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Re: Species diversity
Hi Joel -
The data we have is written up in the Compost Tea Brewing Manual. There is a clear predictive value between direct assessment and ability to prevent disease. When 70% or more of the leaf surface is covered with compost tea organisms (at least 5% of the coverage has to be fungal), we have been able to prevent colonization of the leaf surface by any foliar pathogen.
We see the same thing when it comes to roots.
We also see a great deal of usefulness in direct counts for assessing which organisms are present, right now, in tea, or soil, or compost. We can discern which as ACTIVE, and performing a function, right now.
Plate counts cannot differentiate between spores and dormant stages, and things that are alive and active now. Spores and dormant stages germinate and grow during the several day time period it takes for the colonies to get large enough to count.
Extraction of a soluble nutrient from soil isn't like assessing diversity or trying to predict whether a bacterium will suppress disease. The connection between nitrate and plant uptake and response is easy to see, because nitrate is so soluble and so mobile. But this isn't the case for P, for example. Or Fe. Most likely, many of the bacteria and fungi that have beneficial effects on plants are not culturable.
Mycorrhizal fungi, for example, cannot be cultured on any plate medium. That seems a huge set of beneficial organisms that are completely missed by plate counts. Mycorrhizal fungi play very important roles in both plant uptake and in disease suppression. They are not included in any plate assessment.
There are many instances that a nutrient is present in soil, and yet the plant tissues remain low in that nutrient. Just because we can extract it does not mean the plant will be able to take it up.
Dr. Eric Nelson has mentioned a paper coming out that shows that the pathogen inhibition method means nothing when it comes to predictability of disease inhibition in soil samples. There was no relationship between inhibition on plates and inhibition in soil. I'm waiting patiently to see it in print.
But it makes sense that there isn't any real relationship. The interaction between a pathogen growing on a plate in the lab, and organisms in the tea, growing on a plate, bears no relationship to how either the pathogen or the set of tea organisms will grow in soil, on the leaf surface, on the bark or blossom.
The ways that organisms respond in artificial lab conditions is nothing like the way they behave in the field. So, how predictive can lab trials be?
Now, in the case of trying to detect whether Xanthomonas is present before or after application of compost tea, that's a different story. We know the conditions that allow Xanthomonas to grow on plates, And in this instance, plate counts can be useful at detecting whether the tea has reduced the disease-causing organisms.
I think I made the usefulness of plate counts for that sort of approach clear in my previous post. But using plate counts to assess diversity? No way.
President, Soil Foodweb Inc.