Page:Popular Science Monthly Volume 48.djvu/684

612 below, which show the facts more plainly than would figures; the vertical line to the left representing the soil depth (in feet and inches), while on the horizontal lines the percentages of salts in the soil are entered at intervals representing two, or four one-hundredths of one per cent each. The area within the curve generated by connecting the points of actual observation represents, of course, the total of each ingredient indicated.

Diagram No. 1 represents the natural condition, the soil being at the time covered with the native spring growth of bright flowers. No alkali salts are seen on the surface at this point at any season.

It will be noted that the alkali salts are almost wholly accumulated between the depths of twenty and forty inches. Both above and below these limits impregnation is not strong enough to interfere with vegetable growth of any kind; within them, the subsoil is hardened into a sheet of hard pan, which not only prevents the passage of roots by its resistance, but would corrode them by contact with a mixture of salts containing up to ninety-four per cent of carbonate of soda. But as the native vegetation is mostly shallow-rooted and annual, this does not interfere with its welfare. The moisture imparted to the land by the scanty rainfall (about seven inches) evaporates through the roots and leaves of this vegetation during its growing period; when it dies off it leaves the ground completely dry, so that no rise of the salts to the surface by evaporation can take place during the season, and the seeds dropped can germinate when the rainy season comes, without injury from alkali.

Diagram No. 3 shows, on the other hand, what happens when irrigation is practiced on this land (or when the water rises from below by seepage from leaky ditches), and the ground is left bare. The abundant water then dissolves the alkali salts in the subsoil hardpan; and evaporation continuing through the whole year, the entire salts are in the course of a few seasons carried upward nearer to the surface. Diagram 3 shows the state of things under these conditions at the same date as Diagram 1 (May 3, 1895); No. 4, a and b, shows the condition existing near the surface at the end of the dry season, in September or October. It will be seen that at that time the salts have accumulated so near the surface that by taking the soil away to the depth of six inches, from five sixths to seven eighths of the total mass of salts would be removed, leaving the land with no more than almost any crop can easily resist.

Diagram 2, a and b, shows the state of the irrigated land when sown to barley, it being understood that these samples were taken within ten feet of No. 3. A glance reveals that we have here a case intermediate between Nos. 1 and 3. The upward movement of