Had the following paper forwarded to me by Tom Gordon from UCD for a further understanding from the literature on what it takes for an organic soil amendment to be suppressive:
In summary, this paper:
1- Uses an immense data set (2423 studies) drawn from 252 papers to explore the efficacy of using organic soil amendments (compost, peat, organic wastes, animal and green manures) in inducing soilborne diseases suppression.
2- Looks at the influence of the state of decomposition of the organic matter. Decomposition either increases or decreases suppressiveness, or in other cases (especially compost) shows more complex responses (see Fig 4 in the paper).
3- States that generally speaking when a specific organic amendment was suppressive to one pathogen it was ineffective or even CONDUCIVE (emphasis mine) to other pathogens (important consideration in our emerging Central Coast soil pathogen complex of Verticillium – Fusarium – Phytophthora – Macrophomina).
4- Finds that chemical and physical aspects of an organic amendment are not as strong predictors of suppressiveness as are enzymatic and microbiological aspects. The paper concludes then that, while certain parameters such as FDA activity, substrate respiration, microbial biomass, total culturable bacteria, fluorescent pseudomonads and populations of the fungus Trichoderma are all useful for predicting suppressiveness of organic matter, no one variable can be considered in isolation to be a reliable and consistent predictor of soil suppressiveness. An integration of methods is therefore suggested.
Read the paper, it's well worth the time you put into it.
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