By the weathering of feldspar, mica, and other minerals containing aluminum, kaolin or true clay is formed, which is of the greatest importance in the constitution of the soil. The compounds of aluminum are not so important as plant food as they are as the constituents of the soil, forming a large part of its bulk, and modifying in the most profound degree its physical properties. It is the custom of some authors to use the word clay to designate the fine particles of soil which have in general the same relations to moisture and tilth as the particles of weathered feldspar, etc. In a strict chemical sense, however, the term clay is applied only to the hydrated silicate of alumina formed as indicated above. The fertility of a soil is largely dependent on the quantity of clay which it contains, its relations to moisture and amenability to culture being chiefly conditioned by its clay content. The determination of the percentage of clay in soils is an operation of the highest utility in forming an opinion of the value of a soil on analytical data alone. =16. Calcium= is one of the commonest and most important elements of the earth’s crust, of which it has been estimated to compose about one-sixteenth. It does not occur free in nature, but its most common form is in combination with carbon dioxid, forming the mineral calcite, marble, and the very abundant limestone rocks. In this form it is slightly soluble in water containing carbon dioxid, and hence lime has become a universal component of all soils and is very generally found in natural waters, in which it furnishes the chief ingredient necessary for the formation of the shells and skeletons of the various tribes of mollusca and corals. In combination with sulfuric acid calcium forms the rock gypsum. Lime is not only a necessary plant food, but influences in a marked degree the physical condition of the soil and the progress of nitrification. Many stiff clay soils are rendered porous and pulverulent by an application of lime, and thus made far more productive. On account of its great abundance and low price, it has not commanded the degree of attention from farmers and agricultural chemists which its merits deserve. It forms an essential ingredient of plants and animals, in the latter being collected chiefly in the bones, while in plants it is rather uniformly distributed throughout all the tissues. =17. Magnesium= occurs chiefly in combination with carbon dioxid or with lime and carbon dioxid in the mineral dolomite. It is intimately associated with calcium and a trace of it is nearly always found where lime occurs in any considerable quantity. The bitter taste of sea water and some mineral waters is due to the presence of salts of magnesia. In combination with silica it forms an essential part of such rocks as serpentine, soapstone, and talc. Magnesia is not of much importance as a plant food nor as a fertilizing material.