Soil salinity and alkalinity in the Great Konya Basin, Turkey

In the summers of 1964 to 1968 a study was made of soil salinity and alkalinity in the Great Konya Basin, under the auspices of the Konya Project, a research and training programme of the Department of Tropical Soil Science of the Agricultural University, Wageningen.The Great Konya Basin, some 300 km south of Ankara, at an altitude of about 1010 m, covers about 10 000 sq. km. For centuries it has been an important agricultural area.The Basin is surrounded by mountains, draining into it by rivers, surface run-off and subsurface seepage. Some of the riverwater is used for irrigation. Since the Basin has no outlet, the remaining water collects in the central depression, whence it evaporates.The climate is semi-arid with a dry warm summer and a cool rather wet winter. As annual evaporation considerably exceeds precipitation, salts collect in many places in the soil or on the surface. This causes increasing salinity harmful for agriculture, especially since Na +is the major cation so that most saline soils are alkali-effected as well. The distribution of the salts has been related to hydrology and topography.During the summer of 1964, 1965 and 1966, numerous salt-affected soils have been studied and analysed as basis for a survey of salinity and alkalinity during one week of September 1967. In the survey about 800 samples were taken from over 250 soil profiles and 500 wells at representative sites throughout the Basin were sampled, after analyses of 500 groundwater samples in May 1967 to check variations in depth and electrical conductivity of the groundwater during the dry season. Taking into account the physiography and distribution of the soil types, the collected information has been presented in sketch maps of salinity and alkalinity of the soils, and depth, chemical composition and properties of groundwater throughout the Basin.To study salinity, some new techniques were introduced: the sum of the cations in the saturation extract has been related to its electrical conductivity. This relation shows geographic variation.Salinity in the Great Konya Basin occurs mainly in three areas: the western central part of the Basin, a depression north of the Town of Karapinar, and the central eastern part of the Basin. The north and south are not salt-affected. Generally salinity increases with decreasing altitude, except for the central western part, where this trend is disturbed by irrigation. The low parts also have the highest watertable, with very high electrical conductivity. There salinity is caused mainly by supply of salts from subsurface flow from the higher soils towards the central depression.The salts accumulating in the soil chiefly consist of sulphates and chlorides; sulphates predominate in the west, chlorides in the east.The alkalinity map, based on sodium adsorption ratio (SAR), shows a similar pattern to salinity.Dynamics of the salts in the soil have been examined. Efflorescence has been studied by chemical and X-ray analysis, and by electron-microscopy. It had an influence on evaporation from the surface. A salt crust considerably decreases further salinization because of decreased evaporation. Three types of salt crusts, differing in morphology and structure, were distinguished.Consequences of salinity and alkalinity for agriculture in the Great Konya Basin are discussed. Recommendations for increasing yields and for preventing or decreasing salinity in irrigated soils are given.