The Effect of Tillage Management and Cover Crop Cultivation on the Components of the Soil Compression Curve in Different Matric Suctions

Abstract
Background and Objectives
Soil compaction is a crucial factor that limits plant growth and reduces yield. To assess the level of compaction, the compaction curve is used, which shows the relationship between compressive stress and void ratio. This curve comprises three main components: pre-compression stress, compression index, and swelling index. The soil behaves elastically along the swelling line but becomes plastic and irreversible at the end of the virgin compaction line. The point where these two lines intersect is known as pre-compaction stress. The soil swelling index indicates its resilience and elasticity, while the compaction index reflects its ability to resist or undergo compaction. Some researchers have used the ratio of swelling index to compaction index to express soil's ability to recover after being subjected to stress. As long as applied stress does not exceed pre-compaction stress, soil exhibits elastic properties. To date, no comprehensive study has been conducted on how tillage type and cover crop type affect confined compression curve parameters. Therefore, this research aims to investigate how changing tillage systems and cover crop types impact soil compaction characteristics at different suctions.
Methodology
This study was conducted at Bu-Ali Sina University, Hamedan, Iran, to investigate the effect of tillage type and cover crop type on the components of the confined compression curve. The cover crop factor was tested in three levels: Vicia ash, Lathyrus sativus, and no cover crop. The tillage factor was tested in three levels: conventional tillage, minimum tillage, and no tillage. The experiment was designed as a randomized complete block factorial with three replications over two consecutive crop years. The Experiments were conducted at three suctions of 6, 30, and 1500 kPa. The confined compression curve was measured using a uniaxial device under a loading rate of 1 mm/min. The test consisted of two stages: loading and unloading. During the loading stage, 100 readings were taken at 0.01 mm intervals with an additional force of 10 kPa applied to the sample at each interval. During the unloading stage, 33 readings were taken at 0.03 mm intervals. Finally, the components of the confined compaction curve including pre-compaction stress, compaction index, and swelling index were determined.
Findings
The results indicated that the highest compaction index values for suction levels of 30 and 1500 kPa were observed in the tillage treatment without cover crops, with values of 0.63 and 0.75, respectively. The effectiveness of tillage and cover crop types was found to be dependent on soil suction levels, with significant changes in soil compaction occurring at lower suctions. Conservation tillage was observed to improve soil compaction compared to conventional tillage. At the suction level of 6 kPa, the cover crop had a significant effect (p < 0.05) on the swelling index, while the effect of tillage on this parameter was not significant. The reduction in compaction index due to cover crop application may be attributed to increased binding properties of soil particles, reduced deformation against external forces, and increased soil resilience. At the suction level of 6 kPa, higher moisture content resulted in differences in mechanical characteristics of the soil due to different cultivation systems and plant types. Overall, the results indicated that an increase in soil suction leads to an increase in pre-compression stress. However, no significant changes were observed in the three parameters investigated at suctions of 1500 kPa and 30 kPa. It is possible that the interaction between tillage type, cover crop type, and moisture content at this suction level contributed to the increase in pre-compaction stress observed in the conservation tillage-cover crop treatment. This may be due to improvements in soil structure, which have a significant impact on soil compaction properties. At the matric suction of 1500 kPa, the highest compaction index value was observed in the tillage treatment without cover crops (0.754). At lower suctions, cover crops were found to reduce compressibility by increasing soil elasticity, while treatments without cover crops were more susceptible to compaction.
Conclusion
In conclusion, the study investigated the impact of tillage practices, cover crop types, and soil suction levels on soil compaction properties. The findings revealed that the compaction index values were highest in the tillage treatment without cover crops at suction levels of 30 and 1500 kPa, indicating greater susceptibility to compaction in the absence of cover crops. The effectiveness of tillage and cover crops varied depending on soil suction levels, with notable changes in compaction occurring at lower suctions. It was observed that treatments that increase organic matter in the soil, decrease soil compressibility. Overall, the results have shown that using cover crops and conservation tillage systems in semi-arid areas plays an important role in reducing soil compaction.