The Wisconsin Soils Report is always good reading. I especially enjoyed "Soils Under the Microscope" by Doug Soldat. The article is about soil microbial communities. I found this section on the amount of bacteria in the soil, compared to the amount of bacteria added in compost tea or a microbial additive, particularly interesting:

Microbial products are designed to change the community of organisms in the soil by adding “beneficial” organisms. This sounds great. However, let’s take a minute to address the amount of product added in comparison to the native communities. We’ll start with a compost tea that might have 40,000,000,000 colony forming bacterial units per gallon. That sounds like a lot!

You might apply this compost tea solution at a rate of 1 gallon per thousand square feet. Even though you are adding 40 billion or so bacteria, you’re adding them to a soil that likely has about one quadrillion (10¹⁵) bacteria per thousand square feet. That is one bacterial cell from compost tea for every 250 million native cells. Not very good odds for changing the microbial make-up. Here is another example of a bacterial additive that we’ve tested at the O.J. Noer (which had no visual effects). The label says to apply 0.8 oz/1000 sq. ft. The product contains about 80,000,000 bacteria per oz. This means we are adding 64 million bacteria per thousand sq. ft. which is about one applied bacterial cell per 156 million native cells. These products might have pronounced effects if we apply them at levels where the added amounts can begin to compete with the native levels. But this is not going to happen at labeled rates.

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