In any good system of soil management the farmer has two natural ways of maintaining the nitrogen supply in the soil; (1) by the proper growth and use of inoculated legumes; and (2) by encouraging the development of the non-symbiotic nitrogen fixing microorganisms in the soil. Altho the first method is probably the more important especially in those regions where legumes are commonly grown, undoubtedly many farmers have unconsciously practised the second method. It has been definitely shown that even where legumes are not grown the nitrogen balance in the soil is automatically cared for, to some extent at least, by microorganisms which are capable of fixing large amounts of nitrogen from the air without the aid of a host plant. The exact relation of this process of non-symbiotic nitrogen fixation to soil fertility is an interesting problem of both practical and scientific importance. It has attracted the attention of many soil bacteriologists, but in spite of much accumulated information on the subject, there are many questions still to be answered before it will be known just how much nitrogen is fixed annually per acre of soil by this process
