effects, with the exception of the 50% restriction for N. Voluntary restriction of fertiliser inputs for sugar beet, therefore, would be both environmentally and financially efficient compared with price-induced restriction at the level of a fertiliser tax.

Since distribution of yield deficits was right-skewed, a two-parameter gamma distribution was used to determine the probability of exceeding deficits of 5% and 3%. Only extreme price increases or input restrictions, or excessive use in the case of N, resulted in deficit frequencies of significant proportions; comparatively, P and K were more sensitive to price increase and N more sensitive to direct restriction. Complete input restriction on high-fertility, index 4, soils breached the acceptable tolerance of deficits >5% for N and K but not for P. For the latter, the results implied a possible conflict between environmental good-practice and the achievement of nutrient reserves required for highest yield.