16 THE SOIL OF THE FAIIM. 



exhaustion, while at the same time it operates favorably 

 in sustaining the proper temperature for the ripening of 

 later fruits and other croj^s. The surface heat is often 

 preserved, too, in the autumn by rain; and in the spring 

 rains aid in warming the soil. Emmon mentions an in- 

 stance of rain whose temperature was fifty-four degrees 

 falling when the earth was fourty-nine degrees, and the 

 surface was raised soon after to fifty-one degrees. Dark- 

 colored soils absorb heat more rapidly than light-colored 

 ones. 



CHAPTEE III. 



COMPOSITION AND FERTILITY OF THE SOIL. 



Organic and Inorganic Constituents — Classification — Composition and 

 Texture — Barrenness — Fertility, natural and acquired — Exhaustion 

 and Restoration of Fertility. 



Soil consists of an organic and an inorganic or mineral 

 part ; and we have seen that the former is derived from the 

 roots and stems of decayed plants and from the manure 

 and remains of animals, and the latter from the waste of 

 the rocks forming the earth's crust. 



Orp^anic matter is most deficient in sandy soils and 

 poor clays. Even in fertile soils, however, it often occurs 

 but sparingly. In one sample of fertile mould, the amount 

 of organic matter was ascertained to be only 1.7(> per 

 cent. ; in the famous black soil of Russia it varies from 

 five to twelve per cent. In leaf-mould the amount is 

 much greater, and in peat the carbon alone sometimes ex- 

 ceeds sixty per cent. The carbon in tlie soil tends gradu- 

 ally to oxidize and to disappear, except where water ac- 



