Effect of crop history on consistency of clayey soil of multi-purpose paddy fields
2005
Adachi,K.(National Agricultural Research Center, Tsukuba, Ibaraki (Japan)) | Hosokawa,H. | Yoshida, S.
Consistency of clayey soils in the plow layer of multi-purpose paddy fields was measured in fresh, air-dried, and oven-dried states. Relationship between the measured consistency and the crop history of the fields was analyzed. Plotting liquid limit (LL), plastic limit (PL), and plasticity index (PI) measured for fresh soil as functions of clay content revealed clear difference in response to the summer crops cultivated just before the sampling: the soils after the cultivation of wetland rice had higher values of LL, PL, and PI than the soils after that of upland crops. The LL, PL, and PI decreased by drying treatments. The difference relative to crop history became smaller by air-drying and almost disappeared by oven-drying. Though the LL and PL measured for air-dried and oven-dried soils were still increased with increase in clay content, the PI was nearly independent of clay content. These qualitative relationships supported the finding that activity, defined as the PI divided by clay content, decreased with increasing clay content. Plasticity index ratio, defined as the ratio of PI measured for the fresh to that measured for the oven-dried soil, is a promising index for the evaluation of change in soil properties resulting from the use of paddy fields for cultivation of upland crops. Whereas the plasticity index ratios measured for the soil under the cultivation of upland crops were smaller than 1.85, those of the soil just after the cropping of wetland rice were larger than 1.85. The plowed soils under the cultivation of wetland rice had plasticity index ratios greater than 2.0, which had decreased considerably from the surface layer at 4 years after the start of cultivation of upland crops, and settled to 1.4 through the plow layer at more than 10 years of cultivation.
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