Keywords: Sulphur, Sulphur nutrition, Sulphur deficiency, Sulphur fertilisers.
The incidence of sulphur (S) deficiency has increased in UK crops in the last 10 years. About a quarter to a third of experiments with grass, cereals and oilseed rape responded to S fertilisers during this period, with yield increases varying from 4 to 327%. The requirements of crops for S are updated and related to the inputs. Optimum rates of fertiliser S for winter oilseed rape are 20-30 kg/ha and 10-20 kg/ha for cereals and grass. Quality issues related to S supply can be either negative or positive. Application of S fertilisers to oilseed rape can lead to glucosinolate contents in the seed which exceed the limit for home-saved seed. On the other hand, the benefit of S on bread-making quality of wheat has now been shown under field conditions in this country. Sulphur may also have a positive influence on the quality of grain legumes and sugar beet. The need to maintain a balance and not oversupply S for crop quality and other environmental reasons is stressed.
Major reactions of S in soil are immobilisation, mobilisation (mineralisation) and leaching. These have major effects on how much S is available in soil for use by crops. Losses due to leaching can be substantial. Many different types of S fertiliser are available, containing several different chemical forms of S. This has important effects on the mode of action of each type. Elemental S-containing fertilisers need to be oxidised before they can be taken up by crops. Progress has been made in understanding the factors affecting the rate of oxidation of elemental S and the groups of organisms responsible for this process in different types of soil. Animal manures contain 5, however, its availability to crops has not been quantified yet.
Methods of predicting whether crops are likely to require S fertilisers include computer modelling, soil tests and plant tissue analysis. A risk assessment model indicates that a significant proportion of the UK is potentially deficient in 5, particularly light and coarse textured soils and that this area will increase in future due to decreased atmospheric emissions. Soil testing has been shown to be useful for crops such as oilseed rape with high S requirements but less reliable for other crops such as cereals. Plant tissue analysis is often made on late vegetative growth and is therefore too late for the current crop. Fe give reliable predictions, a combination of these different approaches needs to be considered, particularly when the aim is to give local site and soil-specific diagnosis.
Because S deficiency is a relatively new phenomenon in this country, there are many gaps in knowledge. Research recommendations to enable a more rational use of sulphur fertilisers are presented.
S P McGrath and F J Zhao, IACR Rothamsted, Harpenden, UK
P J A Withers, ADAS Bridgets, Winchester, UK.
47 pages, 9 figures, 5 tables, 108 refs.