The evaluation of plant-available sulphur in soils. II

Summary The availability to oats of adsorbed sulphate in soils and of sulphate impurity in calcium carbonate was studied in pot-culture experiments.When calcium carbonate was added to soils with pH values ranging from 5.7 to 7.4 the uptake of sulphur by oats was increased, due probably to enhanced mineralization of soil organic sulphur. When the calcium carbonate contained sulphate impurity the uptake of sulphur was further increased by an amount comparable with the release of sulphate which could be expected from a reaction of the calcium carbonate with the exchangeable hydrogen of the soil. Sulphate in excess of this amount appeared to be largely unavailable. Uptake of sulphur by oats from calcareous sands containing large amounts of insoluble sulphate associated with calcium carbonate also suggested that soil sulphur in this form had very low availability to plants.Substantial increases in the amounts of sulphur extracted by reagents commonly used for the determination of adsorbed sulphate in soils occurred when soils were airdried at about 20°C. Decreases in adsorbed sulphate in soils following the growth of oats in pot culture confirmed that adsorbed sulphate is readily available to plants.