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AND SOME SOIL MOISTURE CONSTANTS USING PATH ANALYSIS Nutullah Özdemir, Coşkun Gülser, Tayfun Aşkın Ondokuz Mayıs University, Faculty of Agriculture, Department of Soil Science, Samsun/Turkey ABSTRACT The relationships between some soil physical and chemical properties such as, bulk density (rb), clay content (C), organic matter (OM), cation exchange capacity (CEC), lime content (LC) and exchangeable Na (Exc-Na) and some soil moisture constants such as, field capacity (FC), permanent wilting point (PWP) and available water capacity (AWC) were studied using path analysis on 44 surface soil samples (0-20 cm) around Samsun. Soil moisture constants showed positive relationships with C, OM, CEC and negative relationships with rb, LC, Exc-Na. It was determined that the direct effects of some soil properties on AWC were in the following order; C > LC > rb > Exc-Na > CEC > OM. On the other hand, the indirect effects of soil properties varied among soil moisture constants. The indirect effects of the soil properties generally became through clay content and bulk density. Clay content was the most effective soil property that affected water retention in soils. INTRODUCTION Knowledge of soil water relationships is essential for determining the type of plants to be grown, plant spacing, yield and managing soil water systems. Water holding abilities of the soil are related to removing water from the soil by drainage or evapotransporation and storing water in the soil by rainfall or irrigation. Soil water retention is a basic soil property that is influenced by some soil physical and chemical properties and is needed for the study of plant available water, infiltration, drainage, hydraulic conductivity, irrigation, water stress on plants and solute movements. In most soils, optimum growth of plants occurs, when the soil water retention is kept near the field capacity or at least does not approach the permanent wilting point (Brady, 1974). Field capacity (FC) is the percentage of water remaining in a soil after soil is wetted and allowed to drain one to two days. It represents the upper limit of water available to plants, usually defined as 0.1 to 0.3 bars tension. Permanent wilting point (PWP) represents to lower limit of water available to plants, usually defined as 15 bars tension. Water retained by soil between the field capacity and the permanent wilting point is considered available for plant use and defined as the available water capacity (AWC) (Brady, 1974). Although the soil moisture constants are based on an equilibrium established with forces exerting differing degrees of tension on the water, they have utility for many agronomic purposes and for measuring relative differences in available water capacity within and among soils (Bauer and Black,1992). Bahtiyar (1975) studied on relationships between field capacity, permanent wilting point and some soil properties, explained that these constants can be estimated by means of developed regression models. A unit increase in organic carbon concentration caused a relatively larger increase in weight percentage at the field capacity than at the permanent wilting point in coarse and moderately coarse soils. But in medium and moderately fine or fine soils a unit increase in organic carbon concentration caused essentially identical increases in weight percentage at field capacity and permanent wilting point (Bauer and Black, 1992). A soil system can be thought as a network of soil properties. Path analysis may be used to investigate the relationships among these soil properties . The path diagram gives a picture of network of relations among the characters, as quantitative evaluation is possible from the data (Wright, 1968). The objective of this study was to determine the relationships between some soil physical, chemical properties and some soil moisture constants such as field capacity, permanent wilting point and available water capacity, using path analysis. MATERIALS and METHODS Soil samples were taken 44 surface soil (0-20 cm depth) around Samsun. The soils have mostly alluvial, and partly colluvial character. Annual mean of precipitation is 927.6 mm, annual evaporation is 600 mm and mean temperature is 15.2 0C (Anonymous, 1994). Some soil physical and chemical properties were determined as follows; soil particle size distribution by the hydrometer method (Demiralay, 1993); lime content by Scheibler Calsimeter (Soil Survey Staff, 1993); pH in 1:2.5 (w/v) soil-water suspension by pH-meter (Black, 1965);exchangeable Na by ammonia acetate extraction; exchangeable Ca+Mg by titration; cation exchange capacity according to Bower method (U.S. Salinity Lab. Staff, 1954). Soil organic matter was measured by Walkley-Black method (Kacar, 1994). Bulk density was determined by means of the clod method (Demiralay, 1993). Soil water concentration by weight at the field capacity was measured on samples passing a 2 mm sieve, saturated for 24 hours and then equilibrated for 24 hours at 33 kPa on a ceramic plate. The permanent wilting point was measured on samples passing a 2 mm sieve, saturated for 24 hours, and then equilibrated for 72 to 96 hours at 1500 kPa on a pressure-plate apparatus. Available water capacity was calculated from AWC = FC - PWP. Where AWC is available water capacity (g water 100 g-1 soil); FC is field capacity (g water 100 g-1 soil) and PWP is permanent wilting point (g water 100 g-1 soil) (Klute, 1986). The soil moisture constants were selected as dependent variables to determine statistical relationships between some soil properties (rb,, C, OM, CEC, LC and Ex-Na) and the soil moisture constant such as FC, PWP and AWC. Also, direct and indirect effects of the variables were determined with path analysis (Wright, 1968), using TARIST software. RESULTS & DISCUSSIONS Soil Properties : Descriptive statistical results for some soil physical and chemical properties and soil moisture constants (FC, PWP and AWC) are given in Table 1.  According to Table 1, the results can be summarized as; soil samples have mostly fine in texture, light to moderate in pH, low in organic matter (average of 1.76 %), moderate in lime content (average of 4.78 %), and free alkaline problem (ESP<15 %) (Soil Survey Staff, 1993). Relationships Between Soil Properties And The Soil Moisture Constants : The correlation coefficients between some soil properties and the soil moisture constants are given with direct and indirect effects of the variables on the soil moisture constants in Table 2.  According to Table 2, clay content showed significant positive relations with all the soil moisture constants at p < 0.01. Direct effects of clay content on FC, PWP and AWC were found to be higher than that of the other soil properties. Also, the soil properties had higher indirect effects through clay content on the soil moisture constants. It indicates that clay content was the most important soil property that affected water retention in soils. In the indirect effects of clay content through the other soil properties on FC (17.0 %) and AWC (15.5 %), bulk density (rb) was the most effective soil property. However, the indirect of C through CEC (18.2 %) was found to be the most effective soil property on PWP. On the other hand the direct effect of CEC on PWP (20.3 %) was higher than that on FC (7.3 %) and AWC (15.4 %). Thus, the clay content play important roles in the adsorption and desorption of water molecules. The surface adsorptive forces of clay minerals greatly affect water retention because of the permanent negative charge of clay mineral particles and the polar nature of water (Petersen et al, 1996). Other than clay content, CEC and OM gave significant positive relations with all the moisture constants at p<0.01 level. Also, the higher indirect effects of OM on the soil moisture constants usually became through bulk density. Bulk density decreases with increasing organic carbon concentration (Bauer and Black, 1992). Soil organic matter influences water retention because of its hydrophilic character and its influence on soil structure and bulk density (Klute, 1986). Increasing soil organic matter increase plant available water holding capacity (Kern, 1995). Bulk density gave the significant negative correlations with FC, PWP at p<0.01 and AWC at p<0.05 level. As known that bulk density decrease with increasing clay content in soil. It is expected that water retention is also increase with decreasing bulk density. The total porosity of sandy soils is less than that of fine textured soils (Hillel, 1982). For this study it can be expected that decreases in bulk density increase the total porosity and water retention because of the high clay content of the soils. Other than bulk density, lime content (LC) and Exc-Na percent also showed the negative correlations with the soil moisture constants. LC had higher direct effects on all the moisture constants after clay content. Lime content was negatively related with FC and AWC at p<0.05 level and negatively related with PWP as non significantly. The behavior of particle size distribution of LC in soils might be the similar to coarse fractions of soil texture. Macropores are important recharge pathways of water but can eventually became plugged due to accumulation of CaCO3 (Stephens, 1996). Therefore increasing the lime content may decrease water retention due to the decreasing macropores. PWP is expected to related with micropores in soils. Thus, lime content did not give highly significant correlation with PWP. There were negative relations between Exc-Na percent with FC and PWP at p<0.05 level. Increments in Exc-Na percent in soils tent to exhibit very poor physical properties if the clay content is fairly high (Bolt and Bruggenwert, 1978). In conclusion, soil moisture constants gave the significant positive correlations with C, CEC and OM at p<0.01 and negative correlations with bulk density, LC and Exc-Na percent. Clay and lime contents showed the higher direct effects on all the moisture constants. Except lime and clay contents, direct and indirect effects of soil properties varied among soil moisture constants. The indirect effect of the soil properties were generally became through clay content and bulk density. Clay content was found to be the most effective soil property that influenced water retention in all the soil moisture constants. REFERENCES Anonymous (1984). Samsun İli Çevre Durum Raporu. Çevre İl Müdürlüğü Yayınları, No:36. Bahtiyar M (1975). Toprakta su tutulması ve hidrolik iletkenliğin tahmin edilebilme olanakları üzerinde bir araştırma . Atatürk Üniversitesi Ziraat Fakültesi (Doktora tezi). Erzurum. Bauer A., Black A. L. (1992). Organic carbon effects on available water capacity of three soil textural groups. Soil Science Society American Journal 56:248-254. Black, C.A. (1965). Methods of Soil Analysis. Part 1, American Soc. of Agron. , No 9. Soil Survey Staff (1975). Soil Taxonomy. Agr. Hdbk. No.436, Soil Cons.Ser. USA Bolt, G.H. and Bruggenwert, M.G.M. (1978). Soil Chemistry. A. Basic Elements. Second revised edition, Elsevier Scientific Publishing Company. Brady, C.N. (1974). The nature and properties of soils. 8th edition. Macmillan Publishing Co.,Inc. New York. Demiralay, İ. (1993). Toprak Fiziksel Analizleri. Atatürk Üniversitesi Ziraat Fakültesi Yayınları, No:143, Erzurum. Hillel, D. (1982). Introduction to Soil Physics. Academic Press Limited, 24-28 Oval Road, London. Kacar, B. (1994). Bitki ve Toprağın Kimyasal Analizleri:III, Toprak Analizleri, s:149-165, Ankara Üniversitesi Ziraat Fakültesi Eğitim, Araştırma ve Geliştirme Vakfı Yayınları No:3, Ankara. Kern J. S. (1995). Evaluation of soil water retention models based on basic soil physical properties. Soil Science Society American Journal 59:1134-1141. Klute, A. (1986). Water Retention: Laboratory methods. In A. Klute (ed.), Methods of Soil Analysis, Part I, Second edition, Agron. Monogr. p: 635-662, 9 ASA and SSSA, Madison, WI. Petersen, L.W., Moldrup, P., Jacobsen, O.H. and Rolston, D.E. (1996). Relations between specific surface area and soil physical and chemical properties. Soil Sci. Vol. 161, No. 1, 9-20, USA. Soil Survey Staff (1993). Soil Survey Manuel. 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