THE STATUS AND EVALUATION OF PLANT AVAILABLE SULPHUR IN SOILS

The forms of sulphur in soils and their avallability to plant, the methods for determination of available S in soil and the corelationship between available sulphur and plant growth were reviewed in this paper. At present, there haven't been a single effective chemical analytical method to evaluate the status of soil available sulphur. The general chemical extract method to determlne the available S in acid soils is 0. 01mol/L Ca(H2PO4)2 and the method to determine available S in calcareous soils is 0.01mol/L CaCl2. In Australia or some other countries 0. 25mol/L KCl-40℃ has been recommended.Among the methods to determine the S concentration in the soil extractant,turbidimetrlc method is easier to operate, less expensive and capable of analyzing more samples per hour than the reduction methods, however. this method is influenced by the operating condition and the pre-treatment of the solutions derived from soil extractions.The reduction method is sensitive, accurate but it is slow, labor intensive. ICP-AES is the best method to determine the S in the soil extractant because it can determine the lnorganic S as well as organic S in the solution and it is very rapid. Soil organic sulphur is the main source and supplement of soil available sulphur and need to be considered in evaluation of plant available S in soils. Ester sulphate is more available to plant than carbon bonded sulphur in short-term growing perlod. Other soil properties such as soil pH, clay content. organic matter content and the sulphate in the profile are also need to be considered. In the future, we need to further research the following aspects. First. select appropriate testing method of plant available S in Chinese soils and integrated with soil organic matter content, clay content and soil pH etc. to determine the critical level of soil available sulphur. Second, evaluation of bioavailability of different S fractions including soluble, sulphate, adsorbed sulphate, ester sulphate and carbon bonded sulphur. The third, study on the dynamics of soil organic sulphur mineralization and the effects of environment factors such as temperature, moisture,liming, organic carbon added and plant growth on the mineralization.