An investigation into the microbial mediation of soil S-supply to plants
Eight methods were evaluated to assess potential soil S-supply to crops, and particularly to identify deficiencies in S-supply. Of these methods, three novel bioassays (sulphate production from incubation of amino acid-amended and elemental S-amended soil, and tissue S-concentrations of plants) proved better indicators (based on correlation with field crop response to S-fertilization) of S-supply than data from traditional chemical extractions. The above experiment was limited by the few soils (10) for which field S-response data was available. A second experiment was conducted with 54 soils using 6 of the 8 methods tested initially. Although no field data was available, a vast array of statistical procedures were performed which demonstrated that bioassays of soil S-supply produced related data sets but with no relation to data sets from chemical extractions. Consistent outlying of data for 2 Bangladesh soils in this experiment suggested an interesting apparent difference in the S-supply characteristics of temperate and tropical soils. The next phase of research aimed to investigate fertilizer S-supply to crops in soil. Firstly, the performance of gypsum and elemental-S were compared. Increased grain yield of barley was only achieved by gypsum application, although enhanced trace element nutrition from elemental S suggested that S-fertilization of crops influences the quality as well as the quantity of crop product. In a further pot experiment, enhancement of barley grain yield due to fertilizer S application was found only for crops grown on S-deficient soil. By labelling gypsum with 35S, the fate of fertilizer in a soil/grass system was assessed. Over 4 cuts in a 12 week period, grass removed about half of applied 35S with a further quarter left in the soil. The remainder was both in the roots and lost from the system. Less than 1 per cent of applied 35S was recovered from microbial biomass. The final experiment investigatead the method for determining S-content of the soil microbial biomass, highlighting the need for effective chloroform removal from soil and the importance of a suitable control (a non-fumigated but evacuated soil was suggested).