| This study was one part of the project of National Natural Science Foundation of China'The relationship of applied stress duration and soil water potential under different moisture and carbon content soil'. The soil water potentials under different durations of applied stress on soil, especially during 0.1～2.5s period were measured, and the results were compared with Larson(1980) under 30 min duration. The Larson curve was developed. Soil water potential was defined as indictor of soil compaction. The critical load was determined, in which the soil would not over compacted. The effect of soil organic matter content and soil moisture on the critical load was discussed.The conclusions were as followed:1. Larson theory was applicable in the black soil of Northeast of China. The similar curve was found on the relationship of soil water potential and applied stress under 30 min duration. The corresponding applied stress value to the water potential threshold could be defined as the highest load that soil would not be over compacted. The soil water potential increased with the soil moisture increasing, and the corresponding threshold of load decreased, namely, the risk of soil over compaction increased. Compared to SOM 8 g/kg black soil, the thresholds of load in SOM 110 g/kg black soil under 11%, 15%, 19% and 24% moistures were increased 1.04kg/cm2, 0.39 kg/cm2, 0.49 kg/cm2 and 0.62 kg/cm2, respectively.2. The soil water potential was greatly influenced by the duration of applied stress, namely, the relationship of soil water potential and applied stress in Larson curve was different with that in farmland, an error existed. The values of soil water potential changed in four organic matter soils 8g/kg,50g/kg,80g/kg and110g/kg, four soil moistures 11%, 15%, 19% and 24% under three applied stresses and duration from 0.1 s to 1800 s. The water potentials decreased in four organic matter content soils under 0.3 kg/cm2 and 1 kg/cm2 loads and load durations from 0.1 s to 1800 s. And they increased with soil organic matter contents increasing under the same soil moisture condition. Compared to the soil water potential under 0.1s with that of 1800s, the soil water potential increased. All these showed the Larson curve measured under 30 min duration could not show the critical threshold of load in the farmland. The soil water potential decreased with the load duration extended. A warp existed in Larson curve, and it increased with soil moisture decreasing, applied stress decreasing and soil organic matter contents increasing, namely, the warp increased under the decreasing of the risk of soil over compaction. The critical threshold of load under vehicle traffic velocity was lower than that determine by Larson method, which means that increase vehicle traffic velocity could reduce the risk of soil over compaction.3. The applied stress threshold below which soil would not be over compacted predicted by Larson curve was lower than that under field vehicle traffic velocity. Compared with the applied stress threshold measured by Larson method 30 min duration in a 50 g/kg organic matter content and 15% water content black soil, the applied stress threshold under 0.1s, 0.5s and 2.5s durations were lower 0.58 kg/cm2, 0.47 kg/cm2 and 0.26kg/cm2, respectively. In the 30 min duration of applied stress, the threshold of soil bulk density was 1.33g/cm3, and soil saturated conductivity was 0.0584 cm/s. And that the threshold of soil bulk density was from 1.22 g/cm3 to 1.33g/cm3, and soil saturated conductivity was 0.0584 cm/s under the duration of applied stress from 0.1s to 2.5s. These indicated the Larson curve had a low prediction value, all of these induced by the prediction error of threshold below which soil would not be over compacted. |
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