The interaction
between geology and climate serves to create parent soils. Running
water, freezing and thawing cycles and heating and cooling break
rock into smaller and smaller units. Soil microbes, algae and
fungi, chemical interaction and organic matter add to the mix.
Humus, the near-end breakdown of organic matter, is an important
component. Humus becomes ash, gas and minerals that are mixed
into and incorporated into the "top" soil that can eventually
support the herbaceous and woody vegetation of particular region.
That vegetation, in turn, causes changes in the soil mix helping
to create an integrated physical, chemical and biological complex.
Physical
Soils are composed of mineral and organic matter mixed with air
and water and have individual structural and textural characteristics.
Structure concerns soil cohesiveness. A handful of soil might
stick together and form a ball or it might fall apart as it is
squeezed. A good soil for gardening has a crumb structure where
there is looseness and air space but also some clumping of soil
particles into aggregate parts. It is a soil with lots of internal
surface area around and within the aggregates. Texture is created
by the balance of clay, silt, sand and organic matter present.
A well balanced mix is called loam and is considered the best
soil for growth of vegetation. Soils with too much clay do not
have air spaces and soils with too much sand will not retain water.
Healthy soil is loose, friable and aerated.
Chemical
The main measure of soil chemistry is pH which is a measure of
the soil's acidity/alkalinity. Distilled water is the neutral
measure with a pH of 7.0. Lowering the pH makes a soil acid; raising
the pH makes a soil alkaline. A change of one increment in pH
represents a 10X change in acidity or alkalinity. Most living
things like a pH of 6.5 -7.5 with most vegetables being comfortable
in the 6 - 7 range.
Life is a function of cell membrane systems. Too high or low a
pH closes up or opens membranes too much. The membrane reaction
is related to exchange of positive (cation) and negative (anion)
exchanges and soils have various cation exchange capacities. Various
minerals have different ionic charges. Sand, nitrogen and phosphorous
have negative charges. Clay, humus, potassium, magnesium, and
calcium have positive charges. Plants, as accumulators, can change
the pH balance of soils. Having the proper pH balance for plants
allows the proper exchange of minerals for their best growth.
Improper pH can deny a plant the ability to absorb a needed substance
and reduce its vitality, ability to ward off disease or insects,
and lower its productivity.
The major mineral nutrients needed by plants are nitrogen (N),
phosphorus (P) and potassium (K). Nitrogen promotes plant growth
and is a key element in protein building. Phosphorus helps in
the transfer of energy from sunlight to the plant. Potassium is
vital in control of diseases and pests because it thickens the
walls of plant cells. Secondary nutrients needed are Calcium (Ca)
for cell division, root tip growth, genetic coding, and neutralizing
imbalances of other nutrients, magnesium (Mg) for chlorophyll,
and sulphur (S) for the production of flavor and odor compounds.
Trace elements of many other minerals are needed. (Harrison, Lee.
Soil Fertility. in Permaculture Activist, May 1992, pg. 8-11)
Biological
Soil is not just a mineral mixture. It is biologically active
and living. The biological activity in soil helps to stabilize
humus, form colloidal particles, and moderate pH. Biological components
of soil include bacteria, nitrogen fixing bacteria, fungi, algae,
plant roots and root hairs, worms, arthropods, nematodes, grubs,
beetles, and small animals to name a few. Soil fertility is dependent
upon organisms recycling nutrients. Macro-organisms and other
burrowing animals take litter underneath the surface and release
it in feces. As they burrow they make channels for water and keep
the soil aerated. Micro-organisms break down organic matter releasing
nutrients. The interaction of aerobic and anaerobic bacteria in
the soil is a key to nutrient availability and the activity of
plant pathogens. To keep a proper nutrient balance Harrison Lee
suggests returning organic matter to the soil, using minimum tillage,
applying nitrogenous fertilizer in the form of ammonium when needed,
reflecting the natural balance of legumes in the system, and mixing
native with exotic species to keep native soil organisms healthy.
Bombarding plants and soils with herbicides and pesticides can
have serious consequences for the biologic component of soils.