62 FIRST PRINCIPLES OF AGRICULTURE. 



CHAPTER V. 

 Natural Manures. 



A MANURE is, in a broad sense, anything that aids or 

 increases the production of farm crops. Manures may 

 be direct in their effect, by adding to the actual plant- 

 food in the soil, or indirect, by aiding the decay, and 

 making active insoluble plant-food constituents in the soil. 



It was shown in previous chapters, that, of all of the 

 constituents which plants need, but four were liable to 

 be exhausted by any system of cropping; these were 

 nitrogen, phosphoric acid, potash, and lime. Direct ma- 

 nures contain one or two or all of these constituents. Any- 

 thing called a "direct manure," which does not contain 

 one or more of these constituents, cannot add to the stock 

 of true plant-food. 



Essential Fertilizing Elements. Nitrogen, phos- 

 phoric acid, potash, and lime are called the essential fer- 

 tilizing elements, because they are more important in 

 manures than the others that plants require ; and a direct 

 manure is useful in proportion to the amount and avail- 

 ability, or direct usefulness, of these constituents con- 

 tained in it. 



Direct manures may also be indirect at the same time ; 

 that is, they may contain materials which add no plant- 

 food directly, but which act upon the soil constituents. 



