96

THE SOIL.

hydrogen, carbon, and nitrogen in a state of combina-tion, and moreover that these compounds, to be absorbed,must be either fluid or gaseous.

293. The soil consists of silica, alumina, lime, mag-nesia, oxide of iron, small quantities of various alkalineand earthy salts, and a portion of decaying organicmatter. It likewise contains water, and the smallquantity of ammonia and carbonic acid which the rainhas brought down from the air. Plants cannot derivethe elements of organic matter from the earthy consti-tuents of the soil, nor from the organic matter which itmay contain, unless there is air present; by the actionof air these substances decay, and are gradually changedinto gases, which plants can absorb.

294. It is commonly supposed that plants derive thewhole of their food from the soil ; but this is a greaterror : it is a fact well ascertained by chemical experi-ments that plants derive the greater part of their nourish-ment from the air, although the soil is equally essentialto their growth.

295. The earthy substances contained in plants areprincipally obtained from the soil ; it is true that theair contains exceedingly minute traces of various earthyand saline substances which are suspended in it in theform of dust, but the quantity which plants can derivefrom this source is comparatively small. The air nearthe sea-shore, and even to a great distance inland, isfrequently loaded with saline particles derived from thesea ; after a storm the quantity of salts thus suspendedin the air is very considerable.