80 GENERAL CONSIDERATIONS 



from drought. Commercial fertilizers are really 

 indispensable in modern agriculture and when used 

 appropriately they are very profitable, but as a gen- 

 eral rule they should only be used to supplement, 

 not to replace, stable and green manures. It should 

 never be forgotten that vegetable matter is always 

 necessary in order to maintain the soil in a good 

 mechanical condition and that this is fully as impor- 

 tant as chemical composition in the production of 

 crops. 



Commercial fertilizers are valuable, generally 

 speaking, only for the nitrogen, phosphoric acid, 

 and potash which they contain. If a fertilizer con- 

 tains all three of these substances, it is said to be 

 a complete fertilizer. If one or more of them is 

 lacking, it is called an incomplete fertilizer. Most 

 farmers make the mistake of valuing fertilizers 

 according to the price per ton. This is very mis- 

 leading, and means nothing, unless the price per 

 ton is considered with reference to composition; 

 that is, to the number of pounds of each of these 

 substances that the ton of fertilizer contains. Of 

 these substances nitrogen is much the most expen- 

 sive. It usually costs about three times as much 

 per pound as either of the other two. In different 

 localities, and in different years, the nitrogen in 

 fertilizers usually costs from 14 cents to 18 cents 

 per pound, while the potash and soluble phos- 

 phoric acid cost from 3 to 6 cents per pound each. 

 If a fertilizer contains 4 per cent of nitrogen, 8 per 

 cent of phosphoric acid, and 5 per cent of potash, 

 each ton will contain 80 pounds of nitrogen, 160 



