41 



or 



is 



of 



,he 



ine 



in the surface soil tban lias been found in any of the other experi- 

 mental fields ; though determinations made in samples from other parts 

 of the same field, and also in an adjoining field, show considei'ably 

 higher results. The nearest approach to so low an amount in any 

 other field is where the land had been under alternate wheat and 

 fallow, without manure, for more than thirty years. 



It will be remembered that the root-crops gave, with mineral 

 manure alone, a very much higher yield of nitrogen than the cereals 

 in the earlier years, and as low a yield in the later years. That they 

 did not give less still is probably owing to the fact that their growth 

 extends later in the season than that of the cereals, by virtue of 

 which they are probably enabled to arrest the nitric acid formed 

 within the soil during the early autumn months, which in the case of 

 the cereals would be more subject to loss by drainage. 



Both the mechanical conditions of surface soil known to be favour- 

 able for the growth of th'^ root-crops, and the large amount of fibrous 

 root they throw out near the surface, are indications of an active 

 demand on the resources of the upper layers of the soil, and are per- 

 fectly consistent with the supposition that their growth has led to a 

 greater reduction in the stores of nitrogen of the superficial layers 

 than in the case of any of the other crops. 



The evidence afforded, both by the facts of production, and by the 

 determinations of nitrogen in the soil, is indeed strongly in favour of 

 the view that the source of the nitrogen of the root-crops, as of the 

 cereals, is, when grown without nitrogenous manure, the soil itself, 

 and the small quantity of combined nitrogen annually contributed by 

 rain, and the minor aqueous deposits from the atmosphere. It is said, 

 however, that these crops require a certain amount of nitrogen to be 

 supplied by manure, and that they are able to take up the remainder 

 from atmospheric sources. The facts of production recorded at page 11 

 afford no countenance to such a view. We conclude, indeed, that the 

 dependence of these crops for their nitrogen, on the stores of the soil 

 itself, or on supplies by manure, is as clearly established as in the case 

 of the cereals. 



Is THE Soil a Source of the Nitrogen of the Leguminos^ ? 



We have now to consider the bearing of the evidence on the 

 question of the sources of the nitrogen of the Leguminosje ; and here 

 we approach not only the most important but the most difficult part 

 of our subject. 





