Healthy soil and sustainable growing

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

This is part of our series supporting the role of high organic matter in food 
gardening for nutrition and profitability. This relates directly to 
successful fossil fuel-free growing for the home and local marketing.

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

Function of organic matter in soil
http://www.ar.wroc.pl/~weber/rola2.htm

    Organic matter contributes to plant growth through its effect on the
physical, chemical,and biological properties of the soil. It has a :

    * nutritional function in that it serves as a source of N, P for plant
growth

    * biological function in that it profoundly affects the activities of
microflora and microfaunal organisms

    * physical and physico-chemical function in that it promotes good soil
structure, thereby improving tilth, aeration and retention of moisture and
increasing buffering and exchange capacity of soils.

    Humus also plays an indirect role in soil through its effect on the uptake
of micronutrients by plants and the performance of herbicides and other
agricultural chemicals. It should be emphasized that the importance of any
given factor will vary from one soil to another and will depend upon such
environmental conditions as climate and crpping history.

Availability of nutrients for planth growth
    Organic matter has both a direct and indirect effect on the availability
of nutrients for plant growth. In addition to serving as a source of N, P, S
through its mineralization by soil microorganisms, organic matter influences
the supply of nutrients from other sources (for example, organic matter is
required as an energy source for N-fixing bacteria).

    A factor that needs to be taken into consideration in evaluating humus as
a source of nutrient is the cropping history. When soils are first placed 
under
cultivation, the humus content generally declines over a period of 10 to 30
years until a new equilibrum level is attained. At equilibrium, any nutrients
liberated by microbial activity must be compensated for by incorporation of
equal amounts into newly formed humus.

Effect on soil physical condition, soil erosion and soil buffering and 
exchange
 capacity
    Humus has a profound effect on the structure of many soils. The
deterioration of structure that accompanies intensive tillage is usually less
severe in soils adequately supplied with humus. When humus is lost, soils tend
 to become hard, compact and cloddy.
    Aeration, water-holding capacity and permeability are all favorably 
affected
 by humus. The frequent addition of easily decomposable organic residues leads
to the synthesis of complex organic compounds that bind soil particles into
structural units called aggregates. These aggregates help to maintain a loose,
open, granular condition. Water is the better able to infiltrate and percolate
downward through the soil.The roots of plants need a continual supply of O2 in
order to respire and grow. Large pores permit better exchange of gases between
soil and atmosphere.
    Humus usually increases the ability of the soil to resist erosion. First,
it enables the soil to hold more water. Even more important is its effect in
promoting soil granulation and thus maintaining large pores through which 
water
can enter and percolate downward.
    From 20 to 70% of the exchange capacity of many soils is caused by 
colloidal
humic substances. Total acidities of isolated fractions of humus range from 
300
to 1400 meq/100g.As far as buffer action is concerned, humus exhibits 
buffering
 over a wide pH range.

Effect on soil biological condition
    Organic matter srves as a source of energy for both macro- and microfaunal
organisms. Numbers of bacteria, actinomycetes and fungi in the soil are 
related
in a general way to humus content. Earthworms and other faunal organisms are
strongly affected by the quantity of plant residue material returned to the
soil.
    Organic substances in soil can have a direct physiological effect on plant
growth. Some compounds, such as certain phenolic acids, have phytotoxic
properties; others, such as the auxins, enhance plant growth.
    It is widely known that many of the factors influencing the incidense of
pathogenic organisms in soil are directly or indirectly influenced by organic
matter. For example, a plentiful supply of organic matter may favor the growth
of saprophytic organisms relative to parasitic ones and thereby reduce
populations of the latter. Biologically active compounds in soil, such as
antibiotics and certain phenolic acids, may enhance the ability of certain
plants to resist attack by pathogens.