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.

CHAPTER IV

SOIL AS A SOURCE OF PLANT FOOD

Mineral Constituents of the Plant.-There still remains to be considered the mineral matter found in the ash, or that material which remains when the organic part of the plant is destroyed by burning and which corresponds exactly to the ashes left in the stove after burning wood. The substances found in the ash are all derived from the soil. It has not always been thought that they were necessary to plant growth. The earlier writers on agriculture considered only the organic matter of the soil and certain constituents of the atmosphere as of any importance to the plant. These writers thought the presence of mineral matter merely accidental, and due to the fact that the plant took it because it was dissolved in the necessary soil water, and had no way of rejecting, or removing it. Later writers, however, preeminent among whom was Liebig, proved that the ash ingredients are necessary to the plant. A very simple experiment was sufficient to show that at least some of the mineral matter was essential to plant growth. Seeds were planted in quartz-sand in pots, to one of which nitrogen compounds alone were supplied, and to the other, nitrogen and a small amount of plant ash. The plants in the pot which received the ash grew to maturity, while those in the other pot made only a feeble, short lived growth.

Essential and Non-Essential Elements.-The experiment just described proves that there is something in the ash that is required by the plant, but does not show whether only a part or all of the ingredients are

Experiment to show the essential elements of plant food. Numbers 4, 7, and 11 received all the elements of plant food while one element was withheld in each of the other tests.

essential. This question naturally interested a number of investigators, and soon a mass of evidence was at hand. In order to determine which elements are essential, plants were grown, either in specially prepared sand or by the "water-culture method," in such