The main objectives of this research were: to determine the responsability of weight, number of passes and independent variables, on the vertical distribution of subsoil compaction and to obtain evidence to decide upon the convenience of matching equipments with light or heavy tractors, according to their differences in passes on the field. Field tests were carried out on grassland, on a typic Argiudol with a soil moisture below, but near to field capacity. The dependent experimental variable was the induced soil compaction and was related to bulk density (assessed with gamma probe), penetration resistance (measured with an electronic cone penetrometer), and remaining grassland yield, six and eight months after traffic treatments were applied. Data from two tests, with two phases of data logging were analysed, totalizing a three year period of assessment pursuit. Results did not show differences between heavy and light treatments in the shallower depth range. Nevertheless, highly significant differences were shown if the deepest range is considered. Decrease in grassland yields ranging from 7 to 25% were measured in out-of-track areas, and 52 to 76% in intrack areas. It was concluded that the number of repeated passes on the same tramlines of a light tractor, can do as much or even greater damage than the heavier tractor with fewer passes. Ten is the critical number of passes, beyond it, advantages taken from the use of a light tractor are lost.
traffic intensity; bulk density; penetration resistance; grassland yield
