

'' 6  . • o^^,' % r' THE SOIC OF THE FARM. 



^ e o c c f< <' ( t t c f f r ' 



Soil or rotted subsoil; (4) Subsoil or rotting rock; (5) 

 Underlying rock. 



Conversion of Rock into Soil. — Various forces are 

 active in this work. (1.) The atmosphere acts chemically 

 upon rock; its action consisting chiefly in the oxidation 

 of those minerals which can contain more oxygen, and in 

 the production of carbonates and bicarbonates, whose 

 solubility still further aids disintegration. (2.) Changes 

 of temperature have a loosening influence upon rocks, 

 by causing alternate expansion and contraction. We see 

 its effects in the way in Avhich, after a strong frost, the 

 soil of fields and the material of roads is found to be 

 loosened and pulverized. (3.) Ram acts both chemically 

 and mecluinically in the same direction. Its chemical ac- 

 tion arises chiefly from the solvent power of the carbonic 

 acid which it absorbs from the atmosphere, and which acts 

 especially upon rocks containing lime; partly, also, fi*om 

 its oxygen combining with substances not yet fully oxi- 

 dized. Its mechanical action appears in the way in wliich 

 it washes off the finer particles of disintegrated rock or 

 soil from higher to lower ground. (4.) Plants promote 

 the i^rocess of converting rocks into soil, both in their 

 growth and in their decay. Growing plants keep the 

 mineral matters, amidst which they grow, moist, and 

 enable water to penetrate and rot them ; while their roots 

 exercise a double effect, inserting themselves into joints 

 and crevices, and thereby causing fragments to be de- 

 tached, dissolving also and then absorbing minute por- 

 tions of the rock fragments. The action of decaying 

 vegetable matter is still greater. It absorbs much moist- 

 ure and keeps all bodies around it damp; it is constantly 

 yielding carbonic acid, which, being absorbed by the ram 

 water, is carried down through the soil and then acts 

 powerfully upon mineral matters below. Certain organic 

 «cids are also produced during decay, and these and 



