462 THE PRINCIPLES OF SOIL MANAGEMENT 



the boiling point requires 5.3 times as much heat as is 

 necessary to raise its temperature from the freezing point 

 to the boiling point. It is this large absorption of heat 

 which renders evaporation such a large cooling operation. 

 The more evaporation which takes place from the soil 

 moisture, the more will the temperature be kept clown. 

 Any treatment which reduces evaporation, such as the 

 mulch, will favor a higher soil temperature. 



This influence of the moisture content has given 

 rise to popular descriptive terms, such as "warm," 

 and "cold" soils; "early" and "late" soils. A "warm 

 soil" is one which retains naturally a relatively small 

 amount of water, that is, soils of coarse texture. "Cold 

 soils," on the other hand, are those which retain a rela- 

 tively large amount of water, that is, those of fine tex- 

 ture. The difference in the amount of heat required 

 to warm the water contained in the soil, as well as that 

 lost in evaporation, which is of course greatest in the 

 soil containing most water, is the source of their normal 

 differences in temperature. 



An "early soil" is one which retains a relatively 

 small amount of water. It therefore warms up most 

 rapidly under a given heat supply, and is in condition 

 to permit seeding earlier in the season. A late soil 

 retains much water, and, consequently, is slow in 

 warming up. Its planting must therefore be deferred 

 until later in the season. Coarse-textured soils are 

 "early," and fine-textured ones are "late." Wollny 

 concluded, from extensive experiments, that in summer 

 sandy soils are warmest, followed by humus, lime and 

 loam soils. In winter this order is reversed. 



