4 N. H. AGR. EXPERIMENT STATION [Bulletin 210 



per cent, of ammonium and vice versa. The nitrogen per 

 cent, multiplied by 1.21584 gives the per cent, of ammonia. 

 The per cent, of ammonia multiplied by 0.82247 gives the 

 per cent, of nitrogen. Do not be misled by thinking that 

 a tag stating 3.29 per cent, of nitrogen and nitrogen equiv- 

 alent to 4 per cent, of ammonia means that the fertilizer 

 contains the sum of these two. It does not. It means that 

 the fertilizer contains 3.29 per cent, of nitrogen only. 



For convenience, phosphorus is expressed as per cent, 

 of "phosphoric acid" or (PoO-,), phosphorus pentoxide. The 

 tag usually gives the total per cent, of "phosphoric acid," 

 the "water soluble," "citrate soluble" and "insoluble." For 

 the purpose of figuring out the value of these we can take 

 the sum of the "water soluble" and the "citrate soluble" and 

 call it available phosphoric acid. The per cent, of avail- 

 able phosphoric acid is the figure to use in determining the 

 value of the fertilizer. Potassium is expressed as per cent, 

 of available K._.0 or potash. This is water soluble and avail- 

 able to the plant. 



METHOD OF CALCULATING THE RELATIVE COM- 

 MERCIAL VALUE 



Because of the variations in prices during the season 

 and the diflferences in freight costs, etc., it is difficult to cal- 

 culate accurately the values of the different brands of fer- 

 tilizer. The commercial value of a fertilizer is based upon 

 the content of available nitrogen, phosphoric acid, (P,0,,) 

 and potash (K,0). If we know the guaranteed analysis 

 and assign approximate commercial values for a pound of 

 each of the plant foods, we can decide the relative values of 

 fertilizers. 



The terms "unit" of nitrogen, "unit" of phosphoric acid 

 and "unit" or potash are sometimes used to express the 

 amounts instead of the per cent. The term "unit" means 

 20 pounds per ton, 2,000 pounds, or 1 per cent. One unit 

 means 1 per cent, of a ton or 20 pounds. A fertilizer hav- 



