2, MANUFACTURE OF -FERTILIZING MATERIALS 



III. The fertility of the soil remains un- 

 changed if all the ingredients of the crops are 

 given back to the soil. Such a restitution is 

 effected by manure or fertilizers. 



IV. The manure produced in the course of 

 farming is not sufficient to maintain permanently 

 the fertility of a farm; it lacks the constituents 

 which are annually sold in the shape of grain, 

 hay, milk, and live stock. 



These laws cover the whole subject, but to 

 understand them so that they may be applied 

 at work in the field, it is necessary to have a 

 fair idea of the sources of plant food of the dif- 

 ferent kinds, and how best to use these different 

 kinds for different crops. 



It is generally understood that all manures 

 or fertilizers are valuable for the nitrogen, potash, 

 and phosphoric acid they contain. Though other 

 substances are needed for plant growth, they are 

 almost always present in the soil in sufficient 

 quantity. Lime might be made an exception, 

 although its use is largely to improve the me- 

 chanical condition of the soil, and cure it of 

 sourness. Lime also aids in rotting the vege- 

 table matter. 



The influence of nitrogen in its various forms 

 upon plant growth is shown by at least three 

 striking effects. The growth of stem and leaves 

 is greatly promoted, while that of buds and 

 flowers is retarded. Ordinarily, most plants, at 

 a certain period of growth, cease to produce 



