Agriculture. SOILS: From the Chemistry of Agriculture.

Waikato Times, Volume XXI, Issue 1771, 10 November 1883, Page 6

Agriculture. SOILS: From the Chemistry of Agriculture.

By E. W. Emerson Maclvor, P.1.G., F.C.S.

(Continued.) Soils are composed of clay, sand, livie, vegetable matter (humus), and mineral fragments (stones). Their properties, and therefore, their agricultural value, depend upon the proportions in which these materials are blended. Clay. — When pure, this substance consists of silicate of alumina, has a white or grayish color, and feels greasy to the touch. As met •with in most soils, it has a red color, due to the presence of ©xide of iron (rust), and is associated frith the constituents of plant food. When exposed to • a moist atmosphere, it absorbs water and retains it with great obstinacy ; and when wetted with water it forms a compact Bticky mass, through which water cannot easily penetrate. Hence, soils containing it in large proportion are heavy to work, and when once wet remain so for a long time. When a dense clay subsoil — a term applied to that part of a soil which lies immediately below the portion turned over by the plough, working at the ordinary depth — underlies an open, or comparatively open, soil, the rain water, being unable to escape, collects in it, and may render the land wet and even swampy. Its power of absorbing and retaining water, taken together with its general association with the constituents of plant food, entitle clay to rank as one of the most important components of soils. Sand has properties almost exactly the opposite of those of clay. It mainly, and in some cases entirely, consists of more or less finely divided granules of quartz, showing no adherence together, but lying loosely beside and upon one another, leaving open spaces through which water can readily pass. It can absorb only a small proportion of moisture from a damp atmosphere, and this it readily loses when the aiv becomes dry. When in the fine state of division in which it occurs j in some alluvial formations, it approaches clay in properties. It ia then less open in texture, absorbs more water from a moist atmosphere, and, when wet, forms a compact tass. Lime. — This substance occurs abundantly n certain Victorian soils, but we must leave consideration for a future lecture. Humus. — In the second lecture of our course we explained the nature of this substance, so that little need here be said about it. By slow decay in the soil it £>roduces caibonic acid, and also some nitric acid, both of which are of importance to growing plants ; the former, when in tho state of solution in water, acting upon, and rendering soluble, mineral food, the latter being direotly absorbed by the roots. Soils deficient in humus are light-colored, dry, and harsh to the touch ; those containing it in proper quantity are, 6n the other hand, dark, moist, and mellow, and otherwise better suited to maintain young yegetation. Mineral Fragment* (stones). — These are found in all classes of soil, and are, of course, objectionable to the practical farmer. But, like everything else, they are not without good qualities. Pieces of granite, basalt, or other rock, are slowly decomposed by natural agencies, and yield products which in couise of time alter the physical character of the soil (pand increase the amount of mineral food. The stones in our fields may therefore be looked upon as magazines of plant food, the largest of which must ultimately yield up the whole of its riches. After these preliminary remarks, we may proceed to consider what are termed the physical properties of a soil, and the extent to which these are modified by the proportions in which clay, sand, lime, and humus enter into its composition. Ist. Absorption and Retention of Water. — When dry soils are exposed to a damp atmosphere they, as may be concluded from what has already been said, absorb moisture and increase in weight ; and they possess in different degress the power of retaining, or holding, the water so absorbed, or which falls upon them as rain or dew. The more sand a soil contains in proportion to clay and humus the less moistuie it is capable of absorbing, and the more readily it parts with what it may take up. Again, rain-water easily passes through a loose sandy soil, without being to any great extent retained. These statements lead us to the conclusion that the greater the amount of clay and htimtis present, the greater ivill be the abhorptitoe and retentive power of a well cultivated soil. Some fifty years ago, Schiibler, to whom we owe much important information regarding soils, made an elaborate series of experiments on the absorptive and retentive power of soils and their constituents, and, as his results bear out what has been said, we may briefly notice them. He found that after seventy-two hours' exposure to moist air, humus had taken up nearly two-and a-half times as much water as clay, and forty times more than sand, which, under the same circumstances, took up sixteen times less than puie clay. In a second series of experiments Schubler placed weighed quantities of the dried soils in funnels, made them peifectly wet by the gradual addition of water, and then left -ttiem to drain. As soon as the water ceased to drop from them the wet masses were carefully weighed. The difference in weight between the dry and wet soils was taken as representing the amount of water that they would hold after thorough saturation by long continued rains. The retentive power of the different soils was then determined by exposing the saturated masses for about four hours to a dry atmosphere having the temperature at 66 degs. The greater the loss of water experienced under these conditions, the less letentive the soils.

These numbers speak for themselves, and, after what has been said, need no further comment. The subject we hay \ been considering is of great practical importance, for the ability of a soil to resist drought is, in a great measure, dependent upon its absorptive and retentive power for water. Sandy • have this power in the least degree, and hence suffer much more than clay soils from a continuance of dry weather. The latter are, however, often too retentive, and remain wet and unworkable. Again, the continuous and slow evaporation from such soils is objectionable for another reason, viz., it keeps the land "cold," that is to say the heat of the sun is spent in vaporizing the water instead of being employed in warming the soil itself. The cure for " co.dness " in clay land is drainage. 2nd. Absorption and Retention of Soluble Plant Food.—- In 1845 Mr. H. S. Thompson made the interesting and ultimately important observation that soils rapidly remove ammonia from its solution in water, and fix it so that rain cannot easily wash it out. On filtering solutions of sulphate or carbonate of ammonia through a layer of soil it was found that the first portions of the liquid that came through were free from ammonia. Soon after Thompson's observations, Huxtable found that dunghill drainings lost their colour and smell on being filtered through a loamy soil. But the first really complete investigation of this subject was made by "Way, who found that not only do solutions of ammo-nia salts give up the whole of their ammonia to the soil, but that potash, phosphoric aoid, and, to a lesser extent, magnesia and soda are also removed from their solutions and fixed in the time way. Nitric acid is not retained so firmly, as the other substances named, andhence is more r liable to Jbe washed out by heary rains, ' . ■ • > > -, ;-' » ■ ' /- '*.'(. ' i 1 jn

Water absorbed by 100 parts per cent. Sand . . 25 Light clay . . 40 Stiff clay . . 50 Heavy clay 61 Pure clay . . 70 Humus ..190 Bich garden BWsoil ... 96 t Of 100 parts of water absorbed, there evaporate in four hours, at 66 deg. P. 88-4 52-0 45-7 34-9 31-3 20-5 24-5

The following are some of the results obtained by Schubler :—: —