CONDITIONS OF NITEOGEN-FIXATION 177 



8. The earth should be aerated, as is shown by the following table, 

 which gives the results of experiments carried out to show the effect of 

 aeration and non-aeration at different depths in the soil. 



Layer Aerated. Non-aerated. 



Nit. present (%). Nit. present (%). 



i. 1-20 cms. 0-132 0-113 



ii. 20-40 cms. 0-109 0-074 



iii. 40-60 cms. 0-076 0-059 



iv. 60-89 cms. 0069 0-046 



The foregoing description shows that much has been done in this 

 branch, and systematic nitrogen-farming of the atmosphere is almost 

 within our reach. What is still lacking is an exact knowledge of all 

 the factors which are detrimental, and all which are beneficial, to the 

 growth of nitrogen-bacteria. We also require to know more of their 

 relations to other organisms in the soil, for the struggle for existence is 

 as keen here as in other places. It is probable that in a few years all 

 these difficulties will disappear, for most of the important points 

 bearing on the subject have already been mastered. 



§4. NITROGEN-BACTEEIA ACTING IN CONJUNCTION 

 WITH LEGUMINOUS PLANTS. 



There is another group of organisms of this class which have a most 

 peculiar mode of life. They act in conjunction with plants belonging 

 to the Leguminosae (Pea and Bean family). From very early times it 

 was known that plants belonging to this family were able to thrive in 

 soil which contained little or no available nitrogen, and that the soil 

 became, in consequence of their growth, richer in nitrogen than it was 

 before. A farmer does not, for example, sow wheat for two successive 

 years in the same field. He follows wheat with clover, or some other 

 leguminous plant, in order to make up for the deficiency of nitrogen 

 in the soil which has been caused by cropping wheat. This is the 

 principle upon which the Rotation of Crops is based. If the Legu- 

 minosae are thus so different from other plants, there must be some 

 peculiarity in their nature to account for this remarkable difference. 

 It had long been known that the roots of these plants have a swollen 

 or nodular structure. In Lupinus the main root itself is swollen in 

 appearance (Fig. 109), though usually it is the side roots that are 

 nodular. A typical example is seen in Fig. 110, which shows 

 the nodules of Robinia pseudacacia. Unfortunately these structures 



M 



