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1 Mehmet Ali Çullu, 2 İsmail Çelik, 1 Ahmet Almaca 1 University of Harran, Faculty of Agriculture, Dept. of Soil Science, Şanlıurfa-TURKEY 2 University of Çukurova, Faculty of Agriculture, Dept. of Soil Science, Adana-TURKEY ABSTRACT In the research, hydraulic conductivity, structural properties, salinization and clay mineralogy of widely distributed 4 soil in the Harran Plain were investigated following irrigation. At the study, structure stability index values were 30.37-70.67 %, aggregation index values between 0.03-0.28 mm and hydraulic conductivity values were changing from 0.07 to 1.51 cm/h. ESP values were determined between 1,03-45,10 %. Smectite is the dominant clay mineral in the soils of the Harran Plain followed by palygorskite and kaolinite respectively. Results revealed that a slight decrease was observed in structure stability and aggregation indexes after irrigation, however decrease in hydraulic conductivity was significant. Moreover, soil salinity were significantly increased in soils, and there were no change in clay mineral contents. INTRODUCTION Misuse of soils caused degradation of their productivity in relatively short periods. The degradation of physical, chemical and mineralogical characteristics of soils reflects this event to product as much as degradation rate. Özcan and Çetin (1996) stated that, soil is an essential support of human life, not only in relation to our food supply but also for production of fiber and shelter. The rapid increase in the rate of land use as a result of pressures from population, technology and industrial development have caused the acceleration of land degradation. According to Bilgehan (1998), irrigated agriculture enables intensive agriculture while increasing yield. Although, irrigated agriculture provides some benefits such as yield increases, it brings about some problems. The most important of the problems are variations in the infiltration and drainage rates, salinization-alkalinization and degradation of soil structure. When the water is applied, particles of soil with high smectite clay content swell considerably due to hydration of expandable layers (Rolstan et al., 1984). Such swelling reduces the cross-sectional area of soil pores. The process of swelling is more pronounced in the presence of the high sodium or low salt concentration, or both in the soil water. Dispersion of fine soil particles is controlled by a similar mechanism. Dispersion is directly influenced by ions adsorbed on particle surfaces, particularly clay minerals. The presence of high sodium, especially at the low salt concentration in the soil water causes dispersion and movement of the fine particles within the pores. The particles may then became lodged in smaller pore, blocking waters and airs. The impact of salinity is manifested by the degradation of the soil structure (Richards, 1954; Rhoades, 1990). The application of irrigation water to areas with abundant halite (common in arid and semiarid areas) and more than 15 % exchangeable sodium lead to the alkaline hydrolysis. If the soil is low in chloride and calcium, and irrigation water applied rich in exchangeable sodium bicarbonate or sodium carbonate, the clay particles in the soil absorb the sodium and magnesium salt by losing their permeability, water infiltration and oxygen for plant roots. The aim of this study is to investigate the degradation of selected, widely distributed, soil series with high water tables due to irrigation. MATERIALS AND METHODS In the study, 4 disturbed and undisturbed soil samples which were threaten by salinization were taken from widely distributed soils series of the Harran Plain. In each series, soil samples were sampled from the upper 3 horizons belonging to the root zone. The soil moisture regime for the area is Xeric-Aridic and the temperature regime is Thermic (USDA Soil Conservation Service, 1993), the mean annual evaporation is 1945 mm and the average annual rainfall is 390.1 mm (Köy Hizmetleri Şanlı Urfa Araştırma Enstitüsü, 1997). Soil samples were separated according to sample sizes, which would be needed for different laboratory analyses. The analyses conducted were: Texture (Bouyoucos, 1951), CaCO3 (Allison and Moode, 1965); pH, EC, CEC and exchangeable sodium (Richard, 1954), structural stability index (Leo, 1963), aggregation index (Yeşilsoy and Berkman, 1974). The saturated hydraulic conductivity values were measured according to (Klute and Dirksen, 1986) using undisturbed soil samples of 100 cm3 volume. The dry bulk density was determined using the core method of (Blake and Hatrge, 1986). The clay mineral analyses were conducted according to Jackson (1979). RESULTS AND DISCUSSION In this study, 4 soil series of the Harran Plain, which are affected by groundwater, were investigated for determining physical and chemical degradation. Disturbed and undisturbed soil samples were collected from 4 soil series. The results were compared with previous data obtained taken before irrigation. Irrigation of the Harran Plain has started in 1995 and after irrigation, the water table increased and inhibited plant growth due to degradation of the chemical and physical characteristics of the soils.  The hydraulic conductivity of the soils were significantly decreased, also a slight decrease were observed in the structure stability and aggregation indexes. Bilgehan, (1998) stated that, structural characteristics of soils and aggregate stability in water must be determined in soils of rainfed areas before irrigation in order to monitor of changes degradation following irrigation. Chemical analyses showed an increase in ESP and EC with depth (Fig. 1 and Table 2), depending on annual rainfall, evaporation and land use. Çullu et al. (1999), indicated salinity fluctuations during the year because of different agricultural applications to gather with shallow water tables and high evaporations.  There is a negative relation between ESP and hydraulic conductivity (Fig. 1). However the evaporation and leaching depth as well as intensity of salt accumulation has affected this relation. The high value of ESP causes degradation of soil structure, decreases hydraulic conductivity creating variations during the year with soil depth.  Yeşilsoy et al. (1992) suggested that, in order to extrapolate the possible changes taking place in soils after irrigation, to develop a map indicating the present infiltration rates and alkalization tendencies. The examination of these maps reveals that there is an inverse relationship between infiltration rates and alkalization tendencies. These results suggest that improper management practices, which promote alkalization, might cause further decreases in infiltration rates and increase the dimensions of the problems. Table 3 is indicated the clay mineralogy of salt affected widely soil series of the Harran Plain. Results revealed that no more important changes, which is a pleasure for irrigation. However, high exchangeable sodium is threatening the productivity of the soil. Dinç et al. (1988) stated that, with the beginning of irrigation, palygorskite may weather to smectite and this will decrease hydraulic conductivity of the soil.  However dominant smectite may probably cause dispersion and swelling of the topsoil together with high exchangeable sodium (Table 3). The swelling and dispersion of soil clays affect water transmission properties by altering the geometry or continuity of soil pores (Rolston et al., 1984). The hydraulic properties of soil, such as hydraulic conductivity, are highly dependent on both exchangeable sodium percentage and the salt concentration of the percolating soil solution. As a result, the GAP irrigation caused on increase on the height of the water table of Southern part of the Harran Plain, causing salinization. This is affected the physical and chemical properties of some soil series and decreased plant production, gradually. REFERENCES Allison, L. E. and Moode, C. D. (1965). Carbonate (C. A. Black Editör) Methots of soil analysis. Part 2. Agronomy series. No 9. Am. Soc. of Agronomy. Wisconsin Series. P:1379-1396. Bilgehan. G., (1998). Structure characteristics of soils in Harran Plain before irrigation. pp.208-214. M. Şefik Yeşilsoy Int. Sym. On Arid Region. Menemen, İzmir. Blake , G. R. and Hatrge K. H. (1986). Bulk density. In: Methods of soil analysis. Part 1, Physical and mineralogical methods (E. A. Klute). Agr. Monogr. 9. ASA and SSSA, Madison, pp:363-375. 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