THE PHYSICAL PROPERTIES OF SOIL From the standpoint of physics the soil is regarded as a mass of particles of all shapes and sizes, between which are innumerable voids and interstices, serving as channels for the movement of water and air. The physical properties of this moist porous ma terial are important, both in their bearing on problems of soil cultivation and in elucidating the environment of soil micro-organ isms, on which soil fertility so largely depends.
A century ago, when scientific men first seriously turned their attention to agriculture, the physical properties of soil received much attention, and within the limits of existing knowledge, fairly satisfactory explanations were given of the broad differences in behaviour between light, or sandy, and heavy, or clayey soils. Sandy soils drained readily and were susceptible to drought, while clayey soils although resisting drought would lie wet, or even saturated, in rainy weather; the explanation offered was that the soil particles, and therefore the interstices, were largest in sandy soils and thus permitted easy drainage and quick drying out by evaporation of water into the atmosphere. The water was sup posed to rise to the soil surface just as it does in a narrow glass capillary tube ; hence the effect of a mulch of dry soil in conserv ing the moisture below it was attributed to the inability of the water to rise beyond the bottom of the mulched layer into the much wider voids in the mulch itself. This hypothesis, although adequate for a qualitative understanding of some of the broader features, offered no explanation of the supreme characteristic of soil—the ability of individual particles to aggregate into crumbs or compound particles, that every gardener and farmer recog nises as the essential feature of a good tilth. There was no ob vious explanation to hand, and in any case, the brilliant achieve ments of Lawes and Gilbert with their promise of a complete chemical theory of soil fertility focused attention on soil chemis try to the exclusion of all other work. The subject was reopened by competent physicists in the closing years of last century. The underlying hypothesis was that the water was distributed over the surface of the soil particles in a continuous film, that was thinnest where the particles did not touch one another, and thick ened into annular rings around the points of contact of particles. This distribution could be predicted, in fact, from the known laws of pure physics. This theory fitted reasonably well with the experimental facts. The slow movement of water from point to point within the soil, the existence of an optimum moisture content for plant growth, the inability of vegetation to extract the full moisture content from the soil, were phenomena that fell within this new theory, but the compound particle or soil crumb, still remained outside its scope. But the study of the colloidal state of matter that has been so actively pursued in recent years has opened a new field in soil science, for there are many suggestive analogies between the behaviour of collodial systems, and soil. Our present picture of the soil particle differs from the earlier form in that the particle surface is no longer regarded as inert, but as coated with colloidal material derived partly from inorganic weathering products of the soil and partly from the decayed or ganic residues of plants and micro-organisms. This mixed ma
terial has a complex composition, and some of the relationships between soil and its water content deduced from the earlier hy pothesis are profoundly modified by its presence. This is es pecially true for moisture contents below saturation, i.e., over the range of most importance to cultivation.
The Soil Material.—The practical man recognizes many dif ferent kinds of soil and grades them in a succession from heavy to light. The terms refer to the ease of cultivation; heavy soils are sticky when wet, require great skill in cultivation and may dry into hard clods, while light soils can be cultivated at almost any time. Various descriptive terms, such as clays, heavy, medium and light loams, and sandy soils mark the gradation from heavy to light types. When such soils are examined in the labora tory it is found, in general, that the greater the proportion of coarse or sandy particles present the lighter is the soil in the farm ers' sense. A method, known as mechanical analysis, has been devised for sorting out the particles from a soil into groups of dif ferent average fineness, thus giving an impersonal and arithmetical specification in place of the purely qualitative terms, that naturally vary with the individual judgment. The method depends on the fact that the smaller the particle the slower will be its velocity of fall in water. If a mixture of soil and water is placed in a beaker, allowed to stand for a time, and the turbid suspension poured off, a sediment will be left in the bottom of the beaker. Water is added to the sediment, to the same height in the beaker as before, the whole is well mixed and allowed to stand for the same time before the fresh turbid suspension is poured off. Repe titions of this process will eventually clear the sediment of all particles whose effective diameter is less than a certain critical value that depends on the height of the liquid in the beaker and the time of settling. The collected turbid suspension can be sim ilarly divided. In practice only some three or four divisions are made. These have already been given: those now in use in Great Britain are: The sand fractions are separated by sieves, and the remaining two fractions by sedimentation in water as already described. Soils containing more than 25% by weight of the clay fraction are in the heavy class ; the light soils contain at least 50-60% of coarse and fine sand. The silt fraction and the finer part of the fine sand fraction are exceedingly valuable constituents, and when present in amounts not exceeding 3o%-4o% produce the optimum mois ture conditions : the soil drains easily and yet retains a good supply of moisture. The class of soils known as loarns derive their de sirable properties largely from this cause. If the silt fraction is present in greater amount than the clay a very difficult soil type is produced, that is less susceptible to ameliorative cultivation and manurial treatment than even heavy clay soils.