CHAPTER XVII. 



RELATIONS OF SOILS AND PLANT GROWTH TO HEAT. 



The Temperature of Soils. The rapid germination of seeds, 

 as well as the development of plants to maturity, is essentially 

 dependent upon the maintenance of the appropriate tempera- 

 ture. The temperature most favorable to germination or 

 growth, as well as the degree of tolerance of high and low 

 temperatures, varies greatly with different plants, governing 

 mainly what is known as their climatic adaptation. A knowl- 

 edge of these points with reference to the several crops is 

 therefore of no mean importance to the farmer; for, to a cer- 

 tain extent, he can control the temperature in the soil itself, 

 and he can mostly choose for sowing and planting, the time 

 when the soil shall have the proper temperature for rapid 

 germination or maturity. As a rule, it is not desirable to have 

 either seeds or seedling plants in the ground for any length 

 of time when the temperature is too low for active vegetation; 

 for while they rest, other, lower organisms (fungi and bac- 

 teria), adapted to low temperatures, may continue in full 

 activity at the expense of the vitality of the crop plant. 



Water exerts controlling Influence. Since the capacity of 

 water for heat is approximately five times greater than that of 

 the average soil, equal weights being considered, it follows 

 that the temperature of soil-water must exert a controlling 

 influence over that of the soil. Taking the case of a cubic 

 foot of loamy soil, fully saturated with water, in which one- 

 third of the volume may be assumed to be water: the weight 

 of the dry soil being about eighty pounds per cubic foot, cal- 

 culation shows that the amount of heat required to raise the 

 temperature of the water contained, one degree, will be fully 

 twice as great as for that required for the soil itself. It is 

 thus obvious that the control of soil-temperature is largely 

 dependent upon the control of the water-content of the same, 

 which has been discussed in a. former chapter. Even in the 



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