Soil resistance to penetration (RP) was determined by carrying out five in situ cone index measurements (0e50 cm) for each plot,using a Rimik CP40 cone penetrometer to measure soil compaction. The instrument recorded cone index values and used an ultrasonic transducer to measure the depth of insertion. The OM in the top layer of the soil (0e30 cm) was determined by measuring organic carbon using the dichromate oxidation procedure in which residual dichromate is titrated against ferrous sulfate. Some studies indicate organic carbon recuperation of 60e86%, so an adjustment factor was required