6 th International Conference on Trends in Agricultural Engineering 7 - 9 September 2016, Prague, Czech Republic SOIL PHYSICAL PROPERTIES AS AFFECTED BY REPEATED WHEELING F. E. Berisso Institute of Technology, School of Biosystems and Environmental Engineering, Hawassa University, Hawassa Ethiopia Abstract A field experiment was carried out in the spring of 2015 to analyse the effect of field traffic on soil physical properties. A tractor used for wheeling and the experiment had a randomized block design with four replicate plots. Control and four repeated wheeling were used as treatments. Intact cores of 100 cm 3 were collected from and used for determination of soil mechanical and physical properties. The inflicted compaction was signifi- cantly increased bulk density at 10 and 20 cm depths, whereas the saturated hydraulic conductivity was reduced. At -6 and -30 hPa, the air-filled porosities were consistently lower for compacted soil than for control soil; whereas for most soil the values were higher than value proposed as the critical lower limit for plant growth. Our study hence documents that commonly used agricultural machinery may compact the soil to 0.3 m and even deeper with effect on important soil functions. Key words: soil compaction, bulk density air –filled porosity, saturated hydraulic conductivity. INTRODUCTION In recent years, a growth in agricultural economy of Ethiopia has increased the dependence of agricultural operations on modern machineries such as tractors and combines harvesters. An increasing number of farmers are purchasing these machineries and associated drudgeries with farm operation are eased. On the other hand, high costs of these machineries, combined with timeliness of agricultural operations, force the owners to hire these equipments and operate even in wet soil condition, which increases high risk of soil compac- tion. Compaction is a major problem affecting agricultural soil structure and this consequently leads to a reduc- tion to crop production. Direct cause and effect rela- tions appear to exist between the use of machinery and soil compaction, between soil compaction and a plant root environment, and between a plant root environ- ment and crop production (HAMZA AND ANDERSSON, 2005). Compaction by wheel traffic, cultivation equipment, animals or natural processes can affect soil water movement by increasing bulk density and de- creasing porosity and infiltration (ARVIDSSON, 2001). These changes can result in less soil water storage, poor nutrient movement, slowed gas exchange and restricted root growth, all of which can cause a reduc- tion in crop yields (LIPIEC AND HATANO, 2003). Due to these deleterious effects, soil compaction has received greater attention from different stakeholders, including public authorities and international policy makers, especially in developed countries. For in- stance, the European Union (EU) proposed an EU Soil Framework Directive to protect soil against threats that undermine its capacity to perform environmental, economic, social and cultural functions (COMMISSION OF THE EUROPEAN COMMUNITIES, 2006). Soil compac- tion has also been the subject of many studies, which are reported in books, research articles and review papers. Many international series of field experiments and projects have also been initiated at different times in order to address compaction effects on soil and crop yield, for example the Working Group on Soil Com- paction by Vehicles with High Axle Load in 1980 (HÅKANSSON ET AL., 1987) and the POSEIDON pro- ject in 2009 (WWW.POSEIDON-NORDIC.DK). Compared with these numerous studies and efforts worldwide, there are few reports from Ethiopia. Moreover, most of the data concerns the primitive agriculture or few estate farms alone. For instance, TADDESE ET AL. (2002) conducted study in primitive agriculture and reported higher bulk density in heavily grazed than in nongrazed medium grazed plots. TESHOME AND KIBRETE (2009) characterized soils of soil in one of the estate farms in terms of their physi- cal and hydraulic properties and reported variation in soil bulk density. They attribute the observed differ- ences entirely to soil management classes by disre- garding the effect of agricultural machinery. In Ethiopian context, there are a lack of studies that focus on the effect stresses due to agricultural machin- ery on soil compaction. The aim of this project was to analyse how soil compaction during field traffic ef- fects soil pore structure and associated transport proc- 57