THE ALKALI PROBLEM IN IRRIGATION.

By CARL S. SCOFIELD,

United States Department of Agriculture.

[With 3 plates.]

Soon after gold was discovered in California a large number of Americans undertook to reach the new El Dorado by crossing what was then known as the Great American Desert. The two or three trails across this 2,000 miles of wilderness proved difficult enough to try the endurance of the strongest. In the broad plains and dry valleys the dust was annoying and the water was often scarce and bad. These irritating and disappointing features of the region were so important and so serious that they became widely known and the word alkali soon came into general use to describe the dust and water of the arid West.

This word now passes current in our literature as applying generally to the salts that are often found in abundance in arid lands. While the major portion of these salts are not really alkaline in reaction the term is so well known and so widely used that we must recognize its validity.

Following the rush of gold seekers to California, pioneers began to settle in the desert and to practice irrigation. For them the word alkali came to have a special interest and significance. Some of the water that was used for irrigation proved to be detrimental rather than helpful to their crops and the land upon which such water had been used soon became unfit for crop production, even when better water was substituted. Thus in the progress of time, and with improved methods of transportation, alkali dust and water became less of a problem to the traveler but remained a problem to the desert farmer.

Save in a few exceptional localities, alkali is now or is likely to become a serious problem wherever irrigation is practiced. It has been the primary cause of the abandonment of large areas of irrigated land both in this country and in the Old World. The injurious effects of alkali are manifested in several different ways and

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the nature and causes of these different results have not always been easy to understand or to explain.

THE IMPORTANT ALKALI SALTS.

The so-called alkali salts include sodium chloride, or common salt, sodium sulphate, sodium carbonate, and sodium bicarbonate, together with some calcium and magnesium and even some potassium, combined usually as the carbonate or sulphate. There is wide variation in the proportions in which these salts occur, but they are usually all present. When sodium carbonate is abundant, it often causes a dark colored deposit on the soil surface, in consequence of which the term "black alkali" is used in reference to this salt. Sodium sulphate, on the other hand, often produces a white efflorescence on the surface of the soil and is known as "white alkali."

THE ORIGIN OF ALKALI SALTS.

These salts are derived from the disintegration and weathering of rocks in the process of soil formation. They are formed not only in arid regions but in humid regions as well. In humid regions, however, they are leached from the soil by the rain as soon as they become soluble and are carried away by the rivers to the sea In arid regions where the rainfall is insufficient to leach the soil completely the salts remain and accumulate.

In the course of time these soluble salts are moved about by such rain as falls. As the rains gather into small streams that form temporary pools or lakes, the salts are carried into these and left behind when the water evaporates. In some places it is possible to trace very clearly the course of events in the formation of the soil in the dry country. A cross section of the soil will show alternate layers of salt and earthy material. It is clear that this material was laid down in a basin that was subject to flooding after torrential rains. These floods of muddy water spread out over the floor of the basin and the water was soon evaporated, leaving the mud and salt behind. As the last of the water evaporated from the surface of the mud layer it left a crust of salt on this surface. The next flood brought down more mud, which covered the previous layer, and some of the less soluble salts remained between the successive layers of mud, while the more soluble salts were redissolved, only to be redeposited again at the new surface.

With the changing conditions of flood channels and drainage these surface deposits of salt have been reworked many times even in our most recent geological periods. Thus we find in arid regions that the soluble salts which we call alkali are abundant in some spots while virtually absent in large areas of the better-drained land.

A SECTION OF SOIL THAT HAD BEEN SATURATED WITH WATER CONTAINING DISSOLVED SALTS. WHEN THE WATER WAS EVAPORATED FROM THE SOIL THE SALTS WERE DEPOSITED ON THE SURFACE IN CRYSTALLINE FORM.

2. A PORTION OF THE SURFACE OF THE SOIL IN THE DESERT THAT HAS DRIED RAPIDLY AFTER WETTING AND, IN DRYING, HAS FORMED SHRINKAGE CRACKS, FROM THE SIDES OF WHICH THE SOIL WATER IS VAPORIZED. UNDER SUCH CONDITIONS A LARGE PROPORTION OF THE SALTS DISSOLVED IN THE SOIL SOLUTION ARE PRECIPITATED WITHIN THE SOIL MASS RATHER THAN ON THE SURFACE.