SOIL HEAT 291 



It is noticeable that there are here three groups of 

 plants as far as temperature conditions for optimum 

 growth are concerned. Wheat represents the crops that 

 uvrminute and grow at a relatively low temperature. 

 Corn requires a medium high temperature for proper 

 growth, while melons and pumpkins represent crops the 

 temperature requirements of which are very high. These 

 needs must be supplied for a proper development of such 

 plants, and must of course be considered in crop adapta- 

 tion as well as in soil management in general. 



208. Chemical and physical changes due to heat. - 

 In the soil a certain amount of chemical action is going on, 

 no matter what the temperature may be ; but it is with- 

 out doubt true that this activity is greatly accelerated by 

 an increase in soil heat. This arises from two causes : 

 (1) because heat increases the solubility of the soil con- 

 stituents; and (2) because the activity of the soil or- 

 ganisms is stimulated to such an extent as to in 

 turn influence chemical reaction. The increased pro- 

 duction of carbon dioxide is a good example of this re- 

 lationship. The warming of .the soil in spring and 

 summer, therefore, by stimulating the amount of solu- 

 tion, increases to a marked extent the constituents avail- 

 able for plant growth. 



The effect of temperature is less marked in a direct 

 way on the structure of the soil than on its chemical or 

 biological nature unless the freezing point is reached. 

 At this point, if moisture is present, the soil mass is dis- 

 rupted and may become rather granulated if the freezing 

 process is often repeated. The practice of fall-plowing 

 in order to better the tilth of the soil is really taking advan- 

 tage of this natural phenomenon. A change in tem- 

 perature also causes the expansion or contraction of the 



