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This study was in an olive ( Olea europea L.) grove in the Vélez Blanco District of Almería, Spain, where the soil is a typical Aridisol. The aim was to evaluate subsoil compaction caused by three different tractors currently used in olive groves. Measurements were made of (i) the cone index ( CI), (ii) hydraulic conductivity ( HC) and (iii) rut depth after passage of a light tractor ( LT = 22.50 kN), a heavy tractor ( HT = 42.60 kN) and a medium tractor ( MT = 33.30 kN). The CI differed for the topsoil (0-200 mm) for each type of tractor after up to five passes. In this depth soil level, the CI was greatest for LT because the ground pressure (by narrow tyres) was greater than under the MT and HT. For deeper layers, there was a strong positive relationship between number of tractor passes and CI values, and the CI was greater for passes by the HT than the LT or MT. The HT resulted in shallower ruts up to the fifth pass, and the CI values were smaller because there was less ground pressure from this tractor than the others. In all treatments, tractor traffic caused varying decreases in HC in the 0-600 mm depth range. The main conclusion is that subsoil compaction is related directly to tractor weight. For the three tractors, topsoil compaction is caused by ground pressure and not on total axle load. [ABSTRACT FROM AUTHOR]