Soil

 
 
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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.