nitrogen was a matter of controversy for a long time afterwards. The experiments of some investigators showed that with sterilized soil and with all sources of combined atmospheric nitrogen cut off, the free nitrogen takes no part in the food supply of the plant. Other investigators arrived at just the opposite conclusion. These opposite views led to a great deal of discussion, and it was not until 1888 that Hellrigel was able to account for these conflicting results by growing leguminous plants in nitrogen-free soils. One set of plants was watered with distilled water, while to the other set was added in addition small amounts of leachings, containing only a trace of nitrogen, from a cultivated field. The plants watered with distilled water made but a small growth and soon died of nitrogen starvation, but those watered with the leachings reached a full growth and were found to contain about one hundred times more nitrogen than the seed sown. It was observed that the roots of the latter plants were covered with swellings, or nodules, which contained characteristic organisms while those which were watered with distilled water only had none. Furthermore, no nodules appeared and the plants did not develop when the soil leachings were sterilized before using. The experiments thus showed that the plants which were provided with nodules must have obtained nitrogen through the agency of the microorganisms; that these must have come from the soil leachings; and that they must have the property of fixing the nitrogen of the air. For some unknown reason these bacteria, to which the name Bacillus radicicola has been given, do not develop on the roots of non-leguminous plants; consequently, when plants of this kind are grown in the soil and harvested the total quantity of nitrogen present gradually becomes less. The advantage of rotating leguminous plants with crops of this kind thus becomes clear, because when a crop of the former is grown or plowed under as green manure the total nitrogen in the soil is increased. As the plants decay a part of the protein nitrogen of the plant again passes into the elementary state, part changes into ammonia, and a third part changes into nitrates. These changes are brought about by different bacteria, those responsible for the formation of nitrates being called nitrifying bacteria.
The amount of protein nitrogen which is converted into nitrates in the soil by these bacteria varies with conditions and depends on the physical condition of the soil, the quantity of organic matter present, the moisture content and the temperature. A basic element as potash, soda or lime must also be present with which the nitric acid formed may unite. On a limited scale these conditons may be so controlled that large quantities of protein nitrogen may be converted into nitrates, as is still done in India for the production of potassium nitrate to be used in the manufacture of gun-powder. The action of the nitrifying bacteria in thus leading to the formation of nitrates does not bring about any in-
