Sooner or later every gardener, if he is a good gardener, becomes an Organic humus enthusiast. Humus, like chlorophyll, is one of the keys to the mystery of life. It presents problems still unsolved by the probings of science, although sufficient info is known to indicate its vital significance. It cannot be expressed within the terms of a chemical formula, neither can it be reproduced synthetically, but it is the stuff that brings the soil to life, and makes it capable of supporting plant life. No humus, no fertility, no life.
In the simplest terms Organic humus is the final stage of decomposition, rot, or decay of all organic material, vegetable and animal. Humus is the state of matter in which the complex biochemical forms of plant and animal life, bereft of the dynamic principle of life itself are broken down into simple substances capable of being built up anew into plant structures. As such, Organic humus, by its presence, quality, and amount, determines the fertility of the soil.
Humus is all things to all soils. Its primary source is organic vegetable and animal refuse. This is converted into humus by the combined action of soil bacteria and earthworm. If produced on the surface, disintegration of the organic matter produces a light brown or brown mass, containing more nitrogen than carbon.
Decomposition within the top layer of soil produces a humus of black or dark brown colour, rich in carbon, and more active as a basis for plant activity within the soil. Humus acts as a weak cement to the soil. Added to light and sandy soils it serves to hold the coarse particles together. Added to clay and heavy soils it forms aggregates of the fine clay particles and makes its texture more open.
Physically humus is a colloid, with a colloidal property of swelling by absorption of water. In sandy, it improves the ability of the soil to hold moisture and the soil solution for plant roots. In clay, it pushes the aggregated particles apart, opens up the soil, and promotes better drainage.
In all soils humus makes aeration more effective, and oxygen is as necessary to plant roots and soil life as their counterparts above ground. Organic Humus also darkens the ground, enabling it to absorb the sun’s heat more easily and retain it more completely. It makes soils capable of sustaining plant life earlier in the year, and of maintaining a longer period of growth.
Humus betrays its presence by the texture of the soil. A soil well enriched with Organic humus is sweet smelling and elastic, and when squeezed feels like a sponge. It darkens the top soil. Lower down the soil is lighter in colour, owing to the absence of humus. Soils long enriched with humus develop the characteristic dark brown colour in depth.
Valuable as it is to the physical condition of soils, Organic humus is to be treasured even more for its direct and indirect release of plant foods. The raw materials of humus provide nutriment for all manner of soil inhabitants, but chiefly bacteria.
The chemical constituents of humus are highly complex. All we know is that in humus we have a mixture of the products of disintegrating organic refuse, more or less rich in carbon, nitrogen, and oxygen, and the numerous other elements in smaller amounts that make up the former organic and living structures. From the gardener’s viewpoint the essential of Organic humus is its condition.
Humus accumulated under conditions of poor drainage, hard soil surface, moisture, cold weather, and poor aeration, becomes peat-inactive, sour, and in itself infertile.
The best type of humus is produced under conditions of well-aerated cultivated soil, in which drainage is efficient, and a temperature around 45 degrees Celsius the ideal for bacteria activity. Such Organic humus provides the neutral colloidal material that appears to be the primary source of those subtle but vital elements of healthy plant growth, growth hormones, as indispensable to plant life as vitamins are to us.
Humus serves also to hold plant food made available in the soil in reserve, before completely disintegrating itself into nitrogen, carbonic acid, etc.
Unlike the mineral ingredients of the soil humus is exhaustible. Its formation and disintegration are hastened by the pressure of lime or calcium salts. These help to neutralize the acids produced in Organic humus-formation, and so speed decomposition. Humus vanishes fast in chalky, limestone, and gravely soils for this reason. They are humus-hungry soils, and we cannot replenish them with organic material too often or too much.
In all garden soils the act of cultivation serves to exhaust the humus content, and with it the fertility of the soil. Soil bacteria are always more numerous and more active in a cultivated soil, and the plants themselves draw extensively upon the soluble foods afforded by Organic humus and its derivatives.
Without humus living vegetation cannot thrive, and without humus the whole intermeshing activity of soil becomes suspended. Conversely, soil well enriched with Organic humus is well equipped to enable plants to withstand the vagaries of the weather. Being warmer, it permits an earlier start to gardening out of doors, and a healthy resistance to cold spells. Providing a reservoir for moisture and the soil solution of plant foods, it withstands hot weather and drought.
Depletion of the humus factor depends on the type of soil, cropping, cultivation, and manurial policy. Regular renewal through the medium of organic manures and composts result in an accretion of fertility over and above the seasonal expenditure.
Under natural conditions all things organic origin decay to form Organic humus and enrich the earth. Soil bacteria and micro-organisms in themselves, by means of their dead remains, are no mean contributors of humus. In rich, healthy soils, where there activity is encouraged, they furnish as mush as 10 to 12 lbs per 100 square yards annually, of a kind particularly rich in phosphoric acid.
In the garden some discretion must be exercised. Tree trunks, as well as tree leaves, furnish humus, but there’s a great difference in their rates of decomposition. Bones disintegrate much more slowly than blood. All organic remains are of value, but to render immediate service to the cause of gardening, especially food growing, they must be easily disintegrated, and their food values rapidly released.
Every garden, no matter how small, has a constant supply of potential Organic humus in its own plant waste of leaf, stalk, flower heads, hedge clippings, lawn mowings, etc. Seaweed from the shore, spent hops from the brewery, scraps from the household table, stable manure and poultry run, or the rabbit hutch-all provide potential life for the soil.
Materials, slow to decompose in bulk, can be more swiftly disintegrated by breaking then up into small particles. Bones con be ground, twigs chopped, tough cabbage stalks crushes and bruised, and then pre-rotted before incorporated with the soil, by mixing them with finer refuse, and allowing all to lie in a compost heap.
If results are to be swift and gratifying organic matter should not be added to any soil fresh. The first stages of decomposition are essentially fungal, and lead to the production of acids and carbonic acid in excess. This interferes with the manufacture of soluble plant foods in the soil, and restricts the breathing of plant roots, and their growth.
Organic material, well rotted, fits into the scheme of soil activity perfectly, and introduces matter already rich in partially formed Organic humus. If green or fresh manure of any kind must be added, it should be incorporated with the soil when the ground is unoccupied by plants, and well ahead of sowing or planting time. This gives the necessary time for the material to be decomposed and take its place in supplementing the dynamic plant-sustaining functions of the soil.