The value of humus
Humus benefits thein two ways: it acts as a superb soil conditioner, and also supplies small amounts of plant foods. Humus improves the texture of all types of soils, from light sandy soils to heavy clays. When added to a sandy soil, it helps to retain water, so that plant nutrients are not easily washed away and the soil remains fertile for a longer time. When added to a clay soil, humus ‘opens it out’ by separating the tiny clay particles, so that tile soil is made more porous and water flows through it more freely. The improved drainage that results increases the air supply in the soil, which makes for healthy root growth of your crops; also, because the soil is less wet, it is warmer, and seed germination and plant growth are speeded up.
In addition to this, the presence of humus helps to control the heat content of soil generally and prevents violent fluctuations in temperature.
Humus also provides a slow but steady supply of plant foods, of which the most important are nitrogen and phosphates. The former is particularly important because, without humus, soil consists literally of powdered rocks which, although they give a good supply of other plant nutrients, yield no nitrogen.
Nitrogen can be supplied to plants in fertilizers; but these contain it in the form of simple compounds of nitrogen and other chemicals, which are soluble and may be washed or ‘leached’ out of the soil by rain and lost. The nitrogen in humus, on the other hand, is contained in complex insoluble compounds which are only very gradually but continuously released in a form which plants can take in after they have been broken down by the soil bacteria and other organisms. The plants take it up as it is produced, and the waste is negligible.
It is not possible to increase rapidly the amount ofin the soil by green manuring, because digging in a typical crop containing 2.5 tonnes per hectare (1 ton per acre) of dry matter will increase the organic matter in the soil by only about 0.1 percent, and of this small percentage, only a fraction will eventually form humus. Nevertheless, if green manure crops are grown regularly in rotation over a period of years, they will make an appreciable difference to the humus content and structure of the soil.
Green manuring is an excellent method of maintaining the humus content of a good soil, or improving the humus content of a poor soil.