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The primary soil factors or properties which influence crop production are organic matter, mineral composition, soil atmosphere, soil texture, and soil moisture. These factors are closely interrelated in determining the supply and availability of the chemical elements necessary for plant growth. Certain portions of the organic and much of the mineral-clay fractions exhibit a large capacity for absorbing ions within a colloidal complex.
From the Greek word "Kolla," meaning glue-like, we derive the word colloid. When used in soil descriptions it identifies those minute, plastic, sticky portions of the soil having large surface areas in comparison to diameter, high base attraction, and capacities to trade or exchange one base element for another. From the complex interactions of these factors we derive the term "colloidal complex."
So when we speak of the colloidal properties or colloidal complex of the soil, we mean the ability of the soil to hold, by absorption, various plant food elements, such as potassium, calcium, magnesium, zinc, manganese, copper, iron, sodium, etc., and to release or exchange these elements under plant growth conditions.
Soil colloids are made up principally of inorganic material, primarily clays such as montmorillonite, kaolinite, or illite, but highly decomposed organic matter or humus also has colloidal properties. In fact, for a given unit mass, the humus colloids are about 3 to 4 times more active than the clay colloids.
These minute particles of clay and humus have certain properties such as plasticity, aggregation, and the many absorption exchange reactions by ions on the surface of the colloidal particles. These properties are illustrated by the soil being readily molded when wet, by its formation of granular structure in proper moisture, its capacity to absorb many chemical ions out of solutions and conversely plaiting them back into solution from the soil by exchange processes.
It is through this process of absorption and exchange that the colloidal complex can be increased in fertility potential through soil treatments. Plants exchange hydrogen for the nutrient elements they need as their roots make contact with the colloidal surfaces or the surrounding soil solution. Water percolating through the soil exchanges hydrogen ions for other bases or cations and the exchanged cations are lost through leaching or removed by cropping. The quantity of hydrogen ions on the colloids goes up and the quantity of the other elements goes down. Treatment then becomes a problem of replacing the hydrogen ions again with the proper amounts and balance of the other plant food elements.
Soils containing much sand and little organic matter or clay have a low ion exchange capacity. They neither swell nor shrink much and are easily tilled, but they generally do not absorb large amounts of plant food, have low reserve supplies, and do not retain plant food added in large quantities.
Soils with a high clay content (especially montmorillonite types} or containing moderate to high organic matter, have high exchange capacities. They swell and shrink considerably and can present tillage problems, but they absorb larger amounts of added plant food and have greater reserve supplying capacity.
The interactions between the plant root, the soil colloid, the soil water solution, the soil air, and the parent material mineral reserves is diagrammed in Figure 1.
All soil systems contain some salts dissolved in the soil water. The soil water is a dynamic solution which contains at least a trace of every plant food element. These elements, ionized in water, are usually referred to as the active ions, and those absorbed or held on the colloidal surface but capable of exchange are called reserve ions. In most instances, the ions held either as absorbed ions on the colloid surface or as active ions in its surrounding film of water can be considered available to the plant. Unfertilized soils contain varying amounts of these salts in the soil water system. When these soils are treated with commercial fertilizer or manure the salt content of the system increases. This is a desirable situation as long as the nutrient ions remain in balance and do not exceed the toxic limits of concentration of an individual element or total salts. It is usually simpler to fertilize a salt-free soil for increased productivity than a soil which has been fertilized until some nutrient is out of balance due to an excess of one or more fertilizer salts.
Another important factor of soil fertility is associated with the weathering of soil minerals. The silt and sand separates of soils which contain weatherable minerals will, when encouraged by organic matter and clay, transfer ions from the crystals of these minerals to the surface of the colloidal complex. In cases where the sand, silt, and clay separates are composed of mainly quartz, or other extremely resistant minerals, the supply of nutrient ions on the colloid is not restored by "resting of the soil". It can be maintained only by the addition of nutrient elements contained in fertilizers, manures, and plant residues.
A diagrammatic presentation of the constant and continuing interactions going on between the plant root, the plant residue, the soil colloidal complex, the soil solution and the mineral reserves for plant nutrition is given in Figure 1.
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