The most prominent of these is usually supposed to be clay—the hydrous silicate of alumina that in its purest condition forms kaolinite or porcelain earth. Any alumina found in the usual course of soil analysis is generally referred to this mineral, which contains silica and alumina nearly in the proportion of 46% to 40%.
In very many cases, however, the reference of these two ingredients to clay is manifestly unjustified. This is clearly so when (as not unfrequently happens) the amount of alumina found exceeds that which would form clay with the ascertained percentage of soluble silica; it is almost as certainly so when, in addition to the alumina, other bases (notably potash, lime and magnesia), are found in proportions which preclude their being in combination with any other acidic compounds present. The only possible inference in such cases is that these bases, together with at least a portion of the alumina, are present in the form of hydrated, and therefore easily decomposable silicates or zeolites.
The subjoined analysis by R. H. Loughridge, of a clay obtained in the usual process of mechanical soil analysis (by precipitating with common salt the turbid water remaining after 24 hours subsidence in a column of 200 millimeters) from a very generalized soil of northern Mississippi, shows one of the many cases in which the numerical ratios of the several ingredients are incompatible with the assumption that silica and alumina are present in combination as clay (kaolinite) only:
ANALYSIS OF COLLOIDAL CLAY.
| Insoluble matter | 15.96 |
| Soluble silica | 33.10 |
| Potash (K₂O) | 1.47 |
| Soda (Na₂O) | 1.70 |
| Lime (CaO) | .09 |
| Magnesia (MgO) | 1.33 |
| Br. ox. of Manganese (Mn₃O₄) | .30 |
| Peroxid of iron (Fe₂O₃) | 18.76 |
| Alumina (Al₂O₃) | 18.19 |
| Phosphoric acid (P₂O₅) | .18 |
| Sulfuric acid (SO₃) | .06 |
| Carbonic acid (CO₂) | .00 |
| Water and organic matter | 9.00 |
| Total | 100.14 |
If in this case we assign all alumina to silica, as required for the composition of kaolinite or pure clay, there yet remains a trifle over twelve (12.17) per cent of silica to be allotted to the other bases present. Deducting from this the ascertained amount of silica soluble in sodic carbonate, pre-existing in the raw material (.38 per cent), we come to 11.79 per cent as the amount of silica which must have been in combinations other than kaolinite, viz., hydrous silicates, or soil zeolites, formed either with the bases other than alumina shown in the analysis or, more probably, containing some of the alumina itself in essential combination.
We are thus enabled to obtain from the determination of the soluble silica an estimate of the extent to which these soil zeolites, that form so important a portion of the soil in being the repositories of the reserve of more or less available mineral plant-food, are present in the soils of the several regions. A glance at the table shows that the general average of soluble silica is very much greater in the soils of the arid regions than in those of the humid, approximating one to two in favor of the arid division.[134]
Differences in the Sands of the Arid and Humid Regions.—In [chapter 5] mention has been made of the fact that while in the humid regions, “sand” as a rule means quartz grains, mostly with a clean surface and very frequently rounded and polished, in the arid regions even the coarse sand grains consist of, or are covered with, a great variety of minerals in a partially decomposed condition. This is owing to the absence of the abundant rainfall which in humid climates continually washes down the finely divided, half-decomposed mineral matter into the subsoil; while in arid climates the light rains cannot produce any such washing effect and hence the sand grains remain incrusted with the products of either their own decomposition, or of that of neighboring particles; it being therefore not concentrated in the finer portion only, viz., the clay and finest silts. This fundamental difference, which is illustrated in the analytical table below, at once explains why in the arid regions generally, sandy soils are found so highly productive that, owing to their easy cultivation they are preferred to the clayey lands, in which tillage and irrigation are more difficult. It is a well-known fact that on the “sands of the desert” when either irrigated, or wetted by rain, vegetation at once springs up with remarkable luxuriance, even on sand drifts; and this productiveness appears to be quite as lasting as that of “strong” clay soils of the humid regions.
This difference is curiously illustrated on the southern edge of the “black adobe” or prairie soil area which surrounds Stockton, Cal. Here we find on the opposite sides of a small stream (French Camp slough) the two extremes, of heavy clay and the sandy soils which for many years made Stanislaus county the “banner” county for wheat. The grain product of both banks ranked alike in quantity and quality in average years; but in extreme seasons sometimes one, sometimes the other failed, according to the weather conditions which favored one or the other soil. No one would think of sowing wheat on so sandy a soil in the humid States.
Table Illustrating Difference in Sands
of the Humid and Arid Regions.