A Word About

Soil Structure

Soil structure is the way soil particles (sand, silt, clay) arrange, and combine. Many soil structure classes are differentiated on the basis of their aggregate size, shape, arrangement etc.

Selected soil structure classes and description:

Blocky Block type aggregates; several types like cube-block or subangular-block.

Crumbly Small spherical aggregates with large spacing, ideal structure. If spacing between aggregates is small this structure is called "granular."

Platy Plate-like aggregates. Not easily penetrated by water.

Prismatic Columnar aggregates, usually found deeper in soil.

It is a proven fact that the biological characteristics of a soil, the succession and activity of microbes, minute insects and worms, are directly affected by the physical and chemical conditions of that soil. In turn, the activity of life in the soil helps to improve its physical and chemical conditions. Together, these conditions determine whether the soil will produce a minimal harvest or a healthy bumper crop.

It is largely within a farmerís power to control the structure of his soil by providing a suitable environment necessary to encourage high microbial activity which will result in higher yields of healthy crops. To achieve these desired results, the farmer must give careful thought to soil structure in his fields and what he can do to increase its quality. This knowledge is the logical basis for sound soil management.

Good soil structure has to have the following parameters in order to achieve satisfactory plant growth:

The mineral particles, humus substances, and microorganisms have to build small, round aggregates of 50µm to 5mm.

Soil with good structure is then friable and loose, well-aerated, and water penetrable. Friable soil and good tilth are established only when good humus with high activity of soil life is present.

The rain-resistant humus colloids are built by microorganisms of soil life. We could refer to them as soil "builders." Humus and clay hold important roles in the activity of soil life and are of great importance for crop yield ability, efficiency of the soil itself, and equally, the total efficiency of the farm.

Fields with organic matter content of <2% will not bear good yields . Organic matter content of 4-5% can make a farmer satisfied with his field (under same climatic conditions). The bio-organic-chemical system raises humus and organic matter content in the soil to an optimum level.

The process of decomposition causes a darkening of the soil. The soilís small animals make the organic substances smaller, while the microflora take care of chemical degradation through its metabolism. These microorganisms break down the minerals by metabolism and make them soluble, free food for plant availability.

Characteristics Of Soil Structure

The basic characteristic of a fertile soil is its structure. As structure is broken down or damaged, fertility is be lowered. Soils with damaged structure are more difficult and expensive to farm, and consequently produce lower crop yields.

With the exception of sand, soil particles are not usually found alone but rather as aggregates. Soil structure is directly related to the arrangement and shape of these aggregates. Granule size has a direct effect on the type and number of microbes, their evolution and their activity within the soil. Good soil structure also has a high capacity for absorbing and retaining moisture.

The importance of good soil structure is apparent by observing its effect on the activity of soil microbes. The microbes live within the soil solution around the soil granules. They perform their necessary functions most efficiently in the top surface cover of the soil particles, near the surface within the soil. The smaller the particles, the thinner the solution around them. If the solution around the particles becomes too thin it is difficult for microbes to survive. This rather dangerous situation develops when the soil becomes compacted.

Very small soil aggregates (like dust, perhaps <20 µm in size) lay isolated. The soil solution around them is very thin and only very small spaces of air will remain between them.

Large aggregates (up to 3 millimeters is ideal) will consist of bigger amounts of small soil particles sticking together. The soil solutions around ideal size soil particles are thicker, creating larger spaces between particles, which permits more air to enter the area. This "sticking" together is caused by fungal mycelia, humus, and clay type.

Soils with larger aggregates have larger pore spaces. This provides room for a thicker film of soil solution and still allows space for air between the particles. Air, and the oxygen it contains, is required since it directly affects microbial growth. Aerated soil will have a proper balance, or ratio, of water to air, the basic requirement for the multiplication and activity of the helpful microbes.

Pores of more than 10µm are conductive to water (water moves readily through them). If the pores get to be less than 0.2µm their water will be unavailable to the roots. Such soils are clay soils that can not support plant growth due to dryness eventhough they are moist (also called soils with high permanent wilting point).

Soils will adsorb microbes (bind them to the surface of their granules), in varying degrees. Heavy clay soils, being high in colloids, will adsorb microbes at a high level and become firmly attached to other small soil particles. Raising the acidity of the soil will increase the rate of adsorption. The rate also varies with the number of cations, or positive ions, in the soil. Excessive amounts of certain elements in the soil will reduce the adsorptive capacity.

Soils with high humus content adsorb microbes at an accelerated rate. They attract gram positive microbes more readily than gram negative ones.

The adsorption capacity of the soil is constantly changing during the year because of varying humidity, temperature and pH balance. In the spring, the adsorption capacity is low because of high humidity and low temperature. In the summer, the rate is pushed up rapidly by the higher temperature and lower humidity. If a highly humid environment occurs in summer the process is reversed and esorption, a moving away of microbes from the soil particles, can develop. In fall, with the return of high spring-like humidity and low temperature, low adsorption occurs.

For yet unknown reasons, very high adsorption has a negative effect on the activity of aerobic microbes. Increased soil particle size, as found in sandier soils, decreases the adsorption rate.

Stimulating Soil Life By Building Structure

It is possible to increase microbial activity in soil by improving its structure. Studies indicate that the size of the soil granules has a direct relation to the number of microbes in the soil. It was concluded that soil particles smaller than 10 µm were not inhabited. Larger granules, 100 µm in size, favored tremendous microbial activity.

The addition of proper amounts of organic materials such as compost will stimulate microbial activity in a low organic soil. The addition of aluminosilicates like montmorillonite to very high organic soil helps maintain soil structure. Adversely, application of excessive amounts of ionic chemical fertilizers has, and will continue to disturb life in the soil. Chemical fertilizer salts weaken the soil structure, especially if larger amounts are used, and if continued over a period of years. They actually break down and destroy the structure by suppressing vital soil microbes which inhabit the soil granules.

Denitrification is the reduction chain that reduces nitrates to nitrogen gas (N2) or oxides of nitrogen. This occurs in non-aerated (anaerobic conditions when some facultative anaerobes find no oxygen and get their energy from the reduction process. But denitrification may occur also in the presence of oxygen simply if high levels of nitrate are present in the environment of the microorganism capable of nitrate reduction (see discussion on respiration in chapter RESPIRATION). Even so, denitrification is an important process as it returns nitrogen to the atmosphere; if we had no denitrification all nitrogen would end up in the oceans.

A basic fact that is often overlooked is that the soil is unable to eliminate excessive applications of material. Consequently, it behooves the farmer to give careful consideration to what he applies to his land. Once a soilís structure is thrown out of balance, it can be a long and difficult process to rebuild its natural fertility.