10 INTRODUCTORY [chap. 



manure. But in addition to establishing^ the vahie of 

 nitrogenous manures, the Rothamstcd experiments also 

 settled in a practical fashion the question of which of 

 the ash constituents were indispensable to the plant 

 and wore necessary constituents of a complete manure. 

 The fundamental necessity of phosphoric acid and 

 potash, and the non-essential nature of soda, magnesia, 

 and silica as manure constituents were soon established, 

 and the experiments began at once to bear fruit in the 

 way the various artificial manures, then being discovered 

 and put on the market, were utilised by farmers. 

 Contemporaneously the methods of pot cultures in weak 

 solutions of known salts were evolved, and in the hands 

 of Boussingault, Knop, Stohmann, and others, demon- 

 strated with all the precision of a laboratory process 

 that of the elements found in the plant, only compounds 

 of nitrogen, phosphoric and sulj;huric a:ids among 

 acids, and potash and lime with a trace of iron among 

 bases, are absolutely essential to the nutrition of the 

 plant. 



There was, however, still one point which remained 

 somewhat unintelligible — the gain in combined nitrogen 

 which seemed to take place when certain crops of the 

 leguminous order were grown. Cases were recorded 

 where more nitrogen was found in the crop than was 

 supplied in the manure, and yet the soil on which the 

 crop had been grown itself showed an increase of 

 nitrogen. 



Boussingault, in his earliest investigations, had shown 

 that in certain rotations which included clover or lucerne 

 more nitrogen was removed in the crop than had been 

 supplied in the manure, and many of the Rothamsted 

 results could only be explained on the assumption that 

 the roots of such crops ranged exceptionally deep and 

 drew upon stores of subsoil nitrogen unavailable for other 



