PHYSICAL PROPERTIES OF SOILS 49 



annually as the burning of a ton of coal. 9 The addi- 

 tional heat in well-drained and well-manured land is an 

 important factor in stimulating crop growth, particu- 

 larly in a cold backward spring. The production of 

 heat from manure is utilized in the case of hotbeds 

 where manure in rotting raises the temperature of the 

 soil. When soils are well manured, heat is retained 

 more effectually and crops on such land often escape 

 early frosts. 



45. Influence of Exposure upon Soil Temperature. 



Land with a southern slope receives the sun's rays 

 longer and at a better angle for absorbing heat than 

 land sloping to the north. In valleys and low places 

 the soil at night cools more rapidly than on higher 

 ground, and hence crops in valleys may be injured by 

 late spring and early autumn frosts, while those on 

 higher and warmer land escape. 



46. Influence of Cultivation upon Soil Temperature. 

 Thorough cultivation resulting in the production of a 

 fine pulverized seed bed enables the soil to absorb a 

 larger amount of heat than if left in a rough lumpy con- 

 dition. Cultivated land is more porous and allows 

 greater freedom of movement of water into the subsoil. 

 Warm spring rains have a marked effect upon the 

 temperature of cultivated soils by filling the pores with 

 warm water. The influence of temperature upon nitri- 

 fication is discussed in Chapter IV. 



