25 



unless the soil is inoist and has 'free access of air, and some base, gen- 

 erally lime, is present with which the nitric acid can combine. Nitrifi- 

 cation is thus most active in summer and ceases apparently in winter. 



Messrs. Lawes and Gilbert have for some years past been devoting 

 their attention to the sources of the nitrogen of crops, and in the pages 

 of the Journal of the Eoyal Agricultural Society, and of the Jour- 

 nal of the Chemical Society, will be found their reports in full. 



The following is the summary and conclusion which they have just 

 published in a long article on "Some points in the composition of soil, 7 ' 

 in the June number of the Chemical Society for last year, p. 420 : 



(1) The annual yield of nitrogen per acre in various crops, grown for many years 

 in succession on the same land without nitrogenous manure, was found to be very 

 much greater than the amount of combined nitrogen annually coming down in rain 

 and the minor measurable aqueous deposits. 



(2) So far as the evidence at command enables us to judge, other supplies of com- 

 bined nitrogen from the atmosphere, either to the soil or to the plant itself, are quite 

 inadequate to make up the deficiency. 



(3) The experimental evidence as to whether plants assimilate the free nitrogen of 

 the atmosphere is very conflicting ; but the balance is decidedly against the supposi- 

 tion that they so derive any portion of their nitrogen. 



(4) When crops are grown year after year on the same land, for many years in suc- 

 cession without nitrogenous manure, both the amount of produce per acre, and the 

 amount of nitrogen in it, decl.ne in a very marked degree. This is the case even 

 when a full mineral manure is applied; and it is the case not only with cereals and 

 with root crops, but also with Leguminosce. 



(5) Determinations of nitrogen in the soils show that, coincideutally with the de- 

 cline in the annual yield of nitrogen per acre of these very various descriptions of 

 plants, grown without nitrogenous manure, there is also a decline in the stock of ni- 

 trogen in the soil. Thus a soil source, of at any rate some, of the nitrogen of the crops 

 is indicated. Other evidence pointed in the same direction. 



(6) Determinations of the nitrogen as 1 nitric acid, in soils of known history as to 

 manuring and cropping, and to a considerable depth, showed that, the amount of ni- 

 trogen in the soil in that form was much less after the growth of a crop than under 

 corresponding conditions without a crop. This was the case not only with gramineous 

 but with leguminous cropn. It was hence concluded that nitrogen had been taken 

 up as nitric acid by the growing crops. 



(?) In the case of gramineous crop soils, the evidence pointed to the conclusion 

 that most, if not the whole, of the nitrogen of the crops was taken up as nitric acid 

 from the soil. 



(8) In the experiments with leguminous crop soils, it was clear that some at any 

 rate of the nitrogen had been taken up as nitric acid. In some cases, the evidence 

 was in favor of the supposition that the whole of the nitrogen had been so taken up. 

 In others this seemed doubtful, 



(9) Although in the growth of leguminous crops year after year on the same land 

 without nitrogenous manure, the crop, the yield of the nitrogen in it, and the total 

 nitrogen in the surface soil, greatly decline, yet, on the substitution of another plant 

 of the same family, with different root habits and root range, large crops, containing 

 large amounts of nitrogen, may be grown. Further, in the case of the occasional 

 growth of a leguminous crop, red clover for example, after a number of cereal and 

 other crops, manured in the ordinary way, not only may there be a very large amount 

 of nitrogen in the crop, presumably derived from the subsoil, but the surface soil be- 

 comes determiuably richer in nitrogen, due to crop residue. 



(10) It was found that, under otherwise parallel conditions, there was very much 

 more nitrogen as nitric acid, in soils and subsoils down to a depth of 108 inches, where 



