Microbial biomass, S mineralization and S uptake by African millet from soil amended with various composts

Microbial biomass growth, S mineralization after compost amendment (plant seeding) and S uptake by African millet at d 30, 60 and 120 (first, second, and third cutting, respectively) were monitored in an S-deficient soil amended with cattle manure compost (CMC), saw dust compost (SDC) or rice husk compost (RHC) at the rate of 20 t ha−1 in the presence or absence of growing African millet. A chemical fertilizer (CF) treatment at the rate of 30 μg g−1 soil along with a control (CT) was included for comparison. CMC produced a significantly larger microbial biomass-C and -S than SDC or RHC. In the planted soil, during rapid growth of African millet, microbial biomass-S decreased more rapidly than in unplanted soil. Both biomass-C and biomass-S then showed a significant flush particularly at d 60–120 in all the treatments. CMC, RHC and SDC released 20, 10, and 8 μg CaCl2 extractable S g−1 soil, respectively, by d 5. Microbial biomass showed a marked increase in C-to-S ratio across the treatments which eventually reached 154 in the unplanted soil and 291 in the planted soil from an initial value of 64. Substantial mineralization of soil organic-S in all the treatments was observed during the period of greatest plant growth, but not in the absence of plants. Total S uptake was 37, 81 and 76% lower in the CMC, SDC and RHC amendment, respectively, than that of CF. CMC improved the S supplying potential of the soil, but addition of SDC or RHC (high C-to-S ratio) resulted in severe S deficiency of plant due to S immobilization in soil.