Temperature of the air 44.9 37 44.4 50.5 53.4 57.6 60.5 60.5 57.7 51 1 50.9 49-351-5 Mr. Smith has also given * some observations respecting the temperature of some springs on Exmoor which are interesting. It is therefore not surprising that Mr. Smith should have described in the following terms the effect of this warm water being spread over the surface. "Nothing can exceed the effect produced below the 'Picked Stones' spring; and the growth of the succeeding grasses, from stage to stage, clearly demonstrates the cooling of the stream as it becomes assimilated to the air." Facts most undeniably prove that the warmth of the spring adds to its efficacy when used for irrigation. We may therefore safely take it as a principle, upon which successful irrigation depends, that we thereby produce an artificial climate in the soil both by retaining the heat constantly given off from the earth in the cold months of the year, and also by adding to it the supplies of warmth which the spring water may contain. It is on these principles that we must explain the beneficial results arising from the water when the only qualification possessed is that of warmth. Increased produce is another result of the practice of irrigation. This is induced partly by the causes already noticed, and partly by the effects of water as a manure co-operating in the result. This, however, depends upon the composition of the water; its use is simply that of a liquid manure. The ingredients thus presented to vegetation possess fertilizing properties, and the minute proportion in which they are presented is compensated by the large bulk of water used. Some springs are very free from such ingredients, but by far the majority take various matters into solution in their passage through the rocks from which they rise. It is therefore clear that the character of the water would be influenced by the rock through which it may pass. In practice we find a few ordinary tests used by experienced gutterers, and from these they draw tolerably correct inferences as to the adaptation of a spring to purposes of irrigation. Water having a soft and greasy feeling is generally preferred, and persons with only moderate experience can readily recognise that which is suitable to the purpose. This arises from the presence of the alkaline salts (potash and soda), and with these we generally find silica and phosphoric acid. This quality, which is also determined by the power of the water to dissolve soap, is called "softness." The opposite quality is known as hardness, and is very generally objected to, though there are exceptions, as in the Gloucestershire, Wilts, and Hampshire meadows, the chief parts of which are supplied with hard water alone, and yet very marked results are produced from the use of it. Some water contains organic matters which increase its value as a fertilizer, others are entirely dependent on inorganic matters. Whatever may be the substances the water contains, we may anticipate the same results from their use as if we were employing them as ordinary manures, with this exception, that by virtue of their diffusion throughout a large bulk of water, the benefits resulting will probably be much increased. If we have phosphates, alkalies, or organic matters, each has a special action as a fertilizer, and their solution in water only renders their action more prompt. * Journal of the Bath and West of England Society, vol. iv. p. 297. Another criterion of good water for irrigation is the fact of its having good trout. The growth of watercresses is also considered an indication of quality. These probably result from causes already stated. They are, at all events, clear indications of a healthy character of water, alike congenial to animal life and vegetable growth. These are generally found in that clear and bright water, which is much to be preferred for irrigation. There are some instances showing the advantage of turbid streams, but the cases are exceedingly rare in which meadows are not more improved by a clear and bright stream. Thus many streams, which are apparently worthless, contain in them the elements of fertility, which the unpractised eye fails to detect, because they are clear and transparent; but an analysis reveals the treasure they contain, and when judiciously applied to the land, the increased luxuriance of the herbage proves the value of the hidden fertilizer. The value of natural springs has been much increased artificially, by adding to them various manures, in fact, by changing them into streams of liquid manure. In this manner various kinds of fertilizing matters have been distributed over the land. Farmyard dung, guano, and other artificial manures have thus been distributed most economically and effectually. The economy of the distribution results from the motive power being simply the gravity of the water, whilst the effect has been great, because the materials applied, in consequence of being in solution, are immediately available for plants. Instances can be shown of land of the poorest character being thus rendered valuable. For example, a rabbit warren on the sides of the river Mann was converted by the Duke of Portland into the far-famed Clipston Meadows, now producing an annual rent exceeding 111. per acre. The fertility of the land is increased by the passage of water through the soil, as well as over it. A careful inspection of irrigated land will convince any one of the fact that not merely surfacedrainage but under-drainage is essential to success. If the water is to act fully on the land it must pass through the land. Such is the evidence of practice: now what is the cause of this practical necessity? The explanation may be found in Professor Way's 'Researches on the Power of the Soil to absorb Manures,' published in the Journal of the Royal Agricultural Society; * they are deeply interesting and of considerable practical value. He has shown that in the passage of liquids holding manures in solution through the soil, the soils have the power of abstracting these manures from the water, and storing them ready for the use of the plants. Thus water, which is perfectly colourless, and apparently pure, may be deprived of the alkaline salts, the phosphates, &c., contained in it, by this peculiar absorptive power, and when so separated it is jealously preserved from loss until required by growing plants. Now experience has proved that the water should pass over the surface as far only as it is necessary for its distribution, and after this point is gained the more fully it passes through the soil the better is the effect produced. To show the completeness with which Professor Way's experiments illustrate this peculiar property possessed by soils, a few examples are adduced. Experiment 43.-Solution of caustic potash was thrown into a 10-inch filter of Mr. Pusey's soil; the liquid, after percolation, contained no potash in any form, it must therefore have been arrested by the soil. Experiment 45.-20 grains of carbonate of potash were dissolved in 8 ounces of water. It was poured upon a filter bed of Mr. Pusey's soil, 10 inches in depth; the results were very satisfactory, and proved the total separation of the carbonate of potash by filtration. Experiments 52, 53, and 54.-Lime water of moderate strength * Vol ii. p. 313. was poured upon three different subsoil clays of the gault formation obtained from Mr. Paine's land at Farnham; in all these cases a large quantity of lime was arrested. Experiment 62. -The ash of Peruvian guano (which is principally phosphate of lime) was dissolved in diluted sulphuric acid, an excess of the phosphate being employed. The solution was made clear by filtering through paper, and was then thrown upon a filter-bed of Mr. Pusey's soil; when 8 ounces of liquid had passed, the solution was tested, but no phosphoric acid could be detected in it. Experiment 93.--Mr. Huxtable's light soil was mixed with its own weight of white sand to make it more permeable. Upon this filtering-bed a quantity of highly offensive tank-water was poured. The liquid did not pass for several hours, but ultimately more than 1 ounce of it passed quite clear and free from smell or taste, except a peculiar earthy smell and taste derived from the soil. These experiments show how perfectly the soil is able to remove from water the fertilizing matters contained in it, and prove that the important influence of under-drainage arises not only from the ordinary beneficial operation of removing stagnant moisture, but especially from the food of plants which is arrested as the water passes through the soil. There is no reason to believe that liquid simply passing over the surface can do more than protect from the cold, and perhaps add warmth to the soil; but it cannot thus nourish as a fertilizer. We might as well expect food applied to the lungs of an animal to be as nutritious as when passed into the stomach and digested. The food for the plant's growth should enter from the roots, and it is evident that a simple passage over the surface does not fully accomplish the result aimed at. Another important point which receives explanation by this discovery is, that provided the under-drainage is in an equally efficient state in each case, the advantages of irrigation will be evident and beneficial in proportion to the quality of the land. As the increase in the quality of the land is usually accompanied by a large proportion of clayey matter, we find at once a reason for the soil being able the more completely to separate the fertilizing ingredients existing in the water. Thus science confirms the character which practice has given to such soils as regards their power of "holding manures." These appear to be the principles which operate in the irrigation of land; we will now proceed to see how they are applied in practice. In the formation of water-meadows, the first point is the selection of water which will be beneficial to the land for which it is available. Here, as in almost every other case, local experience is necessary; for no fixed laws can be named when the circumstances under which they may be applied vary so entirely. I have already named the qualities generally considered desirable in water to be used for irrigation. If it be a warm spring it is almost sure to be useful, and when, in addition to this quality, it has a soft alkaline character, then there is still less doubt of its being valuable. But it need not be rejected, though possessing neither of these qualities; for I have shown that hard water, as well as that of the ordinary temperature of our streams and rivers, has been found valuable for irrigation. Neither can the possession of the most desirable qualities be relied upon as giving a certainty of success in its employment, since climate may be unfavourable to its use. It is therefore important, before the land is irrigated, that local experience should determine the probable result. A small experiment will probably be satisfactory in districts where irrigation is not general. The districts in which it is practised are chiefly in the west and south-west of England; but there appears no reason why these should not be extended. In Devon, Somerset, Wilts, Dorset, Hampshire, and Gloucestershire, irrigation is very largely practised; and to a smaller degree in Worcestershire, Berkshire, and Nottinghamshire. There is every reason to believe that with judgment and discretion a far larger breadth of land may be brought under this valuable fertilizing agency. Experience has shown that some streams are fertilizing, some harmless, while others are positively injurious. The last character arises in many cases from the presence of lime, and in others it is caused by magnesia and iron. The presence of lime is prejudicial to some soils only, whilst on others it has a beneficial effect; but magnesia and iron are far more constant in their action. Thus the drainage from a bog is very unfavourable, and should always be avoided;* for although it may become less and less injurious from the passage of air through the soil, still it will seldom, if ever, possess valuable qualities. If a stream is harmless it may often be rendered valuable by the addition of manures; and in like manner other springs may be increased in value by the same process. This has been done extensively on Exmoor by Mr. Robert Smith, who has had cheap sheds erected so as to wash the manures over the sides of the hills below; thus making the stream a distributor of manure. This is not only accomplished without horse labour, but the manure is carried where no horse could convey it, and applied in the manner most beneficial to the herbage. In such cases the * Bogs are usually situated below and caused by springs issuing from behind a tight rock. In this case, if the spring be tapped (above the bog) the bog will be cured, and the water will be at once available for irrigation.-R.'S. |