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Cevdet Şeker 1 , A.Ali Işıldar 2 , Saim Karakaplan 1 1 Department of Soil Science, Faculty of Agriculture, University of Selçuk, 42031 Konya-Turkey 2 Department of Soil Science, Faculty of Agriculture, University of S. Demirel, Isparta-Turkey ABSTRACT The aim of this experimental study was to estimate soil compaction caused by wheel traffic using some soil properties. The study was conducted on a sandy loam soil at the experimental area of Agricultural Faculty, University of Selçuk. The soil surface was compacted by passing a tire wheel tractor once, twice or four times. The soil was sandy loam and its water content was approximately at field capacity. Penetration resistance and some soil properties were measured in the soil profile in order to determine their interactions. The results showed a positive relationship between penetration resistance and bulk density, and negative relationships between penetration resistance and total porosity, void ratio, air porosity and drainage porosity. Correlation coefficients of those relationships were 0.874**, -0.850**, -0.856**, -0.852** and -0.708**; respectively. INTRODUCTION Wheel traffic of agricultural prime movers is well recognised as a major contributor to detrimental soil compaction (Hunter, 1991; Wood et al., 1991). Soil penetration resistance, bulk density, and pore size distribution have been used for determination of soil compaction (Gupta et al., 1989; Carter, 1990). Soil penetration resistance may vary rapidly depending on soil water content changes, soil structure and stoniness of soil (O'Sullivan et al., 1987; Busscher, 1990). Soil penetration resistance readings that need to be compared are often taken at different soil water contents. Because, soil water changes may significantly affect soil penetration resistance, it is often difficult to determine the penetration resistance differences caused by water content or treatment (Busscher, 1990). To be able to compare penetration resistance readings, it would be necessary to adjust for differences from water content changes (Busscher, 1990). The relationship between penetration resistance and soil water content depends on soil physical properties, such as bulk density, soil porosity, texture and structure (Gerard et al.,1982; Spivey et al.,1986; Busscher, 1990; Carter, 1990). Calibration of soil penetration resistance readings with regard to soil water content changes takes a long time and is difficult. The purposes of this article are to develop relationships between soil penetration resistance readings and bulk density, total porosity, void ratio, air porosity and drainage porosity, and to estimate soil compaction from cited soil properties if soil conditions are different such as soil water content, soil structure and stoniness. MATERIALS AND METHODS The study was conducted on a Calcic xerosol sandy loam soil at the experimental area of Agricultural Faculty, University of Selçuk. The soil has relatively high sand and calcium carbonate content. Particle size distribution was determined by the hydrometer method (Day, 1965); clay (<2 m): 24 %; silt (2-50 m): 26 % and sand ( 50-2000 m): 50 %. Water content as a percentage of dry weight representing field capacity ( 25 %, w/w ) and permanent wilting ( 12 %, w/w) was determined according to Peters (1965). Soil pH and electrical conductivity in water (1:2.5) was 7.78 and 192 dS/m, respectively ( Peech, 1965; Bower and Wilcox, 1965 ). Organic matter content of the soil was 2.25 % ( Allison,1965 ). The CaCO3 equivalent of the soil was 29 % ( Allison and Moodie, 1965). Particle density was 2.65 g.cm-3 (Blake, 1965a ). Bulk density and total porosity, void ratio, air porosity, drainage porosity were measured according to Blake, (1965b) and Vomocil, (1965), respectively. All measurements were made at three different soil layers ( 0-10, 10-20 and 20-30 cm ) and replicated thrice. Experimental plots ( 15x30 m ) were tilled to a depth of 10 cm. Plots were irrigated to the measurement depth by sprinkler irrigation. The soil water content was determined periodically, sampling holes not affected subsequent measurement of the physical properties. When the soil water content was about the field capacity, the soil surface was compacted by passing a tire wheel tractor once, twice or four times. A two wheel drive Steyr model 8073-70 tractor, with a power of 51.5 kW, a weight of 2900 kg was used. The pressure of the rear and front tires of the tractor were 0.18 Pa and 0.21 Pa, respectively. The tractor forward velocity was 4.5 km.h-1. Penetration resistance of every soil layer was calculated by taking on arithmetic mean of six replications. A soil penetrometer, with a cone angle of 300 and cone diameter of 12.83 mm, was used to determine of soil penetration resistance . It was pushed by hand into the soil to a depth of 30 cm, and penetrometer resistance for each 1 cm depth interval was recorded. Data were subjected to correlation and regression analyses by using SPSS software (SPSS, 1988). RESULTS AND DISCUSSION The effects of different passing number of a tire wheel tractor on soil cone penetration resistance are shown in Fig 1. Passes of the tractor on the soil surface increased cone penetration resistance. Maximum increase of cone penetration resistance occurred at the soil surface (0-10 cm). Bulk density, total porosity, void ratio, air porosity and drainage porosity of the untrafficked soil and of the soil after 1, 2 and 4 passes of the tire wheel tractor are illustrated in Figs 2 to 6. Passes of the tractor increased bulk density and decreased total porosity, void ratio, air porosity and drainage porosity (Figures 2-6). 
Analysis of correlation was done to determine the relationships between cone penetration resistance and bulk density, total porosity, void ratio, air porosity, drainage porosity and available water porosity (AWP; size of between 8.6m-0.2m) Correlation coefficients of relationships between penetration resistance and measured soil properties are given in Table 1. The relationships between penetration resistance and total porosity, void ratio, air porosity and drainage porosity were found take negative and statistically significant at the 1% level. Correlation coefficients of these relationships were -0.850, -0.856, -0.852 and -0.708, respectively.  Statistical analyses and regression equations relating penetration resistance and bulk density, total porosity, void ratio, air porosity and drainage porosity are given in Table 2. Coefficients of determination of regression equations between penetration resistance and bulk density, void ratio, air porosity, total porosity and drainage porosity were 0.83, 0.83, 0.81, 0.80 and 0.60, respectively.  CONCLUSIONS - Penetration resistance was affected importantly by soil water content. Therefore, there were various difficulties to compare each other penetration resistance readings at different soil conditions such as soil water content, soil structure and soil stonier. - There were statistically significant relationships between penetration resistance and bulk density, total porosity, void ratio, air porosity and drainage porosity. - Bulk density, total porosity, void ratio and air porosity which have high coefficients of determination with penetration resistance could be used to compare the soil compaction at different soil conditions. REFERENCES Allison, L.E., (1965). Organic carbon. In: Black, C.A. (ed.), Methods of Soil Analysis. Part II, American Society of Agronomy. Madison, Wisconsin, 1367-1378. Allison, L.E., Moodie, C.D., (1965). Carbonate. In: Black, C.A. (ed.), Methods of Soil Analysis. Part II, American Society of Agronomy. Madison, Wisconsin, 1379-1396. Blake, G.R., (1965a). Particle density. In: Black, C.A. (ed.), Methods of Soil Analysis. Part I, American Society of Agronomy. Madison, Wisconsin, 371-373. Blake, G.R., (1965b). Bulk density. In: Black, C.A. (ed.), Methods of Soil Analysis. Part II, American Society of Agronomy. Madison, Wisconsin, 374-390. Bower, C.A., Wilcox, L.V., (1965). Soluble salt. In: Black, C.A. (ed.), Methods of Soil Analysis. Part II, American Society of Agronomy. Madison, Wisconsin, 933-951. Busscher, W.J., (1990). Adjustment of flat-tipped penetrometer resistance data to a common water content. Transactions ASAE 33: 519 - 524. Carter, M.R., (1990). Relative measures of soil bulk density to characterise compaction in tillage studies on fine sandy loams. Can. J. Soil Sci. 70: 425-433. Day, P.R., (1965). Particle fractionation and particle-size analysis. In: Black, C.A. (ed.), Methods of Soil Analysis. Part I, American Society of Agronomy. Madison, Wisconsin, 545-566. Gerard, C.J., Sexton, P. Shaw, G., (1982). Physical factors influencing soil strength and root growth. Agronomy J. 74: 875-879. Gupta, S.C., Sharma, P.P. DeFranchi, S.A, (1989). Compaction effects on soil structure. Adv. Agron. 42: 311-338. Hunter, A.G.M., (1991). Soil-vehicle interaction. J. Terramechanics 28: 297-308. O'Sullivan, M.F., Dickson, J.W. Campbell, D.J., (1987). Interpretation and presentation of cone resistance data in tillage and traffic studies. J. Soil Sci. 38: 137-148. Peech, M., (1965). Hydrogen-ion activity. In: Black, C.A. (ed.), Methods of Soil Analysis. Part II, American Society of Agronomy. Madison, Wisconsin, 914-926. Peters, B.D., (1965). Water availability. In: Black, C.A. (ed.), Methods of Soil Analysis. Part I, American Society of Agronomy. Madison, Wisconsin, 279-285. Spivey, L.D., Busscher, W.J. Campbell, R.B., (1986). The effects of texture on strength of southeastern Coastal Plain Soils. Soil Till. Res. 6: 351-363. SPSS, (1988). SPSS/PC+V.2.0. Base Manuel for the IBM PC/XT/AT and PS/2, Marija and Moruis. SPSS Inc. Vomocil, J.A., (1965). Porosity. In: Black, C.A. (ed.), Methods of Soil Analysis. Part I, American Society of Agronomy. Madison, Wisconsin, 315-318. Wood, R.K., Morgan, M.T., Holmes, R.G., Brodbeck, K.N., Carpenter, T.G. Reeder, R.C., (1991). Soil physical properties as affected by traffic: Single, dual, and flotation tires. Transactionss ASAE 34: 2363-2369. |