WHAT FARMERS OUGHT TO KNOW.

Temuka Leader, Issue 1141, 19 February 1884, Page 2

WHAT FARMERS OUGHT TO KNOW.

AGRICULTURAL CHEMISTRY. (Written specially for this Paper.) in. Soil. A thorough and intimate knowedge of his soil, -of its chemical composition, of i's mechanical texture, of its retentive oroperties, of its conditions in iclation to the atmosphere, etc., —is quite indispensable to the farmer who works on scientific principles, and does not trust to mere chance and guess-work. The soil being the basis of al! agriculture, and the material dealt with to the greatest extent by the farmer, it naturally forms the first and most important subject treated - on in all books concerning agriculture. It '8 needless to say that the study of the substance we see most of (and which a great many know really very little about) is highly interesting and instructive. Firstly, as to the origin of soils. It is a well known and undisputed geological fact that a constant disintegration or crumbling away of all materials on the face of the earth is taking place. The rocks before our eyes are changing their aspect; the rivers, in time of flood, are visibly carrying away soil; the rocks, forming the bed of the soil, or sub-soil, are crumbling away; soil itself is swept off the sides of mountains, and the faces of the rocks are laid bare; portions of lulls, in the form of landslips, are tumbling down to the level ground ; sand is swept away by the wind and deposited in drifts in different localities ; the sea is making unceasing ravages on the cliffs within its reach. Such are some of the changes silently though unceasingly going on around us. Professor Geikie, in one of his treatises on Geology, says on this subject :—“The work performed by the

various forces employed in modifying the earth’s crust is at one and the same time destructive and reconstructive. Eocks are being continually demolished, and out of their ruins new rocks are being built. In other words matter is constantly entering into new relations, now existing as solid rock, or in solution in water, or carried as the lightest dust on the wings of the wind ; now being swept down by rivers into the sea or brought under the influence of subterranean heat, but always changing sooner or later, slowly or rapidly, from one form to another.” Soil results from the decaying, disintepration and crumbling away of rocks. There are many agents at work to accomplish this. The atmosphere produces the wasting away of rocks called weathering. Oxygen of the air, exerting its tendency to unite with other elements, combines with metals present in the rock and exposed to its influence. A new material, and a heavier one than the metal itself is formed, and it is sufficiently evident that a loosening of the particles of the rock necessarily follows. Rain when falling is frequently charged with carbonic acid gathered from the atmosphere, which receives it from the breath of man and animals, and we have (dmesione being instanced) that water charged with this gas is capable of dissolving many substances insoluble in pure water, and thus rain so charged dissolves many salts present in rocks, the natural result being disintegration. Calcareous or limestone rocks undergo perhaps the fastest dissolution in this way.

It is a well-known law (with the inevitable one or two exceptions) that expands bodies and cold contiacts them. The object in laying the rails of a railway line some shmt distance apart from one another, is to allow for expansion of the steel or iron in hot weather. Why does a *lass crack when very hot water is poured into it ? It results from the unequ-d expansion of the material. The interior expands rapidly because of the heat; the exieiior being uninfluenced refuses to expand, and the glass, being so very brittle, cracks. When the weather is warm the mercury in the thermometer r ses, when cold the metal sinks. Now the alternations of heat and cold exert a great influence on rocks by the alternate expansion and contraction produced. One of the exceptions to this law is that of water forming into ice Water on co ding below 4 d-g. U. expends till it becomes ice. Oce very evident proof of this is that ice floats. Suppose that water is present in the interstices of rocks and that a severe frost comes, the liquid expands (and with tremendous force too), and breaks up the containing walls. The mechanical action of water soon carries away the fragments broken by this and other means. In this way whole rocks get cleft in two. Another force at work is plant life. There are many strong plants that force their roots into the crevices of rocks, and in that way aid the process of breaking up. These are some of the principal methods by which nature accomplishes her object of effecting a demolition of rocks. There are, as previously stated, “reconstructive” agencies at work also, but it is quite unnecessary to touch on them. A disintegration of rock having been accomplished, water—rain and river- acts, as carrier, and conveys the crumbled mass away and deposits it somewhere else. In many cases water will not remove the particles, but soil forms on the top of the rock. The process is thus lucidly described by •Sibson : ‘‘Fragments are loosened and detached ; they then crumble down and accumulate, until a layer of sufficient thickness is formed to retain enough water to preserve it in a porous condition. Mosses and plants of a low organisation will now spring up; these in time will decay, and furnish to the imperfect soil a quantity of decaying vegetable matter ; it will now be in a condition to allow the growth of a higher class of plants, whose seeds may accidentally be conveyed to the spot. Vegetation, being once established (the bmmia we shall presently describe), will steadily increase, until a due proportion is present to constitute a sod. This soil will, of couise, be strictly, dependant on the character of the rock from which it is formed ; all its qualities—its colour, texture, etc., —will resemble those of its parent rock.” All soils, however, are not formed directly on the top of the rock. This disintegrated rock is slowly carried off by the action of water, and this gives rise to sand, gravel, etc. Y lien small particles of rock are swept, away by water, abrasion between the particles so held in suspension must always take place, The rubbing together of rough fragments of rocks induces smoothness ; water assists the action by helping to remove the portions loosened hy contact and afterwards rubbed off— thus in a short time sand, smooth pebbles, gravel are produced. To bring home this remark, it is only necessary to instance the smooth, rounded, level shape of stones on the beach ; this is produ ed by the constant action of water and the abrasion between the stones themselves. Now, it is very plain that when the resulting substances from the disintegration of rocks are carried a n ay by water they have better chances of meeting and mixing with the components of other soils er rocks. Sometimes the composition of rocks in the same valley differ widely, but when the dust and particles of decaying rocks are under the influence of water, it will carry away both or all the varieties that may exist within its range, and the consequence is that all the different kinds get mixed, and a toil much richer in the various necessary constituents will result ; and os a rule the soil so produced by an intermixture of the elements of the various rocks will be considerably better than that produced by the disintegration of one particular kind. From the foregoing short sketch some idea may be got of how soil is produced from rock. To summarise it. Rocks are constantly disintegrating or crumbling away ; the various means of nature to accomplish this end may be divided info chemical and mechanical :—Chemical are — (a) the capability of water to dissolve various substances present in rocks through the agency of the solvent carbonic acid ; (6) the union of oxygen with various materials in rocks. Mechanical are —(a) The power exerted by water in loosening particles; (6) the effects of alternate heat and cold ; (c) the action of water in removing the fragments, howsoever detached ; (d) the power of coarse vegetation to spring up with very small oiguqio matter present in the soil ; («) the

decMj of this rough plant life to afford org-in c material necessary for the growth oi Irgher vegetable life, and the consequent formation of a soil fit for agricultural purposes.

Soil is defined to be “ that part of the ground which can be tilled and in which plants grow. It is much tjjerupper stratum of decayed rock mixed with vegetable and animal remains. It varies in death from three inches to more than a foot.”

The subsoil is immediately below the soil ; below the subsoil again is the rock on which it rests. Even this rock beneath the subsoil is constantly decaying and rotting away, and lienee “soil is said to be be rotted subsoil, and subsoil rotting rook.” The organic and inorganic or mineral matter is the composition of soil. Organic matter consists of various chemical com pounds of carbon and oxygen, carbon and hydrogen, the three together, and also combined with nitrogen. Plant life is composed chiefly of organic matter, and this is afforded partly by the soil, partly by the atmosphere. In a former article we spoke of a substance called “ humus.” Humus results from the partial decaying of vegetable matter in the ground. It is composed essentiallv of carbon, and is black. It is found in all produciive soils, and is almost indispensable, as will be seen by its action. It has tlip property, as previously stated, of absorbing ammonia and retaining it. When rain falls the ammonia dissolves in it, and by th«s means it is conveyed to the roots of plants, by them to be absorbed and appropriated for their use. Ammonia (oitrogen and hydrogen) is exceedingly useful to the plant, since it affords it nitrogen, an element entering very largely into vegetable structures. This gas takes part in forming ths flesh of animals, conit is found in plants, and consequently it is present in various com hinations in the soil. Wheat (grain), of flesh-forming materials, contain, 11.6 per cent, turnips 1.14 per cent, cabbage 4.75 per cent. From this the importance of the presence of humus in the soil to absorb nitrogen (in the shape of ammonia) to furnish it to the plant for the nourishment of man and the animals, is sufficiently obvious. Another office that humus performs in the soil is supplying vegetable life with carbonic acid. Humus consisting essentially of carbon, decay or decomposition takes place by union with oxygen of the atmosphere, thus producing carbonic acid gas. This acid we have already referred ta as a necessary conatiluent in the food of plants. In the presence of sunlight, plants have the power of splitting it up, reserving the carbon to build up its stalks, etc., and rejecting the oxygen. The fertility of a soil does not altogether depend on the amount of humus present in it, since many good crops are grown on land containing but a small amount of it. As we have said before, it results from the partial decay of vegetable mattei. Peaty ground consists- of excess of organic matter—humus—but in this state it does not decompose, and the soil is almost useless.

Humus is not the only organic matter in the soil. The decay of roots, weeds, etc., also affords organic material, other than humus. When a crop is taken off a piece of land, in order to remunerate the soil for the portion removed, sometimes mustard seed, or rape, or other fast growing plant, is sown, and when it is grown it is ploughed in; the plants decay, thus giving back to the soil materials that have been removed in the crop, and winch it is imperative for the soil to have, if it is required to grow grain again—provided of course that the land has been deprived of so much that it cannot afford the growing plants enough. Insects, larvte, etc., are the means of fertilising the soil. We shall now pass on to consider the inorganic portion of soil. The inorganic constituents consist chiefly of metals, but some non-metallic substances are also present. The metals exist principally as salts (composed of a metal and acid). The acids generally present are carbonic, phosphoric, silicic, sulphuric, hydrochloric ; the salts formed from these will be respectively, carbonates, phosphates, silicates, sulphates, chlorides. The fol lowing are the chief ingredients— Silica (oxide of the metcl silicon), alumina (oxide of aluminum), carbonate of lime (chalk or limestone), potash (oxide of potassium), soda (oxide of iron), magnesia, and the non-metallic elements, sulphur and chlorine. For reference and comparison we append the nnalysis of some soils, extracted from Mr Sibson's work on tin's subject. In our next we will consider in more detail the various constituents of soil enumerated above. Percentage Compositon op Soils.

(To be Continued.)

A good Sand j Soil. A fertile Clay Soil. A Lime Soil. Organic Matter, humus, etc. ... .49 3.38 6.33 Oxide of Iron ... 3.19 8.82 | 9.31 Alumina 2.65 6,67 Lime .24 1.44 Car.of L. 54.56 Magnesia .70 .92 Trace. Pot ash and Soda.. .14 2.56 1.03 Phosphoric Acid.. .07 1.51 Trace. Sulphuric Acid ... Trace. Trace. Trace. Insoluble Silicates Clay and Sand.. 92.52 72.83 28.77 Carbonic Acid ... — 1.87 —