THE ORGANIC MATTER OF THE SOIL.

New Zealand Journal of Agriculture, Volume XXVII, Issue 2, 20 August 1923, Page 85

THE ORGANIC MATTER OF THE SOIL.

B. C. ASTON,

F.I.C., F.N.Z.Inst., Chemist to the Department

The various classes of material which go to make up a fertile soil may be broadly classified into five groups, according to the origin of each.

First there is the rock-derived mineral matter, wholly lifeless, but capable of change in composition, character, and appearance — compound reacting with compound, or with the soil-water, or with the substances dissolved in it. Second are the important salts of calcium, mostly derived from pre-existing —the carbonates and phosphates of lime, as they are commonly called. Third is the soil-water or soilsolution, containing, in addition to a certain amount of carbonic-acid gas, small amounts of other soil-constituents. Fourth is the soil-air, about which so much remains to be learnt. Fifth, and 'most important, is that portion of the soil which will burn away when ignited, known as “ organic matter,” derived from the remains of plants and animals dying in or on the soil, forming, ultimately a black spongy mass, the culture medium for nourishing that portion of the soil-life which lives wholly below the surface. .

This organic matter, or, as it is often broadly termed, humus, is the portion which gives character to the soil, and when present in sufficient quantity obliterates all other distinctions. For instance, both sandy and clay soils when altered by the growth of organic matter lose their characteristic features, both loose porous and very dense impermeable soils, when mixed with organic matter, becoming altogether changed in texture. Thus on the Manawatu dune-sands, where the topography will not permit the escape of the copious surface water, aquatic and semi-aquatic vegetable growth develops, organic matter accumulates, and finally, in an area surrounded by sandhills and resting upon sand, occurs an area which when drained and “ brought in ” presents none of the difficulties of treatment which the sand (with which it may be still largely admixed) originally exhibited.

A good supply of organic matter darkens the colour of . the soil, thereby causing the absorption of more sun's rays than is possible in a lighter-coloured soil. This effect in rise of temperature is appreciable, and may be measured by means of a thermometer. Organic matter can withdraw from the soil-solution various plant-foods. It causes the soil to swell up, thereby increasing its pore-space and

mitigating the evil effects of wet seasons on the growth of crops. By increasing the water-holding capacity of the soil in seasons of temporary drought it again benefits the growing plant. Organic matter is a source of energy. With it beneath the soil-surface life is possible to many kinds of visible and invisible plants and animals, some having a beneficent and some a maleficent effect on crop-growth, but the net result on crops must be for good. Organic matter assists the decay of rocks, and in soils of coarse texture fills up the large excess of air-space with colloidal (or gelatinous) vegetable matter, which has a decomposing effect on the mineral particles and renders the whole into a mellow loamy mass highly responsive to cultivation, liming, and artificial manuring, resisting abnormalities of climate—a fit home for the nurture of the seedling crop at its tenderest stage.

The importance of conserving the organic matter in a soil will be seen when the difficulty of replacement and the result, if this is not done, are considered. Usually everything that a soil requires may be purchased in commerce except organic matter. This must be either supplied on the farm or accumulated indirectly by an artificial system of fertilizing and liming, green-manuring, or by laying down in pasture. The history of many of the derelict farms of the United States of America is that of neglect in conserving and replacing organic matter. The rehabilitation of these farms is dependent on giving back those soil-constituents which have been squandered by wasteful and continuous methods of cropping. The replacement of the mineral nutrients removed by crops or washed away in the drainage-water may be accomplished at any time artificially, but the recovery of the nitrogenized organic matter is a much slower process, involving the abandonment of all tillage operations and leaving the soil to be covered by vegetation for a number of years. So by the aid of life the gases of the atmosphere—carbon dioxide, nitrogen, and oxygen—and water are slowly built up into life-giving humus.

Chinese market-gardeners—oldest of husbandmen—know the advantages of organic matter, and not only endeavour by manuring with decomposable materials to maintain a state of high fertility, but in the first place take care to select the site of old swamps black with decayed vegetation as the situation of their gardens.

The organic matter of the soil consists of material in all stages of decomposition, from material which shows organized structure (even the kind of plant which yielded it can be made out in some swamps) to the spongy black material in which all visible traces of organic structure have been lost. The change goes further still, for the oxidation of the humified mass continues towards the stage at which it is resolved into those gases and water whence the organic matter was originally derived. ‘ - -

Among the factors in nature which bring about an accumulation of organic matter are low temperature, excessive water, and limestone. In the subantarctic islands southward of New Zealand a humus soil accumulates as it does in the northern islands of Britain, even in situations where there should be good drainage. One would think that the low temperature would be the main factor in our southern islands, though the absence of bacteria may contribute to the result. In the warmer North Island of the mainland, where peaty soils develop, one

would think that it is the stagnant water and lack . oh oxygen which determines the accumulation ; while in situations where limestone comes near the surface in dry, warm districts one finds a deposit of mellow black soil accumulating which can be due neither to cold nor excessive moisture, nor to lack of oxygen.

In certain areas where swampy soils containing much organic matter have developed and are deficient in mineral matter it may be practicable to transport inorganic or mineral matter, such as sand and pumice or clay, on to the surface, and so improve the soil in its deficiency. Nature has done this for the extensive Te Puke and Rangitaiki Swamps. Layers of air-borne pumice and volcanic ash are to be found at various levels in the former, and calcareous mud, pumice, and ash layers in the latter swamp. These layers greatly facilitate drainage, and ameliorate the conditions physically and chemically to a degree which owners of other swamp lands can only envy. When one reflects on the good which a pumice deposit will effect on a swamp soil it at once suggests what should be supplied to a pumice soil to improve it.

It must not be thought, however, that humus soils are everything that could be desired, for even they have their weaknesses. Frequently they are deficient in mineral plant-food, but respond at once to dressings of phosphates or potash when. these are lacking. Sir John Russell, F.R.S., quotes a pathetic instance of an American farmer who endeavoured to farm on a black soil. “ The land looked rich,” he said, “ as rich as any land I ever saw. I bought it, drained it, and built my home on a sandy knoll.” His first crops were fair, but grew rapidly worse. He and his wife and children wasted twenty years of their lives on this land. It was poverty, poverty always. After he had given up his holding, and a chemist had been at work on the problem, the farmer one day brought his wife and children to see the heavy crops on plots treated with potassic fertilizers alongside the miserable ones on untreated land. In tears he asked, “ How was I to know that this single substance which you call potassium was all we needed to make this land productive and valuable ? ” In the case of humus soils mechanical analysis is inapplicable, so that any knowledge, apart from field and pot experiments, must be gained by chemical analysis.

The importance of increasing the store of organic matter or humus in New Zealand soils has not in the past received that attention merited by the importance of this aspect of manuring. There are three ways in which the organic matter in the soil may be increased in farming practice : (i) By applying dung, stable, farmyard, or other crude refuse of an approved organic nature to the soil; (2) by growing and turning in a green crop, known as -manuring ” (3) by putting the land down in pasture, which allows organic matter to accumulate. The difficulties of supplementing the store of organic matter in the soil are sufficiently indicated by these methods of redress, but the problem must be faced if many of the coarser-grained soils of this country are to be profitably and continuously worked.

At Stanley Brook Valley, Nelson, there is a flat area of gravelly loam and sandy silt concerning which the writer advised the local branch of the Farmers’ Union in March, 1919, that “ the improvement of these soils will depend largely bn increasing the organic matter in

the soil, which may be accomplished either by ploughing in green crops (green-manuring), or growing a larger share of clovers and other leguminous plants in the pasture, or, where possible, by applying organic manures, stable manures, or flax-waste refuse, and generally adopting methods of farming which will conserve the organic matter in the soil.” The Cawthron Institute's chemist, Mr. T. Rigg, has recently pointed out as a result of his investigation of this land that “ every effort should be made to maintain the humus content of the soil. This may be effected by periodical ploughing-in of catch-crops of blue lupins or tares with oats. The latter crop is particularly recommended.”

The pumice gravels and coarse sands of the North Island thermal district show similar improvement, and largely lose their identity as coarse soils when they have been submerged by lake or river, mixed with the remains of aquatic plants, and have finally emerged as a terrace highly fertile in comparison with the material as it existed in its original, state, and still exists in many parts where the beneficial influence of lake or river has not been felt.

Attempts to improve coarse pumice lands should be based upon the methods seen to be successful in nature, but hastened to accord with present needs. In forested areas which have been cleared topdressing methods must suffice until the stumps can be profitably removed, but in areas growing scrub green-manuring and rolling could at least be tried experimentally. In areas suffering from exceptionally severe climatic conditions the planting of exotic forest will mitigate the severity of the cold and add organic matter to a soil greatly in need of it. (A discussion' on pumice soils may be found in the Journal, vol. 4, 1912, page’374.)

CONCLUSION.

It is thus seen that organic matter improves the texture of all soils deficient in that constituent, whether they are, on the one hand, extremely loose and porous, or, on the other hand, stiff, impermeable, and tenacious. However rich a soil may be in mineral plant-food, if it lacks the texture conferred by an adequate supply of organic matter there will be trouble in abnormal seasons ; while for a large number of soils of coarse texture and deficient water - holding power it is essential that prompt attention should be paid to the organic-matter content if fertility is to be maintained. Organic matter tends to accumulate in land under permanent pasture, and to dimmish in land under crop.

List of Qualified, Veterinary Surgeons. The name of Mr. A. Taylor, F.R.C.V.S., was omitted from the list published in the June Journal. Mr. Taylor recently retired from the staff of the Canterbury Agricultural College, Lincoln, and is now located at Christchurch.

The Gold Medal of the Linnean Society (London) has this year been awarded to Mr. T. F. Cheeseman, of Auckland, author of the “ Manual of the New Zealand Flora.”