A N. H, AGR. EXPERIMENT STATION [Bulletin 201 



A part of the phosphoric acid which is insoluble in water is 

 soluble in a certain strength of ammonium citrate solution. 

 This is known as "citrate soluble" or "reverted" phosphoric 

 acid. This is also available to the plant. In the analysis 

 the "available" phosphoric acid includes the "water soluble" 

 and the "citrate soluble." The insoluble may be obtained 

 b}' subtracting the available from the total in the tables of 

 analyses. The "insoluble" phosphoric acid becomes avail- 

 able to the plant only very slowly. 



Potassium occurs in commercial fertilizers usually in the 

 form of a chloride (muriate) or sulphate. Only the water 

 soluble potassium is readily available to plants. The analysis, 

 therefore, expresses the percentage of "water soluble" pot- 

 ash (K2O). 



Statement of the Analysis. There are numerous ways of 

 expressing the amounts of nitrogen, phosphoric acid and 

 potash contained in a fertilizer. The purchaser should not 

 be misled by these statements. For instance, if a fertilizer 

 contains 3 per cent of nitrogen it may be expressed as 

 nitrogen 3 per cent, as nitrogen equivalent to 3.63 per cent 

 ammonia or as nitrogen equivalent to 14.1 per cent of am- 

 monium sulphate. The per cent of nitrogen is the thing 

 which is important and is the figure which should be used in 

 calculating the value of the fertilizer. The following shows 

 how to calculate the per cent of nitrogen from the per cent 

 of ammonia and vice versa. The nitrogen per cent multi- 

 plied by 1. 2 1 584 gives the per cent of ammonia. The per 

 cent of ammonia multiplied by 0.82247 gives the per cent of 

 nitrogen. Do not be mislead by thinking that a tag stating 

 3.29 per cent of nitrogen and nitrogen equivalent to 4 per 

 cent of ammonia means that the fertilizer contains the sum 

 of these two. It does not. It means that the fertilizer con- 

 tains 3.29 per cent of nitrogen only. 



For convenience phosphorus is expressed as per cent of 

 "phosphoric acid" or PoOs (Phosphorus pentoxide). The 

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

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



