Sulphur in Soils: Processes, Behaviour and Measurement

Keywords: Sulphur, Sulphate, Mineralisation, Immobilisation, Organic, Leaching.

The amount, chemical forms and distribution of sulphur varies in soils across different management, climatic and geochemical conditions. Both inorganic and organic forms exist in soils, with organic sulphur normally consisting of the larger pool. However, plants take up the sulphate form of sulphur via roots and this is therefore the most important form in soils from a nutritional point of view. Soils receive sulphur from wet and dry deposition, which either exists or is transformed into sulphate, and sulphate or elemental sulphur can be intentionally added to soil in fertilisers. Sulphate is also the form that is prone to being leached out of the rooting zone. Where elemental sulphur fertilisers are used, their oxidation is a biological process, but the rate is often limited by the physical form of elemental sulphur in the soil. Sulphur in soils undergoes microbial immobilisation and mineralisation concurrently. The net outcome of these two processes varies with time and net mineralisation is difficult to measure and to predict.

The forms of sulphur in soil organic matter are both heterogeneous and complex. It remains unclear how different fractions of organic sulphur are interrelated in the turnover processes. Microbial biomass-sulphur is probably the most active component of soil organic sulphur. Methods which fractionate soil organic sulphur into different pools according to their biological and biochemical lability need to be developed, and these may be more informative. Data on soil sulphur turnover from field studies, which are essential for developing and validating models, are scarce and scattered.

Prof. Steve P McGrath, Dr. Fangjie Zhao and Dr. Mechteld M A Blake-Kalff, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK.

27 pages, 4 figures, 4 tables, 105 references.