It will be seen that there are about 24,000 square feet of surface area in a cubic foot of the subsoil of the pine barrens, no less then 100,000 square feet or two and three-tenths acres of surface area in a cubic foot of the subsoil of the river terrace, and 200,000 square feet of surface area in a cubic foot of the limestone subsoil. These figures seem vast, but they are probably below rather than above the true values, on account of the wide range of the diameters of the clay group. This great extent of surface and of surface attraction, which has been described as potential, gives the soil great power to absorb moisture from the air, and to absorb and hold back mineral matters from solution. A smooth surface of glass will attract and hold, by this surface attraction, an appreciable amount of moisture from the surrounding air. A cubic foot of soil, having 100,000 square feet of surface, should be able to attract and hold a considerably larger amount of moisture. It might have been added that if the potential of the surface, separating the solution from the soil, be greater than the potential in the interior of the liquid mass, there will be a tendency to concentrate the liquid on this surface of separation. It has been shown that certain fluids have greater density on a surface separating the fluid from a solid. On the other hand, if the potential were low there might be no tendency for this concentration, and even the reverse conditions would prevail and the soluble substance could be readily washed out of the soil. =130. Removal of Organic Matters.=—It is probably largely due to this straining power that organic matters are removed from solutions in percolating through the soil. Whitney[95] has observed that the organic matter may be coagulated and precipitated from solution by the soil constituents, and held in the soil in loose flocculent masses, while the liquid passes through nearly free of organic matter. =131. Importance of Soil Absorption.=—The importance of the absorptive power of the soil can hardly be overestimated. By means of this power those mineral ingredients of plant food, of which most soils contain but little, are held too closely to allow of rapid loss by drainage, and still sufficiently available to answer the needs of vegetation, provided the store is large enough. The only important plant food liable to be deficient in the soil which does not come under the influence of absorption is nitrogen in the form of salts of nitric acid, and nature has made a wide provision for this element by binding it in the form of organic bodies which nitrify but slowly, and by supplying each year a small quantity from the atmosphere.