Sub-soil
These Soils contain abundance of mineral food for plants; but it is rendered unavailable by their stiff texture. When this is corrected, by the addition of sand, lime, ashes, or suitable manures, and when the stagnant water is removed, by drainage, clays become very fertile. Calcareous Soils contain above 20 per cent. of Carbonate of Lime, in the form of chalk, or mixed with clay to form marls. These may be dry and friable, or (e.g., marl) may approach the clays in texture. In productiveness they vary greatly. They are less frequent than the two soils already described. Peaty Soils have been discussed above at sufficient length; as have been also the methods of improving them.
Vegetable Moulds (Humus) vary much in percentages of organic matter contained in them. They belong to the more fertile kinds of Soils, and are retentive of water.
Gravelly Soils may belong to any one of the first three classes, seldom to the fourth. The term applies only to the presence of gravel or stones in Soils, without reference to their composition.
Loamy Soils are intimate mixtures of all the first four, in which the clay is under 50 per cent., and the lime under 5 per cent. Loams are productive and excellent Soils, being easily cultivated and fertile.