Rake Angle Effect On Draft Power Requirement: Case Of A Sub-Soiler In Nitisols

Abstract

Draft animals are key to the provision of draft power for tillage in small-holder farms that have limited access to tractors. There is limited information on guidelines for utilization of animal draft, particularly for sub-soiling which is a more recent conservation tillage technology. The limited information on draft requirement for sub-soiling that demands large amounts of draft has resulted in sub-soiling tools that are poorly adapted to local conditions and are neither economical nor environmentally sustainable. Further studies are required to address insufficient on-farm draft for sub-soiling by correctly matching the prime mover with the relevant implement based on accurate prediction of draft power requirement. This may result in reduced time for the operation, less draft power requirement, reduced wear/tear and significant savings on sub-soiling costs. A study was initiated at University of Nairobi, College of Agriculture and Veterinary Sciences, Kabete Campus whose broad objective was to evaluate the effect of varying the rake angle on the draft power requirement for a sub-soiler in Nitisols. The specific objectives were to: Identify the soil physical characteristics at the experimental field pertinent to sub-soiling, Assess the effect of rake angle, speed and depth of tillage on specific draft force tillage, Establish prediction model for specific draft force based on rake angle, speed and depth of tillage and Identify rake angle, depth and speed for optimization of specific draft force. An experimental plot of 50m long by 5m wide was used. A block of 30m long by 5m wide was used as a pilot area prior to the beginning of the experimental runs to enable the tractor and the implement attain the required speed and depth. Two tractors were used in this study. The first tractor towed the second tractor with a dynamometer attached between to measure draft. The Sub-soiler was hitched on the second tractor. Readings from the dynamometer were taken at pre-determined xiv intervals. The parameters investigated for the draft measurement were forward speed and depth of sub-soiling. For the five rake angles, two forward speeds and three depths were used in combination for 30 treatments. Tillage depth was measured as the vertical distance from the top of the undisturbed soil surface to the implements deepest penetration point. Effects of rake angle on draft power were assessed by ANOVA using the linear mixed model in Genstat Statistical Package. Regression was used to fit a model to predict draft force from tillage depth, speed and rake angle. The difference in draft power between tillage speeds was not significant (P=0.088). Rake angle and tillage depth had significant main (P<0.001) and interactive effects on draft power requirement (P=0.023). For small rake angles (<400), the draft power increased linearly with sub-soiling depth while at greater rake angles (>40o) the relationship was also linear but skewed to the left. The least square fit for second order relationship coefficient between rake angles and draft force at the 300 mm was 0.956 and significant (P<0.05) allowing prediction of the draft requirement for the sub-soiler on clay soils. In conclusion, the critical tillage depth for sub-soiling on clay soils is at least 300 mm. Sub-soilers in Kenya are currently produced with a general rake angle of 50o for all types but for clay soils, there is need to increase the rake angle to 72o for optimal sub-soiling. Further studies should focus on assessing the effect of sub-soiler rake angle on power requirement in Nitisols at tractor ploughing speeds and multi-locational studies for other soil types such as Ferralsols which are prone to compactions. Keywords: Subsoiler‟s Rake Angle, Critical Depth, Draft Power Requirement, Sub-soiling,