These distinctions are closely connected with the chemical composition of the particles.
Coarse sand, fine sand, and probably also silt consist in the main of the unaltered mineral particles : complex silicates such as orthoclase, the ferrosilicates; and quartz; these can be identified by the mineralogical methods of Delage and Lagatu, van der Kolk, Steinriede and others. These are chemically almost inac tive, and their composition plays only a subordinate part in deter mining their properties: the chief factor is their size, hence the grouping according to size.
Clay (below .002 mm.) differs from sand and silt in two important respects (I) it is a colloid, (2) it differs from the original mineral and consists of much changed silicates, now regarded as compounds of insoluble complex alumino-silicic acids. The particles appear to consist of aggregates of much smaller particles, perhaps as little as io //A in diameter, which however, are only with difficulty kept from joining up to form particles of 5o AA or more. This fine material is isolated by means of the supercentrifuge and is called "Ultra clay"; it appears to be of the same general nature in all soils, the difference being in degree only; its physical properties are related to its chemical composi tion, varying approximately as the ratio A1203+Fe203 which varies in different samples between 1.2 and 3.2. As obtained by R. Bradfield from the prairie soils of Missouri the ultra clay contained 24.5% A1203; 45.5% Si02; 7% 1-7% CaO; 1.4% MgO; 0.3% it easily loses bases especially calcium. When these are gone it behaves like a weak acid showing all the usual titration phenomena with sharp end points. The equivalent weight of this "clay acid" is estimated by D. J. Hissink to be between l,000 and 2,000, while Bradfield assigns it a dissociation constant of the order of Pc', the same as carbonic acid. The bases and their chemical interchanges have been much studied by Gedroiz in Russia and Hissink in Holland. They are readily dis placed by dilute acids and other bases, hence they are called exchangeable bases: they consist mainly of calcium, magnesium, potassium and sodium. From the circumstance that the whole of the calcium of the clay can be so displaced, but only part of the others, it is inferred that the calcium is present entirely as exchangeable base while the others occur partly in some other form. Clay thus behaves like a complex acid and its interactions with soluble salts follow the ordinary stoichiometric laws and can be expressed by the usual type of equation: Among its basic radicals or cations there is usually hydrogen i.e., the clay is not a saturated salt, hence in any interaction of this kind some acidity appears : it seems strange that an alumino silicic acid should be able to displace so strong an acid as hydro chloric but the fact is easily demonstrated.
Little is known about the constitution of the acidic part beyond what is stated above. It is much larger than the ordinary acid
molecule : the relative sizes are probably as a wheat grain is to a football, hence surface phenomena come much into play. The rapidity of exchange of cations when the clay is brought into a salt solution suggests that the clay cations are all on the surface, which would imply that the anions should be immediately below the surface of the ultimate particle, the same conception as the Helmholtz double electric layer, the inner being negative, and the outer positive. The flocculation produced by salts or acids under certain conditions accounts for some of the advantages of liming and it is discussed later.
The clay of normal fertile soils is mainly a calcium clay, cal cium forming 8o per cent or more of the exchangeable bases : all present agricultural methods and varieties of crops have been developed for this. Two other clays occur fairly commonly in nature : acid or hydrogen clay, and sodium clay. Acid clay is formed in wet regions where, as the result of continual leaching with rain water containing always some carbonic acid, the calcium tends to be displaced by hydrogen : it differs in physical properties from calcium clay and is not amenable to the usual agricultural treatment used for calcium clay : it is less suited for plant growth partly because of its acidity, partly because it competes with the plant for available bases. It is less stable than calcium clay, and tends to give up iron and aluminium oxides as its acidity increases. The best methods of dealing with acid clays are (I) to find crops tolerant of the conditions, e.g., rye, oats, alsike, etc. and to breed more tolerant varieties : (2) to add sufficient calcium oxide or carbonate to reform the calcium clay. Sodium clay may form whenever sodium chloride comes in contact with soil: when the sea breaks in and floods the land, or, in arid regions, whenever drainage is not good and when the irrigation water contains sodium salts in solution, which it frequently does. As compared with calcium clay it is more easily deflocculated, it has more affinity for water, becomes more sticky and impervious when wet and much harder when dry. It is also less stable and tends to lose iron and aluminium oxides. Its worst property, so far as the plant is concerned, is that in presence of water it readily hydrolyzes and the sodium may combine with the CO2 in the soil to form sodium carbonate, which not only intensifies its stickiness and imperviousness, but is toxic to plants. It can be reconverted to calcium clay by treatment with calcium sulphate and then flooding to wash out the sodium sulphate, but considerable im provement is effected by treatment with sulphur which oxidises to form sulphur acids and so reduces or neutralises alkalinity, a trouble that cannot be overcome by suitable cultivation.