In countries of heavy rainfall, the alkaline materials are leached out in the drainage waters. In arid countries there is very little or no leachage; the water passes off by evaporation, and the alkaline and other materials in solution are left at or near the surface of the ground.

The normal condition of arid lands is illustrated in the table below (Means). The first part gives the percentage of total soluble salts in two soils from central Montana, where neither soil originally contained enough alkali within the zone of root action to be detrimental. The second part shows the condition of these soils after a few years of judicious irrigation, and the third part displays the condition after a few years of irrigation without drainage: —

Table showing percentage of alkali in soils

Alkali Lands 17

Percentage composition of alkali in arid soils1 (Lyon and Fippin)

Alkali Lands 18

1 Compiled from analyses made by the Bureau of Soils of the United States Department of Agriculture and by the California Experiment Station.

The following table shows the quantity of gypsum required to neutralize sodium carbonate in an acre-foot of soil: —

Per Cent Sodium Carbonate

Gypsum per Acre-foot1

Per Cent Sodium Carbonate

Gypsum per Acre-foot l

Per cent

Pounds

Per cent

Pounds

.01

640

.06

3840

.02

1280

.07

4480

.03

1920

.08

5120

.04

2560

.09

5760

.05

3200

.10

6400

1 An acre-foot of soil weighs 4,000,000 pounds.

Very often the black alkali is accompanied by other soluble salts, and the change in kind of salt brought about by the gypsum leaves more white alkali than plants will stand. The economic use of gypsum is therefore restricted to localities having only small amounts of total soluble salts. As a general rule, drainage can be properly applied, and the land freed of both black alkali and white alkali at less expense than by the application of gypsum. Gypsum costs $4 to $10 per ton in the regions where it is needed in black alkali reclamation, and when it becomes necessary to apply sufficient to neutralize 0.1 per cent of sodium carbonate in two or three acre-feet of soil per acre, the cost is seen to be prohibitive.