| Design and operation of smallholder irrigation in South Asia |
|Chapter 6 - Irrigability|
The following brief description of the principal factors influencing the behavior of soils under irrigation is given as background to discussion of particular soils problems. For more detailed treatment, reference should be made to Richards (1954), the classic original text on salinity and alkalinity, and to Tanji (1990).
Formation of soils from the parent material produces an array of constituents ranging from relatively unweathered resistant components (notably silica) to fully weathered material, part of the latter being in the form of clays. Organic material is usually also present. From the agronomic viewpoint, the soil may be grouped into relatively inert components, material still in the process of breaking down (a source of nutrients), days, and organic material. Soil moisture is also an essential ingredient.
The clay fraction plays a very important role, due to its ability to absorb ions on its surface. Positively charged ions (cations) of principal significance are calcium, magnesium, sodium, and to a less extent potassium. Although tightly bonded to the clay mineral by electrostatic forces, they may be exchanged with other cations in the soil solutions and thus constitute a source of plant nutrients. The adsorption sites not occupied by these cations may be occupied by hydrogen ions. The ability of a soil to absorb cations is referred to as its Cation Exchange Capacity (C.E.C). The extent to which that capacity is occupied by calcium, magnesium, sodium and potassium is termed the percentage of base saturation.
As cation absorption is a surface phenomenon it is primarily associated with clays, which due to the lamellar nature of the clay mineral have very high specific surface. The "2:1" clays such as mon-tmorillouite and illite which have both "internal" and "external" surfaces have C.E.C of some 100 milk equivalents per 100 g. The "1:1" clays such as kaolinite have C.E.C of 10 to 15 meq/100 g. Colloidal organic matter (humus) has C.E.C of up to 200 meq/100 g. Fine textured non-laminar minerals (e.g. fine silts) also have adsorptive capacity, but to a much lesser degree than clays.
Values of Cation Exchange Capacity for composite soils commonly range up to 30 milliequivalents per 100 g, the actual figure depending upon the clay content. A relatively high value of C.E.C., particularly with a high degree of base saturation, usually signifies high fertility. However, soils with C.E.C as low as 4 or 5 meq/100 g can grow irrigated crops provided that sufficient fertilizer is applied and that the interval between irrigations is short.
The undesirable cation to have on the exchange complex, if in excess, is sodium. Particularly at low levels of soil moisture salinity, sodium on the adsorption complex above a certain limit may hydrolyze, resulting in an alkaline condition. This can cause deflocculation and dispersion of the clay, with drastic reduction in soil permeability, hence the interest in the percentage of sodium on the exchange complex and in means of controlling it. The concentration of a particular cation on the exchange complex is influenced by the concentration of the same, and other, cations in the soil moisture with which it is in contact. In the long-term an equilibrium is achieved. The equilibrium concentration of sodium on the complex, corresponding to prolonged irrigation with water of a particular chemical make-up, is obviously a matter of primary importance. The relationship is an empirical one, which has been determined by study of a wide range of soils and irrigation waters. It relates the value of a function referred to as the Sodium Absorption Ration (S.A.R.) of the saturation extract of the soil moisture, to the Exchangeable Sodium Percentage (E.S.P.) on the soil exchange complex. It is noted that soil moisture is referred to, rather than irrigation water, as the exchange complex is in contact with soil moisture, not directly with irrigation water (other than at ground surface). The concentration of cations in the soil moisture at plant root level is two to three times that of the incoming irrigation water (averaged over a period of time), due to extraction of water by the plant. In determining the S.A.R. value of the soil moisture from data on the chemistry of the irrigation water, this increase in cation concentration is taken into account.
It is noted that the term "Sodium Absorption Ratio" causes some confusion as "absorption" occurs on the soil complex, not in the solution. However, it is the term customarily applied to the above-defined function of the soil solution.
Determination of the Exchangeable Sodium Percentage on the exchange complex, the associated S.A.R. of the soil moisture, and the S.A.R. of the proposed irrigation supply, is of interest for three reasons. First for classification of the soil in terms of its alkalinity hazard, second for assessment of the effect on the soil of long-term application of the particular irrigation water proposed to be used, and thirdly for design of remedial treatment if needed.
The adjustment of the E.S.P. of a soil to come into equilibrium with the S.A.R. of an irrigation supply can be a very slow process due to the large quantity of cations held on the exchange complex compared with the relatively small concentration in the irrigation water. In fact, amelioration of an alkaline condition (as distinct from saline) in the course of normal irrigation, or by leaching with irrigation water, is unlikely to be rapid enough to be of practical significance, except under special conditions (e.g. presence of gypsum or lime in the soil). However, in the opposite circumstances in which the nature of the irrigation water is such that it slowly increases the amount of sodium on the complex this would cause serious alkalinity over a long-term period, and historically has done so in some areas. Chemical and other remedial treatment of alkaline soils is discussed in the next section.