SOIL AIR AND SOIL TEMPERATURE 149 



beginning growth slightly above that point, while others 

 need several degrees higher temperature. This is also true 

 of the germination of seeds. The optimum temperatures 

 for both plants and seeds are considerably higher. A tem- 

 perature may be reached at which both plant growth and 

 seed germination may be inhibited, but soils rarely reach 

 such a degree of heat, except at the immediate surface. 

 The problem with soils usually consists in bringing them to 

 a sufficiently high temperature in the spring. 



The indirect influence of temperature is exerted through 

 the germs that affect plant growth. These, like higher 

 plants, require a certain degree of warmth before growth 

 begins and a still higher temperature before they reach their 

 full activity. It often occurs that crop growth is well under 

 way before the soil is sufficiently warm for germs to function 

 actively, and consequently growth is checked by the need 

 of nitrates, which have not been formed in sufficient quantity 

 on account of the low temperature. This condition is often 

 demonstrated by the yellow color of the leaves. 



193. Sources of soil heat. — The greater part of the heat 

 that enters the soil comes directly from the sun. The other 

 possible sources are the organic matter in the soil and heat 

 from the interior of the earth. Heat produced by the de- 

 composition of organic matter may sometimes be a factor 

 w T hen the proportion is large, as is the case in hotbeds and 

 gome gardens, but ordinarily it may be left out of considera- 

 tion, as may also the heat transmitted from the center of 

 the earth. 



194. Relation of soil temperature to atmospheric tem- 

 perature. — Changes in temperature of the atmosphere are 

 transmitted to the soil, although the extremes are never so 

 great in the soil as in the atmosphere, except at the im- 

 mediate surface, and the extremes become less as the depth 

 increases. In summer the temperature of the surface soil is 



