32 FERTILIZERS 



The phosphoric acid of fertile soils is practically 

 always insoluble. This is true of new lands, the rich- 

 est and most productive known. It is nature's 

 method. All fertile soils contain such bases as lime, 

 iron and others that hold phosphorus in insoluble 

 compounds from whence it is released only by the 

 processes of plant growth and the chemical activities 

 of fertile soils, due to a sufficient supply of humus. 

 (35) (36). 



THE PURCHASE OF FERTILIZERS. 



52. Fertilizers should be purchased by the unit of 

 plant food contained, with due consideration of its 

 source, and not simply by the ton or brand, as is 

 usually the case. Each twenty pounds of a ton is 

 called one unit or \% ; 5% is five units or one hun- 

 dred pounds. The value of a fertilizer depends en- 

 tirely upon the amount and source of plant food con- 

 tained. High grade analyses are worth more than 

 low grade. Freight, sacking, storing and handling 

 are fixed expenses on low or high grades. Therefore, 

 high grades are cheapest. 



All the simple forms of fertilizer material, such as 

 bone meal, tankage, blood, sulphate of potash, ni- 

 trate of soda, etc., carry definite amounts or per- 

 centages of their respective plant foods. (28). 



53. If, for instance, blood is pure and cleanly 

 handled, its nitrogen content is from 13% to 14%, 

 or 260 Ibs. to 280 Ibs. per ton. Pure bone meal (raw 

 and steamed) varies in nitrogen from \% to 5% and 

 in phosphoric acid from 20 % to 30%. If the nitro- 



