vi.] SOLVENTS FOR AVAILABLE PLANT FOOD 159 



particular soil which yielded very poor crops unless man- 

 ured with superphosphate, though it contained o-i per 

 cent, of phosphoric acid soluble in strong hydrochloric 

 acid. Hence he concluded that dilute acetic acid 

 forms a solvent only capable of attacking the avail- 

 able phosphoric acid. A solution of carbonic acid has 

 been suggested as akin to the natural soil water; 

 other solutions have been employed because they will 

 dissolve certain of the compounds of phosphoric acid 

 in the soil, but not all — the calcium phosphates, for 

 example, but not the phosphates of iron and alumi- 

 nium ; other solvents, again, are recommended as 

 akin to the acid cell sap. However, experience seems 

 to show that the 1 per cent, solution of citric acid pro- 

 posed by Dyer in 1894 gives results that are most in 

 accord with what is known of the soil, either from its 

 past history or by cropping experiments. 



The method of conducting the analysis is as follows : 

 — 200 grams of the " fine earth " that has passed the 3 

 mm. sieve, in its air-dried state, is placed without any 

 further grinding in a dry Winchester quart bottle with 

 20 grams of pure crystallised citric acid and 2 litres of 

 water. The bottle should either be one previously used 

 for the storage of strong acids or should have a pre- 

 liminary soaking in dilute hydrochloric acid. The 

 mixture of soil and dilute acid is thoroughly shaken from 

 time to time, as often as may be convenient, during the 

 seven days the solvent action is allowed to proceed. 

 After seven days the solution is filtered, and two aliquot 

 portions of 500 c.c. each are evaporated to dryness and 

 ignited to get rid of the citric acid and other organic 

 matter. The residues are dissolved in hydrochloric acid, 

 again evaporated, and heated for a time to I05°C. to 

 render all the silica insoluble. In one portion the 

 phosphoric acid, and in the other the potash, are deter- 



