THE EFFECT OF THE TRACTOR PASSES ON THE SOIL BULK DENSITY IN SILTY CLAY SOIL

Silty clay soil was plowed by deep digger mold board plow at two moisture content (M.C) levels of 15% and 24.5%. The soil was plowed at three plowing depths (10, 15 and 20cm). Tractor of weight of 32.3kN was passed once to five times on the soil surface for both moisture content levels. The soil bulk density at M.C. level of 24.5%was 1.11, 1.16, 1.20, 1.25 and 1.30ton/m^3 while at M.C. level of 15% was 1.22, 1.27, 1.32, 1.35 and 1.42ton/m^3 for depths 0-5, 5-10, 10-15, 15-20 and 20- 25ton/m^3 respectively. The plowing operation reduced the bulk density (average) of the soil from 1.31 and 1.20ton/m^3 to 1.07 and 1.02ton/m^3 for moisture content levels 15% and 24.5% (21% and 17.6%) respectively. When the tractor was passed once on the soil surface the bulk density of the plowed soil of M.C. level of 15% increased from 1.08 to 1.32ton/m^3 to become greater than that of unplowed (1.31ton/m^3). While for M.C. level of 24.5%, the bulk density increased from 1.02 to 1.12ton/m^3 but it remains lower than that for unplowed soil (1.20ton/m^3). However, when the tractor was passed five times, the bulk density of plowed soil for M.C. level of 15% increased to1.45ton/m^3. While for M.C. level of 24.5%, the bulk density increased to1.23ton/m^3 and in this M.C. level the bulk density of the plowed soil exceeded the unplowed soil at the fourth passes. The soil compaction was more sever at the shallower plowing depth than the deeper operating depth for both M.C. levels. For operating depth of 10cm the bulk density for M.C. of 15% increased from 1.10 to 1.35ton/m^3 for one tractor pass which exceeded the bulk density of unplowed soil (1.27ton/m3). However, for operating depth of 20cm the bulk density increased from1.06 to1.30ton/m^3. For M.C. of 24.5% the bulk density increased from 1.03 to 1.14ton/m^3 but remained lower than that for unplowed soil 1.21ton /m^3 but for operating depth of 20cm the bulk density increased from 1.019 to 1.10ton/m^3.