Soils and fertilizers . — The arrange-ment of the soil particles determines to a considerable degreethe amount of free or pore space within the soil, especially inloams and clays. Merely for the purpose of illustrating thislet us suppose that the soil particles are perfect spheres ofequal size, which, of course, they are not. There would be twoarrangements possible, if each sphere were independent ofevery other: (1) in columnar order, in which each particleis touched on four places by its neighbors; (2) obliqueorder, in which each particle is in contact with six of its 38 SOILS AND FERTILIZERS
Soils and fertilizers . — The arrange-ment of the soil particles determines to a considerable degreethe amount of free or pore space within the soil, especially inloams and clays. Merely for the purpose of illustrating thislet us suppose that the soil particles are perfect spheres ofequal size, which, of course, they are not. There would be twoarrangements possible, if each sphere were independent ofevery other: (1) in columnar order, in which each particleis touched on four places by its neighbors; (2) obliqueorder, in which each particle is in contact with six of its 38 SOILS AND FERTILIZERS neighbors. The calculated pore space in the first arrange-ment is percent. That in the second case is (See Fig. 5.) It is not actually the case, however, that soil particlesare of the same size in any natural soU. Consequentlysmall particles fit in between large ones, thus decreasinggreatly the actual pore space. These three cases, of whichonly the last may occur in nature, illustrate pore space. Fig. 5. — If all soil particles were spheres they could be arranged asshown above, in which case the pore space would vary in volume as ex-plained in the text. when the separate grain structure obtains, as in a dry sandor a puddled loam or clay. The granular structure is the one most likely to be foundin nature, although all of the particles may not be in gran-ules. The granules being of irregular form, with manyangles, there is likely to be a large amount of space betweenthem. It would be possible under this arrangement for asoil to have a pore space of 72 percent. The weight of a given volume of soil, including the porespace, as compared with an equal volume of water is termedthe apparent specific gravity. This it will be seen is not thesame as the absolute specific gravity because the amount ofpore space is the important factor in determining the ap-parent specific gravity. Neither do the terms light soiland heavy soil bear any definite relation to the appar
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