108 THE ESSENTIALS OF AGRICULTURE 



nor the meaning of fertilizer formulas, there is much fertilizer 

 used that is not suited to the soil or crop. 



131. What a fertilizer formula means. One of the most 

 common types of fertilizer on the market is one having a 

 composition of about 2 per cent ammonia, 8 per cent of so- 

 called available phosphoric acid, and 2 per cent of water-soluble 

 potash. It is common in the trade to speak of such a fertilizer 

 as " two-eight- two." In the same way, one containing 3 per cent 

 of ammonia, 10 per cent of available phosphoric acid, and 5 per 

 cent of water-soluble potash is called a " three-ten-five." 



Fertilizers usually contain a large amount of inert matter, 

 commonly known as a " filler," along with a comparatively small 

 amount of plant food, since it is necessary to have a certain 

 bulk in order to apply the fertilizer evenly. 



132. States control the sale of fertilizers. All states in which 

 fertilizers are used to any extent have laws regulating the sale 

 of commercial fertilizers. The manufacturer is required to stamp 

 the composition of the fertilizer on each bag, and penalties 

 are provided for false branding. Formerly it was the custom 

 among manufacturers to make the statements of analyses more 

 complicated than is necessary. 



The important things are the ammonia (which contains the 

 nitrogen), the available phosphoric acid, and the potash, and the 

 statement should indicate clearly the amounts of these things. 



133. How to compute fertilizer values. The market value of 

 a fertilizer is based upon the per cent and availability of the 

 plant-food elements it contains. In calculating values it is com- 

 mon among fertilizer dealers to speak of " units " of plant food. 

 A unit represents 20 pounds of a given element per ton, or 

 I per cent. Nitrogen can be delivered to the farmer in quickly 

 available form for 20 cents a pound, or $4 a unit. On the 

 same basis available phosphoric acid is worth 6 cents a pound, 

 or $1.20 a unit. Insoluble phosphoric acid is worth 2 cents a 

 pound, or 40 cents a unit, and water-soluble potash is worth 

 6 cents a pound, or $1.20 a unit. Thus the value of a fertilizer 

 of known composition would be determined readily as follows : 



