II. Between I. and 111.—Still Less Leaching: Less Water but Higher Temperature: "Moderate Moisture Conditions" Steppe Soil Group.—There are several sub-groups of these. (a) The black earths or Chernozens. Here there is good grass vegetation but very incomplete decomposition of the dead residues there fore much accumulation of humus which, however, being sat urated with lime is not carried down as in the podsol group. The carbonic acid forms some alkaline carbonates as before and they wash down but the calcium carbonate does not get far before it is deposited. The calcium sulphate, however, washes farther down. These soils are neutral and fertile. (b) The chest nut coloured soils, formed in drier conditions where there is not much vegetation and little humus formation : in the cooler regions the surface layer contains humus and differs in colour from the layer below : in the warmer regions, however, the humus layer is either very thin or nod-existent. Neither calcium carbonate nor calcium sulphate have washed down as in the preceding soils.
There has been but little decomposition of the rock and con sequently little or no transfer of bases or of silica from one level to another. Other sub-groups include grey soils and red soils of the desert steppes. These soils are neutral or alkaline.
III. Dry Conditions: Soil Material Brought Up: Saline Soils with Tendency to Alkalinity.—These soils occur only in arid regions : they are characterised by the presence of soluble salts in quantities that may be sufficient to interfere with vegeta tion. They fall into two groups: 1. those liable to occasional flooding: the "solonetz" group, 2. those not liable to flooding: the "solonchak." The former possess structure, the latter do not. In the solonetz group the sodium salts form a sodium clay which is less stable than the calcium clay and therefore lose iron and aluminium oxides which wash down leaving the surface layer richer in silica. Some of the humus is also carried down along cracks which, once formed, tend to be planes of weakness : the soil therefore, on drying breaks into columns or irregular prisms. The solonchaks (no flooding) contain in their upper layers chlo rides, sulphates and carbonates of calcium, magnesium and sodium : the group is subdivided according to the preponderance of the various cations: there is no washing down of humus, the different levels have somewhat similar textures, and there is no definite structure.
Finally there are the desert soils.
This method of classifying soils necessitates the study not only of the surface but also of the lower layers. The material dis solved out from the upper layer, or "A horizon," is washed down to the lower layer, or "B horizon," and is there and then pre cipitated more or less unchanged, or it interacts with other sub stances. Lower down the unaltered material, the "C horizon" is formed. The three horizons constitute the "profile" of the soil.
In a continental area where the character of the climate varies regularly in passing from one region to another the soils also vary regularly in the same way. The best illustration is in Russia itself : in the north are the frozen tundra: then farther south the podsols under forest: farther south still the grey soils all tending to be acid; then the black earths; then the chestnut coloured steppe soils all neutral, and finally in the dry arid regions come the alkali and the desert soils. Western Europe does not show so simple a zoning, but there still remains the relationship with the climate.
III. Organic soils. (i) Peat: acid. (2) Fen or calcareous peat: neutral.
I. Mineral Soils.—The properties of the mineral soils are determined largely by the proportions of particles of different sizes. The grouping is only approximate, there being no rigid definitions or boundaries, and sandy soils shade off into loams, and loarns into clays, through a continuous series where no sharp lines can be drawn. By international agreement among soil in vestigators it is now usual to divide the soil particles into groups of the following diameter sizes: This grouping, like that of the soils, is in the main arbitrary, but it has ample justification, even though the actual figures for the limits could not be rigidly defended.
Sandy soils usually contain more than 6o% of coarse and fine sand and less than io per cent of clay : loams contain 8 to 15 per cent of clay and more silt : clays contain still more clay— occasionally in England up to 4o per cent and not infrequently 20 per cent or more. The physical properties are discussed in detail later; the agricultural properties are :—sandy soils: easily permeable by air and water hence the plant roots are well aerated but may suffer from drought; these soils are easily cultivated and hence are much used for fruit, market garden and nursery work., loams: less easily permeable but sufficiently to allow ample aera tion of roots and to prevent waterlogging : these are suitable for all types of crops; clays: much less permeable, liable to be water logged, sticky when wet, hard and difficult to cultivate when dry, and usually difficult for plant roots to penetrate. They are greatly improved by lime and by organic matter, usually in Great Britain they are left as grassland, cultivation being too costly, but they are well suited to wheat and beans.