Fig. 13.—Expansion on Wetting and Contraction on Drying of heavy clay soils.
Contraction on Wetting.—In the case of alkali soils containing much carbonate of soda, a very notable contraction occurs in wetting the loose, dry soil. The cause is here obviously the collapse of the crumbs, formed in dry tillage or crushing, into single grains, closely packed. The same result is observed in the naturally depressed “alkali spots” ([see chapt. 22]).
“Hog-wallows.”—In the field the wetting of cracked clay soils produces some very curious effects. The effect of the first light rains usually is to crumble off the edges or angles near the surface, the materials thus loosened falling into the lower portion of the cracks. This is repeated at each successive shower followed by sunshine, the crevices thus becoming partly filled with surface soil. When, subsequently, the heavier and more continuous rains wet the land fully, also causing the consolidated mass in the crevices to expand, the latter cannot close on account of the surplus material having fallen into them; the result being that the intermediate portions of the soil are compelled to bulge upward, sometimes for six or more inches, creating a very uneven, humpy surface, well-known in the southwestern United States as “hog-wallows.”[33]
Such a surface is always therefore an indication of an extremely heavy soil, difficult to cultivate; yet embracing some of the most highly and permanently productive lands known in the United States, and in India, where the “regur” lands of the Deccan are of this character; they have been cultivated without fertilization for thousands of years. The subjoined physical analyses of lands of such extreme character as to be almost uncultivatable will serve to exemplify their physical composition.
| PHYSICAL ANALYSES OF HEAVIEST CLAY SOILS. | ||||
|---|---|---|---|---|
| No 242 Miss. | No. 643 Cal. | |||
| Hog-wallows soil. Jasper Co. Mississippi. | Black Adobe Contra Costa Co. California. | |||
| Weight of gravel over 1.2 mm. diameter | .83 | |||
| “ “ between 1.2 and 1 mm | ||||
| “ “ between 1 and 0.6 mm | 1.19 | |||
| Fine earth | 97.98 | 100.00 | ||
| 100.00 | 100.00 | |||
| FINE EARTH. | ||||
| Hydr. Value. | Diameter. | |||
| Clay | <.0023 mm | ? | 48.00 | 45.96 |
| Silt | <0.25 mm | .010 | 35.18 | 37.64 |
| 0.25 mm | .016 | |||
| 0.5 mm | .025 | 5.50 | 2.74 | |
| 1.0 mm | .036 | 3.74 | 3.31 | |
| 2.0 mm | .047 | 2.54 | 2.95 | |
| 4.0 mm | .072 | .20 | 2.39 | |
| Sand | 8.0 mm | .120 | .27 | 1.68 |
| 16.0 mm | .160 | .90 | .79 | |
| 32.0 mm | .30 | 1.67 | 2.36 | |
| 64.0 mm | .50 | 2.00 | ||
| 100.00 | 100.00 | |||
It will be noted that in both these extremely heavy soils the sum of the clay and finest sediments is a little over 83%.
It should be stated that both these soils after being thoroughly wetted become so adhesive that it is almost impossible to travel over the tracts occupied by them, and that they are practically almost untillable, being too adhesive when wet; yet if allowed to dry to a certain extent (varying within very narrow limits) they turn up by the plow in large clods, which after a few hours of sunshine become of stony hardness and will resist all efforts at pulverization or the production of tilth.[34]
Calcareous Clay Soils crumble on drying.—The heavy clay soils of some of the calcareous prairies of the Southwest, instead of contracting into a stony mass on drying, on the contrary resolve into a mass of crumbs, thus producing excellent tilth. This occurs even though the land may have been plowed when wet, and of course is a great advantage. The most striking exemplification of this peculiarity occurs in the heavy but profusely fertile “buckshot” clay lands of the Yazoo bottom, in Mississippi, where it is usual to plant corn and sweet potatoes in the semi-fluid mud left after an overflow, after turning a shallow furrow, then covering by turning another. To the onlooker it seems impossible that such plantings could be successful; but within a short time the muddy surface becomes a bed of crumbs (“buckshot”), forming a seed-bed not readily excelled by any made by artificial means. Hence, largely, the almost invariable success of crops in the Yazoo region.
Port Hudson Bluff.—The same clay produces a most unpleasant result at the foot of the Port Hudson bluff, where it crops out some feet above low water. When after a freshet the water level falls below this stratum, on drying the clay disintegrates into crumbs just as does the Yazoo buckshot soil; with the result that at the next rise, the loose mass subsides into the river as a flood of mud. Thus the foot of the bluff is being constantly undermined, and the falling of the bluff scarp has obliged the town above to recede many hundreds of feet from its original historic site.
The exact proportions of lime carbonate necessary to produce this phenomenon, and its necessary relations to clay substance and other physical soil ingredients, yet remain to be investigated.[35]