Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/383

IRRIGATION 367 conductors into built conduits formed at right angles to them in parallel lines through the fields ; it rises upwards in them as high as the surface of the ground, and again subsides through the soil and the conduits into the ditches as main drains, and thence it passes at a lower level either into a stream or other suitable outfall. The ditches may be filled in one or other of several different ways. The water may be drainage-water from lands at a higher level ; or it may be water from a neighbouring river ; or it may be drainage- water accumulated from a farm and pumped up to tho necessary level. But it may also be the drainage -water of the field itself. In this case the mouths of the underground main pipe-drains are stopped up, and the water in them and the secondary, drains thus caused to stand back until it has risen sufficiently near the surface. Of course it is necessary to build the mouths of such main drains of very solid masonry, and to construct efficient sluices for the retention of the water in the drains. Irrigation of the kind now under discussion may bj practised wherever a command of water can be secured, but the ground must be level. It has been success fully employed in recently drained morasses, which are apt to become too dry in summer. It is suitable for stiffish soils where the subsoil is fairly open, but is less successful in sand. The water used may be turbid or clear, and it acts, not only for moisten ing the soil, but as manure. For if, as is commonly the case, the water employed be drainage-water from cultivated lands, it is sure to contain a considerable quantity of nitrates, which, not being subject to retention by the soil, would otherwise escape. These coming into contact with the roots of plants during thSir season of active growth, are utilized as direct nourishment for the vegetation. It is necessary in upward or subterranean irrigation to send the water on and to take it off very gently, in order to avoid the displacement and loss of the finer particles of the soil which a forcible current would cause. Warping. -In this variety of irrigation the suspended solid matters are of importance, not merely for any value they may have as manure, but also as a material addition to the ground to be irrigated. The waters of the Nile and the Ganges afford con spicuous examples of rivers rich in suspended matter, which occasionally amounts to one hundredth of their volume, and fre quently to more than one part in two hundred parts of water. The warping which is practised in England is almost exclusively con fined to the overflowing of level ground within tide mark, and is conducted mostly within the districts commanded by estuaries or tidal livers. The best notion of the process of warping may be gained by sailing up the Trent from the Humber to Gainsborough. Here the banks of the river were constructed centuries ago to pro tect the land within them from the encroachments of the tide. A great tract of country was thus laid comparatively dry. But, while the wisdom of one age thus succeeded in restricting within bounds the tidal water of the river, it was left to the greater wisdom of a suc ceeding age to improve upon this arrangement, by admitting these muddy waters to lay a fresh coat of rich silt on the exhausted soils. The process began more than a century ago, but has become a system in recent times. Large sluices of stone, with strong doors, to be shut when it is wished to exclude the tide, may be seen on both banks of the river, and from these great conduits are carried miles inward through the flat country, to the point previously prepared by embankment, over which the muddy waters are allowed to spread. These main conduits, being very costly, are constructed for the warp ing of large adjoining districts, and openings are made at such points as are then undergoing the operation. The mud is deposited, and the waters return with the falling tide to the bed of the river. Spring tides are preferred, and so great is the quantity of mud in these rivers that from 10 to 15 acres have been known to be covered with silt from 1 to 3 feet in thickness during one spring of ten or twelve tides. Peat-moss of the most sterile character has been by this process covered with soil of the greatest fertility, and swamps which used to be resorted to for leeches are now, by the effects of warping, converted into firm and fertile fields. The art is now so well understood that, by careful attention to the currents, the expert warp farmer can temper his soil as he pleases. When the tide is first admitted, the heavier particles, which are pure sand, are first deposited ; the second deposit is a mixture of sand and fine mud, which, from its friable texture, forms the most valuable soil ; while lastly the pure mud subsides, containing the finest particles of all, and forms a rich but very tenacious soil. The great effort, there fore, of the warp farmer is to get the second or mixed deposit as equally over the whole surface as he can, and to prevent the deposit of the last. This he does by keeping the water in constant motion, as the last deposit can only take place when the water is suffered to be still. Three years may be said to be spent in the process, one year warping, one year drying and consolidating, and one year growing the first crop, which is generally seed hoed in by hand, as the mud at this time is too soft to admit of horse labour. The immediate effect, which is highly beneficial, is the deposition of silt from the tide. To ensure this deposition, it is necessary to surround the field to be warped with a strong embankment, in order to retain the water as the tide recedes. The water is admitted by valved sluices, which open as the tide flows into the field, and shut by the pressure of the confined water when the tide recedes. These sluices are placed on as low a level as possible, to permit the most turbid water at the bottom of the tide to pass through a channel in the base of the embankment. The silt deposited after warping is exceedingly rich, and capable of carrying any species of crop. It may be admitted in so small a quantity as only to act as a manure to arable soil, or in such a large quantity as to form a new soil. This latter acquisition is the principal object of warping, and it excites astonishment to witness how soon anew soil may be formed. From June to September a soil of 3 feet in depth may be formed under the favourable circumstances of a very dry season and long drought. In winter and in floods warping ceases to be beneficial. In ordinary circumstances, on the Trent and Humber, a soil from 6 to 16 inches in depth may be obtained, and inequalities of 3 feet filled up. But every tide generally leaves only g- inch of silt, and the field which has only one sluice can only be warped every other tide. The silt, as deposited in each tide, does not mix into a uniform mass, but remains in distinct layers. The water should be made to run completely off, and the ditches should be come dry, before the influx of the next tide, otherwise the silt will not incrust, and the tide not have the same effect. Warp soil is of surpassing fertility. The expense of forming canals, embank ments, and sluices for warping land is from 10 to 20 an acre. A sluice of 6 feet in height and 8 feet wide will warp from 60 to 80 acres, according to the distance of the field from the river. The embankments may be from 3 to 7 feet in height, as the field may stand in regard to the level of the highest tides, After the new land has been left for a year or two in seeds and clover, it produces great crops of wheat and potatoes. Warping is practised only in Lincolnshire and Yorkshire, on the estuary of the Humber, and in the neighbourhood of the rivers which flow into it the Trent, the Ouse, and the Don. The silt and mud brought down by these rivers is rich in clay and organic matter, and sometimes when dry contains as much as one per cent, of nitrogen. The Management and Advantages of Water-Meadows. Constant care is required if a water-meadow is to yield quite satisfactory results. The earliness of the feed, its quantity, and its quality will all depend in very great measure upon the proper management of the irriga tion. The points which require constant attention are the perfect freedom of all carriers, feeders, and drains from every kind of obstruction, however minute ; the state and amount of water in the river or stream, whether it be sufficient to irrigate the whole area properly or only a part of it; the length of time the water should be allowed to remain on the meadow at different periods of the season ; the regulation of the depth of the water, its quantity, and its rate of flow, in accordance with the temperature and the condition of the herbage ; the proper times for the commencing and ending of pasturing and of shutting up for hay ; the mechanical condition of the surface of the ground ; the cutting out of any very large and coarse plants, as docks ; and the improvement of the phj-sical and chemical conditions of the soil by additions to it of sand, silt, loam, chalk, &c. Whatever may be the command of water, it is unwise to attempt to irrigate too large a surface at once. Even with a river supply fairly constant in level and always abundant, no attempt should be made to force on a larger volume of water than the feeders can properly distribute and the drains adequately remove, or one part of the meadow will be deluged and another stinted. When this inequality of irrigation once occurs, it is likely to increase, from the consequent derangement of the feeders and drains. And one result on the herbage will be an irregularity of composition and growth, seriously detrimental to its food-value. The adjustment of the water by means of the sluices is a delicate operation when there is little water, and also when there is much ; in the latter case the fine earth may be washed away from some parts of the meadow ; in the former case, by attempting too much with a limited water current, one may permit the languid streams to deposit their valuable suspended matters instead of carrying them forward to enrich the soil. The water is not to be allowed to remain too long on the