ORGANIC MATTER OF THE SOIL 95 



Aside from its direct use to crops, soil air is essential 

 in several indirect ways. When air is excluded from the 

 soil, the beneficial soil organisms cease to be active. It 

 is from the air in the soil that these organisms and the 

 leguminous plants secure free nitrogen for the use of crops. 

 Not only does the fixation of atmospheric nitrogen cease 

 when air is excluded from the soil, but under these con- 

 ditions the organisms that break down nitrogen com- 

 pounds are very active, so that the nitrogen that was 

 fixed is lost by being returned to the air. One of the first 

 effects of having the soil too wet is the yellowing of the 

 leaves. This appears to be due to the lack of nitrogen. 



Some soils are too well aerated, just as some are too 

 well drained. Usually it is the same soils in each case. 



ORGANIC MATTER OF THE SOIL 



103. The Uses of Humus. All productive soils contain 

 decaying roots, leaves and animal life. This partly decayed 

 organic matter is called humus. It is the humus that 

 gives soils their dark color. Humus is necessary for the 

 growth of good crops. Plants may be grown in fine sand 

 if all the plant-food elements are supplied. Under field' 

 conditions, humus is necessary if these foods are to be 

 supplied for the successful production of crops. 



Humus has many functions in soils. It increases the 

 water-holding power, which is particularly important on 

 sandy land. It loosens heavy soil and promotes ae'ra- 

 tinn, which are of special importance on clay soils. It 

 furnishes food for bacteria. These, acting on the humus, 

 change nitrogen to nitric acid so that it is ready for 



