| Clay. | Per cent in Soil. | Potash. | Lime. | Magnesia. | |
|---|---|---|---|---|---|
| Mississippi[135] | 21.64 | .32 | .03 | .29 | |
| California | 1281 Chino[136] | 7.60 | .16 | .14 | .17 |
| “ | Jackson[137] | 16.43 | .13 | .12 | .08 |
| Silt | .06-.016 mm. diam. | ||||
| Mississippi | 35.10 | .41 | .15 | .36 | |
| California | (Chino) | 18.53 | .24 | .53 | .29 |
| “ | Jackson | 34.90 | .10 | .04 | .08 |
| Silt | .016-.025 mm. diam. | ||||
| Mississippi | 13.67 | .12 | .09 | .10 | |
| California, | Chino | 5.49 | .05 | .11 | .02 |
| “ | Jackson | 9.96 | .08 | .04 | .10 |
| Silt | .025-.036 mm. diam. | ||||
| Mississippi | |||||
| California, | Chino | 3.92 | |||
| “ | Jackson | 7.68 | .06 | .02 | |
| Silt | .036-.047 mm. diam. | ||||
| Mississippi | |||||
| California, | Chino | 6.40 | .05 | .18 | |
| “ | Jackson | 8.21 | .04 | .01 | |
| Coarse Silt | .047-.072 mm. diam. | ||||
| California, | Chino | 7.92 | .06 | .23 | |
| “ | Jackson | 5.91 | .01 | .01 | |
| Fine sand | .072-.12 mm. diam. | ||||
| California, | Chino | 11.87 | .06 | .26 | |
| “ | Jackson | 4.03 | .01 | .01 | |
| Sand | .12-.50 mm diam. | ||||
| California, | Chino | 36.11 | .11 | .69 | |
| “ | Jackson | 10.10 | |||
| Clay. | Phosphoric Acid. | Soluble Silica. | Alumina. | Summation. | |
|---|---|---|---|---|---|
| Mississippi[138] | .04 | 7.17 | 3.97 | 11.82 . | |
| California | 1281 Chino[139] | .04 | 1.70 | 1.35 | 3.56 |
| “ | Jackson[140] | .05 | 2.83 | 2.13 | 5.34 |
| Silt | .06-.016 mm. diam. | ||||
| Mississippi | .07 | 2.87 | 1.36 | 5.22 | |
| California | (Chino) | .06 | 4.96 | 1.76 | 7.84 |
| “ | Jackson | .02 | 2.50 | 2.44 | 5.18 |
| Silt | .016-.025 mm. diam. | ||||
| Mississippi | .02 | .32 | .17 | .82 | |
| California, | Chino | .01 | .80 | .51 | 1.50 |
| “ | Jackson | .007 | 1.01 | 1.01 | 2.25 |
| Silt | .025-.036 mm. diam. | ||||
| Mississippi | .36 | ||||
| California, | Chino | lost | |||
| “ | Jackson | .006 | 0.82 | .74 | 1.70 |
| Silt | .036-.047 mm. diam. | ||||
| Mississippi | .55 | trace | |||
| California, | Chino | .01 | .80 | .64 | 1.66 |
| “ | Jackson | .001 | .43 | 1.12 | |
| Coarse Silt | 047-.072 mm. diam. | ||||
| California, | Chino | .02 | .89 | .59 | 1.79 |
| “ | Jackson | .003 | .42 | .30 | .77 |
| Fine sand | .072-.12 mm. diam. | ||||
| California, | Chino | .03 | .98 | 1.43 | |
| “ | Jackson | .003 | .28 | .09 | .40 |
| Sand | .12-.50 mm diam. | ||||
| California, | Chino | .04 | 2.43 | 1.59 | 4.98 |
| “ | Jackson | Not detd | |||
It thus appears that while in the Mississippi soil, solubility of plant-food practically ceased at grain-diameter of .036 mm, in the arid California soils, as large an amount was found in the sand-grain sizes between .12 and .50 millimeters as in the fine silt .016 to .025 mm. in Mississippi.
Hydrous Silicates are More Abundant in Arid than Humid Soils.—This predominance of hydrous silicates in the soils of the arid regions should not be a matter of surprise when we consider the agencies which are brought to bear upon these soils with so much greater intensity than can be the case where the solutions resulting from the weathering process are continually removed as fast as formed, by the continuous leaching effect of atmospheric waters. In the soils of regions where summer rains are insignificant or wanting, these solutions not only remain, but are concentrated by evaporation to a point that, in the nature of the case, can never be reached in humid climates. Prominent among these soluble ingredients are the silicates and carbonates of the two alkalies, potash and soda. The former, when filtered through a soil containing the carbonates of lime and magnesia, will soon be transformed into complex silicates, in which potash takes precedence of soda, and which, existing in a very finely divided (at the outset in a gelatinous) condition, serve as an ever-ready reservoir to catch and store the lingering alkalies as they are set free from the rocks, whether in the form of soluble silicates or carbonates. The latter have another important effect: in the concentrated form at least, they, themselves, are effective in decomposing silicate minerals refractory to milder agencies, such as calcic carbonate solution; and thus the more decomposed state in which we find the soil minerals of the arid regions is intelligible on that ground alone.
It must not be forgotten that lime carbonate, though less effective than the corresponding alkali solutions, nevertheless is also known to produce, by long-continued action, chemical effects similar to those that are more quickly and energetically brought about by the action of caustic lime. In fact, the agricultural effects of “liming” are only in degree different from those produced by marling with finely pulverized carbonate; and in nature the same relation is strikingly exemplified in the peculiarly black humus that is characteristic of calcareous lands, but which can be much more quickly formed under the influence of caustic lime on peaty soils.
In the analysis of silicates we employ caustic lime for the setting-free of the alkalies and the formation of easily decomposable silicates, by igniting the mixture; but the carbonate will slowly produce a similar change, both in the laboratory and in the soils in which it is constantly present. This is strikingly seen when we contrast the analyses of calcareous clay soils of the humid region with the corresponding non-calcareous ones of the same. In the former the proportions of dissolved silica and alumina are almost invariably much greater than in the latter, so far as such comparisons are practicable without assured absolute identity of materials. That is, calcareous clays or clay soils are so sure to yield to the analyst large precipitates of alumina, that experience teaches him to employ smaller amounts for analysis than he would of non-calcareous materials, in order to avoid unmanageably large bulks of aluminic hydrate. It is but rarely that even the heaviest non-calcareous soils yield to the acid usually used in soil analysis more than 10 per cent of alumina; while heavy calcareous clay (prairie) soils commonly yield between 13 and 20 per cent.[141] It would be interesting to verify this relation by artificial digestions of one and the same clays with calcic carbonate at high temperatures, as it must always be extremely difficult to insure absolute identity of all other conditions in natural materials.
In most of these cases, what is true of alumina is also true of the soluble silica. But since the latter is constantly liable to be dissolved out by solutions of carbonated alkalies, it is not surprising that this relation is not always shown.
Aluminic Hydrate.—In numerous cases, the amount of alumina dissolved in analysis is greatly in excess of the soluble silica, so as to force the conclusion that a portion of the latter must be present in a different form from that of clay (kaolinite); the only choice being between that of complex hydrous silicates (none of which, however, could contain as large a percentage of alumina as clay itself) and aluminic hydrate. The latter is alone capable of explaining the presence of more alumina than silica in easily soluble form;[142] and the visible occurrence of “gibbsite” and “bauxite” in modern formaations renders this a perfectly simple and acceptable explanation. Since these minerals are known to be incapable of crystallization, we are moreover led to the presumption that it will as a rule be found in the finest portions of the soil, viz., in the “clay” of mechanical analysis.
Some illustrations of these conditions are given below, for soils from Mississippi and California. The soluble silica being all assigned to kaolinite, the rest of the alumina must be assumed to be present as hydrate, since no other compound could fulfil the stoichiometrical requirements.[143] The table therefore shows the differences between the amounts of alumina found by analysis, and those assignable to kaolinite, calculated to the mineral bauxite—the most abundant, as well as the one containing the medium proportion of water, among the three naturally occurring aluminic hydrates.
TABLE SHOWING EXCESS OF
ALUMINA OVER SILICA IN SOILS;
CALCULATED AS BAUXITE.