Packning av jordbruksmark

Compaction is a big problem in agriculture today since it has negative effects on both yield and environment. The change in the environment caused by compaction will mean decreased quality of atmosphere, surface- and ground water and soil resources. It is therefore very important to know how different agricultural practices and technical solutions of machinery affect the soil and its functions in order to be ab1e to decrease the negative effects from farming. Three field experiments were carried out during the autumn 200l. Vertical soil displacement and vertical soil stress was measured at 15, 30 and 50 cm depth in three different tractor comparisons; wheel- and tracked tractor, single and dual wheel and pulling and non pulling tractor. For the pull and the track experiments macroporosity, saturated hydraulic conductivity and bulk density were also measured. Calculations of the vertical soil stress were also made. For the single- dual wheel experiment a John Deere 4640 with single wheel and a wheel load of 2500 kg and a Steiger with dual wheels and a wheel load of 2200 kg were used. Both tractors had 60 kPa tyre inflation pressure. The John Deere was also used, together with a cultivator, for the pull experiment. These two experiments were carried out at Staby Säteri, Uppland. The experiment with a tracked vehicle was carried out at Krenkerup Gods, Lolland, Danmark with a rubber tracked tractor weighing 18,5 tons and a wheel tractor weighing 9,7 tons. The tracked tractor was balanced before comparing it to the wheeled tractor to get the weight distribution as even as possible. Vertical soil stress and vertical soil displacement right under the wheels of the single and dual wheels did not differ significantly. The soil stress was lowest between the dual wheel at 15 cm depth but ii was evenly distributed under the wheels at 50 cm depth. No significant differences were found between a non pulling and a pulling tractor regarding vertical soil stress and soil displacement. There was however a tendency that the saturated hydraulic conductivity was lowest after the pass of a pulling tractor. It is possible that a higher pull would have affected the soil more. The soil stress was higher at 15 and 30 cm depth for the wheeled compared to the tracked tractor. There were no significant differences in soil displacement, saturated hydraulic conductivity and bulk density but there was a tendency that the bulk density was higher for the tracked than for the wheeled tractor. The most important conclusions were: (1) The two tyres in a dual wheel arrangement acted as two separate wheels which makes it more relevant to talk about wheelload than axle 1oad regarding the risk for subsoil compaction. This is a very important result because the recommendation today is to avoid an axle load over 6 tons. (2) Even for a well balanced tracked tractor the maximum soil stress will be considerably higher than the theoretically calculated. In spite of these problems and a higher total weight, a tracked tractor can have a smaller impact on the soil than a wheeled tractor. (3) The model calculations correlated well with the measured values in the subsoil but not in the topsoil!. A better approximation for the ground pressure than the tyre inflation pressure is however' needed since the ground pressure of ten was as much as twice as high as the tyre inflation pressure.