372 SOILS 



The "reverted phosphoric acid*' on fertiliser 

 tags is intermediate between the soluble and the 

 insoluble, thus: 



Lime ^^^ 



Lime ^Phosphoric acid 

 " 



Reverted means turned back; this is phosphoric 

 acid which was once soluble but is gradually be- 

 coming insoluble, since more lime has been added 

 to it. If soluble phosphoric acid in the soil is not 

 quickly used by plants it tends to revert. In this 

 condition it is not quite as readily used by plants. 

 However, the reverted phosphoric acid given in 

 fertiliser analyses may be considered about as valu- 

 able as the soluble. Sometimes a tag will read 

 " phosphoric acid soluble in ammonium citrate." 

 This is reverted phosphoric acid, ammonium 

 citrate being the weak acid used by chemists 

 to dissolve it. 



Points that the fertiliser buyer should remember 

 when studying guarantees are: 



1. Look for the percentage of nitrogen. If the 

 analysis gives the percentage of ammonia, remem- 

 ber that it is but four-fifths nitrogen. If the 

 analysis says "equivalent to nitrate of soda," 

 remember that but 15 per cent, of nitrate of soda 

 is the plant food nitrogen. 



2. Look for the percentage of potash. If the 

 tag says, "equivalent to sulphate of potash," or 

 "equivalent to muriate of potash," remember that 

 but half of sulphate or muriate of potash is the 

 plant-food potash. 



3. Look for water-soluble phosphoric acid or 

 available phosphoric acid. Insoluble phosphoric 

 acid has half value; reverted phosphoric acid is 

 slightly less valuable than soluble. 



