This table exhibits also another standing characteristic of alkali soils, which is to be anticipated from the conditions of their formation; viz, high lime-content, which sometimes rises to the extent of marliness.
In phosphates, also, alkali soils are almost always high; and an unusually large proportion is found to be readily soluble.
In presence of much carbonate of soda, nitrates are usually scarce or altogether absent; while owing to the action of the alkaline solution upon the humus, ammonia salts, or even free (or carbonated and therefore readily dissociated and assimilated) ammonia may be present, so as to be perceptible to the senses by its odor in hot sunshine. But in the case of “white alkali,” more especially of the sulphate in moderate amounts, nitrification is exceedingly active and nitrates may sometimes rise to as much as 20% of the soluble salts. As alkali spots are usually low in the central portion and therefore more moist than around the edges, we sometimes find ammonia salts in the middle of a spot, while nitrates are abundant along the margin of the same. These differences, first demonstrated by an investigation made by Colmore,[170] illustrate some of the reactions that are essentially concerned in the agricultural availability of alkali lands. A summary of Colmore’s results is given in the table below.
Cross Section of an Alkali Spot.—The spot examined lies outside of Tulare, California, substation; it being late in the season, when the bulk of the salts is found near the surface, the samples were taken to the depth of one foot only, at points four feet apart, from the center out.
AMOUNT AND COMPOSITION OF SALTS IN ALKALI
SPOT FROM CENTER TO CIRCUMFERENCE.
4 FEET APART, 1 FT. DEPTH.
| Mineral Salts. | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Center of spot. | Four feet. | Eight feet. | Twelve feet. | Outer margin. | |
| Potassium sulfate | 6.70 | 9.55 | 11.92 | 19.26 | 13.95 |
| Sodium sulfate | 19.84 | 12.85 | 23.72 | 23.97 | 16.96 |
| Magnesium sulfate | 3.07 | .07 | .95 | 2.05 | 8.29 |
| Sodium chlorid | 13.80 | 23.73 | 24.12 | 24.23 | 29.69 |
| Sodium carbonate | 50.72 | 50.96 | 37.55 | 35.49 | 29.94 |
| Sodium phosphate | 5.57 | 2.88 | .87 | ? | 1.04 |
| Sodium nitrate | 30 | ? | .87 | ? | .13 |
| Totals | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 |
| Organic matter | 30.00 | 24.80 | 19.48 | 23.36 | 20.31 |
| Total soluble in soil | .78 | .54 | .70 | .37 | .34 |
| Mineral salts | .38 | .40 | .54 | .25 | .23 |
While the table shows an obvious irregularity in some of the data at the eight-foot point, arising doubtless from an irregularity of surface or of texture overlooked in taking the samples, we find a very remarkable regularity of progression in the cases of potassium sulfate, sodium chlorid, sodium carbonate and sodium phosphate in the other four samples. The maxima of the “black alkali” and the soluble organic matter (humus) coincide, as does that of the phosphate; the total mineral salts at the outer margin are only a little over half of what is found at the center. This is natural, as owing to the deflocculating effect of the black alkali, the center is nearly a foot lower than the margin. The lowering of the nitrate-content at the outer margin is obviously due to the luxuriant vegetation growing adjacent.
Reactions between the Carbonates, Chlorids and Sulfates of Alkalies and Earths. That a soluble earth-salt, such as the sulfate or chlorid of calcium, will react upon an alkaline carbonate solution so as to form an alkali sulfate, and e.g. lime carbonate, is well known; the neutralization of the sodic carbonate in the soil by means of gypsum, above referred to, is based upon this reaction. It is not so well known that the latter may be reversed, partly or wholly, by the presence of carbonic acid in the solution of the soil. Although observed as early as 1824 by Brandes, and again in 1859 by A. Müller, this reaction is not mentioned in text-books and attracted no attention as a source of naturally occurring alkali carbonates which in the past have formed the basis of extensive commerce from the Orient, until in 1888, the writer together with Weber and later with Jaffa, investigated it quantitatively.[171] It was found that up to .75 grms. per liter, the entire amount of sodic sulfate present in solution is transformed into carbonate in presence of calcic carbonate, by a current of carbonic dioxid; but the amount so transformed does not continue to increase beyond about 4 grams per liter. A corresponding amount of calcic sulfate is formed. In the case of potassic sulfate, the transformation also occurs, proportionally to the molecular weight. This relation is shown in the subjoined diagram, which also shows in the curves on the left, the residual alkalinity left after evaporation and drying the residue at 100° C.
Fig. 69.—Progressive Transformation of Alkali Sulfates into Carbonates.
(The figures along upper line represent tenths of one per cent.)