PLANT FEEDING. 23 



72. A fertilizer containing 7 per cent phosphoric acid, 

 2 per cent nitrogen, and 2 per cent potash sells for $25 

 a ton. How much phosphoric acid should a ton of a dif- 

 ferent fertilizer contain, which is also sold at $25, pro- 

 vided it must be 2 per cent nitrogen and 1 per cent 

 potash ? 



Phosphorus and phosphoric acid. Phosphoric acid 

 means a compound whose composition is e^cpressed by 

 the symbol P:: O- and contains 43.79 per cent of phos- 

 phorus. To change phosphoric acid into terms of phos- 

 phorus it is necessary, therefore, to multiply the number 

 representing phosphoric acid by 0.437, since only 43.7 

 per cent phosphoric acid is phosphorus. 



On many accounts it would be preferable to use the term phosphorus in- 

 stead of phosphoric acid. The present use is so firmly established that 

 many practical difficulties would be encountered in making the change. 



Potash and potassium. Potash is almost universally 

 used in referring to potassium compounds. Strictly 

 speaking, potash means potassium oxide (K2O). Potas- 

 sium carbonate (K2 COs) or carbonate of potash, con- 

 tains 56.6 per cent of potassium which is equivalent to 

 68 per cent of potash (K=0). Potassium chloride 

 (KCl) contains 52.7 per cent of potassium, which is 

 equivalent to 63.5 per cent of potash. Potassium sul- 

 phate (K2 SOO contains 45 per cent of potassium, 

 which is equivalent to 54 per cent of potash. Potassium 

 nitrate (KNO3) contains 38.6 per cent of potassium, 

 which is equivalent to 46.6 per cent of potash. 



Ammonia and nitrogen. Ammonia is often substi- 

 tuted for nitrogen in giving the analysis of a fertilizer. 

 This is done simply to increase the size of the figure of 

 the nitrogen content. Nitrogen is the food element, not 

 ammonia, hence ammonia percentage when used must 

 always be changed into nitrogen percentage. This ma}- 

 be done by multiplying the ammonia percentage by .82, 

 since ammonia is 82% nitrogen. 



