SIGNIFICANCE OF CARBONATES IN IRRIGATION WATERS

Abstract

Though black alkali occurs commonly in soils irrigated by the Nile in Egypt, none has been found in the valleys of the Tigris and Euphrates in Iraq. The Nile water contains residual Na2CO3 (i.e., CO3 in excess of Ca plus Mg) whereas this is not the case in the waters of the Tigris and Euphrates. Agreement is indicated, also, between the occurrence of black alkali in the soils of the San Joaquin Valley of California and the presence of residual Na2CO3 in the water supplies. Relations are outlined between the salt constituents of irrigation waters (particularly the concns. of Ca, Mg, and CO3) and the formation of Ca saline, Na saline, and black alkali soils. The usefulness of % Na values for irrigation waters as indices to the possibility of Ca replacement and impermeability conditions in soils is greatly enhanced if the "% Na possible" also is computed. The latter value is the one that best applies to the soil soln. after CaCO3 precipitation. The abandonment of millions of acres of once irrigated but now Ca saline land along the Tigris and Euphrates has been the consequence of salt accumulation (largely Ca and Mg salts). Drainage facilities have never been constructed in Iraq. Only drainage works and leaching would be required for the reclamation of what at one time was evidently a flourishing Mesopo-tamian irrigation agriculture.