singault grew plants in sterile soil free from nitrogen, the plants being so protected that they came in contact with no nitrogen save that of the air. The plants grew for a short time only, and upon analysis showed that they contained no more nitrogen than was present in the seed. Similar experiments conducted by Ville gave contrary results. To decide the matter, a great number of painstaking experiments were carried out at Rothamsted, England, all of which confirmed the results obtained by Boussingault, and the question was considered settled by most experimenters. About the same time field tests were conducted at Rothamsted which indicated that when clover and other leguminous plants were grown, there was an actual gain of nitrogen in the soil, in addition to that removed by the vegetation, while the growth of cereals resulted in a loss of nitrogen. Other experimenters also arrived at the conclusion that clover has the power of procuring nitrogen from some unknown source. Farmers had known for some time that wheat grown after clover does as well as when manured with a nitrogenous fertilizer. Some writers tried to explain this fact by assuming that the clover roots bring up the nitrogen from the deep subsoil and leave it near the surface, but the explanation was never satisfactory. The conditions under which the pot tests were conducted were not normal, as the plants were grown in prepared soils that had been heated to kill any bacteria they might contain. It occurred to Atwater that plants grown under natural conditions might use free nitrogen. even though they did not under the conditions of these experiments. He, therefore, grew plants in pots in the 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 |