Soils 63 
their best growth; their action is practically discon- 
tinued when the air supply is reduced greatly. 
SOIL HEAT 
The temperature of the soil is important because of its 
influence on the germination of seeds and on the growth of 
plants, and also because of its effect on chemical changes 
and bacterial action in the soil. When a soil is cold, life 
in it is dormant and chemical action is reduced. The 
earlier a soil is warmed in spring and the later it is kept 
warm in fall, the longer is the growing season. This is 
‘very important for sugar-beets, since there is not time 
during a short season to store large quantities of sugar. 
Soil heat is derived largely from the sun, the rays of 
which are most effective when striking perpendicularly. 
A south slope, therefore, is considerably warmer than one 
facing the north. A sandy soil is also warmer than a 
clay. On account of the high specific heat of water, a 
wet soil is much slower to warm up in spring than a well- 
drained soil. The high evaporation from a wet soil also 
reduces the temperature. Such factors as colors, specific 
heat, and tillage play an important réle in regulating soil 
temperature. 
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ORGANIC MATTER 
The chemical, physical, and biological conditions of the 
soil are greatly influenced by organic matter because it 
reacts favorably on the tilth, the water-holding capacity, 
and the temperature of the soil. Through its decay, or- 
ganic matter increases the availability of mineral matter 
