THE SOIL. 41 



RELATION OF THE SOIL TO HEAT. 



The temperature of the soil has a great influence on 

 the growth of vegetation. The specific heat of the soil 

 is a point of minor importance, for the temperature of the 

 soil under cultivation depends chiefly on the wetness of 

 the land. Up to a certain point, a high temperature, if 

 accompanied by sufficient moisture, is conducive to the 

 rapid growth of the crop and to the activity of the micro- 

 organisms which are preparing plant food. The plant 

 roots are more active, more water is transpired and more 

 food is obtained. Cotton seed sown early is always planted 

 on the warm side of the ridge as the heat ensures rapid and 

 consequently better germination. In different parts of 

 the soil, different temperatures prevail and this tends to 

 maintain a circulation of the soil air. The soil derives its 

 heat chiefly from the sun and, in a less degree, from the 

 hot interior of the earth and the fermentation of the organic 

 matter it contains. Dry rough sandy soil is usually 

 warmer than soils which are capable of retaining more 

 moisture. The coldness of wet soils is due partly to the 

 high specific heat of water, and partly to the loss of heat 

 consequent on the evaporation of water from the surface 

 of the land. On the other hand, sandy soils become coldest 

 at night and the crops growing on such soils are subject 

 to great extremes of heat. The conduction of heat through 

 the soil is most rapid in compact iion porous soils. The 

 soil loses its heat by radiation, evaporation of water, and 

 the heating of cold irrigation water. Under the earth's 

 surface there is a belt having a nearly constant temperature, 



