40 AGRICULTURE 



being represented by 1, that is to say, 

 being taken as unity, that of the other 

 constituents of the soil will be represented 

 by a fraction of 1. Thus the specific heat 

 of humus is about 0'5, the specific heat of 

 carbonate of lime and clay about 0*25, while 

 the specific heat of sand is no more than 0*2. 

 This means that, whereas 1 Ib. of water would 

 rise in temperature 1 when exposed to a 

 certain amount of heat, 2 Ib. of humus, 

 4 Ib. of clay or carbonate of lime, and 5 Ib. 

 of sand would rise to the same temperature 

 when exposed to the same amount of heat. 

 In other words, substances of low specific 

 heat, like sand, are readily warmed when 

 exposed to the rays of the sun, whereas a 

 soil containing much humus is not so easily 

 heated, and a soil that holds a large quantity 

 of water remains coolest of all. Wet soils, 

 therefore, are cold soils, while sandy soils 

 are warm, so that it is evident that the 

 specific heat of soil must have a marked 

 influence on the rapidity of germination of 

 seed, as well as on the growth of plants, and 

 the time of harvest. Within limits it is an 

 advantage that a soil should be warm, but in 

 the case of such a soil as sand the tempera- 

 ture may rise to too high a point, with the 

 result that crops may ripen prematurely, and 



