ALKALI in Irrigated Districts

The occurrence of alkali salts on the surface or in the root zone of irrigated soils is an abnormal condition indicating that the irrigation water is not passing downward through the soil. These alkali salts are easily dissolved in the soil moisture, so that if there is a gradual or even an occasional movement downward of the soil solution, the dissolved salts are carried with it below the root zone.  This is the only way that alkali salts can be removed from the soil.  Likewise, the only way that the accumulation of alkali salts in the soil can be prevented is to apply enough irrigation water, at least occasionally, to leach the root zone.

The reason for this is that irrigation water always contains some dissolved salts, sometimes rather large quantities. This salt that is brought into the soil by the irrigation water is left there when the water is absorbed by plants or lost by evaporation. If the water applied by irrigation never penetrates below the root zone, then the salt brought in by it continues to accumulate in the root zone, where it remains in solution in the soil moisture until the soil solution becomes such a concentrated salt solution that crop plants can not absorb from it the water they need for growth. The injurious effect of alkali salts in the soil solution is due to the fact that the salts in that solution prevent the absorption of water by plants rather than that the salts are absorbed by the plants and act as poisons in the plant system. Most crop plants can absorb water rapidly enough to supply their growth requirements from an available soil solution that contains less than 1.5 per cent of mixed salts.  When the salt concentration of the soil solution gets much above that point, crop plants begin to show symptoms of injury. These symptoms of alkali injury closely resemble the symptoms of drought injury because the plant can not absorb water normally from a strong salt solution nor from the soil when the moisture content is below the wilting point.

The aim of careful irrigation should be not only to supply the water needed by the crop for its growth requirements but enough in excess to insure a cumulative downward movement of water through the root zone in order to prevent the accumulation in that zone of injurious quantities of soluble salts. It is not necessary or even desirable that this leaching of the root zone should go on continuously or that more than a small part of the water applied should pass out below. But it is essential to the sustained productivity of irrigated land that the root zone should be leached to some extent at least occasionally.

It would not be a difficult matter to apply irrigation water in sufficient quantity to leach the root zone and thus prevent the injurious accumulation of alkali salts in the soil solution if the soil and subsoil were both readily permeable to the movement of water.  Where both soil and subsoil permit excess water to move freely in response to the force of gravity there is no alkali problem. It is only where a condition of impermeability exists either in the soil or in the subsoil that alkali troubles occur.

When the soil is permeable and the subsoil is impermeable to the movement of excess water, artificial drainage must be provided. In many irrigated sections subsoil impermeability occurs in the form of subsurface bars or dikes of tight or cemented material. These bars or dikes interfere with the free movement of excess subsoil water in the direction of the natural drainage outlets, such as creeks and rivers, and it becomes necessary to cut artificial channels through these barriers to afford relief. These artificial channels may be open ditches or covered lines of tile.

In planning a drainage system for a tract of irrigated land it is desirable to recognize the fact that the excess of subsoil water may originate from either of two sources. It may come chiefly from percolation through the root zone of irrigated fields or it may be due chiefly to seepage losses from irrigation canals. The importance of ascertaining the chief source of troublesome accumulations of subsoil water lies in the fact that one system of drainage is required for one of these situations and another system is required for the other. Where the excess of subsoil water comes chiefly from root- zone percolation, the indicated method of relief is to locate the impermeable subsoil barriers that hold the water back and cut these with artificial outlets. Where the water comes chiefly from canal seepage it is usually more efficient and economical to ascertain just where, in the canal system, these seepage losses occur and then either line the canal to prevent seepage or install a drain to intercept the seepage and prevent the water-logging of the irrigated land by excessive subirrigation. In many irrigated districts the water-carrying capacity of the subsoil is adequate to permit the outflow, to natural drainage channels, of all of the excess water that percolates through the root zone of the irrigated fields. In such situations, if the canal losses can be intercepted, no further drainage is necessary.

The importance of subsoil permeability in relation to the alkali problem becomes apparent, when it is realized fully that with irrigation the root zone must be leached and that with a saturated or impermeable subsoil it is not possible to leach the root zone. In fact when the subsoil is saturated, a process which is the reverse of leaching sets in. The solution that accumulates in a saturated subsoil is more salty than the irrigation water. With the progress of time it becomes still more salty as it gives up water to crop plants and loses water by evaporation. Furthermore, a saturated subsoil is an effective barrier to the downward movement of irrigation water through the soil, so that any additional salt brought in by irrigation water is added to the soil solution at any spot where the subsoil is saturated. Thus a field or a spot in a field where the subsoil is saturated soon becomes too salty to support normal crop growth and in the ordinary course of irrigation the soil of the root zone may also become saturated or water-logged.

It is a natural assumption that where land is regarded as irrigable, the surface soil that is to constitute the root zone for crop plants will be sufficiently permeable to permit the ready penetration of irrigation water.

As a matter of fact, the attempt is often made to grow crops on land where the surface soil absorbs water very slowly and where in consequence the root zone is shallow and because of its impermeability it can not be leached. Such land may have good surface topography and be easy to prepare for irrigation. It is ultimately disappointing, however, and the experienced irrigation farmer avoids it. Spots of such impermeable soil often occur in fields where most of the soil is good.

The condition of soil impermeability here referred to constitutes a phase of the alkali problem somewhat different from that associated with an impermeable subsoil. The condition itself is due to certain physical properties of the soil, which in turn are the result of its chemical composition. The chemical composition of the soil at any time is the result of reactions of base exchange that normally take place between the soil material and the salts contained in the soil solution.

When the salts contained in the soil solution are preponderantly salts of sodium, the reaction tendency is in the direction of an exchange by which sodium from the soil solution enters into combination with the soil and an equivalent quantity of some other base, usually calcium, passes from combination with the soil into the soil solution. A soil that has thus absorbed an appreciable quantity of sodium, properly designated an alkaline soil, becomes relatively impermeable to the movement of water through it.

On the other hand, a soil in which the replaceable bases are chiefly calcium and magnesium manifests those physical properties that are associated with friability and the ready penetration of water. When a soil solution becomes concentrated with sodium salts, the soil becomes alkaline as a result of reactions of base exchange. In this condition it tends to become impermeable to the movement of water whenever the concentration of its solution is reduced as a result of leaching.

An impermeable soil may occur as the result of conditions existing independently of irrigation. Many examples of such soil are found in the arid regions. On the other hand, a soil that is naturally friable and permeable to water may become impermeable as a result of irrigation if the conditions of irrigation are such that alkali salts (the salts of sodium) are allowed to accumulate in the soil solution. Such accumulation can be prevented by a system of irrigation and of drainage that insures a periodical leaching of the root zone.