RE: [compost_tea] Inclusion of clay in compost

From: Stephen Barrow <barrow_at_telkomsa.net>
Date: Sun, 6 Feb 2005 11:37:42 +0200

Clay is also added to compost to stabilise the humus fraction - the
clay-humus complex is more stable than humus alone, and therefore more
stable and more resistant to weathering in arid and semi-arid regions. The
climatic conditions in South Africa are such that on average, carbon (not
clay stabilised) mineralises 5 times faster than in temperate regions, so,
you need stabilise it as much as possible. This is most probably the most
important reason to add clay to compost in arid and semi arid regions.

Stephen

-----Original Message-----
From: soilfoodweb_at_aol.com [mailto:soilfoodweb_at_aol.com]
Sent: 04 February 2005 11:08
To: compost_tea_at_yahoogroups.com
Subject: Re: [compost_tea] Inclusion of clay in compost

There are circumstances where adding clay to compost helps and there are
conditions where adding clay to compost makes no difference what so ever.

What are the conditions?

That is what is critical to understand. When does clay help, and why?

Think about organic matter in compost. How could there not be adequate
absorbing capacity of cations just in the organic matter?

Organic matter typically has a net negative charge, so it is perfect for
absorbing cations. Why would clay be needed?

During composting, humus will be produced, and humus has a much higher
ability to absorb both positive and negative ions than any clay.

Clay of course has higher surface area than silts or sands,but humus has
clay beat. So, that argument for clay doesn't hold water for me.

Does clay moderate temperature? Why would it do that?

Temperature in compost piles is a result of the growth of bacteria and
fungi. If you dilute the food resources for the bacteria and fungi, then
you slow decomposition. Maybe that is needed if the starting materials
weren't correctly balanced to begin with, but then learn from that situation
that you nee to adjust the starting material.

I personally think that clay is added to the compost pile to increase the
weight of the product. More volume, more to sell.

So,compost mixes that are too high in salt, too high in N (so temperature
gets out of control rapidly), need clay. Absorb salt, dilute the N so
temperature doesn't peak so fast.

If you have a compost with the right mix of high N, green, and wide C:N
ratio materials, then clay is not needed.

When you make compost tea, you clearly do NOT want a compost that had to be
"fixed" by adding clay. There may be a salt issue, there is a too high
temperature issue. Fungi have been taken out by too much turning. Piles
should never have to be turned more than 5 to 6 times, if made properly.

Properly means the biology was purpose in making the compost. The biology
is present and functioning to suppress disease, retain nutrients, produce
plant-available N, build aggregates and structure.

Hope this helps -

Elaine


A question for someone in the group;
I intend using a compost for extraction which has about 10% clay. Both
these arguments appear plausible. This composter has done the CNC composting
course in Holland where it would seem the inclusion of clay is recommended.
The rationale of the composters for including the clay is that they believe
the clay serves firstly to adsorb the cations and secondly to moderate the
temperature in the compost pile so that it doesn't build up excessively.

CNC are a company involved in the mushroom business and it would appear they
specialise in preparing the feedstock for the mushroom farms namely a mix of
lime, horse manure and straw. You can check their web site if you like,
Google CNC.

The problem is that the extraction process commonly used here consists of an
airpump (I think its specs are 2500lt per minute at 2bar) with a membrane
bubbler in the bottom of the 1500lt tank. Extraction times are about 20
hours. No mechanical agitation is used - again it's a system adopted from
the Dutch. I have noticed though that there is a build-up of clay in the
bottom of the tank leading me to suppose that the system is not stripping
the cations and biota from the clay and that the clay is simply settling
below and between the bubblers without being agitated.

I am having a sample of the compost tested through the kind offices of SFI.
I'm sure some knowledgeable person amongst you would like to comment on the
inclusion of the clay. I guess I should have a sample of the tea tested and
maybe test the clay from the bottom of the tank.

Your comments/suggestions would be much appreciated.

Steve Steyn ssteyn_at_wcaccess.co.za <mailto:ssteyn_at_wcaccess.co.za>

Elaine R. Ingham
Soil Foodweb Inc., Corvallis, Oregon
Soil Foodweb Inc., Port Jefferson, New York
Soil Foodweb Institute, Lismore Australia
Soil Foodweb Institute Cambridge, New Zealand
Laboratorios de Soil Foodweb, Culiacan, Mexico
Soil Foodweb Inc., Jerome, Idaho
Soil Foodweb Inc., South Africa

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Received on Sun Feb 06 2005 - 07:38:10 EST

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