Humus is the term used to describe the organic matter in the soil. Its presence in adequate quantities is a vital factor in building up and maintaining soil fertility. Humus may consist of decayed vegetable or animal refuse, still showing traces of leaves, stems, stubble, bones, and so on, or it may have been completely broken down into a blackish, powdery substance.
Garden soil which has been regularly treated with humus, whether from the compost heap or from organic manures, has a rich dark color. This factor is of benefit in itself, for dark soils, rich in humus, tend to warm up more quickly in spring, so promoting early seed germination and earlier cropping.
Humus aids fertility in two main ways. First, the organic matter stores nitrogen, which is converted into ammonia by the micro-organisms in the soil and released to the plants in the form of nitrate. Second, humus can hold and store considerable quantities of moisture. This moisture-holding capacity helps to break the soil down into crumbs, giving sufficient space for the retention of moisture, but allowing excess water to drain away. The good soil structure and aeration, obtained in this way, assist root development and, together with the water holding capacity, enable plants to stand up better to drought conditions. A humus-rich soil also produces the fine tilth necessary for seed sowing.
All cultivated soils lose organic matter and this loss is increased when the ground is limed. Some humus is left in the soil by plant residues, but this is not enough and the losses must be made good. The more rapid and thorough the cultivation, the more rapid the loss of humus. Where farmyard manure is available, it is still the easiest and most practical way of maintaining the organic content of the soil. It should be spread and dug into the ground as soon as possible, for some of the nutrients will be lost if it is allowed to stand in heaps exposed to the rain. In clay soils it should be dug in during the autumn.
The old-time gardeners dug manure into the second spit, but it is far better to incorporate it in the top spit within easy reach of the growing plants. Other manures (from pigs, rabbits, poultry, etc) and spent mushroom compost can all be used to add humus. All gardens should have a compost heap where most of the garden refuse can be broken down (the roots of diseased plants and roots of persistent weeds such as bindweed and couch grass should be burned).
Humus can also be added in the form of peat, leafmould, spent hops, seaweed, shoddy, straw, fish waste, or in prepared fertilizers made up from hoof and horn and bone meal. Blood and offal from a slaughter house can be an equally valuable source. The presence of adequate humus in the soil does not necessarily mean that fertilizers can be dispensed with.
While most crops benefit from high quantities of organic matter in the soil, some flowers, for instance nasturtiums, do better on poorer soils.