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[SANET-MG] Fw: [USCC] organic matter stability
Forwarded with permission, please CC Bill with replies;
----- Original Message -----
From: "William (Bill) Carter" <WCARTER@tceq.state.tx.us>
Sent: Thursday, August 05, 2004 11:55 AM
Subject: [USCC] organic matter stability
At a presentation in Austin last week, an NRCS representative said that the
residual organic matter in soil is 90% humus and that it has an average loss
rate of 3% a year. In contrast, the organic matter in raw crop residues have
a first-year loss rate of about 50%. Thus the common soil science statements
about how long it takes to build up soil organic matter levels. When I asked
whether incorporation of compost into soil affects the loss rate of its OM,
he surprisingly said he though the reverse would be true -- possibly more
with regard to how tilling affects soil oxygenation and respiration than
with the effect of partially burying the compost to reduce its exposure to
This led me to ponder the typical organic matter specifications for compost.
They generally only look at total OM, whether it is 90% humus or raw
vegetative debris (but presumably somewhere in between). I've generally
thought that the organic matter contributed by compost could be considered
to immediately become part of the soil organic matter content. But the
compost's organic matter will be lost at a rate faster than 3% in the first
year, although one would hope less than 50% in the first year.
Stability measured through respirometry should give some indication of where
the product is within this range of loss rates. How well can we predict
those rates, though? Should we assume that there is no significant
difference in the loss rate or decay rate of the organic matter in any
compost meeting minimum stability requirements?
Humic and fulvic acids seem to remain somewhat mysterious although we seem
to be confident of their importance and unique functions in the soil
environment. The TMECC method for determining humic & fulvic acid content is
"proposed" and apparently little used. Standard cooperative extension soil
tests rarely if ever address it. Should we assume that the humic/fulvic
content of all stable composts is equivalent, or unimportant? Or should we
be moving toward testing and specifying for it?
Thanks for any comments on these questions.
Watershed Management Team MC 147
Texas Commission on Environmental Quality
P.O. Box 13087
Austin, TX 78711-3087
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