Monitoring Populations of Soil Microorganisms during a Conversion from a Conventional to an Organic System of Vegetable Growing

Abstract

ABSTRACT Changes in soil populations of fungi, total bacteria, fluorescent pseudomonads, gram negative bacteria and actinomycetes and the number and diversity of fungal species in an organic conversion area were estimated. The effect of past cropping history and different compost rates in this area was examined for these groups. Comparison of microbial populations in the organic conversion area was also made with a conventionally cultivated area. Soil samples were taken six times (June 91-March 92) under three crops and assessed for microorganisms using the dilution-plate method. In the organic conversion area, plots that were cropped previously with pasture had higher fungal, total bacterial and actinomycete populations than plots previously cropped with vegetables. High compost rates increased the total microbial population and number of fungal species. Microbial populations were higher in the organic conversion area than in the conventional area. Soil in the organic conversion area supported approximately twice the number and a wider range of fungal species than soil in the conventionally cultivated area. Species abundance and richness of zygomycetous fungi and occurrence of fungi potentially antagonistic to plant pathogens were greater in the organic conversion area. A number of soil-borne plant pathogens found in the conventionally cultivated area were not isolated in the organic conversion area.