THE study of the physical properties of soil has fundamental place in the application of science to agriculture. It occupied an important position in the early days of agricultural science, and, after a lengthy eclipse in the latter half of the nineteenth century, when Liebig, Lawes and Gilbert, and others were establishing the modern agricultural chemistry and biology, it again came into prominence, owing to the recognition of the colloidal properties of the soil. The older concepts have been examined from this point of view, and it appears that the soil must be regarded not as a mass of comparatively inert grains over which water is distributed in a thin film, but as particles the surface of which is coated with colloidal material. The composition of this material is complex. It is a mixture of organic and inorganic substances derived from the decomposition of organic matter, and the weathering of clay, respectively, and it modifies very largely the deductions on the relations between soil and its moisture content made from the older hypothesis.