14^ THE TEMPERATURE OF THE SOIL [chap. 



one that will ensure a subsequent rapid growth of 

 the seedling plants. 



Temperatures of Germination. 



For example, turnips will germinate at almost as 

 low a temperature as barley, but the optimum tempera- 

 ture is higher for turnips ; they are therefore sown much 

 later in the spring, when the ground has more nearly 

 reached this temperature, because the seed is small and 

 the young plant very susceptible to insect attacks, so 

 that the turnip seed must germinate and grow away 

 rapidly if it is to succeed. 



Under ordinary field conditions much of the 

 nutrition of the crop depends upon the activity of 

 certain bacteria in the soil, which break down organic 

 compounds containing nitrogen, and ultimately resolve 

 them into the nitrates taken up by the plant. Most 

 bacteria are active within about the same limits of 

 temperature as have been indicated above for the 

 higher plants ; the nitrification bacteria, for example, 

 cease their work below 41 F. and above I30F., their 

 optimum temperature being about 99 F. 



The way a low temperature will check the production 

 of nitrates until they are inadequate for the needs of 

 the crop is often seen in spring, and may be connected 



