Page:Popular Science Monthly Volume 48.djvu/678

606 value of vast land areas thus far considered either irreclaimable or adapted only to scanty pasturage.

Without going into technical details or figures, the case may be stated thus: Soils are formed from rocks by the physical and chemical agencies commonly comprehended in the term weathering, which includes both their pulverization and chemical decomposition by atmospheric action. Both actions, but more especially the chemical one, continue in the soil itself; the last named in an accelerated measure, so as to give rise to the farmers' practice of "fallowing"—that is, leaving the land exposed to the action of the air in a well-tilled but unplanted condition, with a view to increasing the succeeding year's crop by the additional amount of plant food rendered available, during the fallow, from the soil itself.

This weathering process is accompanied by the formation of new compounds out of the minerals originally composing the rock. Some of these, such as zeolites and clay, are insoluble in water, and therefore remain in the soil, forming a "reserve" of plant food that may be drawn upon gradually by plants; while another portion, containing especially the compounds of the alkalies, potash and soda, are easily soluble in water. Where the rainfall is abundant, these soluble substances are currently carried into the country drainage, and through the rivers into the ocean; which shows in its saline portion (about three and a half per cent) the average composition of the matters permanently leached out of the land. Most of this is common salt—chloride of sodium—but a large portion, if not all, of the other elements known are represented in sea water in a greater or less proportion. Among these, potash, lime, magnesia, sulphuric and a trifle of phosphoric acids require mention here.

Where, on the contrary, the rainfall is insufficient to carry the soluble compounds formed in the weathering of the soil mass into the country drainage, those compounds must of necessity remain and accumulate in the soil. They then constitute what in the western United States is now universally known as "alkali."

"Alkali" is not, then, as is popularly supposed, something foreign to the soil, imposed as a special affliction upon the dwellers in the arid or irrigation regions. It is the normal product of soil-formation and soil-weathering everywhere; but in the humid regions it appears only in the bottom and stream waters, and is not perceived in the soil itself.

Nor does it in either climatic region consist only of salts injurious or useless to vegetation. Its origin, as well as the chemical nature of sea water, proves that it should contain the useful or plant-food ingredients as well; and direct analysis amply confirms