A "HARD SPOT" IN AN IRRIGATED FIELD WHERE THE SOIL HAD BEEN PUDDLED BY THE IRRIGATION WATER AND HAD BAKED ON DRYING OUT. THE HARD CRUST IS OFTEN TWO OR THREE INCHES THICK. WHEN SUCH A CRUST IS FORMED ON NEWLY SEEDED LAND THE SEEDLINGS ARE PREVENTED FROM COMING UP.

214-3

The way in which the soluble salts are deposited from evaporating water at the surface of the soil is shown in plate 1. In this case a deep layer of soil that had been soaked with salty water dried out by surface evaporation. The water moving upward carried the dissolved salts with it until it was vaporized at the surface, when the salts were deposited in crystalline form.

Another example of the surface deposition of salts in the dry country may be seen in the flat valleys of rivers or creeks. Many desert streams meander through such valleys and where the subsoil on either bank is readily permeable to water there is an appreciable lateral seepage from the stream bed, particularly in times of high water. Some of the water from this underflow is drawn upward through the layers of soil and evaporated, leaving its dissolved salts behind. In the course of time, these surface accumulations of salt reach such concentrations as to prevent the growth of plants and thus render useless large areas of potentially fertile and easily irrigable land.

From these examples it may be seen that the alkali salts of arid lands owe their origin chiefly to the natural processes of soil weathering. The marked irregularities of their distribution are due to subsequent transportation in water and their deposition at the places where the water evaporates.

The processes of soil formation from the breaking up of rock masses proceed very slowly, so that the release of soluble salts to the soil water takes place gradually. The abundant deposits of alkali salts that are now to be found in spots in the desert may represent the accumulations of many centuries of time and the drainage from many square miles of surface. If these salts were evenly distributed throughout the region in which they were formed they would seldom be harmful, but when the salts from many acres are all deposited in a small spot they are likely to prove troublesome if that spot is selected for crop production. An example of the accumulation of salts in a low spot in the desert is shown in plate 2, figure 1. Such a low spot as this may serve for many years as an evaporating basin for the flood waters that bring in the salts from the surrounding land. After a time such a basin may be completely filled and covered by drifting soil only to be discovered later when an attempt is made to use the land for crop production. Often these spots are small in area, covering only a few acres, but in other cases such as the Great Salt Lake in Utah the evaporating basin covers many square miles.

ALKALI SALTS AND THE GROUND WATER.

The alkali salts of the desert have little practical significance except in relation to the ground water. It is only when they are dis

solved in the ground water that they can be transported from place to place, and it is only as they occur in solution that they have a harmful effect on the growth of plants or on the physical character of the soil. For these reasons the alkali problem is always a problem of the ground water or the soil solution.

To understand the alkali problem one must understand something of the conditions which influence the movements and the reactions of the soil solution in relation to the soil. Water exists in and moves through the soil in the liquid form and as a vapor. It is only in the liquid form that it is capable of dissolving and transporting salts.

The water content of the soil varies within wide limits. A saturated soil may hold a quantity of water equal to half its own weight. A soil that is thought of as being dry when examined in the field may have as much as 8 or 9 per cent of water, in case of a very fine soil, or not more than 2 or 3 per cent of water in the case of a sandy soil. When in good condition to support plant growth, ordinary soils contain from 10 to 30 per cent of water. These figures are given to show that if the water contained in a certain quantity of soil has dissolved in it a certain quantity of salts, the concentration of the solution may vary within wide limits as the soil approaches the air-dry limit on the one hand or the saturation limit on the other.

It is probably because of this variation in the moisture content of the soil that it is customary to refer to the proportion of alkali salts as a percentage of the dry weight of the soil rather than to speak of the concentration of the soil solution. Thus in classifying or mapping alkali soils, it is the custom to describe the different areas as containing 1 per cent or 2 per cent of salts, meaning by this that the proportion of water soluble material is equal to 1 per cent or 2 per cent of the dry weight of the soil.

The concentration of the salts in the soil solution is a very different thing from the percentage of salts in the soil. Thus if a soil contained 1 per cent of soluble salts and 25 per cent of water, the concentration of the soil solution would be equivalent to 4 per cent, while if the same soil were merely permitted to dry out until it contained only 10 per cent of water, the concentration of the soil solution would be equivalent to 10 per cent. These figures give something of an idea of the concentrations of salt that may be tolerated by certain desert plants which are able through special adaptations to grow in soils containing as much as 3 per cent of soluble salts, and which are subject to periods of drought during which the moisture content may be reduced to 10 per cent or less.

It has been noted above that the soluble salts are dissolved in and move with the liquid water of the soil, and that when the soil solution is evaporated from the surface of the soil, the salts are left be

hind, forming a crust of crystals. As a matter of fact, under field conditions, much of the water lost by evaporation from the soil is actu ally vaporized within the soil mass, passing upward to the free air through cracks and interstices of the soil. Where this takes place the salts are, of course, left behind at the point where vaporization takes place, which may be well below the actual surface of the soil. When saturated soil dries rapidly, particularly if it is rich in clay, it shrinks and cracks as is shown in plate 2, figure 2. In such a soil the proportion of the dissolved salts finally deposited on the surface may be very small.

In some irrigated sections conditions are such that the subsoil becomes saturated with water and there exists what is known as a ground-water table. This condition occurs naturally in some of the alluvial valleys of desert streams such as the Nile in Egypt and the Colorado and Rio Grande in this country. In other cases the groundwater table comes into existence as the result of the downward percolation of some of the water applied in irrigation, together with the seepage from canals. When the ground-water table is high-that is, when the plane of saturation is within a few feet of the surface of the soil-there may be established a capillary connection with the surface so that some of the ground water is lost by evaporation. Where this condition exists there is almost certain to be an accumulation of salts at or near the surface and consequent injury to crop plants or to the physical condition of the soil.

As a matter of fact, in the great majority of cases in which alkali salts cause trouble in irrigated lands, this trouble is associated with a high ground-water table. It is obvious that this must be so, for with a permeable soil and no ground-water table it would be very easy to wash the excess of soluble salts out of the surface layer of the soil and well below the root zone of crop plants by the simple expedient of a short period of heavy irrigation.

From these considerations it may be concluded that the troublesome accumulation of alkali salts in irrigated land is due to one of two conditions, either the ground-water table is too close to the surface or the soil is not readily permeable to water.

INJURIOUS EFFECTS ON PLANTS.

When the concentration of the soil solution becomes excessive, the alkali salts which occur in irrigated lands have a toxic or injurious effect on crop plants. The limits of toleration for crop plants is variable, depending on the nature of the salt, the kind of plant, and the stage of growth of the plant when the high concentrations In some cases the injury caused by the alkali salts is probably due to the purely physical effect of interfering with the osmotic action by which the plant roots absorb water from the soil solution.

occur.