CHAPTER XIII. 

 THE NITROGEN-BACTERIA. 



§ 1. INTRODUCTION. 



The relation of bacteria to nitrogen is the most important problem 

 wbich presents itself to the agriculturist, the reason being that while 

 nitrogen forms a very large proportion of the constituents necessary to 

 build up a plant, it is present in the soil only in a limited quantity, 

 and consequently, constant cropping soon exhausts the supply. To 

 remedy this defect manuring in some form or other has to be resorted to. 

 To ensure the growth of plants the presence of ten elements is neces- 

 sary : carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, iron, and magnesium. All of these, with the exception of 

 nitrogen, phosphorus, and potassium, are present in abundance in the 

 soil. Of these three, the amount of phosphorus and potassium which 

 is required by plants is small, and an abundant supply is easily 

 obtained. With nitrogen, however, the case is different. In the 

 first place, the higher plants cannot assimilate this element unless it 

 is presented to them in the form of nitrates. When, therefore, 

 constant cropping has produced a dearth of available nitrogen in any 

 given area, that area must be supplied either with nitrates or with 

 some material like guano or dung which can be readily converted into 

 nitrates by the soil-bacteria. But the supplies of manure we at present 

 obtain from the deposits of nitre and guano, which nature has given us, 

 cannot last much longer ; so that some means must before long be 

 adopted to supplement these supplies. Within the last twenty years 

 the efforts of agricultural investigators have been attended with a large 

 measure of success. It has been found that nature has means at her 

 disposal for remedying the defect, and efforts are being made towards 

 helping nature along her own lines. The importance of the fact that 



