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Re: Methods for assessing compost tea quality



Sorry, Joel, you were correct.  I was thinking of endomycorrhiza, since I was focused on row crop plants.

So, now I'm confused about why you brought up extractable nutrients as a good model for testing compost teas. 

If in fact there is little correlation between what can be extracted and plant growth, then why would the incredibly tiny portion of the actual species diversity that you can get from plate counts be expected to give you useful information? 

Extraction of the soluble pools of inorganic nutrients from soil gets a high percentage of those pools.  But if the correlation between the measured nutrient concentration them and plant growth is not all that clear, as I think you just objected back to me. 

Why is that an argument, then, to suggest that we could use assessment of the few species of bacteria that grow on plates to be representative of whether a compost tea is going to be successful as suppressing disease?   

I think you in fact argued against the use of plate count approaches.  If we do a good job of extracting most of a nutrient from soil, and that still isn't a good way to assess if plants will benefit from that nutrient, why would an assay, that barely gives us much understanding of the whole community give us any understanding of the ability of that tea to influence plant growth?

There are no data showing that plate count information relates to benefits to plants, as least that I'm aware.  Maybe the experiment that J Reiten will give us data to show the relationship, but at least so far, there are no data to show any relationship. 

On the other hand, let me summarize an experiment we have finished and are writing up for publication.  We had ten randomly drawn tomato plants that had been in a greenhouse where blight was taking over.  So, all ten plants had equal chance to be infected, and all the plants in the greenhouse succumbed to blight in the next week.  These ten plants were split randomly into two groups.  Five plants were treated with Compost Tea #1, and five with compost tea #2. 

Plate counts showed no significant difference between the two treatments. 

All five plants treated with tea #1 died of blight within the next week.  None of the plants treated with tea #2 died, and we picked tomato from those plants for a long time.  We repeated this test, essentially with field grown tomato at Sunbow Farms. 

What was the difference between tea #1 and tea #2? 

Direct counts showed us the difference.  Tea #1 had high numbers of bacteria, no fungi, no protozoa, no beneficial nematodes.  Tea #2 had high bacterial numbers, good fungal biomass and activity, good protozoan numbers, and some beneficial nematodes.

Tea #1, sprayed on the plants with the same sprayer as used for tea #2, only covered 27% of the leaf surfaces with bacteria and bacteria alone.  Tea #2 covered 87% the leaf surfaces with bacteria and 5% with fungi. 

So, which way of assessing tea quality do you want to use? 

Elaine Ingham
President, Soil Foodweb Inc.
www.soilfoodweb.com