A Word About

Soil Fluids

Microbial activity not only depends on structure and oxygen but also on the mixture of various gases and water in the soil. The gas mixture in the topsoil has about two percent less oxygen than the air. At greater depths the air contains still less oxygen as a result of microbial activity. Yet, the amount of carbon dioxide increases and, in fertile soil, may reach ten times the level that is in the air above. The highest amount of carbon dioxide is recorded when microbial activity is at its maximum level, which occurs in the spring. The carbon dioxide content of the soil increases with depth. (In marsh-like soil with no structure the carbon dioxide content may easily reach five percent, a level that is highly toxic to most plants).

There are also other gases in the soil which are the products of microbial metabolism. The gases being created by microbial activity directly influence the life and growth of other types of microbes; in some cases favorably and in others detrimentally. This is one of the ways in which nature controls the balance of life in the soil. In balanced fertile soil, some of the organic gases aid in protecting plant roots from parasites while still other gases stimulate the growth of roots. The gases create an improved soil environment in which microbes can best propagate and function.

In a well-aerated fertile soil, aerobic and anaerobic microbes live together, each within microsites. The aerobic microbes live near the surface of the soil solution film on the soil particles. In soil with structure, the aerobic microbes use up the available oxygen creating a favorable environment for anaerobic microbes.

As pointed out earlier, the film of soil solution surrounding the granules must not be too thin or desirable microbial activity will not take place. Fertile soils maintain a much thicker film than compacted soils. When moisture in the soil decreases, the thickness of the film over the granules decreases until microbial activity finally ceases to exist. This will occur much more quickly in depleted soils than in soils with proper structure.

The necessary ingredients for establishing and maintaining the proper environment in which soil biology can achieve and maintain activity include:

Air oxygen, nitrogen, carbondioxide, etc.

Minerals such as iron, manganese, magnesium, zinc, etc.

Soil Moisture provides the soil solution wherein the microbes function. As the moisture decreases, the microbial activity decreases.

Temperature high enough to stimulate activity

The gas mixture (atmosphere) in the soil contains somewhat less oxygen; somewhat more (in practical terms the same amount) nitrogen, and at least ten times the carbon dioxide of the air above the soil. Also, the humidity (water vapor capacity) of the soil atmosphere is usually close to 100%. The carbon dioxide in the soil is given off by microflora, fauna, and plant roots.

Gases in soil move mostly due to diffusion; oxygen moves from air to soil and carbon dioxide from soil to air. Good structure and texture are needed for proper aeration of the soil. At times when soil activity reaches a certain peak the soil porosity is of great importance in controlling the toxic accumulations of carbon dioxide which create oxygen starvation and ethylene production.

Water in soils is important for microorganism survival and movement, also for solubilization and dispersion of nutrients. Its movement in the soil is controlled by cohesion and adhesion. In large pores water is moved mostly by gravity. Cohesion is responsible for water supply to plants.

Water molecules are themselves neutral in electrical charge but they have charged fractions; one side is positively and the other negatively charged. These charges make it possible for water molecules to bind together in what is called cohesion. When water molecules adhere to solids it is called adhesion. A third form of water connection is known as hydration (hydrated lime); this is where a water molecule is attached to a charged part of another molecule or ion.

Water-holding capacity is the amount of water in the soil after drainage. This is when the large pores empty and the medium pores contain water and air. The holding capacity is important, but a better measure of water releasable to plants is what is called the plant-available moisture. This depends not only on the holding capacity but subtracts from it the water held in the soil at the permanent wilting point. When plants are no longer able to extract water from the soil, the water remaining is the permanent wilting point. This depends on solutes in the soil solution and on the size of the pores.

Soils are a mixture of living organisms, organic matter, humus, clay, silt, sand, and mineral fraction. The term "clay" is somewhat misleading as it can refer to two different elements in soil characteristics. In texture it refers to particles smaller than 2µm. In soil mineralogy it is a mineral grouping exhibiting specific properties. Not necessarily all particles <2µm are clays, but for practical purposes it is close.

The different soil materials (fractions) bind together to form aggregates in the soil. These aggregates define the fluid movements (gas and liquid) within soil and lead to differentiation of soil structure.

Between the aggregates are spaces for gases and water (soil solution, soil plasma) movement. The quality and quantity of these spaces is highly important for soil processes.

In this series we are going to examine the soil's character and give you more understanding of it.