PRACTICAL IRRIGATION

Otago Witness, Issue 3019, 24 January 1912, Page 21

PRACTICAL IRRIGATION

By John M Keague.

THE SOIL

11. (Copyright.) The soil is a product made or manufactured bv the natural agents—the atmosphere, water, the loots of plants, cold and the grinding action of ioo and of tides. The action of the atmosphere, and chiefly its carbonic a< id, in wearing down iocUs, is well known. Kxpose the surface of afreshly >')tia tried rock to tire air, and soon its fresh and generally bright appearance will disappear. The carbonic _ acid ha.-* overcome the adhesive, the binding power or force that kept the particles so firmly together. I f the rock bo so porous as to admit the smallest portion of water a further powerful agent in the crumbling process gets to work. The carbonic acid in the rain water acts, as it acted in the case of the atn esphere. But, further, should the temperature fall to or below tho freezing point the water in the pores and interstices of the rock freezes, and, expanding, forces the particles, or even portions of the rock, asunder. The grinding action of moving ice, of rivers, and of the tides is further example of the grinding down and crumbling of rocka In procoso of time the boundary of organised matter crossed, the loots of tho lowest forms of plants take root in the newly-formed soil, grow, die, and add organic matter to tho soil. If the rock, from which tho coil has been formed be linestonc, the soil formed will bo calcaieous; if feldspathic, tho soil will be clay; if silicious. the soil will bo sandy or gravelly. As tho growth of the vegetable world increases. masses of vegetable n atter die, decay, and form peaty soil and loam. The decayed vegetable matter is known as the organic portion of the soil; and crumbled rock as the inorganic. In tho course of countless centuries of time, the soil, as we know it, has been formed; and a fertile soil generally ontains about nine-tenths inorganic or irineral matter and about one-tenth organic matter. The similarity between the constituents of the soil and of plants may be set forth thus:-

THE SOIL AND CHOPS. Portion Of Soil. Of Crops. Carbon Woody fibre Hydrogen Starch Oxygen Sugar Nitrogen Gum Sulphar Albuminoids Phosphorus Gluten Oil Fats Portion^ Of Soil. I Of Crops. 6an3 j Silica Olay , Clay Lime Lime Potash Potash Soda Soda Magnesia Magnesia Oxide of iron Oxide of iron Oxide of manganese Sulphuric acid Sulphuric acid Phosphoric acid Phosphoric acid Chlorine Chlorine lodine lodine Bromine Bromine Fluorine Fluorine While it Ls true that ohemiste can often detect traces of substances in plants that they cannot detect in the soils in which the plants grow, it is, nevertheless, true that the inorganic substances of the ooil and of the crops are practically the same; and also that the organic, portions of the crops may be reduced to the organic portions of the soils. Tho crops simply feed cn the prepared portions of tho soil that they require. But the proi>ortions of organic matter and of-inorganic matter are very different in different classes of soil. In burning an average soil, out of every 1001 b it would be found that from 901 b to 981 b are inorganic; and only from 4!b to 101 b organic. But in a peat soil the proportions would be about 601 b to £o!b organic and from 201 b to 401 b inorganic. Nature in her laboratory in the soil is ever busy during tho months while plant life is active and growing, in preparing food for the nourishment of the vegetable kingdom. The increasing warmth of spring is the magician’s wand that sets tho soil laboratory at work. The rain water filters through the soil, and aided by the carbonic acid of the rain water, dissolves the plant foods. When the hairs on the footlets of tho young plants have attached themselves to tho particles of the soil, they exude a further chemical substance. which enables further plant food to be dissolved from the soil, and to be reduced to a condition mutable for tho roots to absorb in much tho same way as oil will pass through a thin membrane. It is plain that it tho soil bo in the condition of a compact, liard mass, tho rainwater will not be able to percolate through the soil, and tho air will not he able to circulate between tho particles that form the soil. The soil must lx? thoroughly tilled that the water may, in its passage through the roil, perform its part in dissolving the plant food, and tliat the air, with its accompanying carbonic acid, may also have the opportunity to do its requisite work. If tho water remain permanently in the soil, stagnation will result, injurious gases will be formed, and plants will sicken and die. The need of drainage to carry away the water which has done its work is apparent. The I'-eight at which the water remains or stands at some distance from the surface in called tho water tale, and it is higher or lower as the rainfall varies. Plants, as a rule, ilourisli best when the water-table Ls several feet from the surface. If too dose to the surface the rain-water and the air cannot percolate and circulate through the soil, and stagnant water with injurious gases result. The temperaure is also lowered. It is important that tho soil bo capable «-f holding and ) (-mining for a time a ’arge supply of rain-water. A clay soil retains the rain-water t<x> long; a sandy

foil not long enough. Then, again, a good deal depends upon tho vuhsoih The drainage must bo capable ojf allowing the used-

up rain-water freely to leave the sell lying between the surface and the water table. In that layer of soil arid subsoil rain-water has two movements: —

(1) Percolation downwards towards the

sea, the river, or the lake. (2) A movement in a’l directions in those parts of the soil which arc above the water-table. Percolation is a slow movement in any case, and the water finds its way through the soil much slower in wooded country and through clay soil than from a bare country and sandy soil. A dry soil is like a dry sponge: it absorbs the water from the water-table much as a wick diaws up the oil in a lamp. (1) The particles and pores of the soil absorb and hold rain-water that comes to them from above. (2j The particles of soil and the pores above the water-table suck up

moisture from it. Hence, while the used-up rain-water is drained away, each particle of the soil is kept constantly moist, and as this moisture, in conjunction with the carbonic acid :n the rain-water and the chemical acids exuded by the hairs on the rootlets of the plants, is used up by the plants in the shape, in part, of the manufactured plant food fresh supplies take its place. The further movement of the water, in the shape of moisture, may be by evaporation from the soil, or by exhalation from the leaves of plants and the foliage of other vegetable plant life. The movements of the water through the soil are greatly facilitated by keeping the soil in a fine, jxvrous condition by frequent tillage. The air Will also circulate more freely through such a soil. To assure the fertility of the soil it thus seems necessary to plough deep. If this be done the air can circulate freely through it, and is free to play its part in converting parts of the soil into piant food. Besides, with deep 'tillage, the roots of the young plant can descend deeper into the soil, can obtain more plant food, and can withstand the effects of the drought much better. Even in cases where the subsoil contains substances injurious to the young plant, tho subsoil plough ought to bo used to stir up the sub-oil without bringing it to the surface. In this way free play is given to the action of both air and moisture in tho layers of tho soil that furnish food for the young and growing plant. But the best tillage is largely useless without efficient drainage. And this applies with added force in the oases of heavy or clay land, and where any soil is irrigated, depth of drains depends on the surrounding circumstances —the outlet for the water and tho nature of the soil. Peat land requires deeper diains than light land. As a rule, deep drainage is preferable, because then the water and tho air, in their passage through the soil, act on a larger body of tho soil, and convert more of its substances into plant food. In a favourable soil tho roots of crops, such as clover and turnips, go down deep into the soil, and with shallow drainage a sufficient supply of plant food would not be available. Deeprooted crops often look well at first, and afterwards sicken and fail; and this very frequently happens in the presence of unskilful irrigation. The cause is not hard to find. Water has collected and become stagnant; unhealthy compounds, and henoq unhealthy plant food, have been formed, and when the roots of the growing plants have descended into the soil to the depth at which this unhealthy plant food exists* the sickening of the plant commences, much the same as schoolboys or .schoolgirls would sicken and die if compelled to feed on lampblack and stagnant water A fertile soil will have these properties:— 1. The water tables will bo at a depth from tho surface of the soil to allow plenty of soil and subsoil for tillage and for the roots of all ordinary farm crops above the water table. 2. Density. 3. Capacity for absorbing and retaining moisture without impeding capillary circulation. 4. Porosity, or the capacity for allowing water to pass through it. 5. Temperature suited to the needs of the growing plant.

If tho density of the soil, such as stiff clay, be too great, it may possess other de.drable properties, and yet be useless for agricultural purposes. If its capacity tor absorbing and retaining moisture bo too great or too little, the soil may possess other desirable properties, and yet be usole.ss. If its temperature be too high or too low, it may possess other desirable properties, and yet lx; unfitted to grow farm crops. Decomposing vegetable matter is an example of the former; water-logged clay or peat of the latter. 1. Pure sand will hardly retain 5 per cent. of its weight of water. 2. Stiff clay will retain 50 per cent, of its weight of water and yet appear dry. 3. Clay soil will retain throe times, or more, as much water as soil of a sandy nature. 4. In dry. hot weather, soil of a sandy nature will give off the same weight of water, in the form of vapour, in one-third of tho time necessary to evaporate the same volume from stiff clay, peat, or rich mould. 5. In drying, sandy soils do not contract. 6. In drying, peat land and strong clay

land shrink one-fifth of their bulk. With a knowledge of the various properties of the various classes of soils, many useful deductions may be made. It Ls easy enough to see why it is desirable to mix sandy soil with clay; why, after rain or irrigating, soils of a sandy nature should be left untouchel as far as possible; why peaty soil and rich mould, if tho crop allow of 'tillage, .should be cutivated after rain; and why strong, clay land should be tilled for tho double purpose of getting rid of the superfluous water and loosening tho soil around the roots of tho plants. All these conditions may be complied with, and yet without an adequate supply of water available at the proper time sterility may result. With an intermittent supply at improper times the best that can result is stunted crops, barren fields, and serious pecuniary loss to a discontented and disheartened people— as in India and parte of America prior to the advent of irrigation in those countries All this may be

avoided by a proper system of irrigation. It is important that_ the land bo properly prepared for applying irrigating water. Should the surface of the soil be dotted by hillocks, hollows, or holes or other inequalities in height of the surface, the irrigating water will probably do more harm than good. The preparation of the land, which includes the levelling, is as necessary as securing the water for irrigating. In the preparation of the field for irrigation it is necessary first to determine

in what direction the irrigating channels will run. The land is then ploughed in the same direction. When the land has been ploughed and sufficiently harrowed, should any irrcgularites exist, a levelling Deadline should bo used. One form of this machine consists of two longitudinal beams, each beam 12ft in length and 6in by Sin. Three pieces, each 9ft in length and 6in by 3Ln, are bolted on in front and underneath to the longitudinal beams in flooring-board fashion. Four additional and similar pieces are bolted to the longitudinal beams behind and underneath, but. in weatherboard fashion, with the edges set forward. Halfway between these two sots of pi an las a wooden scraper ia placed, faced underneath and in front with a steel plate Sin by iin. In front of this scraper there are three eyebolts which swing on a round bar of iron. There is also a sft lever bolted on to the back of this scraper, with a crank underneath it. The horses are yoked to this levelling machine, and when it comes to a olay the driver stands on the lever and forces the scraper into the earth, which is pushed forward until it comes to a hollow. The driver then steps on the crank, which turns upwards, and lifts the lover, when the scraper swings backwards and deposits the earth. The weatherboard edges of the planks behind thorn level the neap and break any lumps. This levelling machine can be worked by four horses and one man. and will level from five to six acres a day In every country in which irrigation has been practised it has boon found, sooner or later, that the soil becomes saturated; and hence the imperative need of a system of drainage deep enough to carry off all superfluous water and prevent the permanent level of the underground water rising higher than the lowest level to which the roots of growing plants descend. Should this happen, the plants sicken, and will ultimately die. The permanent standing water has already exhausted its capacity for dissolving plantfocd, and in addition to being useless for this purpose, it prevents the passage of the air through the soil. When the water abends to the surface, or when, from defective drainage, the irrigating water is allowed to remain in pools on the surface, the effects on plant-life are disastrous.