JWSS - Journal of Water and Soil Science

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  This study was undertaken to investigate the effects of soil compaction due to wheel traffic on corn growth and yield, in summer 2004 and 2005 at the Badjgah Research Station, Shiraz University located in North West of Shiraz, Iran. The treatments consisted of axial load in two levels and wheel traffic location treatments at four levels, including none, between rows, on rows and on entire plot area. The corn growth rate, root density and yield were not significantly influenced by axial load, but they were significantly affected by wheel traffic in all plots. Corn yield with wheel traffic on entire plot area averaged 3.97 Mg/ha which was significantly lower than the yields with the other treatments which averaged 6.65 Mg/ha with no traffic.

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For prevention of soil compaction, knowledge of allowable compression stress limit (compaction strength) in soil is important. Pre-compaction stress ( σ_pc) was introduced as soil compaction strength and often used as a criterion for evaluation of soil susceptibility to compaction. In this research, pre-compaction stress was measured for a sandy loam soil with plate sinkage (PST) and confined compression (CCT) tests. To prepare soil samples with different initial compactness, two soil water contents (17 and 19%db) and six pre-loading stresses (0, 25, 50, 100, 150 and 200 kPa) were used. The effects of soil water content and pre-loading stress on estimated pre-compaction stress were studied using a factorial experiment in a completely randomized design with three replications. The σ_pcvalues were significantly influenced by loading combination and soil water content. For PST, pre-load increase and higher soil water content resulted in higher and lower values of σ_pc , respectively. However, predicted σ_pc value increased with higher soil water content for CCT. The results also showed that the σ_pc predicted with PST was accurate, whereas the values obtained with CCT were 4.5 (at 17 %db) and 8.5 (at 19 %db) times higher than the applied pre-loads. Overall, the findings indicated that σ_pc prediction depends on the compression test, and PST could be a suitable method for soil pre-compaction stress (compaction strength) determination in sustainable soil management, i.e., soil trafficability and tillage. The PST method is also suitable to assess the effect of managing factors on pre-compaction stress.

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Forest future depends on forest stands regeneration. Soil is one of the principal capitals in the forest area. It is important to minimize damages to the forest ecosystem during logging operation. In forestry and forest management, it is significant to know the effect of forest logging damages on regeneration, sapling and soil compaction. This study was carried out in Asalem Beech forest area, in the north of Iran. The forest was harvested by selective cutting method. A systematic and randomized sampling method was used for data collection. Analysis of variance (ANOVA) techniques and binomial distribution were used for data analysis. The results showed 14.5 ± 0.99 percent of regeneration and saplings of forest stands were destroyed and damaged by logging operations with 20 m3/ha intensity per year. 9.12 ± 0.83 percent of saplings were totally destroyed and 5.3 ± 0.63 percent were damaged. These damages were caused due to felling, accumulating, skidding and landing. Soil bulk density in range of 0-10 cm depth, logging and skid trails areas were increased 17.54 and 35.61 percent, respectively. To reduce logging damages and soil compaction in forestry, and manage the forest, we need a suitable organization of timber extraction operation, careful designing and standard construction of skid trails on the basis of logging maps before felling the trees down.

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One important parameter in determining the allowable stress to prevent soil compaction is pre‌compaction stress (σpc). If the stress induced into the soil due to agricultural machinery traffic is lower than the σpc, the possibility of the irreversible (plastic) deformation is low. In this study, plate sinkage test (PST) and confined compression test (CCT) were used to determine the σpc of the disturbed soil samples obtained from the topsoil of a long-term organic amendment experiment. In the organic amendment experiment, organic manures (sewage sludgeو compost, farmyard manure) at three rates (25, 50 and 100 t ha-1) and one inorganic fertilizer combination (250 kg urea ha-1 and 250 kg ammonium phosphate ha-1) were added to a silty clay loam soil for seven years under wheat-corn rotation. The effect of organic and inorganic fertilizers and gravimetrical water contents (17.1 and 20.9% db) on the σpc values was determined using two statistical designs: 1) factorial design and 2) orthogonal contrasts in a completely randomized design with three replications. The σpc values were estimated by the following methods: 1) Casagrande, 2) maximum curvature and 3) intercept of virgin compression line (VCL) with the x-axis at zero strain. The results showed that in both tests, the σpc values for treatments amended with organic manures had over-estimations and the values of the over-estimation were increased as the manure application rate increased. The values of the over-estimations for CCT were higher than for PST. Therefore, it can be concluded that the results of the PST are more reliable. The σpc values estimated by any of the three procedures depend on the manure treatment as well as the initial water content of the soil. For determining the stress at the threshold of the compaction for the un-manure soils (control and inorganic fertilizer) or the treatments amended with low application rate (25 t ha-1) of the organic manures, the PST and the Casagrande procedure can be recommended. In contrast, for the soils amended with high application rates (50 and 100 t ha-1), the PST and the maximum curvature procedure can be used.

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