It will help us to understand this matter if we bear in mind that the home of these bacteria is the tubercle upon the clover root. It is quite evident that they will continue to live upon the decaying tubercles or roots for three or four years after the clover plant has been killed. On the other hand, we have some notable evidence that the bacteria do not continue to live in a soil after five or six years’ continuous cropping with absolutely no clover growing on the land during those years. It is a simple matter for any one to determine whether the bacteria are present or not, for the tubercles which are formed if the bacteria are present are plainly seen attached to small roots. They look somewhat like miniature potatoes, varying in size from pinheads on clover to peas on soy beans or cowpeas. (See Plates 2 and 4.) It is important to remember that the bacteria live in the soil and not in the seed.
When clover is cut for seed, it is frequently left to lie upon the ground until the straw becomes half rotten and very dirty; and, consequently when it is threshed, it practically always happens that there is at least some small amount of dust and dirt taken with the seed. This dirt is almost sure to carry with it some bacteria from the soil. If these few bacteria are scattered with the clover seed when it is sowed they will inoculate at least a few plants, and if they are allowed to multiply on these plants, and especially if the same field is repeatedly seeded with clover, the soil will ultimately become thoroughly infected with the clover bacteria. Of course they may be carried from one part of the farm to another, or even from one farm to another, by various agencies, as dust or wind storms, surface drainage or flood waters, manure made from clover hay, implements used in cultivating the soil, etc., etc.
Many of the older farmers of Illinois have stated to the writer that when this country was very new it was commonly found difficult to get a “catch” of clover on new land. After a good “catch” was once gotten, then it was easier to get clover to grow on that land the next time. There was a saying among the farmers that clover would not do well until they got the “wild nature” out of the land. Their final success was undoubtedly due, not to getting anything out of the land, but rather to getting the bacteria into the land. Several Illinois farmers have reported some quite remarkable results from very light applications of the clover chaff or straw (obtained in hulling clover) in its beneficial effect on clover on land where it was otherwise difficult to get a “catch.” There is a somewhat general belief among farmers of long experience that clover straw or chaff has some special value in getting a catch of clover aside from its value as manure or for the seed which it sometimes contains.
Manager F. A. Warner of the Sibley Estate, Ford County, recently stated to the writer that they had had very great difficulty to get clover to grow when they first began growing clover on that large estate, some six or eight years ago, although, after a good crop was once secured, they rarely had any further difficulty in getting a catch of clover on the same land.
On the common gray prairie soil of the Lower Illinoisan Glaciation, in southern Illinois, the commonest type of soil in more than twenty counties, practically no red clover is grown. In the spring of 1903 we seeded red clover on that type of soil in three places; namely, on the University of Illinois soil experiment fields near Edgewood, Effingham County, near Du Bois, Washington County, and near Cutler, Perry County. On certain plots the soil acidity had been corrected with lime and an abundant supply of phosphorus (in bone meal) had been provided, potassium also having been added on some plots. These fields were carefully examined the latter part of June, and at Cutler and Du Bois the clover was found to be dead or dying, and no tubercles could be found upon the clover roots, although on the clover which had been seeded at about the same time on the University fields at Urbana the root tubercles were found in great abundance. At Edgewood a few tubercles were found and the clover appeared to be growing fairly well. Infected red clover soil was at once procured and scattered over the fields at Edgewood, Du Bois, and Cutler, but it was evidently too late to be of any marked benefit. At Cutler and at Du Bois the clover was a complete failure. (It will be tried again next year.) At Edgewood it continued to grow fairly well, and its progress next season (1904) will be watched with much interest. It should be stated that the Experiment Station has been growing clover for several years with varying degrees of failure on land adjoining the present clover field at Edgewood, and it is possible that this year’s apparent success from the start is due in part at least to the bacteria which have been incidentally introduced and multiplied year after year and scattered over the adjoining land by wind and dust storms. Before the close of the season the tubercles developed in abundance on the roots of the clover at Edgewood.
An experience reported by Professor Herbert W. Mumford, of the Animal Husbandry Department of this university, will be of interest and value in this connection. Professor Mumford commonly grows clover in his rotations on his own private farm, but he states that at one time one particular field was cropped continuously with timothy, oats, and corn for some six years or more without any clover whatever. It was then again seeded to clover, but the crop made a complete failure, although on other land where clover had been grown more recently a successful clover crop was grown from the same kind of seed seeded at about the same time. The following year this particular field was again seeded to clover. This time the “catch” was not a total failure, but it was too poor to save, and it was plowed up and the land again seeded to clover the next year, and an excellent catch of clover resulted. After this, clover was frequently grown on this field, and no special difficulty was had in getting good crops.
While the failure of clover may often be due to drouth, and in some places due to soil acidity (lack of lime), and sometimes even due to an insufficient supply of available phosphorus or of potassium, we now know with certainty that it sometimes fails because of the absence of the nitrogen-gathering bacteria, especially on land which has never grown clover, and probably also on land which has not grown it recently. We should always remember that the bacteria do not thrive in strongly acid soils. Even though they may sometimes live in such soils and perhaps produce some tubercles upon the roots of certain hardy, strong growing legumes, like cowpeas, nevertheless we are obtaining some strong evidence that in such acid soils they have but little power to gather nitrogen from the air. That ground limestone is the most economical and satisfactory material to use in correcting the acidity of soils is strongly indicated by the information we have thus far obtained. On the upland prairie soils of the Lower Illinoisan Glaciation where red clover has never been grown successfully, largely because of the acidity of the soil, it will undoubtedly be helpful and profitable not only to correct the acidity of the soil with ground limestone, but also to secure infected soil from some field of timber land or bottom land where red clover is growing, well provided with root tubercles, and inoculate the field with it. This soil should be collected to a depth of three or four inches and scattered over the prairie land at the rate of a few hundred pounds per acre at the time the clover is seeded or before.