FIXATION OF FREE NITROGEN. 165 



in progress too short a time to judge how far it will be nitrogen- 

 successful in a series of years, or of rotations. consuming 



There is, of course, nothing new in the fact that after the 

 growth of a leguminous crop, such as red clover, for example, 

 the soil is left in a higher condition for the subsequent growth 

 of a grain crop ; and that, in fact, the growth of such a legu- 

 minous crop is to a great extent equivalent to the application 

 of a nitrogenous manure for the cereal. Indeed, history tells The Ro- 

 us that more than two thousand years ago it was recognised mar }f ^ Me 

 by the Eomans that the occasional growth of plants of the dag. 

 leguminous Order had the effect of increasing the growth of 

 the gramineous crops with which they were alternated ; and 

 it was stated that the effect was equivalent to that of apply- 

 ing manure. Thus Varro says that " certain things are to be 

 sown, not with the hope of any immediate profit being derived 

 from them, but with a view to the following year, because 

 being ploughed in and then left in the ground, they render 

 the soil afterwards more fruitful;" and the plants used for 

 this purpose were lupins, beans, vetches, and other legumes. 



Now, however, that the character of the action is more 

 clearly understood — and it is certain that there is actual gain 

 of nitrogen from sources external to the soil itself — it seems 

 desirable that at any rate tentative trials should be made 

 on different descriptions of soil, with a view of ascertaining 

 whether more advantage cannot be taken of this source of 

 nitrogen than our established practices of rotation at present 

 secure. 



To sum up — the experimental results which have been Summary 

 brought forward clearly establish that there is great gain of °f resuUs - 

 nitrogen under some conditions. It has also been clearly 

 shown that due infection of the soil, and of the plant, is an 

 essential to success. The evidence at the same time points 

 to the conclusion that the soil may be duly infected for the 

 growth of one description or some descriptions of leguminous 

 plant, but not for some other descriptions. The field experi- 

 ments on such plants at Eothamsted have further shown that 

 land which is, so to speak, quite exhausted so far as the 

 growth of one leguminous crop is concerned, may still grow 

 very luxuriant crops of another description of the same 

 Order, but of different habits of growth, and especially of 

 different character and range of roots. This result, though 

 undoubtedly more or less due to other causes also, is, never- 

 theless, in some cases doubtless dependent on the existence, 

 the distribution, and the condition, of the appropriate microbes 

 for the due infection of the different descriptions of plant. 

 In fact, it is pretty certain that success in any system involv- 

 ing a more extended growth of leguminous crops in our 



