Nutrient cycling in grazed pastures. I. A preliminary investigation of the use of [35S] in gypsum.

Abstract

[35S]Gypsum was applied to six strips covering a total of approximately 1/32 of the area of a pasture grazed by sheep. The specific activity of samples of soil, plants, ingesta, faeces, and wool was measured. The results were as would be expected from uniform labelling of the whole paddock, and enabled translocation to untreated areas to be measured. They indicate that doses of the order of 1–10 mc should be detectable on paddocks up to 4 ha in area for periods of up to 1 year. When animals were put onto or removed from the treated pastures, changes in the specific activity of wool indicated that 100–150 days were required for the sulphur in the sheep and pasture to reach equilibrium. Animals continuously maintained on the pasture were still producing wool at 60% of the maximal specific activity after 600 days, which indicated a continuing recycling of the applied sulphur. From calculation of the isotope dilution it appears that only a small fraction of the total soil sulphur was available for plant uptake, and the continuing cycling of this over a period exceeding 600 days indicates a high residual value of the initial application.