Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/384

368 IRRIGATION ground at a time. The soil must get dry at stated intervals in order that the atmospheric air may come in contact with it and penetrate it. In this way as the water sinks down through the porous subsoil, or into the subterranean drains, oxygen enters, and supplies an element which is needed, not only for the oxidation of organic matters in the earth, but also for the direct and indirect nutrition of the roots. Without this occasional drying of the soil the finer grasses and the leguminous plants will infallibly be lost ; while a scum of confervae and other algse will collect upon the surface, and choke the higher forms of vegetation. The water should be ran off thoroughly, for a little stagnant water lying in places upon the surface does much injury. The practice of irrigating differs in different places with differences in the quality of the water, the soil, the drainage, &c. As a general rule, when the irrigating season begins in November, the water may flow for a fortnight con tinuously, but subsequent waterings, especially after December, should be shortened gradually in duration till the first week in April, when irrigation should cease. It is necessary to be very careful in irrigating during frosty weather. For, though grass will grow even under ice, yet if ice be formed under and around the roots of the grasses the plants may be thrown out by the expansion of the water at the moment of its conversion into ice. The water should be let off on the morning of a dry day, and thus the land will be dry enough at night not to suffer from the frost ; or the water may be taken off in the morning and let on again at night. In spring the newly grown and tender grass will be easily destroyed by frost if it be not protected by water, or if the ground be not made thoroughly dry. Several other important matters in the management of water- meadows have to be noticed. Among these the times for depas turing with sheep and other stock are of considerable moment, not only because one of the main services rendered by a water-meadow is the early and valuable feed which it ought to afford, but for securing the health of the animals, particularly their immunity from sheep-rot. A water-meadow cannot be trusted late in the season, especially in view of what is now known concerning the liver-fluke of sheep. It seems to be judicious to depasture the early grass on water-meadows with ewes and lambs at the end of March and in April, and to have it eaten down bare before May with a heavy stock. On good land and in good seasons a second and even a third crop of feed may be got before the 1st of May, the water being let on after each feed. After that the grass is allowed to stand for hay, but it should be irrigated for a few days to clean the pasture. Further particulars as to the management of irrigated meadows may be gathered from the two accounts which follow, which embody, though in a very condensed form, the system pur sued in the district which is perhaps the most noted for its water- meadows, namely, that of the Christchurch Avon. Some of these afford characteristic examples of the usual English system of irri gation. They consist in the main of alluvial soil, often very shallow, lying upon gravel. Professor "Wrightson, of the College of Agriculture at Downton, near Salisbury, gives the following particulars concerning the water-meadows in his own neighbour hood. They are very valuable as they assist to keep sheep from Lady Day until the end of April, a time when green food is scarce ; at that season they never rot sheep. After sheep have been pas tured on the water-meadows, these are shut up for hay, of which they yield in fair seasons about 2 tons per acre. The hay is cleared off in July, and the meadows are then fed off by cows until about the first week in October. At this time the work of clearing out the water-carriers and ditches is proceeded with ; banks, stops, sluices, &c. , are repaired ; and holes and deep hoofprints filled up or laboriously stamped out. As soon as possible the water is let on, the irrigation being continued throughout November, December, January, and February. On the Downton College farm the water, during the above four months, is shared, on alternate weeks, with the neighbours. The water is caused to flow regularly over all the meadows, and the "meadman" is almost constantly employed in "watering" and "drowning." In March the water is shut off, and the meadows are ready for sheep during the first week in April. In about four weeks time the sheep are taken off, and the meadows are again watered on alternate weeks up to mid-June. At this time the ground is allowed to become dry and firm so as to permit of grass-cutting (with scythes) and of hay-making. The hay is good and of agreeable flavour, but not equal to upland hay. The Avon meadows begin at Britford, just below Salisbury ; and here the results of irrigation are as good if not better than anywhere else in England. They continue from Britford to Fordingbridge, but below the latter place down to Kingwood and Christchurch they degenerate into mere flooded meadows and marshes abounding in wild duck, and yielding a very coarse and innutritions herbage. The Avon valley waters are derived from the Chalk, the Upper Greensand, and the Upper Oolite. The late Mr J. Combes gave, in a paper read before the Royal Agricultural Society, some remarkable instances of the value of the grass produced on some of these Avon water-meadows. He mentioned the fact that 7 or 8 per acre had been given for the spring feed when there had been a failure of the turnip crop ; once under such circumstances the spring feed of 6 acres fetched no less than 80. He cited an instance of a meadow of 20 acres, depastured by sheep in spring, as keeping eight hundred sheep twenty-five days, and as yielding after this, in the first and second cuttings, no less than 40 tons of hay. The following directions for the management of water-meadows given by the late J. Combes of Tisbury (whose observation as a practical irrigator was exact, and whose experience was very exten sive), though in the first instance applicable to the Wiltshire Avon meadows, are of general value. Let the meadows be ready to receive the water in the first week in November, that the manurial matters present in the first freshet of the river after the autumnal rains have commenced may be caught and utilized. Water as much as possible during November and December. In January let the water on six days out of seven, in February three out of four, in March two out of four, in May and June two out of seven, in July and August one out of six ; and shut off the water entirely during September and October. The young grass coming up where sheep have just fed olf a portion should not be immersed ; but generally thin watering is bad, and, if there is not enough water for the whole meadow, let one portion be generously treated at a time. Such sections, in Wiltshire called steins, may be watered for five days at a time in winter and two days at a time in summer. It is better to water by night than by day, and in shady rather than in sunny weather. Assuming that the sluices are in working order, and the conductors or carriers, the feeders, and the drains sound and clear of all obstructions, then actual irrigation begins thus. The sluice is drawn up, and if the water be abundant the conductor and feeders will be filled in about half an hour. The motion of the water should first be adjusted in all the conductors, then in the feeders nearest the upper part of the meadow, and then successively in those which are lower. The sluices regulate the water in the conductors, and the position of the " stops " regulates the water in the feeders. The stops should be so placed as to cause the water to overflow the sides of the feeders, by so adjusting the stops as to make the openings or waterways at either side of them wider or narrower as required. The first general inundation will show any irregularities in the levels and meadow surfaces ; these should be noted for rectification in the ensuing summer. It will in general need three trial adjustments of the sluices and stops before an experienced irrigator can satisfy himself that the meadow is properly irrigated with the requisite depth of 1 inch of water. During each period of irrigation the meadow should be visited and inspected at regular intervals to see that obstructions are removed and accidents repaired. In Scotland irrigation is generally continued all April, though in reduced amount towards the end of the month. The average annual repairs of a water-meadow have been estimated at 5s to 6s. an acre ; the greatest expense will be incurred for levelling, &c., in the second year after laying out the ground. Mention has been made already not only of the general advantages resulting from that variety of irrigation practised in water-meadows, but also of particular examples of profitable resiilts. It would not be difficult to accumulate many further examples of the latter sort, but they must always be received as applying to the particular circumstances of the case, and very often to seasons and commercial and agricultural conditions different from those which have ruled. An example or two of favourable results obtained by irrigation of water-meadows may be cited here. The late Mr Pusey, after having converted a field of 2 acres on his Berkshire home farm into a water-meadow, was able to obtain from it five months keep for seventy-three sheep. The grass of the meadow had previously become hardly worth cutting, from the land having got out of condition ; but by irrigation 2 acres of it had become equal to 5 acres of superior grazing land unwatered. The late Mr Stephens quoted in his Practical Irrigator a case of the conversion of 5 acres (valued at 8s. per acre) of a peat bog into a bedwork water- meadow. The expense was 6 per acre, and the crop of hay was 4 tons llj cwts. per acre, with an aftermath valued at 18s. per acre. Theory of Irrigation. Although in many cases it is easy to explain the reasons why water artificially applied to land brings crops or increases their yield, the theory