Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/356

Rh 330 AGRICULTURE [DRAINING at which they often deposit manure, and the stress which they lay upon thorough drainage. On the other hand, it is well known that soils which soonest become saturated, and run from the surface in wet weather, are precisely those which parch and get chapped the soonest in drought. The effectual way to secure our crops at once from drown ing and parching, is to put the land in a right condition with respect to drainage. All soils possess more or less the power of absorbing and retaining water. Pure clays have it in the greatest degree, and gritty siliceous ones in the smallest. In dry weather this power of attracting moisture is constantly operating to supply from below the loss taking place by evaporation at the surface. In heavy rains, as soon as the entire mass has drunk its fill, the excess begins to flow off below ; and therefore a deep stratum, through which water can percolate, but in which it can never stagnate that is, never exceed the point of saturation is precisely that in which plants are most secure from the extremes of drought and drowning. If a perfect condition of the soil with respect to drainage is of importance for its influence in preserving it in a right condition as respects moisture, it is still more so for its effects upon its temperature. All who are conversant with rural affairs are familiar with that popular classification of soils in virtue of which such as are naturally dry are also invariably spoken of as warm and early ; and conversely, that wet soils are invariably described as being cold and late. This classification is strictly accurate, and the explana tion of it is simple. An excess of water in soil keeps, down its temperature in various ways. In passing into th e state of vapour it rapidly carries off the heat which the joil has obtained from the sun s rays. Water possesses also a high radiating power ; so that, when present in the soil in excess, and in a stagnant state, it is constantly carrying off heat by evaporation and radiation. On the other hand, stagnant water conveys no heat downwards ; for although tho surface is warmed, the portion of water thus heated being lightest, remains floating on the surface, and will give back its heat to the atmosphere, but conveys none downwards. When the surface of stagnant water becomes colder than the general mass, the very opposite effect immediately ensues ; for as water cools its density increases, and thus causes an instant sinking of the portion that has been cooled, and a rising of a warm portion from below to take its place this movement containing until the whole has been lowered to 40, at which point water reaches its maximum density, while, if the temperature be reduced a few degrees more, water will begin to freeze. It is thus that soil surcharged with water is kept at a lower temperature than similar soil that has a sufficient natural or artificial drainage. But while the presence of stagnant water in a soil has this injurious power of lowering its temperature, a very different effect ensues when rain water can sink freely into it to a depth of several feet, and then find a ready exit by drainage ; for in this case the rain water carries down with it the heat which it has acqiiired from the atmosphere and from the sun-heated surface, and imparts it to the subsoil. There is as yet a lack of published experiments to show the ordinary increase of temperature at various depths and in different soils, as the result of draining wet land. Those conducted by Mr Parkes, in a Lancashire bog in June 1837, showed, as the mean of thirty-five observations, that the drained and cultivated soil at seven inches from the surface was 10 warmer than the adjoining undrained bog in its natural state at the same depth. It is understood that later experiments conducted by the same gentleman on an extended scale fully establish the fact, that an increased temperature of the soil is an unfailing accompaniment of thorough draining. The importance of this result cannot well be over-rated. The temperature and other conditions of the atmosphere, which we call climate, are placed beyond human control ; but this power of raising the temperature of all wet, and consequently cold soils, becomes tantamount in some of its results to a power of improving the climate. There are, accordingly, good grounds for stating that in numerous cases grain crops have ripened sooner by ten or twelve days than they would have done but for the draining of the land on which they grew. The points which we have thus briefly touched upon are so essential to an intelligent appreciation of the subject, that we have felt constrained to notice them, however meagrely. But our space forbids more than a mere enumera tion of some of the many evils inseparable from the presence of stagnant water in the soil, and of the benefits that flow from its removal. Wet land, if in grass, produces only the coarser grasses, and many sub-aquatic plants and mosses, which are of little or no value for pasturage ; its herbage is late of coming in spring, and fails early in autumn ; the animals grazed upon it are unduly liable to disease, and sheep, especially, to the fatal rot. When land is used as arable, tillage operations are easily interrupted by rain, and the period always much limited in which they can be prosecuted at all ; the compactness and toughness of such land renders each operation more arduous, and more of them necessary, than in the case of dry land. The surface must necessarily be thrown into ridges, and the furrows and cross-cuts duly cleared out after each process of tillage, on which surface expedients as much labour has probably been expended in each thirty years as would now suffice to make drains enough to lay it permanently dry. With all theso precautions the best seed-time is often missed, and this usually proves the prelude to a scanty crop, or to a late and disastrous harvest. The cultivation of the turnip and other root crops, which require the soil to be wrought to a deep and free tilth, either becomes altogether impracticable, and must be abandoned for the safe but costly bare fallow, or is carried out with great labour and hazard ; and the crop, when grown, can neither be removed from the ground, nor consumed upon it by sheep without damage by poaching. The dung, lime, and other manure, that is applied to sncli land is in a great measure wasted ; and the breaking of the subsoil and general deep tillage, so beneficial in other circumstances, is here positively mischievous, as it does but increase its power of retaining water. Taking into account the excessive labour, cost, and risk, inseparable from the cultivation of wet land, and the scanty and precarious character of the crops so obtained, it would in many cases be wiser to keep such lands in grass, than to prosecute arable husbandry under such adverse circumstances. These very serious evils can either be entirely removed, or, at the least, very greatly lessened by thorough draining. It often happens that naturally porous soils are so soaked by springs, or so water-logged by resting upon an impervious subsoil, or, it may be, so drowned for want of an outfall in some neighbouring river or stream, that draining at once effects a perfect cure, and places them on a par with the best naturally dry soils. In the case of clay soils, the improve ment effected by draining is in some respects greater than in any other class, but still it cannot change the inherent properties of clay. This has sometimes been overlooked by sanguine improvers, who, hastily assuming that their strong land, when drained, would henceforward be as friable and sound as the more porous kinds, have proceeded to treat it on this assumption, and have found to their cost that clay, however well drained, will still get into mortar and clods, if it is tilled or trodden on too soon after rain. It is entirely owing to such rash and unskilful management that an opinion has sometimes got abroad, that clay lands are injured by draining. They merely retain the qualities