open, analyzing the soil before the experiment and the soil plus the plant at the end of the growing season, correcting for the nitrogen carried down in the rain. water. He found that while in most cases there was no gain of nitrogen, in some cases there was a decided increase. Those plants which produced a gain in nitrogen invariably belong to the same family as the pea, bean, clover, etc., or in other words to the socalled "legumes" or "leguminous plants." It remained for Hellriegel to explain this phenomenon. He repeated the experiments of Boussingault with this variation that to the soil in some of the pots he added a small quantity of water leached from a natural soil so as to introduce any bacteria that might exist naturally in the earth. He found that in the perfectly sterile soil there was no gain in nitrogen by any of the plants, but that in the pots to which the soil leachings had been added the legumes grew vigorously, while the cereals produced only feeble and short-lived plants. Upon examination of those legumes which made marked growth he found that they all had numbers of small nodules or tubercles on their roots, and these nodules on inspection were found to contain innumerable bacteria.

Further tests have demonstrated that when leguminous plants are grown in soils containing the proper bacteria, they can indirectly make use of free nitrogen, and are practically independent of the nitrogen in the soil. This property is not a function of the legume itself, but of the bacteria that produce the nodules, and in the absence of these organisms the legumes are quite as dependent upon the supply of nitrates as are

the other orders of plants. It may be further said that so long as the leguminous plant can procure in the form of the nitrates all the nitrogen it needs the nodules

Showing the power of clover to obtain nitrogen by means of the bacteria in the root-nodules. Both pots received all the elements of plant food except nitrogen The pot on the right was inoculated with the proper bacteria, while that on the left was not

will not be formed. For that reason, in a soil rich in nitrogen the root tubercles may not be found on the legumes, even when the proper bacteria are present. Yet for all practical purposes it may be taken for granted that clover, peas, beans, alfalfa and other

legumes derive the bulk of their nitrogen from the air, and that in growing them the farmer is not decreasing the nitrogen content of the soil, but may actually be adding thereto.

Inoculation of the Soil.-Experience has shown that all soils do not contain the bacteria necessary to

Root tubercles on soy beans. The left inoculated and the other uninoculated. Tubercles appear only when the proper bacteria are present in the soil

the fixation of free nitrogen by legumes. They may be introduced into a field by sowing with the seed a small quantity of soil from a field in which the legume has been successfully grown. This has been done so often as to leave no doubt of its practicability. Late investigations have shown that the same species of bacteria will not do for all legumes; so that a soil, for instance, may grow clover to perfection, when soy beans or alfalfa will not thrive on it at all. This fact explains many of the disappointments experienced by farmers in the trials of some of the more recently

Effect of inoculation on yield. The plant on the left came from a plot where all the plants had nodules on roots; the other from a plot where practically none of the plants had nodules. The yield was in the ratio of the size of the plants shown in the illustration

introduced leguminous crops. While inoculation of the soil is of undoubted use in some cases, there is danger of overestimating its value. It must not be regarded as a panacea for all the ills of the soil. Inoculating a soil simply introduces the nodule forming bacteria, and if the failure of the leguminous crop was due alone to absence of these bacteria the results will be beneficial. It will in no wise overcome failure due to bad seed, improper preparation of the ground, adverse weather conditions, acidity of the soil, etc., and the farmer should assure himself that the soil conditions are as favorable as possible before he attempts inoculation.

Other Ways in which Nitrogen is Fixed.-Within the last few years a number of bacteria have been discovered in the soil which have the power, when cultivated in the laboratory, of using free nitrogen, and which do not grow in connection with the higher plants. These bacteria are found in most soils, and may be an important factor in maintaining the supply of nitrogen in the soil. At the present time it is impossible to say whether the nitrogen added to the soil in this way is of any considerable moment.