TRANSFORMATIONS OF SULFUR IN SOIL AND SUBSEQUENT UPTAKE BY SUBTERRANEAN CLOVER

For a soil test to be effective it must sample pools of sulfur (S) tha t are immediately available to the plant and organic pools that turn o ver rapidly and supply S in the longer term. An experiment was conduct ed in two phases, the first being over 96 days to study the incorporat ion of S from Na2 35SO4 into various soil components at two temperatur es [18/11-degrees-C (day/night) and 27/22-degrees-C] and two S applica tion rates (10 and 50 mug S g-1 soil) and its uptake by subclover grow n in pots. At intervals the pots were destructively sampled and the co mponents radioassayed. In the second phase, soil sulfate was removed f rom the soil used in the first phase and the soil was potted and repla nted with subclover. Soil and plant samples were taken 70 days later a nd the plant uptake and recycling of S-35 from the labelled soil organ ic pools were measured. Within 14 days in phase 1, at least 40% of the applied S-35 sulfate had been incorporated into the hydriodic acid-re ducible (HI-S) fraction. Subsequently, the S in the newly formed H-S f raction was recycled with mean net release rates of S-35 being 26% and 38% over the next 21 and 28 day periods respectively. The mean S-35 c ontent of the carbon bonded (C-S) fraction reached a peak of 22% after 63 days, indicating a very much slower recycling rate than the H-S fr action. These results show the importance of the HI-S fraction and str ess that any reliable soil testing method must at least include some e stimate of the potential contribution from the H-S fraction as well as the 'available' inorganic S.