THE SULPHUR CYCLE IN GRASSLAND SOILS

Abstract

In well drained grassland soils, most of the sulphur occurs in the organic matter and the average C : N : S ratio in the A horizon of normal soils is probably 100 : 8 : 1. In the absence of outside sources of sulphur, grasses in a grass/legume association would be almost completely dominant as they compete intensively with legumes for sulphur and could utilise almost all the mineral nitrogen and sulphur made available by the mineralisation of the organic matter. Legumes are thus dependent on outside sources of sulphur without which they will fix little nitrogen. The amount of sulphur needed will depend on the factors governing the amount of nitrogen fixed—climate, species, soils, management—it will be roughly a tenth to a fifteenth of the nitrogen fixed and will vary between 1–2 lb. per acre where the only addition of nitrogen to the soil is from rain or by non‐symbiotic fixation to about 60 lb. per acre under the best New Zealand pastoral conditions. In the absence of fertiliser sulphur, the atmosphere is the most significant, but a highly variable, source. An interesting situation arises in New Zealand where in one rather vigorous climatic zone where nitrogen fixation by legumes is limited to about 100 lb. nitrogen per acre per annum, so little sulphur comes from the atmosphere (probably less than 1 lb. per acre) that sulphur must be applied to enable clovers to make any appreciable growth. Under more favourable conditions where clovers may fix 600 lb. nitrogen per acre it is calculated that some 50 lb. of sulphur is obtained from natural sources and further responses may be obtained from fertiliser sulphur. Less sulphur may be needed when grass/legume associations are grazed than when they are cut for conservation, owing to the retum of sulphur in the urine—but much must depend on which system of utilisation most depresses clover growth and nitrogen fixation. The fate of sulphur when herbage is grazed is closely parallel to that of nitrogen; most of it is excreted in the urine as sulphate. Sulphate (even if applied as gypsum) may be readily lost by leaching from the A horizon and care must be exercised in the choice of forms of sulphur, rates and times of application.