Healthy soil and sustainable growing

["TradingPostPaul" <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.