IMPROVEMENT OF SOIL BY MANURING. 51 



It should be kept in mind that a part of the plant 

 remains on the soil in almost all cases. The roots 

 and stubble of grain and hay crops do much to keep 

 up the supply of humus in the soil and keep the sur- 

 face in a good physical condition. 



Classification of ]flaniires. — To manure land, 

 according to the original meaning of the word, was to 

 cultivate it by manual, or hand labor. Tillage or cul- 

 tivation, properly done, does increase the fertility of 

 the soil. Probably crops were grown for centuries 

 before the practice of applying plant food to enrich 

 the land was thought of. Now, by manuring we mean 

 the application of something to increase the fertility 

 of the soil. 



Manures may aot in three ways: 



1. They may directly add plant food to the soil. 

 Manure is sometimes defined as plant food and this is 

 the most common thought in regard to it and its 

 action. 



2. They may hasten decomposition and chemical 

 changes in the soil by which available plant food is 

 prepared from material which has been lying dor- 

 mant. 



3. They may improve the physical condition of the 

 soil. The application of pare sand to a clay soil, or 

 pure clay to a sandy soil, does not add plant food, 

 nor directly affect chemical action in the soil, but by 

 making the one soil more open, and the other more 

 compact, the fertility of each may be largely increased. 



Manures may be divided into two classes: General 

 and special; or complete and partial. They are also 

 sometimes divided into "mineral" and "organic" 

 classes. 



