302 THE PRINCIPLES OF AGRICULTURE 



Manures are of two general characters; those 

 that improve the texture of the soil, and those 

 that add plant-food. Barn manures usually per- 

 form both offices, and this is one reason why 

 they give excellent results. 



As a matter of farm -practice, we may divide 

 all fertilizers or manurial substances in two great 

 classes: those that are produced on the farm, 

 and those that are bought from the market. 

 The best agriculture is that which aims to pro- 

 duce a good part of the necessary fertilizing 

 materials on the farm itself. These materials 

 are by-products (see definition in glossary). 



In Chapter V we discuss three general cate- 

 gories: (1) what these farm manures are, (2) 

 the enriching of soil by means of crops that are 

 plowed under, (3) direct application of farm 

 manures to the land. 



The following questions will tend to bring out 

 the various points in the chapter: 



What is the real fertility of the land? Has it to do alone with 

 plant- food? What is the first step toward increasing the produc- 

 tiveness of any soil? What are the means by which this step may 

 be taken? What is humus (review paragraph 33) ? How is humus 

 secured? 



What are green-manures? How much of the weight of a 

 clover crop may be left in the ground (108a)? Name the three 

 classes of green-manure crops, and explain them. How may 

 green-manuring crops be classified, with reference to their nitro- 

 gen-gathering power? Name some of the nitrogen-gatherers. 

 To what family of plants do they belong? Name some of the 



