available to plants. While the amounts of these materials removed by the crop seem insignificant when compared with the total plant food in the soil, they may be very large in comparison with the available part. The unavailable, or "potential," plant food is gradually being made available, but not with sufficient rapidity to replace that removed from the field at harvest. It will thus be seen that the present fertility of the soil depends not upon the potential plant food it contains, Diagram illustrating the formation of a soil on a limestone hill but upon that which is immediately available to the plant, and that the yield will be limited by the element of this available plant food present in least quantity. Continuous cropping of the soil with the removal of everything from the field results in the exhaustion of the plant food which has been rendered available during the past ages. It will be interesting to study the origin of the plant food, and the manner in which it became available to the plants. CHAPTER V ORIGIN OF THE SOIL The Primary Soils. All soils are derived primarily from the igneous, or original rocks, of which the granites and trap are good examples. Geology teaches that the earth was once a molten mass, and that upon cooling it solidified into rocks, of which those mentioned are types. These rocks must have contained all of the mineral or ash elements of plant food, as no other source of them is conceivable. This plant food, however, was present in insoluble compounds, and in this form was not available to plants. The conversion of this potential plant food into available forms was brought about by a number of agencies. Fortunately these changes can be studied at first hand in the lava beds resulting from volcanic eruptions. These beds have been transformed in an incredibly short time from The freezing of the water in the rock crevices helps to break the rocks into small particles beds of solid rock into more or less fertile soils, by a series of changes much like those to be described. The Rocks Must be Pulverized.-Evidently the first step toward the conversion of the solid rock into soil must have been the act of pulverization. A number of natural agencies have taken part in the grinding of the original rock into the small particles in which they are found in the ground. The rocks have been disintegrated through the influence of heat and cold, freezing and thawing, and by the action of air, water and ice. Such rocks as the granites, for example, can easily be seen to consist of several different minerals. These substances are differently affected by heat and cold, expanding and contracting at different rates, and for this reason the ef fect of changes in temperature is to separate the rock into its component parts. All rocks are more or less porous, and consequently, absorb water, and the expansion of this water when frozen tends to break the mass into fragments. Perhaps more important in this. grinding process than either of these factors is the action of running water and moving ice in the form of glaciers. There is no need to discuss these forces in detail, for it will be sufficient for the present purpose if it be kept in mind that all of these influences combine to disintegrate and grind the surface rocks into smaller and smaller fragments, until they are reduced to the finest particles found in what is called the soil. Microphotograph of a section of granite magnified 30 diameters. (By courtesy Geological Department, Columbia University) Plant Food Must Be Made Soluble.-A soil produced by mere pulverization of the rocks would not furnish proper food for the higher plants, as one can readily imagine if he thinks how unsuitable pulverized granite would be for plant production. The essential elements locked up in these insoluble compounds must be transformed into materials that the plant can assimilate, and water is an important factor in bringing about these chemical changes. Pure water has very little Running water is constantly changing the face of the earth, cutting out ravines and grinding the rock to powder solvent effect upon the minerals of which the igneous rocks are composed. The water that enters the ground has dissolved in it small amounts of carbonic acid gas derived from the air, and water containing this gas will dissolve these minerals in appreciable quantities. A Fertile Soil must contain Nitrogen.-All the processes enumerated unite in transforming the mineral |