THE NATURE OF SOIL 33 



loams are among the earliest and warmest of soils. 

 In Europe, gardeners sometimes put loose gravel 

 around grape vines to keep them warm during the 

 night. But a soil in which the particles are very 

 small, as in clay, warms much faster than sand 

 because the particles lie so close together that the 

 heat passes more readily from grain to grain than 

 in sand where the grains lie loosely. For the same 

 reason a clay soil loses more heat by radiation than 

 a sandy soil. Moreover, a clay soil holds more 

 water than a sandy soil and so loses more heat be- 

 cause of the larger amount of evaporation. Hence, 

 fine-grained soils, though they absorb more heat 

 than coarse-grained soils, are colder. Sandy soils 

 are "warm," clay soils are "cold." 



Draining a Soil Warms it. The warmth of a 

 soil comes chiefly from the sun and incidentally 

 from the fermentation and decay of the vegetable 

 matter and other refuse that it contains. The 

 temperature of a soil is modified most by the 

 amount of water it contains. Wet soils are cold. 

 The wetter a soil is the colder it is, at least during 

 the summer, when warmth is needed most. It is 

 the coolness as much as the excess of moisture and 

 lack of air that makes corn with "wet feet" grow 

 poorly. The chief reason for this is that it takes 

 a large amount of heat to evaporate the excess 

 water from a soil, and also much heat to warm the 

 wet soil that remains, water being a poor con- 

 ductor of heat, the evaporation of one pound of 

 water from a cubic foot of clay soil makes it 10 

 degrees cooler. There may be,a difference of 7 

 to 10 in the temperature of a well-drained 

 loam and a poorly drained soil of the same 

 character. There is one exception to the state- 

 ment that the wetter a soil is the cooler it is. 



