Keywords: Sulphur cycling, Nutrient interactions, Molybdenum, Selenium, Cyanide, White Muscle Disease (WMD), Goitre.
The primary objective is to highlight some of the important factors that must be considered when selecting a fertiliser that will supply the required amount of sulphur to plants efficiently while maintaining a nutrient balance, and exploiting or limiting the effects of interactions between other nutrients on the end user. The relative inputs and losses from the system are considered together the dynamics of the recycling processes in the determination of how much fertiliser is needed to sustain the system. The results from [##^s2##35##^s3##S]gypsum studies of sheep at pasture are used to show the domination of the sulphur cycle by the organic matter. A diagrammatic representation of nutrient turnover in a carbon cycle is used, together with plant protein and total nitrogen:sulphur ratios, to emphasise the difficulties of maintaining a balanced supply of nutrients to the plant and estimating the amount of fertiliser needed. The relative merits of sulphate containing fertilisers and elemental sulphur are considered in regard to leaching, sulphate release rates and the effects of mixtures and placement. The utilisation of sulphur by ruminants and the ratio of nitrogen to sulphur retained from a variety of different feeds are used to show the probable optimum ratio of nitrogen:sulphur in feed for efficient use by sheep. The interactions that take place between sulphur and other nutrients in the various steps of the consumer food chain show the need to keep fertiliser applications to the minimum required to balance losses. Even if a balance is achieved in one part of the chain the tangled web of primary and secondary interactions between sulphur and molybdenum, copper, selenium cyanide, iodine etc. are such that additional adjustments will be needed to overcome induced deficiencies and toxicities in the other parts of the production system. To overcome problems like these will require more focus on managing the pathway to the particular end user or product. This might be achieved by an integrated approach to develop special fertilisers and/or strategic supplementation together with the use of alternative species or genetically modified plants and rhizobia that are more efficient and/or more tolerant of nutrient levels.
Dr. Ray Till, Centre for Sustainable Farming Systems, Agronomy and Soil Science, University of New England, Armidale, NSW, 2350, Australia.
27 pages, 8 figures, 2 tables, 47 references.