Mobilization of nutrients in tropical soils through worm casting: availability of macronutrients

The influence of soil ingestion by earthworms on nutrient mobilization was studied in an incubation experiment using a red soil (Fluventic Ustropepts) and a black soil (Typic Chromusterts). Analysis of casts and soil samples revealed that both the organic C and total N content of the red and the black soil casts were significantly higher than in the bulk parent soils. The C-to-N ratio of the casts gradually narrowed relative to non-ingested soil. The worm casts also contained significantly larger amounts of NH4-N, extractable P and SO4-S, exchangeable and non-exchangeable K and extractable Ca and Mg than the parent soil. The pH of the casts increased in the red soil over non-ingested soil while it decreased in the casts of the black soil. The selective feeding behaviour of earthworms and repeated feedings on the casts may be responsible for the increased organic C and total N content of the casts, while increased enzymatic activity may have been responsible for the increased amounts of extractable P and S in the casts.