The effect of sulphur on the growth, sulphur and nitrogen concentrations, and critical sulphur concentrations of some tropical and temperate pasture legumes

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Abstract
Seven tropical and five temperate pasture legume species were grown in pots of three sulphur-deficient soils with varying additions of sulphate. Growth responses and the chemical composition of the plant tops were recorded, and from the latter, tentative critical concentrations of sulphur were established. These were compared with data from two field sites (six species only). Critical concentrations of sulphur in the tops of species sampled at the immediate pre-flowering stage of growth were: Macroptilium lathyroides 0.17%, Macroptilium atropurpureum 0.15%, Desmodium intortum 0.17%, Desmodium uncinatum 0.17%, Stylosanthes humilis 0.14%, Lotononis bainesii 0.15%, Glycine wightii 0.17%, Trifolium repens 0.18%, Trifolium semipilosum 0.17%, Medicago sativa 0.20%, Medicago truncatula 0.20% and Medicago denticulata 0.20%. Sulphate concentration and nitrogen/sulphur ratios were investigated as alternative diagnostic indices, but were not considered as suitable as total sulphur indices for the assessment of sulphur deficiency in legumes. Nitrogen concentrations in the plant tops were increased by sulphate supply, and excellent correlations were established between the nitrogen and sulphur concentrations in the plant tops.

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