ESTIMATING THE QUANTITATIVE VARIABILITY OF SOIL STRUCTURE UNDER THE EFFECT OF DIFFERENT LEVELS OF COMPACTION

Field traffic is the main cause of soil compacting, especially on the topsoil, and its effect may extend to the subsoil, which reduces the biological activity of the soil and plants. In order to evaluate changes in soil structure, this study was conducted in silty clay loam soil (Typic Torrifluvents) plowed to a depth of 20 cm under the effects of tractor passes (without Passes (C-0), once passes (C-1), twice passes (C-2) and four-times passes (C-4) on the same track) and for three soil depth ranges, (0-10, 10-20 and 20-30 cm). Soil structural properties, bulk density, total porosity, air-filled porosity, and soil penetration resistance are used as indicators to assess changes in soil structure due to compaction. The passes treatments were conducted at optimum moisture content for the compaction. The results indicated that with the increase in the number of passes, the average values of bulk density and penetration resistance increased until it reached its highest value of 1.51 Mg m(-3) and 1.843 MPa, respectively, and this was accompanied by the highest decrease in the values of total porosity 43.020% and the air-filled porosity 5.24% were occurred at the effect of the passes repetition treatment was four times. The same effect also occurred for soil depth in the mentioned traits. The highest value of apparent density was 1.47 Mg m(-3), the penetration resistance was 1.632 MPa, the highest decrease in the total porosity value is 44.528% and the air-filled porosity was 6.20%, were found at the depth 20-30 cm. The results showed that the one-time passes treatment achieved the highest percentage of change in the values of the study indicators for all depth ranges. The highest soil compaction rate was observed within the depth range of 10-20 cm for all passes treatments based on the percentages of change in the studied traits except for penetration resistance. The depth of the effective compaction reached the soil within the depth range of 20-30 cm when dealing with one-time pass treatment and its amount increased with the passes sequence. The results also showed that the values of bulk density and total porosity exceeded the critical value under the influence of the passes treatment twice at the depths of 10-20 and 20-30 cm and the passes treatment four times for all ranges of soil depth. While the air-filled porosity and penetration resistance overtaken the critical limits under the influence of the one-time passes treatment at the depths of 10-20 and 20-30 cm, and the passes treatments twice and four times for all ranges of soil depth. The studied soil seems to be very sensitive to compaction, has a weak structure, and less organic matter content, so it needs an efficient management system to avoid the risk of compaction.