plant-food in the rich soils of the arid regions, is fully as essential in order to maintain the humus supply. (B.) MINERALS UNESSENTIAL OR INJURIOUS TO SOILS. The minerals heretofore mentioned contribute to soil formation either one or several ingredients, important to plant growth either by their mechanical or chemical action. It remains to consider some not intrinsically desirable, but frequently present in certain soils, which should be known to the farmer in order that he may be enabled to counteract or remove their injurious effects. Leaving aside such as are of only casual or rare occurrence, the following may be mentioned as among those which not unfrequently affect soils desirable for culture to such extent as to make them unavailable for general farming purposes: _Iron Pyrite_; sulphid of iron containing two molecules of sulphur to one of iron; a mineral exceedingly common in deposits of metallic ores, and whose deceptive gold-like color has caused it to be mistaken for gold so often as to cause it to be designated as “fool’s gold” among miners. While it frequently does contain some gold and is often associated with valuable ores, it is practically valueless when occurring outside of mineral veins, in rock-masses; and more especially in sedimentary rocks, such as sandstones, limestones, shales and clays. When present in soils it sometimes becomes a source of trouble to the farmer, because in contact with air it is soon transformed into ferrous sulfate or copperas, which, like the carbonate referred to above, is injurious to plants. Sometimes indeed iron pyrite is actually _formed_ in badly-drained soils alongside of the carbonate of iron, when much sulfate (such as gypsum) is present; and then its injurious effects subside more slowly than do those of the carbonate (see above, p. 46). _Recognition of Iron pyrite._—The mineral is easily recognized by its golden or brass-yellow tint; the latter color being the one most commonly shown in the “sulphur balls” occurring in marls or soft limestones. A very easy test is to pulverize it and then heat it on a shovel over a fire, when it will soon itself take fire, burning with a blue sulphur flame, and upon more complete roasting, leaving behind a red powder, viz., “Venetian red” or red ochre; that is, ferric oxid. In clays it commonly occurs in large, well-defined cubes, which do not readily form copperas but rather become covered with a crust of limonite or brown iron ore.