370 SOILS 



phosphoric acid in this fertiliser, as shown in No. 2, 

 out of the total amount of phosphoric acid it con- 

 tains, 15 per cent., indicated in No. 3. We are 

 not concerned about the bone phosphate in No. 4, 

 because this is merely a repetition of the figures 

 given for phosphoric acid; 46 per cent, of bone 

 phosphate is actual phosphoric acid, and this has 

 already been stated in Nos. 2 and 3. There is 

 12 per cent, of actual potash, which is that found in 

 the muriate of potash and repeated in No. 6. The 

 guarantee of this fertiliser might better be: 



Per Cent. 



Nitrogen 3 



Available phosphoric acid .11 



(furnished in bone phosphate) 



Insoluble phosphoric acid 4 



Potash 12 



(furnished in muriate of potash) 



Always take the lowest per cent, given. Rarely 

 does a fertiliser contain more than the minimum 

 amount of plant food stated in the guarantee. 



THE FORMS OF PHOSPHORIC ACID 



The way in which the amount of phosphoric 

 acid is stated is one of the most common sources 

 of confusion. The nitrogen and potash in com- 

 mercial fertilisers are mostly soluble and available 

 to plants. But the phosphoric acid in fertilisers 

 is more complex. It is usually not alone but com- 

 bined with different amounts of lime, making 

 "phosphates of lime" or "phosphates." On fer- 

 tiliser tags one will find these terms: "available 

 phosphoric acid," "soluble phosphoric acid," "in- 

 soluble phosphoric acid," "reverted phosphoric 

 acid." 



"Available" and "soluble" phosphoric acid 

 are the same, for only plant food that is soluble in 



