Page:Popular Science Monthly Volume 62.djvu/143

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THE soil may truly be regarded as a vast laboratory. The many processes normally taking place in cultivated soils lead to the gradual formation of plant-food, to the solution of the mineral constituents, to the breaking down of the organic molecules into simpler forms, such that are in a condition to furnish the chlorophyl-bearing plants the material for the building up of plant tissue. The cycle of transformation from the simple to the complex and the falling apart of these complex molecules involve the activity of higher plant life, on the one hand, and that of lower organisms, on the other. Primarily it is the energy derived from the sun that, with the cooperation of the living protoplasm, impels the atoms to enter one or another of the innumerable combinations. These atoms are, as Carlyle would put it, 'but the garment of the spirit,' and the atom of carbon or nitrogen, which to-day is in the leaf of the oak or in the brain-cell of man, may on the next day become a structural part of some bacterial spore that is scarcely visible even with magnification of 1,500 or 2,000 diameters. The different kinds of atoms whose presence is essential in order that living tissue may arise, are not many. Among the less than one dozen of these, it is the migration and transmigration of the nitrogen atoms that undoubtedly form the most interesting, as well as the most important, phase of agricultural research. Of all the elements that enter into the composition of vegetable and animal substances, nitrogen is the most expensive, the most evasive, the most difficult to replace. And every person who at all concerns himself with questions as to the origin and the development of the various forms of life, can not be indifferent as to the source of nitrogen in the soil, and the factors that in one way or another affect the store of nitrogen at the disposal of the living world. Whence is the soil nitrogen derived? What conditions are most favorable for the maintenance of an adequate supply of this precious material? What means have we at our disposal for replacing the losses that occur, that in the nature of things must occur?

We should remember of all things, that the great aerial ocean, containing as it does more than 78 per cent, by volume of gaseous nitrogen, does not directly offer that element to the plant world. In order that this nitrogen may become available, it must be combined with other