Keywords: soil, plant, animal, fertiliser sulphur.
During the last two decades the amounts of sulphur (S) added to agricultural land in the U.K. have declined appreciably. In part this reflects a shift from the use of S-containing fertilisers to high-analysis, compound fertilisers, which contain zero or low amounts of S. Also, there has been a significant reduction in inputs of sulphur dioxide from atmospheric pollution during this time. When taken in conjunction with the higher yields of grassland and crops it is not surprising that S deficiency has assumed importance in Ireland and Scotland, and is becoming important in parts of England and Wales. What has often been regarded overseas as the fourth major plant nutrient is beginning to appear as a neglected nutrient in the U.K., at least in terms of our understanding of the soil-plant and soil-plant-animal relationships of S. The S cycle forms a framework within which the various aspects of S in agricultural systems can usefully be discussed; it also forms a logical basis for developing a model which predicts the S requirements of crops. This review is concerned with soil, plant and animal aspects of the S cycle and their interactions, and with inputs of S from fertilisers and the atmosphere. The dynamics of soil S are discussed in relation to the chemical, biological and physical processes involved, particularly as they influence the pool of plant-available S. Plant requirements for S vary and current information on the response to S of grassland and crops in the U.K. is considered, along with quality aspects and chemical criteria as indices of S shortage. Sulphur plays a vital role in animal nutrition, being of particular importance in microbial protein synthesis in the rumen. The effects of S on animal production and the requirements of ruminants for S are discussed. Atmospheric S is still the major source of S in many situations in the U.K., with dry deposition of S (sulphur dioxide absorbed by soils and plants) being particularly important in drier areas. Several sulphate-containing fertilisers are available in the U.K. but the high mobility of sulphate in many soils raises a question as to their efficiency. Elemental S, of appropriate particle size, is seen to have good potential as an S source because of its slow-release characteristics. The information required to develop a model which predicts fertiliser S requirements is outlined. This has highlighted several priorities for further research.
J Keith Syers and D Curtin, Department of Soil Science, University of Newcastle upon Tyne, UK, and
R J Skinner, ADAS, Cardiff Sub-centre, Wales.
43 pages, 9 figures, 6 tables, 153 references.