_Reduction of Sulfates._—Should such a soil contain any considerable amount of sulfates, especially in the form of gypsum or calcic (or magnesic) sulfate, the reductive process results in the formation of iron pyrite (ferric sulfid, chap. 5, p. 75); while at the same time the soil is often sufficiently impregnated with sulfuretted hydrogen as to be readily perceived by the odor, or by the blackening of a silver coin. This is very commonly the case in sea-coast marshes, where a hole made with a stick thrust into the mud will be found to give forth both carburetted and sulfuretted hydrogen, while a careful washing of the soil will reveal the presence of minute crystals of iron pyrite. Hence the need of prolonged aeration of marsh soils, effecting the peroxidation of the ferrous compounds, and the conversion of the pyrite first into ferrous sulfate, and subsequently into innocuous, yellow, insoluble ferric oxy-sulfate. _Ferruginous Lands._—The injurious effect of the swamping of ferruginous lands has been especially conspicuous in some of the irrigated rolling lands of the Sierra Foothills of California, where orchards planted in relatively low ground and in full bearing have succumbed to the poisonous effects of the ferrous carbonate formed in the subsoil, long before the water had risen so high that, had the trees been grown afterwards, they would have adapted their root system to the existing conditions and fared moderately well at least. Underdrainage of the lower lands is, of course, the only possible remedy for this state of things, although even then the root-penetration is much more restricted, and therefore natural fertility of much shorter duration, than would have been the case without the rise of the irrigation water. It is thus clear that in the practice of irrigation, the liability of injury to the lower ground by “swamping” through the rise of the ground water should always be kept in view; that, in fact, irrigation and provision for drainage should always go hand in hand. The legal provisions facilitating the rights-of-way for irrigation ditches should be made equally cogent with respect to drainage. CHAPTER XIII. WATER OF SOILS (_Continued_). THE REGULATION AND CONSERVATION OF SOIL MOISTURE. In view of the commanding importance of an adequate supply of water to vegetation, the possible and available means of assuring such supply by utilizing to the best advantage both rainfall and irrigation water, require the closest consideration.