258 POWER OF THE SOIL TO ABSORB SALTS [chap. 



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 1 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. 185) that if the extraction 

 with citric acid be repeated, practically the whole of the 

 phosphoric acid applied as manure and not removed in 

 the crop can be recovered from the top 9 inches of these 

 Rothamsted soils. It is clear, then, that soils well 

 provided with calcium carbonate, as the Rothamsted soil 

 is, will precipitate very near the surface any soluble 

 phosphoric acid applied, and retain it for a long time in 

 a form easily redissolved and obtainable by the plant. 

 It follows, therefore, that superphosphate, the most 

 soluble of the phosphatic manures, can be applied to 

 normal soils in the winter or early spring without any 

 fear of the phosphoric acid being washed out. 



The Composition of Drainage Waters. 



Further evidence of the fate of the various substances 

 applied as manures, their retention or otherwise by the 

 soil, can be obtained by studying the composition of the 

 water flowing from land drains. 



