Bulk soil properties as the determinants of soil surface shear strength of puddled lowland rice soils of Sri Lanka affected by weed controlling methods

Soil shear strength (SS) is an important parameter for designing appropriate farm machinery. This study was aimed to estimate the surface SS of lowland puddled rice soils in Sri Lanka through the assessment of bulk soil parameters including soil moisture content (MC), bulk density (BD), soil texture and organic matter content (OM) under different weed control methods. Vane shear tester was used to measure the surface SS. All soil parameters were measured throughout the Maha Season of 2013/14, with four treatments; (1) bare lowland, (2) no weed control, (3) chemical weed control and (4) weed managed by “Asakura” wooden clog in three replicates. After identifying the effect of weed control method, measured values were evaluated to identify the suitable determinants to explain the variation of surface SS and ultimately built up their relationships. Results reveled that measured SS significantly varied with weed control method and time. Bulk soil properties did not significantly vary with weed control method (excluding clay %) and showed significant temporal variation (excluding sand% and OM) at p<0.05. Surface SS showed a significant relationship with BD and MC at 0.05 probability level, it could be estimated by 107.053 – 1.06 MC – 42.738 BD in chemical weed control fields and other fields by 110.643 – 1.06 MC – 42.738 BD. It is suggested to conduct future studies including depth variation of SS, weed and plant growth parameters which may provide sound information to improve this finding.