SOIL. Soil is the surface layer of earth on which the land plants grow. It is derived from, and therefore is made of the same constituents as the rocks, but it has been subjected to the action of air and water which have altered and removed some of the original components so that the proportions of the various substances in the soil are not the same as in the parent rock. The mineral particles constitute the basis or foundation of the soil, but not the whole of it. In any region where rainfall and tem perature conditions are favourable, vegetation rapidly springs up, obtaining its mineral nutrients and its nitrogen from the soil. The plants build up complex organic matter from the carbon dioxide of the air, using for this purpose the energy of sunlight, and when they die and their dead remains fall back on the soil there is introduced a new group of constituents : organic sub stances containing nitrogen, phosphorus, calcium, potassium, etc., and also stored up energy. These two components, the mineral substances derived from the rock, and the organic substances derived from previous generations of plants, constitute soil.
The obvious method of classifying soils would be to group them according to the rock from which they are derived. This method answers over restricted areas : it was used by Fallou in Central Germany and by Hall and Russell in their survey of Kent, Surrey and Sussex, but it is inapplicable over wide areas where the soil properties are much affected by the changes they have undergone during their formation. The first division, there fore, is: I. Soils whose properties are determined mainly by the charac ter of the parent rock, called by Glinka endodynamomorphic soils because internal factors have been the chief agents in making them.
2. Soils whose properties are determined mainly by external factors, rainfall, temperature, etc., called by Glinka ectodyna momorphic soils.
To the first division belong the soils of recent origin which have not yet had time to go through much of the decomposition they will still have to suffer: examples are furnished by many British soils which have been in existence only since the last glaciation. In the second group belong many of the continental
soils which have been exposed to climatic agencies for long periods. These soils have been much studied by the Russians Dokuchaiev followed by Sibirtzev, Wysotski, Glinka, Neustruev and others who have shown that they are largely the product of climatic conditions: so much so, in fact, that the soil type can be predicted when the climatic type is known. They are subdivided by Glinka according to the amount and nature of the leaching they have suffered through the operation of climatic factors.
I. Wet Conditions: Material Washed Down: Tendency to Acidity.--I. Efficient Leaching: Much Water and High Tem perature. "Optimum Moisture Conditions." The Laterite Group.
—The organic matter rapidly oxidises, the carbonic acid dissolves out the sodium and potassium from the rock forming weak alkali carbonate solutions which dissolve out the silica formed by the hydrolysis of the silicates : finally so much of the silica and of the alkaline bases have gone that the residue consists chiefly of iron and aluminium oxides, the so-called sesquioxides. This group in cludes the laterites, found in the rainy region near the equator: possibly also the red soils of the Mediterranean region.
2. Less Efficient Leaching: Less Water and Low Temperature. "Average Moisture Conditions." The Podsol Group.—The organic matter does not decompose completely: alkali carbonate solu tions are not formed: the silica formed by hydrolysis of silicates therefore does not wash out. Weak organic acids are, however, formed which dissolve out both the alkalis and the sesquioxides leaving a residue mainly of silica. This group includes the podsols, thoroughly leached acid forest soils: the grey clay soils of de ciduous forests mainly oak (these contain some calcium carbonate) and the Brown earths of Ramann. Where the water cannot easily drain away the soil becomes waterlogged for longer or shorter intervals and the bog soils result. The material leached out is deposited not far below the surface where the conditions are some what different and usually form a compact layer or pan which is almost impervious to water.