4 • N. H. AGR. EXPERIMENT STATION [Bulletin 206 



Phosphorus occurs in the various fertilizer materials 

 principally as phosphoric acid in combination with calcium 

 (lime.) In order to be available to plants these compounds 

 of lime and phosphorus must be soluble or made soluble. 

 The amount of lime combined with the phosphoric acid 

 determines the solubility; the material containing the 

 smallest amount of lime is the most soluble. That part 

 of the phosphoric acid which is readily soluble in water is 

 immediately available to the plant during the growing sea- 

 son. This is known as "water soluble" phosphoric acid. 

 A part of the phosphoric acid which is insoluble in water 

 is soluble in a certain strength of ammonium citrate solu- 

 tion. 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 by subtracting the available from the total 

 in the tables of analyses. The "insoluble" phosphoric acid 

 becomes available to the plant only very slowly. 



Potassium occurs in commercial fertilizers usually in the 



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



soluble potassium is readily available to plants. The 



analysis, therefore, expresses the percentage of "water 



. soluble" potash (K,0). 



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 

 ammonium 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 a fertilizer. The following 

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

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

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



