HUMIC SUBSTANCES AND THEIR INFLUENCE ON SOIL FERTILITY
Humic substances are a good source of energy for beneficial soil organisms. Humic substances and non humic (organic) compounds provide the energy and many of the mineral requirements for soil microorganisms and soil animals. Beneficial soil organisms lack the photosynthetic apparatus to capture energy from the sun thus must survive on residual carbon containing substances on or in the soil. Energy stored within the carbon bonds functions to provide energy for various metabolic reactions within these organisms. Beneficial soil organisms (algae, yeasts, bacteria, fungi nematodes, mycorrhizae, and small animals) perform many beneficial functions which influence soil fertility and plant health. For example the bacteria release organic acids which aid in the solubilization of mineral elements bound in soil.
Bacteria also release complex polysaccharides (sugar based compounds) that help create soil crumbs (aggregates). Soil crumbs give soil a desirable structure. Other beneficial soil microorganisms such as the Actinomyces release antibiotics into the soil. These antibiotics are taken up by the plant to protect it against pests. Antibiotics also function to create desirable ecological balances of soil organisms on the root surface (rhizoplane) and in soil near the roots (rhizosphere). Fungi also perform many beneficial functions in soils. For example, mycorrhizae aid plant roots in the uptake of water and trace elements. Other fungi decompose crop residues and vegetative matter releasing bound nutrients for other organisms. Many of the organic compounds released by fungi aid in forming humus and soil crumbs. Beneficial soil animals create tunnel like channels in the soil. These channels allow the soil to breath, and exchange gases with the atmosphere. Soil animals also aid in the formation of humus, and help balance the concentration of soil microorganisms. A healthy fertile soil must contain sufficient carbon containing compounds to sustain the billions of microscopic life forms required for a fertile soil and a healthy plant. A living soil is a fertile healthy soil.
Humus functions to improve the soil’s water holding capacity. The most important function of humic substances within the soil is their ability to hold water. From a quantitative standpoint water is the most important substance derived by plants from the soil. Humic substances help create a desirable soil structure that facilitates water infiltration and helps hold water within the root zone. Because of the large surface area and internal electrical charges, humic substances function as water sponges. These sponge like substances have the ability to hold seven times their volume in water, a greater water holding capacity than sod clays. Water stored within the top soil when needed, provides a carrier medium for nutrients required by soil organisms and plant roots.
Available water is without doubt the most important component of a fertile soil. Soils which contain high concentrations of humic substances hold water for crop use during periods of drought. This is why growers who apply humate based fertilizers and integrate production practices which preserve humic substances, can frequently harvest a crop during periods of dry weather.
Humic substances are key components of a friable (loose) soil structure. Various carbon containing humic substances are key components of soil crumbs (aggregates). Complex carbohydrates synthesized by bacteria and humic substances function together with clay and silt to form soil aggregates. As the humic substances become intimately associated with the mineral fraction of the soil, colloidal complexes of humus-clay and humus silt aggregates are formed. These aggregates are formed by electrical processes which increase the cohesive forces that cause very fine soil particles and clay components to attract each other. Once formed these aggregates help create a desirable crumb structure in the top soil, making it more friable. Soils with good crumb structure have improved tilth, and more porous openings (open spaces). These pores allow for gaseous interchange with the atmosphere, and for greater water infiltration.
The mean residence times of these organo mineral complex aggregates varies with different humic substances. The mean residence time of humic substances within these aggregates, based on radiocarbon dating, using extracts from non disturbed soils, is as follows: humin, 1140 years; humic acid, 1235 years; and fulvic acid, 870 years. Man has shortened the residence time of humic substances by excessive fertilizing and by using tilling practices that cause excessive weathering of sods. Soils abused by applications of anhydrous ammonia and by other destructive practices (those which destroy humic substance) can shorten residence times by several hundred years. The turnover time of organic carbon added each year from plant and animal residues averages approximately 30 years, under ideal conditions. In order to retain humic substances within the soil growers need to implement production practices which prevent their decomposition. Growers need to develop practices which retain the residence time of humic substances. It is essential to avoid destructive fertilization practices, rotate crops, minimize pesticide usage, deep plowing, and mix crop residues in the top soil by using minimum tillage practices. Soils which contain adequate humic substances have improved tilth (workability) and are thus more efficiently maintained for crop production.