70 ELEMENTS OF AGRICULTURE 



This water, containing carbon dioxide, enters the cracks 

 and joints in limestone rocks, and, dissolving portions 

 of them, gradually forms larger cracks and fissures 

 which in time. grow into caves. 



71. Other Soil Builders. — Plants also aid in the for- 

 mation of soils. The line root hairs of plants penetrate 

 the cracks and crevices of rocks, and the root sap dis- 

 solves away portions of the rocks' surfaces. The small 

 plants which grow on the surface of rocks, mosses and 

 lichens, are especially active in forming soils. Their 

 roots draw all their mineral food from the rock, and so 

 gradually wear away the surface. 



The oxygen and carbon dioxide of the air combine 

 with many substances contained in rocks and, forming 

 other compounds, help to wear away the original rock. 



The minute bacteria in the soil also aid in breaking 

 up rocks, but their action is chiefly on the organic mat- 

 ter which all soils contain.. 



72. Organic Matter in Soils. — So far we have consid- 

 ered only the mineral matter of soils, but it is by no 

 means all of the soil. A soil made up exclusively of 

 mineral fragments could no more grow crops than the 

 original rocks from which the fragments came. Mixed 

 with the mineral matter of soils are the decaying por- 

 tions of plants and animals which make up its organic 

 matter. As the organic matter of soil decays it forms a 

 dark substance called humus, which gives to many soils 

 their dark color. The organic matter of soils is of great 

 •importance, and we shall have more to say of it. 



Soils are then made up of two kinds of matter — or- 

 ganic and inorganic. The organic matter results from 



