CH. iiij Nitrogen Fixation 45 



Azotobacter. These require considerable quantities of 

 decaying plant residues as a source of energy: for the 

 process is not one that will continue by itself once it is 

 started like the burning of a bonfire, but rather it re- 

 sembles hauhng a load up a hill and requires the con- 

 tinuous application of energy. The fixation through the 

 root nodules proceeds vigorously during the growth of 

 clover, trifolium, lucerne, sainfoin, vetches, etc., and 

 these crops therefore enrich the soil considerably. Both 

 processes take place in land laid down to grass. The 

 gain does not go on indefinitely: after a time it is 

 counterbalanced by losses; but the net result is that 

 grass land contains considerably more nitrogen than 

 arable land. The nitrogen comes from the atrnosphere, 

 and thus represents an absolute gain to the stock in the 

 soil. The following simple rule should be remembered 

 by the student : land in sod gains nitrogen, land in fallow 

 gains nitrate. The gain in nitrogen is absolute, but the 

 gain in nitrate is not, it only represents a change of one 

 form of soil nitrogen into another. When the grass land 

 is ploughed up the gain in nitrogen ceases, and the gain 

 in nitrate begins; sufficient may be produced to yield 

 corn crops so heavy as to justify the ploughing up of 

 pasture which is not of first rate grazing quality. 



The whole chain of processes we have been describing 

 is of the greatest possible importance to soil fertility 

 because it consists in the conversion of the undecom- 

 posed plant residues, which are of little value to the soil 

 except to open it up, into valuable humus material and 

 plant food. The process is, in short, a release of stored- 

 up fertility and it has to be encouraged by every means 

 in the cultivator's power. It is mainly brought about by 

 living agencies; earthworms play a preliminary part by 



