Water
Water is so indispensable to the very existence of plants, that its use and modes of action deserve to be more fully understood than they are by many gardeners. It is made up of a combination of two gases, Oxygen and Hydrogen, in the proportion of sixteen (by weight) of the former to two of the latter. By volume, the proportion is one of Oxygen to two of Hydrogen, the chemical formula being H2O??. The properties of Water are, for the most part, so well known as to render it needless to dwell upon them. When pure, it has neither colour, taste, nor smell; nor does it leave any solid matter when it is allowed to evaporate by heat or by exposure to the atmosphere. That Water is converted into ice, and rain into snow, at a low temperature (32deg. Fahr.), is a fact familiar to every-one in such a climate as ours. While it is freezing, the Water frees itself from by far the greater part of mineral substances that may be dissolved in it; so that ice consists of almost pure Water, even when formed from the salt Water of the sea. Pure Water has the power of dissolving many mineral substances and gases in greater or less amount, the greatest quantity of any particular substance or gas that it can dissolve varying with its temperature. The solutions thus formed are of very great importance in horticulture, since it is in this form that plants absorb the elements found in their ash, and which are mostly essential to their support. These solutions of minerals in the soil are almost always extremely dilute. In such weak solutions minerals can be absorbed with much greater readiness by the root-hairs of plants than they could be were the solutions stronger, so that they are well suited to supply the requirements of plants. Some minerals-e.g., Carbonate of Lime (whether in the form of marble or of chalk)-are scarcely, if at all, soluble in pure Water; but they become dissolved in water which has Carbonic Acid Gas already dissolved in it. Probably, no natural Waters are wholly deficient in this gas, and they can dissolve small quantities of even marble or Phosphates of Lime. The roots of plants can themselves also dissolve these minerals when in close contact with them. In all cases, the minerals necessary for the nutrition of the plants pass into them from the soil in these weak solutions. There is a constant passage of the fluids into the roots, and thence into the leaves, in order to replace the Water that is at all times escaping from all the green parts into the air in the form of invisible vapour.
The amount and nature of the water-supply for a garden is a matter of very great consequence; and it is necessary to inquire a little into the various natural sources from which it can be obtained, and the relative merits of the Water from each. The sources may be grouped under (1) rain, (2) ponds and streams, and (3) springs. Though it is impossible to draw sharply-defined distinctions between the Waters from these sources, yet they differ in several respects.
Rain is, in a sense, the source from which all Waters are derived, and from which all streams are fed; but the term Rain-water, in the ordinary sense, is restricted to that collected from the atmosphere--usually off the roofs of houses-and carried into a tank, in which it is stored till required. Pure Rain-water may contain a small quantity of Nitrates and Ammonia, which it dissolves out of the atmosphere, and carries with it to the earth. But, owing to the dust and impurities on the roofs or other surfaces from which it is usually collected, it always has also an appreciable, though very small, amount of various mineral substances dissolved in it; and is thus able to supply to plants at least a part of the mineral food that they require. It is fitted also to dissolve from the soil in which plants grow such substances as Carbonate of Lime, as it almost always contains a good deal of Carbonic Acid Gas, and also some Oxygen, dissolved while falling through the atmosphere. It is heated to the same average temperature as the air, so that in summer it helps to warm the soil to that temperature; and it thus stimulates the growth and power of absorption of the roots, and fits them to supply Water to the plants as quickly as it evaporates from the leaves. Rain-water is preferred for watering plants on account of its temperature being nearly the same as the air, and of the gases dissolved in it.
The Water in streams and in ponds contains a larger proportion of mineral substances than occurs in pure Rain-water, the gases are often present, only in smaller amount, and the average temperature--except in very shallow streams and ponds--is usually lower than that of the air in summer; and this is especially the case with Water conveyed from a distance in underground pipes. Spring Water resembles that from streams in the amount of mineral substances in it; or it may even contain these substances in such amount that some of them, especially Carbonate of Lime, may be deposited in a crust on any bodies in the water, owing to evaporation of Carbonic Acid Gas from it, rendering it no longer able to keep them dissolved. Petrifying springs are of this nature. Some springs give Water that is actually injurious, as it contains compounds of Iron, or other substances that are poisonous to plants if present in more than very small amount. The Water from ordinary springs is almost always a good deal colder than the air in summer. If Water from streams, ponds, or springs is to be used in watering plants, it should be kept for some time previously in a tank small enough to allow of its being warmed to the ordinary temperature of the air in summer.
The amount of Water usually present in any soil has a very important influence on its fertility. Light soils with open, sandy subsoil are apt to suffer from want of Water; and a moderate drought may prove very prejudicial, or even fatal, to the plants cultivated in such. On the other hand, clays (and, in a less degree, other soils), over a close, impervious subsoil, retain Water too strongly, so that it stagnates; or they may allow the rain to run off the surface, and, if shallow, may actually suffer from want of Water during continued droughts, after the supply in the surface soil has evaporated. Light soils are much benefited by careful irrigation. Stiff soils, on the contrary, are usually in need of well-considered drainage. The latter operation is of wider utility than is recognised by many agriculturists. That it removes superfluous Water is obvious, and it is admitted by all that stagnant Water in the soil is hurtful to most plants, and in more ways than one. It promotes the formation, from decaying organic remains in the soil, of substances prejudicial to many plants; and where such substances are abundant, only certain weeds will grow. Waterlogged soils are deficient in the Oxygen that is required by roots to permit of the healthy discharge of their functions; the roots are unable to exist under the conditions present a few inches below the surface, or to penetrate deeply; the plants therefore are ill-nourished, as the roots spread less than in more open soils, and they derive their mineral food only from a limited area below the surface. In case of severe drought, the Water may all evaporate down to the depth reached by the roots, while the stiff subsoil prevents the passage of Water from below to take its place; and the plants cultivated on waterlogged land may thus die for lack of Water. Moreover, waterlogged soil is always considerably colder than the average temperature of the air, owing to the heat lost by the constant evaporation of Water from its surface. The crops cultivated on it are thus rendered backward in their growth, and may not reach maturity till a week, or even a fortnight, after those on well-drained soils. All this is changed when drains are well placed and well made, and the impervious subsoil is broken up. The rain no longer runs off the surface, nor does Water stagnate in the soil: it sinks into the subsoil, and there forms a reserve from which the surface soil can obtain supplies as required by the plants in it. The rain, while falling, becomes heated to the temperature of the air, or nearly so, and is thus able to warm the soil in sinking through it, and to supply the roots with Water at nearly the same temperature as the air--a condition most favourable to the due performance of the vital functions, and rapid and healthy growth in plants. Free evaporation from the soil is checked; and, this cause of coldness being removed, the crops are found to ripen earlier. As the rain sinks into the soil, the air follows into the interspaces vacated by the Water: thus the roots are supplied with the gases they require, and, as a result of this, and of the absence of the injurious organic substances formed in stagnant Water, they penetrate deep into the subsoils, beyond the reach of ordinary droughts. In this way they, at the same time, obtain a more reliable source of Water, and draw their food from a wider area.
The means by which Water enters plants, to form the crude sap, the changes that this undergoes, and the channels by which it is conveyed through the tissues, are treated of elsewhere in this work. See Sap, Vascular System, and Vessels.