Conclusions.
In general agriculture in Illinois, whether it be grain farming or ordinary livestock farming, the growing of legumes is absolutely essential as a part of any economic system which shall maintain the fertility of the soil; and for the successful growing of legumes the presence and assistance of the proper species of nitrogen-gathering bacteria are also absolutely essential. These facts being granted, it certainly follows that when sowing any legume on land, where the same legume has never been grown before, or perhaps where it has not been successfully grown within recent years, we should always consider the matter of inoculation; and, unless there is good reason to believe that the soil has been inoculated by the washing from other higher lying land where these bacteria are known to be present or by applications of manure made from that legume, or by some other such incidental means; or unless there is evidence that the bacteria are carried with the seed in sufficient quantity to effect a satisfactory inoculation (as appears to be the case with the cowpea), then we should inoculate the soil directly with the specific bacteria required by the legume which we desire to grow.
While some Illinois soils are becoming deficient in phosphorus and in lime, especially in the southern part of the state, and while phosphorus[8] and ground limestone can be applied to such soils with marked benefit and profit, especially for the growing of legumes, there is abundant evidence that one of the dominant causes for the failure or unsatisfactory growth of some of our most valuable legumes, and on some soils the sole cause of failure, is the absence of the proper nitrogen-gathering bacteria.
There is no reason to believe that any of the different species of nitrogen-gathering bacteria will live in the soil for more than a few years[9] in the entire absence of any legume upon which they naturally live, and the accumulating evidence strongly indicates that the bacteria which are present in places in our soils, such as the red clover bacteria, now found abundantly in many places in the state, especially in northern and central Illinois, the cowpea bacteria more common in southern Illinois, and the alfalfa or sweet clover bacteria, which are becoming prevalent in some sections—that all these have been, and are being, gradually introduced and extended almost entirely by mere chance. Of course if the wagon-wheel, which carries the mud along the road, carries with it sweet clover seed from one place to another, it may also carry the sweet clover bacteria which live on the sweet clover roots.
It now seems absurd to suppose that there were red clover bacteria in Illinois soil before red clover itself was grown on Illinois soil, unless the same bacteria live also upon some other legume which was native to our soils. There is some evidence that the vetch bacteria are native to our soil, possibly living upon the native wild vetches. At any rate, tubercles commonly develop on vetch roots without artificial inoculation. Investigations are in progress to ascertain whether the notorious failure of crimson clover in Illinois may not be due in part, at least, to the absence of the proper bacteria. (It has been stated by some writers that the bacteria of crimson clover and those of red clover are identical, but we already have some reason to doubt the accuracy of this statement.)