vii.] NITROGEN FIXATION IN VIRGIN SOILS 217 



per annum. The annual withdrawal in the crop would 

 thus be closely balanced by the loss experienced by the 

 soil and the additions, were there not other unknown 

 withdrawals in the weeds which are removed from the 

 plot, and in the nitrates which are washed down into the 

 subsoil and the drains. Doubtless, neither of these two 

 withdrawals are large, but because of their existence, un- 

 balanced by any corresponding falling off in the nitrogen 

 content of the soil, it must be concluded that even on 

 the arable land some small restorative action is going 

 on. A portion, however, of the same field has been 

 covered with a wild vegetation of weeds and grasses for 

 the last twenty-five years, and this is never cut or 

 harvested, so that all the debris fall back on the land 

 *)ust as it would on a virgin soil. Analysis of samples of 

 this soil taken in 1881, when it ceased to be under cultiva- 

 tion, and in 1904, showed an annual accumulation of 

 nitrogen of more than 100 lbs. per acre. The enormous 

 difference in the fixation on this plot as compared with 

 the unmanured plot carrying wheat, must be set down 

 to the difference in the supply of non-nitrogenous carbon 

 compounds to the two plots ; in the one case the wheat 

 is all removed except a small portion of root and stubble ; 

 in the other the whole of the vegetable growth falls back 

 on the land. The wild vegetation on this plot did include 

 a considerable proportion of leguminous plants, but a 

 similar, though smaller accumulation of nitrogen was 

 observed in another plot of land which had been allowed 

 to run wild in the same manner, but which carried no 

 leguminous vegetation in consequence of the small 

 amount of calcium carbonate in the soil. These two 

 plots present a very close parallel to the actions which must 

 have been taking place in all virgin soils where the soil 

 similarly contains the Azotobacler organism. Doubtless 

 also some of the value of laying down land to temporary 



