298 



SOIL PHYSICS AND MANAGEMENT 



perature of 40 to 50 degrees F., the zero point of growth, is reached 

 by the soil. Growth is most vigorous at from 80 to 90 degrees F. 

 This means that the temperature must reach that point during the 

 day, even if it does fall below this during the night. The amount 

 of growth depends upon the proportion of the day that is above the 

 zero point of growth or the heat hours. It will be seen- from the 

 following table that corn requires a medium high temperature before 

 growth begins, while melons require a still higher one. 



Temperature of Soil for Growth 6 



3. Temperatures Favorable for Osmosis and Diffusion. 

 Osmosis is a process upon which germination of seeds and the 

 growth of plants depend. The seed coat is the osmotic membrane, 

 and the rapidity with which water passes through this depends upon 

 the temperature. Osmotic pressure is the power that sends the soil 

 moisture into the roots of plants. At low temperatures plants may 

 wilt, and Sachs found that at 55 degrees F. pumpkin and tobacco 

 plants did not receive sufficient moisture to compensate for even- slow 

 transpiration. 



Diffusion of substances in solution is influenced by tempera- 

 ture in the same way, being much more rapid at high than low 

 temperatures. 



4. Temperatures for Nitrification. Our ordinary crops 

 depend to a large extent upon the activity of bacteria in the soil, 

 which by means of the process of nitrification use the nitrogen in 

 the organic matter to produce soluble nitrates. The soil bacteria do 

 not work to any large extent if the temperature of the soil is below 

 41 degrees F., nor above 130 degrees F. They are most active 

 at temperatures between 60 and 85 degrees F. 



Conditions Affecting Soil Temperature. 1. Specific Heat. 

 It is a very interesting as well as an important fact that the same 

 amount of heat applied to different substances raises the temperature 



