Written by Niels Harmuth (Agronomist)
Soil consists of four important parts
Organic matter: What is it and why is it important?
A typical agricultural soil has 1% to 6% organic matter. In some cases it can even be lower than 1%. It consists of 3 distinctly different parts.
Well decomposed residues
These three parts of soil organic matter have been described as the living, the dead and the very dead.
The living part of soil organic matter includes a wide range of microorganisms such as, bacteria, viruses, fungi, protozoa and algae. It also includes plant roots, insects, earthworms and larger animals such as moles, rabbits, etc. The living portion normally represents about 15% of the total soil organic matter. The microorganisms, earthworms and insects feed on plant residues and manures for energy and nutrition, and in the process they mix organic matter into the mineral soil. They also recycle plant nutrients. Sticky substances on the skin of earthworms and other substances produced by fungi, help bind soil particles together, forming soil aggregates which make up good soil structure.
The fresh residue or ‘dead’ organic matter consists of recently deceased micro-organisms, insects, earthworms, old plant roots, crop residues and recently added manures. This part of the soil organic matter is the active or easily decomposed fraction. This active fraction is the main source of food for various micro-organisms, insects and earthworms living in the soil. As organic materials are decomposed by the ‘living’ they release many of the nutrients needed by plants. Organic chemical compounds produced during the decomposition of fresh residues also helps to bind soil particles together and therefore aiding in the development of soil structure.
The well-decomposed organic material in the soil, the ‘very dead’, is called humus. Humus is very stable and complex. Humus is however, not food for organisms. Humic substances play a vital role in soil fertility and plant nutrition. Plants grown on soils which contain adequate humic substances (humus, humates, humic acid, fulvic acid and humin) are less subjected to stress, are healthier, produce higher yields, and nutritional quality of harvested foods and feeds are superior. Humus is the major soil organic component, making up 65% to 75% of the total.
Man became distracted from the importance of organic compound cycling when it was discovered that soluble acidic based NPK ‘fertilizers’ could stimulate plant growth. Continuous use of these acidic fertilizers in the absence of adequate humic substances in the soil has caused many serious sociological and ecological problems. Man needs to reconsider his approach to fertilization techniques by giving higher priority to soil humus.
The urgency to emphasize the importance of humic substances and their value as fertilizer ingredients has never been more important than it is today. Humic substances are recognized by most soil scientists and agronomists as the most important component of a healthy fertile soil.
Humic substances can form naturally within soils that are properly managed. Practices such as crop rotation, using balanced fertilizer programs, planting legumes, ploughing under green manures, returning organic matter to the soil, application of compost and using minimum tillage practises can all help build humus. Any production practice which damages the activities of living organisms within the soil should be avoided. Humus building practices are slow, time consuming and can be costly, however, they pay large dividends over time. The most rapid and practical solution to improve soil fertility is the addition of humates (mineral humic substances) directly to the soil or as foliar fertilizer. In most soils the application of humate based fertilizers is more important than applying the traditional NPK fertilizers. Humic substances will maximise the efficient use of residual plant nutrients, reduce fertilizer costs and help release plant nutrients presently bound as minerals and salts.
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