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Mustafa Canbolat, Taskin Oztas, Kenan Barik, Ekrem L. Aksakal Ataturk University, Faculty of Agriculture, Department of Soil Science, Erzurum, Turkey Abstract The objective of this study was to determine the compactibility of two soil aggregate fractions at different moisture contents, and to evaluate their effects on soil properties. Six soil samples different in organic matter content and soil texture were used. Two sets of aggregate size fractions (< 4.76 mm and < 2.0 mm) were prepared, and drinking water was added to samples until they reach the levels of different moisture tensions. Standard Proctor test was used for defining compactibility at three different compaction levels (5, 15 and 25 blows). Maximum bulk density, optimal moisture content, porosity, pore volume, and penetration resistance were evaluated. Results indicated that soil type, the levels of compaction and moisture content had significant effects on measured soil properties. Maximum dry density increased with increasing number of hammer blows for all soils studied. Introduction In many studies it was reported that soil compaction caused an increase in soil bulk density and soil strength, therefore, resulted in poor hydraulic and physical soil properties (Carter, 1990; Smith, 1997; Arridsson, 1998). Soil compaction, which refers to an increase in soil bulk density or decrease in soil porosity, also reduces crop yield. The objective of this study was to evaluate the effects of different compactive loads at different moisture contents on the maximum dry density, porosity, and penetration of soils with different texture and organic matter content. Material and Methods Two sets of aggregate size fractions (< 4.76 mm and < 2.0 mm) from six soil samples different in organic matter content and soil texture were subjected to different levels of compaction (5, 15 and 25 blows) using a Proctor hammer at 5 different moisture levels (pF4.5, pF4.2, pF3.3, pF2.7, pF1.8, and pF1.0) in the laboratory. The dry densities, optimal moisture contents, porosities, and penetration values of soils were determined. Results and Conclusions The soils used in this study had different texture and organic matter content. Clay content of soils varied from 19 to 34 %, and organic matter content between 2.54 and 7.36 %. Our results indicated that the dry density of soils increased with increasing in compaction level for all soils, and the 25 hammer blows produced the highest density values. However, the optimum moisture content, which is the moisture content where the maximum dry density was reached, decreased with the number of hammer blows increased.  Figure 1 shows the relationship between moisture content and dry bulk density for one of the soils studied. As it was seen the dry density increased with increasing in moisture contents until the maximum dry density was reached and decreased after that point. The results of the analysis of variance showed that the effects of soil type, and the number of blows on maximum dry density, porosity, and cone resistance were statistically significant at p< 0.001 level. Soil with highest organic matter content had the lowest mean of maximum dry density. This may indicate that organic matter makes soil more resistant to compaction as stated by Arridsson (1998). There were small differences in the mean of the optimal moisture contents of the soils, except the soil that had the highest amount of organic matter content. In general, soils with high amounts of sand had the highest maximum dry density and the lowest optimum moisture content, but soils with finer textures had the highest optimum moisture and lowest dry density values. The LSD mean comparison test results indicated that soils with fine texture and having higher amounts of organic matter had the highest mean porosity values. The effect of the number of hammer blows on soil porosity was very clear. While the mean porosity value was 54.43 at 5 hammer blows, it went blow to 50.62 and 49.02 at 15 and 25 blows, respectively. References . Arridsson, J. 1998. Influence of soil texture and organic matter content on bulk density, air content, compression index and crop yield in field and laboratory compression experiments. Soil&Tillage Research. 49:159-170. . Carter, M.R. 1990. Relative measures of soil bulk density to characterize compaction in 7tillage studies on fine sandy loams. Canadian J. of Soil Sci.. 70:425-433. . Smith, CW, Johnston,MA&Lorentz, S. 1997. Assessing the compaction susceptibility of South African forestry soils. II. Soil properties affecting compactibility and compressibility. Soil&Tillage Research.43:335-354. |