Biological Agriculture and Horticulture 10(1):9-27.

1993 This research project examined the effect of past cropping history, rate of compost application, and current farming practices on changes in soil populations of fungi, total bacteria, fluorescent pseudomonads, gram negative bacteria and actinomycetes. The experiment was carried out on two field sites, about 1.5 km apart, and of similar soil type and climatic conditions. The organic area was composed of four blocks, with each block divided into two plots according to whether they had been cropped for ten years previously with either pasture or a vegetable crop. Each plot consisted of five raised beds each 1.5 meters wide and 42 meters long and was divided into four subplots. These experiments started in the third year of the organic conversion.
Data were also collected from a conventionally cultivated area, cropped previously for ten years with vegetables using regular synthetic fertilizer and pesticide treatments. This site consisted of one block (42 x 15 meters) subdivided into eight subplots.
During these experiments the same crop was sown on the same day at both sites. Soil samples were taken six times between June 1991 and March 1992 under three crops and, using the dilution-plate method, assessed for the number and type of microorganisms. The major findings from this investigation agree with other studies that have been conducted along similar lines. Specifically:

• Plots previously cultivated as pasture (for ten years in this case) supported higher microbial populations than those previously cropped with vegetables. This effect lasted two to three years after the pasture was incorporated into the soil, and after numerous cultivations in vegetable and green manure crops.
• High compost rates (120 tons per hectare) increased the total microbial populations and the number of species of fungi compared to the lower compost rate (80 tons per hectare).
• Microbial populations were generally higher in plots that were undergoing a conversion to organic production, a result the researchers say is most likely due to the higher additions of organic matter.
  

In addition to the above, this project makes a significant contribution to our understanding of the soil system by reporting significant detail about changes in microbial populations over time and the widely varied and rich fungal flora that may be involved in promoting plant growth and development in farming systems dependent upon organic matter management.
Acremonium butyri, Chaetomium globosum, Gliocladium roseum, Trichoderma hamatum, and Zygorrhynchus moelleri in particular were all found in greater frequencies in the area being converted to organic vegetable production than in plots managed conventionally. These species have all been found to possess antagonistic activity against certain pathogens such as Sclerotinia sclerotiorum, and two (Trichoderma and Zygorrhynchus) have been found to have some plant growth enhancement properties.

For more information: A. Sivapalan, Dept. of Agriculture, Institute for Horticultural Development, 621 Burwood Highway, Knoxfield, Victoria 3176, Australia.

(DEC. 543)

Contributed by David Chaney