- Whether on a forest floor or in a compost pile, accumulations of shed organic matter attract decomposers. As decomposers break down the matter, they exude resins, waxes and fats and often leave behind tough-to-break-down lignin and resins from plant material. All of these materials are still carbon-rich, stable organic materials.
- On a molecular level, the positive surface charge of humus attracts ions of ammonium, potassium, copper, magnesium and zinc. This means that humus can hold onto these valuable plant nutrients until the plant is ready for them, so they don't leach out of the soil. Humus also retains soil moisture, improves soil oxygen capacity, acts as a pH buffer and can prevent metal toxicities.
- Adding nitrogen fertilizers to soil year after year, decade after decade degrades soil structure. Microbes in the soil need a balance of nitrogen and carbon to feed and reproduce. Humus is a carbon-rich material but hard to break down. When nitrogen is added, microbes suddenly have excessive "fuel" and rapidly try to break down any carbon source available -- usually the humus in soil and the roots of the plants you're trying to fertilize.
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