Re: [compost_tea] ask your questions!!

From: <soilfoodweb_at_aol.com>
Date: Wed, 07 Jan 2004 14:33:22 -0500

Hi John -

I expect every soil will have a somewhat different "toxic threshold" for different salts, but I also expect that the threshold won't vary that much between soils. Most of the time, we see salt effects begin to reduce microbial biomass at around 100 pounds of the salt added per acre.

Doesn't really matter if it is gypsum or lime or anything else. Salt effects occur when water is "taken away" from the organisms by the binding effect of the disaasociated salt on H and OH. Variation occurs as a result of the disassociation constant for the particular salt, which you can look up in standard chemical literature.

And it is per application. If you know the salt from the last application has been moved into the biology, then it's fine to put the next salt dose on. The simplest way to tell is to test by assessing biology before, add the salt at different concentrations, and test again.

When we (I am speaking in the sense of the whole soil ecology community here, not as SFI alone, ok?) start looking at direct effects of sulfate on soil biology, we're seeing impacts at levels lower than 35 ppm. Especially if we are looking at specifically beneficial species.

A couple interesting caveats in this - when you have been using copper sulfate for a few years, the diseases start to become resistant to the sulfur, and it takes higher conentrations to have an effect on the pathogens. The more you use the sulfur, the faster you get resistance. Why is it that the same thing does not select for sulfur tolerant species of beneficials?

It probably does, but then we neglect to feed those good guys, and they don't survive both a lack of food, and the sulfur stress.

Why do pathogens manage to survive - ah, but we do put in food in for them - it's the plant. So, the things we do in agriculture select for the problem organisms, and select against the beneficials.

Elaine Ingham
President, Soil Foodweb Inc.
SFI Corvallis, OR
SFI Port Jefferson, NY
SFI Lismore, NSW, Australia
SFI Hilversum, The Netherlands
SFI Cambridge, New Zealand
www.soilfoodweb.com

 

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Received on Wed Jan 07 2004 - 15:28:34 EST

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