It is thus that the luxuriant vegetation of the San Joaquin plains, dotted with occasional alkali spots, is maintained; the spots themselves being almost always depressions in which the rain water may gather, and where, in consequence of the increased evaporation, the noxious salts have risen to the surface and render impossible all but the most resistant saline growth; particularly when, in consequence of maceration and fermentation in the soil, the formation of carbonate of soda has caused the surface to sink and become almost water-tight.
Upward Translocation from Irrigation.—[Fig. 65] shows the corresponding profile of the same soil after several years’ irrigation. The upward movement of the salts is clearly seen by comparison with the [previous figure]; and the surface soil has become so charged with salts that the seeds of culture plants refuse to germinate.
Ten feet from this bare alkali ground, on which barley had refused to grow, a crop of barley four feet high was harvested the same year, without irrigation. Investigation proved that here the condition of the soil was intermediate between the two preceding diagrams. The irrigation water had dissolved the alkali of the subsoil, and the more abundant evaporation had brought it nearer the surface; but the shading by the barley crop and the evaporation of the moisture through its roots and leaves had prevented the salts from reaching the surface in such amounts as to injure the crop, although the tendency to rise was clearly shown. By the use of gypsum, moreover, the injuriousness of the alkali had been somewhat diminished.
The same season, grain crops were almost a failure on alkali-free land in the same region; and in connection with this result it should be noted as a general fact that alkali lands always retain a certain amount of moisture perceptible to the hand during the dry season, and that this moisture can be utilized by crops; so that at times when crops fail on non-alkaline land, good ones are obtained where a slight taint of alkali exists in the soil. Actual determinations showed that while a sample of alkali soil containing .54% of salts absorbed 12.3% of moisture from moist air, the same soil when leached absorbed only 2.5%—a figure corresponding to that of sandy upland loams.
Fig. 65.—Diagram showing amounts and composition of alkali salts at various depths in bare alkali land, where barley would not grow; irrigated. Taken September, 1894.
Tulare Experiment Substation, California.
Fig. 66.—Distribution of Alkali Salts in Sandy Lands.
Alkali in Sandy Lands.—In very sandy lands, and particularly when the alkali is “white” only, the tendency to accumulation near the surface is much less, even under irrigation. In the natural condition the salts are in such cases often found quite evenly distributed through soil columns of four feet, and even more. This is an additional cause of the lesser injuriousness of “white alkali.” An illustration of the distribution of the salts in very sandy lands, from the Tulare substation, is given in [Fig. 66]. Here we see that the maximum is not at, but some distance below the surface, the entire saline mass is lower down than in the more clayey loam of the same locality, and is more widely distributed in depth.