What is Humus?

The need for constantly replacing organic matter in soil lies in its vital role in building up the humus content. But what is humus? Actually, nobody really knows. It is somehow tied up with lignin (a substance from wood), with protein and a colloidal complex - that about states what we know about it. The three or four lignin chemists who know more about it than anyone else differ and argue about its composition.

But what humus does is far from mysterious. It is so thirsty that it will absorb between 80% and 90% of the water in a saturated atmosphere. In comparison, the best clay can do is to absorb 20%. It makes soil nutrients much more available. It breaks down slowly, releasing its protein in the form of nitrogen over a period of from five fifty years. Thus it is a "built-in" source of plant nutrients available even when other sources fail.

An important function of humus is to pull tiny soil particles such as clay into clumps or "crumbs," which make a heavy clay more porous, easier to work, and a better place in which to grow plants.

The breakdown of organic matter into humus is slower when fats and waxes are present, but the final release of the desirable elements in humus is correspondingly slowed up. Because of the more rapid destruction of humus in tropical and subtropical climates, it has little chance to accumulate. This means that organic matter is used up faster in such climates and must be replaced more often. Humus in warm climates is lighter in color; in northern soils it is often a deep blackish brown.