EXPERIMENT STATION BULLETINS 109 



of a corresponding profit by its use on the farm. It may serve, liowever, as a basis 

 of comparison between different brands of fertilizers. The reliable test of the 

 value is determined by its use on a given soil and crop — worth more than all 

 theoretical discussions and deductions. Yet there are certain general considerations 

 which go to show the probable benefit of any fertilizing element when used on a 

 certain kind of soil and crop, which will be of some benefit in selecting a fertilizer. 



TERMS EXPLAINED. 



In the table giving the results of analysis of commercial fertilizers there are 

 certain terms used which require explanation: "Available nitrogen, estimated as 

 ammonia; " under "Phosphoric acid " the terms " Available P2 O5; "' " Insoluble P2 

 O5;" '"Total P2 O5" and ''Potash soluble in water, estimated as Ko O." 



Nitrogen may exist in available form either as a salt of ammonia, a nitrate, or 

 in organic material such as dried blood, and be equally valuable in these several 

 forms. But for ease of comparison in different manures chemists estimate it as if 

 all these forms of nitrogen existed in the form of ammonia. Chemists estimate the 

 compounds of phosphorus in manures as anhydrous phosphoric acid, Po O5; that 

 Is phosphoric acid deprived of water; when this anhydride combines with water, 

 the acid is formed. Thus if three molecules of water are added to one of P2 O5 two 

 molecules of phosphoric acid are formed 3H2O + Po O^ = 2 Ho PO4 = phosphoric 

 acid The P2 O5 in separate form does not exist in any soil, but this expression 

 is chosen because the amount of this combination of phosphorus is easily indicated, 

 without considering what material the acid may be combined with. "Available 

 P2 Os " signifies the per cent of the acid in a form directly usable by the plant: 

 '• Insoluble Po Or,."" the per cent of acid in the condition of phosphate rock. By 

 adding together the "Available" and "Insoluble" we get the "Total P2 O5." 



The "Potash soluble in water estimated as K2O," is the soluble potash salt esti- 

 mated only as regards the oxide of potassium = K20=oxide of kalium or potassium. 



