278 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Gains of Organic Matter. The soil gains organic matter 

 through residues of crops, weeds, green crops, and manure. Crops 

 are divided into two classes with reference to their effect upon 

 the organic matter of the soil namely humus-decreasing and 

 humus-increasing. Those crops which leave enough, or more 

 than enough vegetable matter in their roots and stubble to restore 

 the loss which takes place during their growth, are called humus- 

 increasing crops. They are generally crops which receive little or 

 no cultivation, and which leave large amounts of roots and 

 stubble. Crops such as clover and grasses are humus-increasing. 



Those crops which do not leave enough organic matter to re- 

 place the loss by oxidation during their growth are called humus- 

 decreasing. On account of the cultivation which these crops 

 receive, there is a greater loss of organic matter than with the 

 first group mentioned, and less organic matter is contained in the 

 stubble. Cotton, corn, potatoes, and beets belong to this group. 



There are some crops which belong to the one or the other of 

 these groups according to the disposition made of the residues 

 therefrom. For example, if rice straw is removed or burned, it 

 is a humus-consuming crop, while if the straw is plowed under 

 there may be little or no loss of humus. 



The quantity of organic matter lost when humus-consuming 

 crops are grown also depends upon the disposition of the residues. 

 Any treatment which results in loss of organic matter that might 

 be plowed under, such as burning off of corn stalks or grass, etc., 

 increases the loss of organic matter from the soil, and vice versa. 



Green crops, when plowed under, increase the organic matter 

 of the soil. Their chief use is to secure nitrogen from the air. 

 Plowed under very green, they decompose rapidly and may sour 

 the soil. If allowed to mature, decomposition takes place much 

 more slowly. Manure is one of the best means of maintaining 

 the organic matter of the soil, but many farms do not make 

 enough manure to suffice for this purpose. 



Formation of Humus. The accumulation of humus takes place 

 more largely under conditions of reduction than conditions of 

 oxidation. Climate, weather, nature of soil, etc., are all of in- 



