BIOCHEMICAL REACTIONS IN SOIL 283 



study of the processes taking place in field soils is also in 

 progress, and frequent determinations are made of the quantity 

 of nitrate present. But as soon as we turn to the field 

 conditions and try to. follow the production of nitrate in the 

 soil, matters are complicated by the fact that the nitrates 

 produced do not all remain in the soil, but are liable to be 

 washed out or taken up by plants. Analytical determinations, 

 therefore, only give the difference between the amount formed 

 and the amount lost ; they do not show how much is actually 

 ' produced, nor give the rate of production that we desire to 

 obtain. For some time we could see no way of getting over 

 this difficulty, but a simple solution was ultimately found. It 

 is evident that if the curves showing the amount of some other 

 substance produced in the same way as the nitrate, but lost in a 

 different way, are of the same general shape as the nitrate 

 curves, then the shape is due mainly to the production factors ; 

 if, on the other hand, the two sets of curves are different in 

 shape, then the loss factors control the situation. The carbon 

 dioxide in the soil air satisfies these requirements ; it is 

 produced- like the nitrates, by bacterial action, but it is lost 

 largely by gaseous- diffusion, and only in very bad weather by 

 leaching. Carbon dioxide was therefore determined simul- 

 taneously with nitrates, and the curves show a marked 

 similarity except that the increases in nitrate come later. 

 Thus we may conclude that the curves both for nitrate and 

 carbon dioxide are in the main production curves. 



The amount of carbon dioxide in the soil air, which, as 

 we have just seen, indicates the rate at which it is produced, 

 follows the soil temperature during the winter months, but 

 not during the summer ; indeed, during hot weather the 

 amount is distinctly low. It shows some connection with 

 the moisture content, but it is more closely related to the 

 rainfall. Rain is a saturated solution of oxygen, and when it 

 falls on the soil it not only supplies the needful water but also 

 renews the stock of dissolved oxygen, and thus gives the 

 micro-organisms and the plant roots new leases of activity. 



