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  • From: "TradingPostPaul" <tradingpost@riseup.net>
  • To: livingontheland@lists.ibiblio.org
  • Subject: [Livingontheland] Some important excerpts from ATTRA
  • Date: Sun, 02 Jul 2006 23:37:41 -0600


Here's some excerpts from SUSTAINABLE SOIL MANAGEMENT
By Preston Sullivan, NCAT Agriculture Specialist
May 2004 ©2004 NCAT
http://attra.org/attra-pub/PDF/soilmgmt.pdf

Research supports our central ideas behind this list. This is basic to the
most productive, sustainable growing for home or market - and the survival
of the family farm.

paul, tradingpost@riseup.net
---------------

Organic matter and humus are terms that describe somewhat
different but related things. Organic matter refers to the fraction of
the soil that is composed of both living organisms and once-living residues
in various stages of decomposition. Humus is only a small portion of the
organic matter. It is the end product of organic matter decomposition
and is relatively stable. Further decomposition of humus occurs
very slowly in both agricultural and natural settings. In natural
systems, a balance is reached between the amount of humus formation
and the amount of humus decay (11). This balance also occurs in most
agricultural soils, but often at a much lower level of soil humus. Humus
contributes to well-structured soil that, in turn, produces high-quality
plants. It is clear that management of organic matter and humus is
essential to sustaining the whole soil ecosystem.

The benefits of a topsoil rich in organic matter and humus are many. They
include rapid decomposition of crop residues, granulation of soil into
water-stable aggregates, decreased crusting and clodding, improved
internal drainage, better water infiltration, and increased water and
nutrient holding capacity. Improvements in the soil.s physical
structure facilitate easier tillage, increased water storage
capacity, reduced erosion, better formation and harvesting of root crops,
and deeper, more prolific plant root systems.

A soil that drains well, does not crust, takes in water rapidly, and does
not make clods is said to have good tilth. Tilth is the physical condition
of the soil as it relates to tillage ease, seedbed quality, easy seedling
emergence, and deep root penetration. Good tilth is dependent on
aggregation.the process whereby individual soil particles are joined
into clusters or .aggregates..

Aggregates form in soils when individual soil particles are
oriented and brought together through the physical forces of wetting and
drying or freezing and thawing. Weak electrical forces from
calcium and magnesium hold soil particles together when the soil
dries. When these aggregates become wet again, however, their
stability is challenged, and they may break apart. Aggregates can
also be held together by plant roots, earthworm activity, and by glue-like
products produced by soil microorganisms. Earthworm-created aggregates
are stable once they come out of the worm. An aggregate formed by
physical forces can be bound together by fine root hairs or threads
produced by fungi.

Aggregates can also become stabilized (remain intact when wet) through
the by-products of organic matter decomposition by fungi and
bacteria.chiefly gums, waxes, and other glue-like substances. These
by-products cement the soil particles together, forming water-stable
aggregates (Figure 2). The aggregate is then strong enough to hold
together when wet.hence the
term .water-stable..

When USDA soil microbiologist Sara Wright named the glue that holds
aggregates together .glomalin. after the Glomales group of common
root-dwelling fungi (12). These fungi secrete a gooey protein known as
glomalin through their hair-like filaments, or hyphae. When Wright
measured glomalin in soil aggregates she found levels as high as 2% of
their total weight in eastern U.S. soils. Soil aggregates from the
West and Midwest had lower levels of glomalin. She found that tillage
tends to lower glomalin levels.

A well-aggregated soil allows for increased water entry, increased
air flow, and increased water-holding capacity (13). Plant roots occupy
a larger volume of well-aggregated soil, high in organic matter, as
compared to a finely pulverized and dispersed soil, low in organic matter.
Roots, earthworms, and soil arthropods can pass more easily through a
well-aggregated soil (14). Aggregated soils also prevent crusting of the
soil surface. Finally, well-aggregated soils are more erosion
resistant, because aggregates are much heavier than their particle
components. For a good example of the effect of organic matter additions
on aggregation, as shown by subsequent increase in water entry
into the soil, see Table 5.

(see link for complete article)







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