CHAPTER V 



THE TEMPERATURE OF THE SOIL 



Causes affecting the Temperature of the Soil — Variation of 

 Temperature with Depth, Season, etc. — Temperatures 

 required for Growth — Radiation — Effect of Colour — Specific 

 Heat of Soils — Heat required for Evaporation — Effect of 

 Situation and Exposure — Early and Late Soils. 



The life of a plant is practically suspended below 

 a certain temperature, which is about 41 ° F. for the 

 majority of cultivated plants ; all the various changes 

 which are essential to the development of the plant 

 such as germination, vegetative activity, and the bac- 

 terial processes in the soil, show a similar dependence 

 upon temperature. 



These vital actions cease below a certain minimum, 

 above which they usually increase with the tempera- 

 ture until an optimum is reached, when the action is 

 at its greatest ; beyond this point the action decreases 

 until a superior limit is reached, which again suspends 

 all change. It therefore becomes important to study 

 the manner in which heat enters and leaves the soil, 

 because upon the temperature acquired depend such 

 practical questions as the suitability or otherwise of 

 the land for particular crops, the season at which to 

 sow, and the earliness or lateness of the harvest 



The surface soil receives heat in four ways : — 

 (1) By direct radiation from the sun, whose rays 

 both of light and invisible heat are absorbed 



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