Leaching.—­Fully as important as any of the differences above outlined are those which depend definitely upon the leaching power of a heavy rainfall.  In countries where the rainfall is 30 inches or over, and in many places where the rainfall is considerably less, the water drains through the soil into the standing ground water.  There is, therefore, in humid countries, a continuous drainage through the soil after every rain, and in general there is a steady downward movement of soil-water throughout the year.  As is clearly shown by the appearance, taste, and chemical composition of drainage waters, this process leaches out considerable quantities of the soluble constituents of the soil.

When the soil contains decomposing organic matter, such as roots, leaves, stalks, the gas carbon dioxid is formed, which, when dissolved in water, forms a solution of great solvent power.  Water passing through well-cultivated soils containing much humus leaches out very much more material than pure water could do.  A study of the composition of the drainage waters from soils and the waters of the great rivers shows that immense quantities of soluble soil constituents are taken out of the soil in countries of abundant rainfall.  These materials ultimately reach the ocean, where they are and have been concentrated throughout the ages.  In short, the saltiness of the ocean is due to the substances that have been washed from the soils in countries of abundant rainfall.

In arid regions, on the other hand, the rainfall penetrates the soil only a few feet.  In time, it is returned to the surface by the action of plants or sunshine and evaporated into the air.  It is true that under proper methods of tillage even the light rainfall of arid and semiarid regions may he made to pass to considerable soil depths, yet there is little if any drainage of water through the soil into the standing ground water.  The arid regions of the world, therefore, contribute proportionately a small amount of the substances which make up the salt of the sea.

Alkali soils.—­Under favorable conditions it sometimes happens that the soluble materials, which would normally be washed out of humid soils, accumulate to so large a degree in arid soils as to make the lands unfitted for agricultural purposes.  Such lands are called alkali lands.  Unwise irrigation in arid climates frequently produces alkali spots, but many occur naturally.  Such soils should not be chosen for dry-farm purposes, for they are likely to give trouble.

Plant-food content.—­This condition necessarily leads at once to the suggestion that the soils from the two regions must differ greatly in their fertility or power to produce and sustain plant life.  It cannot be believed that the water-washed soils of the East retain as much fertility as the dry soils of the West.  Hilgard has made a long and elaborate study of this somewhat difficult question and has constructed a table showing the composition of typical soils of representative states in the arid and humid regions.  The following table shows a few of the average results obtained by him:—­