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Taskin Oztas, Ali Kilic Ozbek, Ekrem Lutfi AKSAKAL University of Ataturk, Faculty of Agriculture, Department of Soil Science, Erzurum, Turkey Abstract Soil structural behavior varies with soil management practices and input of soil amendments. Polysaccharides stabilize soil aggregates because of their contribution as cements and glues. The objective of this study was to determine the effects of polyvinylalcohol (PVA) on aggregate stability and other structural properties. The PVA with the rates of 0.001 and 0.003 and 0.005 w/w were applied on soils with different texture. Changes in structural parameters such as bulk density, porosity, dispersion rate, wet aggregate stability, mean weight diameter, and aggregate strength were determined. Soil structural development was evaluated by comparing the structural parameters in soils with or without PVA. Result of our study indicated that the PVA application to soil had a significant effect on aggregate stability and strength. Increase the rate of application, the higher the aggregate stability and strength of individual aggregates against crushing forces. Introduction Synthetic polymers added to soil as soil conditioners improve soil's physical properties that are important for plant growth and increase soil's resistance against disruptive forces and erosion. The formation of soil structure requires both physical rearrangement of particles and the stabilization of the new arrangement. Aggregate stability, an important soil physical property, affects crop production indirectly through its effects on water, aeration, temperature, mechanical resistance, and soil erosion (Letey, 1985). It influences a soil's ability to transmit or retain water and response to erosive forces. The presence or absence of water-stable aggregates at the soil surface has a direct effect on the potential for sheet erosion, crust formation, and excessive runoff during storms (Shouse et al. 1990). Soils with stable surface aggregates resist water and wind erosion better than soils with unstable aggregates (Lehrsch, 1998). Organic polymers have been used quite effectively to stabilize soil structure in recent years. Many researchers have shown that the application of polyacrylamide maintained high infiltration rate during rainfall and reduced soil surface sealing and runoff soil losses (Smith et al. 1990; Uysal et al.. 1995; Ben-Hur and Keren, 1997; Sojka et al. 1998; Green et al. 2000). The objective of this study was to determine the effects of polyvinylalcohol (PVA), a hydrophilic polymer, on aggregate stability and other structural properties of soils different in texture. Material and Methods Three soil samples different in texture (clay, sandy clay loam, and sand) were collected and carried to the laboratory. The samples air-dried and passed through a sieve with openings of 2 mm. About 500 g soil samples were put into plastic pots and treated with hydrophilic polyvinylalcohol at three different rates of 0.001, 0.003 and 0.005 w/w. The PVA solutions were prepared at 70 0C and applied to the surface of soil samples with soil moisture content near field capacity, and mixed using a spatula. After 48 h, changes in structural parameters such as bulk density, porosity, dispersion rate, wet aggregate stability, mean weight diameter, and aggregate strength were determined. Soil structural development was evaluated by comparing the structural parameters in soils with or without PVA. The analysis of variance (ANOVA) was performed for determining the treatment effects, and the Duncan's multiple comparison test procedure was used for mean comparisons. Results and Discussion Some physical and chemical properties and structural parameters of the soils studied were given in Table 1. Soils with different textures were chosen for this laboratory experiment in order to determine the effectiveness of PVA application in different soils. Clay soil had higher amount of organic mater content than sandy and sandy clay loam soil. Because of differences in texture and organic matter content, clay soil had the highest wet aggregate stability, mean weight diameter and aggregate strength values, but the lowest dispersion rate, initially.  Table 2 shows the changes in measured structural parameter values after PVA application. As it was seen bulk density of samples decreased with increasing doses of PVA application. This may due to micro structural development in soils with PVA, which caused in high pore volume in these soils as supported by porosity values. The rates of decrease in bulk density values were 14, 9, and 20 % for SCL, S, and C textured soils, respectively. There was a statistically significant relationship between PVA doses and bulk density values, which was explained by a linear function (Fig.1). The PVA application to soils caused significant increases in wet aggregate stability. In SCL and S textured soils, aggregate stability increased up to 95 %. However, in a clay soil it was only 72 %. That means may be that all particles in silt and clay fraction in SCL and sandy soils were aggregated by the highest dose of PVA, and most of the aggregates in these soils were water stabile. However, in clay soil the highest dose of PVA may not be sufficient to aggregate all of silt+clay. The effect of PVA doses on wet aggregate stability of soils was given graphically in Fig.2.  Mean weight diameter of soils increased at least twice in SCL and S textured samples at the first dose application of PVA. In clay soil the rate of increase was about 60 % at the first dose, and reached up to 100 % at the highest dose. The most interesting results were obtained in aggregate strength measurements that were determined by crushing individual aggregates between two parallel plates using a modified pocket penetrometer (Oztas et al. 1999a,b). Individual aggregates from the samples treated with PVA needed higher mechanical stresses for breaking down. Higher aggregate strength indicates that bonding mechanisms linking primary particles together in an aggregate are very strong. Even in sandy samples, aggregates resisted against some degree of mechanical stress. In clay samples, the aggregate strength values were greater than 5 kg cm-2. Aggregate strength increased with increasing application dose, but there was no statistically significant difference between 0.001 and 0.003 doses, but 0.005 dose was significantly different than the others with a highest mean value. The standardized aggregate strength values (kg cm-2 g-1) increased with increasing rate of long/short diameters in both sandy and sandy clay loam texture samples (Fig.3).  Data were subjected to analysis of variance (ANOVA) to determine the treatment effect. The ANOVA results indicated that the treatment effects were statistically significant at p > 0.001 significant level considering the bulk density, aggregate stability, mean weight diameter, and aggregate strength as dependent variables. The Duncan's multiple comparison test results also showed that the control sample has completely different means of bulk density, porosity, dispersion rate, wet aggregate stability, mean weight diameter, and aggregate strength than the sample means treated with PVA. Because of the limited page number, ANOVA and the Duncan's multiple comparison test results were not presented in this paper.  References . Ben-Hur, M.& Keren, R. 1997. Polymer effects on water infiltration and soil aggregation. Soil Sci. Soc. Am. J. 61:565-570. . Green, V.S., Stott. D.E., Norton, L.D. & Graveel, J.G. 2000. Polyacrylamide molecular weight and charge effects on infiltration under simulated rainfall. Soil Sci. Soc. Am. J. 64:1786-1791. . Lehrsch. G.A.1998. Freeze-thaw cycles increase near surface aggregate stability. Soil Sci. 163(1). 63-70. . Letey. J. 1985. Relationship between soil physical properties and crop production. Advances in Soil Sci. 1: 277-294. . Shouse, P.J., Gerik, T.J., Russell, W.B. & Cassel, D.K. 1990. Spatial distribution of soil particle size and aggregate stability index in a clay soil. Soil Sci. 149 (6). 351-360. . Smith, H.J.C., Levy, G.J. & Shainberg, I. 1990. Water droplet energy and soil amendments: effect on infiltration and erosion. Soil Sci. Soc. Am. J. 54:1084-1087. . Sojka, R.E., Lentz, R.D. & Westermann, D.T. 1998. Water and erosion management with multiple application of polyacrylamide in furrow irrigation. Soil Sci. Soc. Am. J. 62:1672-1680. . Oztas, T., Sonmez, K. & Canbolat, M.Y. 1999a. Strength of individual soil aggregates against crushing forces: I. Influence of aggregate characteristics. Tr. J. of Agriculture and Forestry. 23:567-572. . Oztas, T., Sonmez, K. & Canbolat, M.Y. 1999b. Strength of individual soil aggregates against crushing forces: II. Influence of selected soil properties. Tr. J. of Agriculture and Forestry. 23:573-577. . Uysal, H., Taysun, A. & Kose, C. 1995. The effects of some polymers depending on soil properties on erosion under the laboratory conditions. I. Akalan Toprak-Çevre Sym. II. Cilt. pp: C101-C111. Menemen, Turkey. |