191 1.] Value of Different Crops as Green Manures. 973 



organism, which is present in Rothamsted as in most culti- 

 vated soils, is capable of effecting considerable fixation of 

 nitrogen ; but in order to do this it must be supplied with 

 organic matter, by the oxidation of which it derives the 

 energy necessary to bring the nitrogen into combination. 

 Although it has been possible in the laboratory to raise the 

 proportion of nitrogen in the soil by merely adding organic 

 matter containing no nitrogen, and thus giving the Azoto- 

 bacter material to work upon, the evidence that this process 

 goes on in the field is still very scanty. Samples of soil, 

 however, taken from this Rothamsted field at the beginning 

 of the experiment, have been preserved, and further analyses 

 after a few more green crops have been ploughed in may be 

 expected to throw more light upon this question. 



If the Rothamsted results, that vetches and crimson clover 

 form good preparations for wheat because of the nitrogen 

 they accumulate, are only in accord with what might have 

 been expected, there still remains the entirely contradictory 

 result at Woburn to explain. Dr. Voelcker has indicated that 

 at Woburn the question is probably one of water supply ; 

 though the vetch crop does contain about twice as much 

 nitrogen as the mustard which is turned in, it seems to 

 leave the land in a drier and more open condition, and this 

 on the light Woburn soil seems more to affect the crop than 

 the extra nitrogen. It would, however, be unsafe to conclude 

 that either the amount of nitrogen brought in by the two 

 crops, or the effects upon the physical conditions of water 

 supply of the soil are the only factors concerned. The pro- 

 cesses of decay which the two materials have to go through 

 before the nitrogen they contain can be available for the wheat 

 crop, are very complex, and are likely to be different in two 

 such contrasting soils as the cool, close Rothamsted land 

 and the drier and warmer Woburn soil, and one is quite 

 ignorant of the possible influence of the intermediate products 

 upon the growing plant. It should be remembered that the 

 opinions of practical men as to whether vetches form a good 

 preparation for wheat are absolutely contradictory in different 

 parts of the country. Some men have found that vetches are 

 always followed by a good crop of wheat, while others hold 

 that the result is invariably poor. It is interesting to find 



