INTERPRETING RESULTS FOR PLANT GROWTH PROMOTION AND DISEASE SUPPRESSION BIOASSAYS USING COMPOST

Applying high rates of thermophilic compost and vermicompost to a soil depleted in organic carbon has the potential to improve soil structure and fertility, and to suppress soilborne disease. Published bioassay methods were used to assess the disease-suppressing properties of three cured cotton trash composts, and the plant growth promoting properties of filtrate produced from pig manure vermicompost. The chemical properties of the composts and the Fusarium wilt-infested receiving soil were analysed, to assess other soil health parameters likely to affect plant growth. Results were not as expected. Cotton seedling survival in the compost treatments was only marginally better than the control, with all seedlings showing disease symptoms. In the plant growth promotion assay, seed germination was highest in the water-only controls, with the vermi- and mesophilic (no worm) compost filtrates inhibiting root growth. The common factor responsible for the unexpected results was excessive salinity. To realize the soil health benefits of organic amendments, application rates must account for the concentration of plant-available nutrients and soluble salts that they contain.