The Soil 43 



into their burrows, and they pass large quantities of the soil 

 through their bodies and deposit it on the surface at a rate which 

 has been estimated on the average to be about ten tons per acre 

 per year. 



Ants in some hot countries, as for example Africa, perform 

 much the same work as earth worms, though perhaps on even a 

 larger scale. Ingle says that in many parts of South Africa, the 

 veld is thickly studded with the hills of the white ant, usually 

 about two feet high and about two to three feet in diameter, 

 though much larger ones are often found. The ant hills are full 

 of cavities and chambers inhabited by the insects and much 

 vegetable matter is stored in them. The material of the ant hills 

 consists of the smaller parts of the surrounding soil, the par- 

 ticles being cemented together and the whole made practically 

 water tight. When the veld is plowed and sown, it is always 

 noticed that where ant hills had formerly been the crop is heavier 

 than elsewhere. 



Plants act as soil formers in several ways : Mechanically the 

 roots penetrate the rocks or soils, rendering them porous and thus 

 admitting air and water. They also exert a tremendous lateral 

 force, breaking apart rocks and stones when once they have ob- 

 tained a foothold in a crevice. The roots penetrate the soil some- 

 times to great depths, and as they decay, afer the death of the 

 plant, they leave in the soil little channels, which serve to carry 

 down water laden with carbon dioxide, as well as the oxygen 

 of the air, which as previously pointed out are important factors 

 in soil making and the production of available plant food. 

 Chemically plants act during life through the corrosive action 

 of the carbon dioxide excreted by the roots and root hairs and 

 after death by producing carbon dioxide and various vegetable 

 acids, which have solvent properties upon certain constituents 

 of soils. 



The formation of a mass of pulverized rock, however, is not 

 all that is necessary for producing a fertile soil. A fertile soil 

 must contain nitrogen. It has been shown that to grow crops the 



