[168] In this designation are included, in this volume, both the normal (mono-) carbonate and the two other compounds, the bi- or hydrocarbonate and the intermediate (so-called sesqui-) compound or trona; all of which are commonly present simultaneously, but in utterly indefinite relative proportions, varying from day to day and from inch to inch of depth, inasmuch as their continued existence depends upon the greater or less formation of carbonic acid in the soil, and the access of air. Hence their separate quantitative determination at any one time is of little practical interest. All naturally occurring carbonate of soda contains, and sometimes consists of, these “super-carbonates,” according to the greater or less exposure to air and solar heat. They are much milder in their action on plants than the monocarbonate, which unfortunately, in the nature of the case, always predominates near the surface, and thus injures the root-crown.
[169] A wholly different kind of “black alkali” exists in some regions, especially in the delta lands of the Colorado of the West and in the Pecos and Rio Grande country in New Mexico. In these cases the dark tint is due, not to a humic solution, but simply to moisture, which is tenaciously retained by the chlorids of calcium and magnesium impregnating the land, thus contrasting strongly with the gray tint of the general dry soil.
[170] Report of the California Exp’t. St’n. for 1892-94, p. 141.
[171] Proc. Am. Soc. Agr. Sci., 1888; ibid., 1890; Rep. Cal. Expt. Sta., 1890, p. 100; Ber. Berlin, Chem. Ges., 1893; Am. Jour. Sci., August 1896.
[172] Farmer’s Bull. No. 88, U. S. Dept. Agr., 1899.
[173] For a general statement and discussion of the physiological effects of saline solutions on plants, see chapter 26.
[175] Bull. 133, Cal. Expt. Sta., by R. H. Loughridge.
[176] Bulletins Nos. 128, 133 and 140, Calif. Expt. Station.
[177] The several columns of figures are independent of each other; the “total” alkali is not the summation for the three salts in the same line.