plant growth. It is, in fact, an immense laboratory in which millions of tiny workmen are bringing about marvelous chemical changes. The principal factors concerned in these transformations are bacteria, of which, it is estimated, there are present in the neighborhood of one hundred fifty millions in each ounce of surface soil. Some of these bacteria cause the fermentations and decay that return the carbonic acid to the air. Others, and these are of particular interest here, bring about the decomposition of the nitrogenous organic matter with the ultimate production of nitrates.

The transformation of organic nitrogen into nitrates undoubtedly results from the action of more than one species of bacteria, and takes place in three or more different steps. The organisms necessary to produce these changes are ordinarily present in all soils. Nitrification takes place only when the temperature is more than 5° above freezing, and becomes more rapid with rise of temperature. Hence, it ceases during the winter months, and is most vigorous during the hot months of midsummer. The nitrifying bacteria can not live without a sufficient supply of oxygen, and, for this reason, stirring up the soil, and thus introducing air, increases the rate of nitrification. Nor can these bacteria thrive in a soil that is acid, so that the presence of carbonate of lime, or some other substance that will neutralize any acid produced in the soil, is essential to nitrification. All of these points will be discussed. in greater detail later; for the present it is sufficient to emphasize the importance of the process of nitrification to the growing crop. So vital indeed, is the subject that successful agriculture may be said to de

pend largely upon providing proper conditions for rapid nitrification.

Denitrification.-While the nitrifying bacteria may be said to be the farmer's friends, there are, unfortunately, in the soil other organisms which produce evil results. One class of these, known as denitrifying bacteria, decompose the nitrates, and perhaps some other nitrogenous compounds, with the final result that the nitrogen is set free and returned to the air in its elemental condition. This process of course, robs the soil of a part of its nitrogen, and is especially unfortunate because it removes the part that was most readily available to the crop. The conditions that are detrimental to nitrification (i. e. lack of oxygen, presence of acidity, etc.) are those that favor denitrification, so that the farmer in producing proper conditions for the former desirable process is at the same time preventing the injurious denitrification.

Can Plants Use Free Nitrogen of the Air?—About four-fifths of the volume of the air consists of the element nitrogen, so that if this were generally available to plants there could be no such thing as "nitrogen starvation." Perhaps no question in the realm of agricultural chemistry, or plant physiology, has received. so much attention as the relation of the plant to the nitrogen of the atmosphere; but many points still remain to be investigated. The question heading this paragraph can best be answered by a very brief historical review of the subject. At one time it was generally believed that the air was the sole source of the nitrogen supply for the plant. The first important experiments that indicated the contrary were those in which Bous

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