Viii.] PHOSPHORIC ACID 221 



subsequent liming on soils containing phosphates of 

 iron or alumina will help to bring them into a more 

 available form, because a double decomposition result- 

 ing in calcium phosphate and aluminium or ferric 

 hydrate, will proceed to an extent dependent on the 

 mass of lime present in the medium. 



Further evidence of the precipitation of phosphoric 

 acid within the soil is afforded by Dyer's examination 

 of the Rothamsted wheat soils at various depths, after 

 fifty years' continuous manuring with and without super- 

 phosphate. By comparing the amount of phosphoric 

 acid contained in the soil of the unmanured plot 

 with that contained in the soils of the plots receiv- 

 ing superphosphate every year, and knowing also the 

 amount removed by the successive crops in each case, 

 it is possible to calculate the surplus that should 

 remain in the manured over the unmanured plots, on 

 the assumption that the soil was uniform at starting. 

 Calculating in this way, Dyer found that no less than 

 83 per cent, of the phosphoric acid which six of the 

 plots should possess after fifty years' manuring was 

 still present in the top 9 inches of soil, whereas the 

 subsoils from 9 inches to 18 inches, and 18 inches to 

 27 inches, showed no accumulation of phosphates. 

 Dyer further determined the phosphoric acid which 

 was soluble in a I per cent, solution of citric acid, 

 and found that on the manured plots the top 9 inches 

 of soil contained about 39 per cent, of the estimated 

 surplus of phosphoric acid so combined as to be 

 soluble in this medium, whereas in the subsoils the 

 "available" phosphoric acid was, if anything, less for 

 the manured than for the unmanured plots. It has 

 already been pointed out (p. 163) that if the extraction 

 with citric acid be repeated, practically the whole of the 

 phosphoric acid applied as manure and not removed in 



