Manure and the Humus Supply 153 



depleted stores. The roots and stubble added every year 

 are a further addition to the soil-humus. Hence, the 

 losses and gains bear a certain relation to one another. 

 The crop residues and small or moderate applications 

 of manure on heavy soils may be sufficient to maintain 

 them in a satisfactory physical condition for many years. 

 The supply of commercial fertilizers alone may prove 

 adequate under such conditions for profitable crop-pro- 

 duction. 



On the other hand, the crop residues and small 

 applications of manure on sandy soils are not adequate 

 in replenishing their humus. Humus-forming material 

 must be applied in large amounts if the soil is to remain 

 fertile. Hence, the practice of market-gardeners on such 

 soils, of buying manures, or of raising green-manuring 

 crops. It is because of the more rapid work of soil 

 bact'eria in the well-aerated, light soils that the latter 

 have shown themselves capable of profitable cultivation 

 under systems of green-manuring. They digest the 

 organic matter rapidly and can, therefore, in a com- 

 paratively short time, unlock the insoluble plant-food 

 contained in it, and place it at the disposal of the grow- 

 ing crop. 



As the fresh roots and stubble, turned under from the 

 green crop, or the animal manures apphed, are attacked 

 by the soil bacteria, they begin to change rapidly at 

 first, and more and more slowly as time goes on. ' From 

 practical experience and from vegetation experiments 

 it is known that these materials usually benefit the crop 

 most in the first year after their application. In the 

 second growing season the benefit is less pronounced, 



