_Effects of Alkali Carbonates upon Clay._—The carbonate of potash and soda, when in very dilute solution (.01 to .05%) exert upon diffused clay an effect the reverse of the acids and neutral salts. They destroy the flocculent aggregates formed by precipitation with these, or naturally existing in the soil, and tend to puddle the clay so as to render it impervious to water. It is thus that in the alkali lands of the arid regions we often find the soil or subsoil consolidated into a very refractory “hardpan,” difficult to break even with a sledge hammer and impossible to reduce to tilth until the alkali carbonate is destroyed by means of a lime salt, such as gypsum. (See chapt. 23). Ammonia water also helps to cause the diffusion of clay in water, but its effect of course disappears upon drying. It is probable that this property of sodic carbonate can be utilized in rendering earth dams firmer and more secure against the penetration of water. CHAPTER V. THE MINOR MINERALINGREDIENTS OF SOILS; MINERAL FERTILIZERS; MINERALS INJURIOUS TO AGRICULTURE. (A.) MINERALS USED AS FERTILIZERS. Of minerals important in soil-formation, not usually present in large amounts in rocks, but extensively used in fertilization, the following require mention: _Apatite_; phosphate of lime containing more or less of the chlorids and fluorids of the same metal; the mineral from which the phosphoric acid of the soil is mostly derived. In the crystallized condition when perfectly pure it is colorless; but it is mostly of a greenish tint (hence “asparagus stone”). The pure crystalline mineral rarely occurs in large masses (as in Canada); but small to minute crystals are found widely disseminated in many rocks (granites, “basalts” of the Pacific Northwest), thus passing into the soils formed from these rocks. These crystals are readily recognized, being regular six-sided prisms with a flat or obtusely pyramidal termination (distinction from quartz), and do not effervesce with acids (distinction from calcite). By far the largest deposits of this mineral occur in connection with carbonate of lime, in the rock materials known as _phosphorites_. Lime phosphate being, like the carbonate, soluble in carbonated water, the two naturally frequently pass into solution, and are subsequently deposited together. Most limestones contain a small proportion of lime phosphate, being, as already stated, formed from the shells and the framework of animal organisms usually containing also phosphates. But the content of phosphates in limestones is not readily apparent to the eye, and the richest deposits, save such as contain animal bones, have long passed unsuspected as to their being anything else but limestone. Systematic search has now revealed the presence of phosphate rock in numerous localities, chiefly where limestone formations occur. In the United States, in South Carolina, Florida, Alabama, Tennessee, Kentucky,