Healthy soil and sustainable growing

["TradingPostPaul" <tradingpost@riseup.net>]

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)