57 



On the other hand, it is known that when growing on ordinary 

 arable soil, the clover plant throws out a large amount of roots in the 

 lower layers, and although in the case of so rich a surface soil, the 

 plant may derive a larger proportion of its nutriment from that 

 source, we must at the same time suppose that it has also availed 

 itself of the resources of the subsoil. Unfortunately, we did not 

 sample deeper than 9 inches in 1857, so that we can make no com- 

 parison of the condition of the subsoil at the two periods. It may, 

 however, be observed that, in 1879, the second 9 inches showed about 

 three times as high a percentage as the subsoils of the arable fields at 

 the same depth ; indeed, not far from twice as high a percentage as 

 several of the exhausted arable surface soils. It cannot be doubted, 

 therefore, that the subsoil of the garden plot has contributed to the 

 yield of nitrogen in the crop. 



If, then, we have not here absolute proof that the source of the 

 whole of the nitrogen of the clover growing on the garden soil was 

 the soil itself, we have surely very strong grounds for concluding that 

 much, and perhaps the whole of it, has been so derived. 



General Conclusions. 



After this review of the evidence which the determinations of 

 nitrogen in the soils of our experimental plots afford, we end, as we 

 began, by saying that, although we admit the facts of production are 

 not yet conclusively explained, we maintain that there is, to say the 

 least, much more of direct experimental proof of the soil than of the 

 atmospheric source of the nitrogen. Moreover, we submit that this 

 rnay be said, not only c . the source of the nitrogen of the cereals, but 

 also of that of the root-crops, and of the Leguminosse. 



If, on the other hand, the atmosphere is the main, if not the ex- 

 clusive, source of the nitrogen of the Leguminosoe, we would ask here, 

 as we have asked elsewhere — why those leguminous crops which take 

 up the most nitrogen can be less frequently grown on the same soil ? 

 Why we entirely failed to grow clover successively on ordinary arable 

 land, which was nevertheless in a condition to yield fairly good cereal 

 crops ? Why the only condition under which we have been able to 

 grow clover continuously was where the soil was very much richer in 

 nitrogen (and of course in other constituents also) than the arable 

 land ? And lastly, why its growth under such circumstances has been 

 accompanied by a rapid diminution in the amount of nitrogen in the 

 soil, and with this a marked decline in the produce ? 



It will not for a moment be supposed that because in the foregoing 



