Transformations of sulfur in soil and subsequent uptake by subterranean clover

Abstract

For a soil test to be effective it must sample pools of sulfur (S) that are immediately available to the plant and organic pools that turn over rapidly and supply S in the longer term. An experiment was conducted in two phases, the first being over 96 days to study the incorporation of S from Na2 35SO4 into various soil components at two temperatures [18/11�C (day/night) and 27/22�C] and two S application rates (10 and 50 �g S g-1 soil) and its uptake by subclover grown in pots. At intervals the pots were destructively sampled and the components radioassayed. In the second phase, soil sulfate was removed from the soil used in the first phase and the soil was potted and replanted with subclover. Soil and plant samples were taken 70 days later and the plant uptake and recycling of S-35 from the labelled soil organic pools were measured. Within 14 days in phase 1, at least 40% of the applied 35S sulfate had been incorporated into the hydriodic acid-reducible (HI-S) fraction. Subsequently, the S in the newly formed H-S fraction was recycled with mean net release rates of 35S being 26% and 38% over the next 21 and 28 day periods respectively. The mean 35S content 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 fraction. These results show the importance of the HI-S fraction and stress that any reliable soil testing method must at least include some estimate of the potential contribution from the H-S fraction as well as the 'available' inorganic S.