Soil Compaction Mechanisms Of Black Soil And Brown Earth In The Northeast Of China

Soil compaction occurs in wide range of soils and climates. Changes in a soil's structure due to compaction can lead to reduced porosity, increased strength and reduced permeability. Consequently, these adverse effects will reduce soil fertility.Brown earth and black soil were taken as representative materials to investigate the effects of mechanical compaction, soil water content, bulk density, incorporated organic matter and clay content on indicators of soil compaction evaluation, pre-compression stress and compression index using field and laboratory experiments. Compression curves and water retention curves of brown earth in different incorporated organic matter and clay content were tested. The effects of deep ripping and compaction on pores distribution and infiltration were determined by measuring soil strength and infiltration rate. Based on Soil physical quality S value evaluating soil quality after compaction, mechanism of soil compaction was discussed. Improvement solutions and suggestions were provided.1. Factors effecting on soil compressive propertiesThe factors include initial water content, bulk density, organic matter, clay content and applied stress.(1) Initial water content and bulk densityPre-compression stress higher and compression index lower, soil is less sensitive to soil compaction. The pre-compression stress of brown and black soil showed a positive correlation with bulk density and a negative correlation with water content. By contrast, the compression index for both soils showed a prominent negative correlation with bulk density, and positive correlation with water content. However, compression index of brown earth showed an insignificant dependence on water content.(2) Organic matter and clay contentOrganic matter content increased with the amount of incorporated rice hulls. Pre-compression stress of all treated soils was positively correlated with incorporated organic matter. The incorporated organic matter increased the pre-compression stress by approximately 20%for receiving every incorporated organic matter 20g/kg increment. Compression index slightlv decreased and later increased as the amount of incorporated organic matter content increased from the same initial level. Incorporated organic matter changed compressive properties. The mutual effects of the amount of incorporated organic matter content, initial levels of water content and bulk density significantly influenced the compression properties (P<0.01).Soil texture was changed after adding artificial clay content. Pre-compression stress and compression index increased with incorporated clay content increasing. The mutual effect of the amount of incorporated clay content, initial levels of water content and bulk density significantly influenced compressive properties (P<0.01).2. The effects of deep ripping and compaction on water infiltration and retention characteristicsTillage and mechanical compaction significantly influence soil water retention and infiltration properties in the deeper depth of soil, and it will last a long time.(1) Soil strengthBy comparing the values of soil strength for deep ripping, compaction and control treatments, it decreased in the sequence compaction> control> deep ripping at the depth of 0-25 cm and deep ripping>compaction> control at the depth of 25-40 cm. However, the influence of all treatments on soil strength weakened with time. After 90 days, the influence of three treatments still existed at the depth of 0-25 cm, but there were no significant differences at the depth of 25-40 cm.(2) Water infiltration propertiesDeep ripping had the greatest initial infiltration rate, whilst control treatment had the intermediate one, and compaction had the lowest. The stable infiltration rates were in the order control> compaction> deep ripping. After 150 days, the representative curves of deep ripping and compaction were got by using calibration theory. The results showed that compaction decreased the macro-pores of the upper depth of soil. Deep ripping can not only improve the infiltration capacity, but also restrain deep percolation.(3) Water retention characteristicIncorporated organic matter, clay content and applied stress significantly influenced soil water retention characteristics. Soil water holding capacity increased with the incorporated organic matter content, clay content and applied stress increasing under the higher matrix potential. Water content was higher and decreased slowly?(4) Soil physical qualityThe index of soil physical quality was used to evaluate the physical quality. The results showed that the index of soil physical quality increased with incorporated organic matter and clay content increasing. There was a negative correlation between the index of soil physical quality and applied stress.3. Soil compaction prevention should be on the base of soil typeReasonable tillage and increasing organic matter are two main solutions to avoid soil compaction.(1) The sensitivity of brown earth and black soil to soil compaction was different. Compared to brown earth, black soil had higher pre-compression stress and lower compression index. That means black soil had lower possibility to be compacted. But brown earth was in a high risk of compaction.(2) The solutions of soil compaction should be different. We should increase organic matter content of brown earth. For black soil, the strategy of avoiding soil compaction should be focused on controlling organic matter decreasing. On the other hand, tilling at appropriate water content and bulk density was another efficient solution. Brown earth has a short till suitable time, so the key to prevent brown earth from compaction was tilling at the right time and condition.