ABSTRACT

During agricultural operations such as bush clearing, soil preparation, planting, and crop harvesting the soil is subjected to high compaction stresses. However, the magnitude of stress-induced on the soil by agricultural machines is seldom studied, especially in the humid tropics. This study examines the response of Zea mays and Phaseolus vulgaris on loamy clay soil and loamy sand soil compacted at different levels. The soils were compacted at the following levels: 0 blow (control), 5 blows, 10 blows and 15 blows. Numerical simulation was performed using PLAXIS 2D to ascertain the stress distribution in the soil due to vehicular load. Results show that the coefficient of permeability of the soil was affected by compaction with more than a 70 % reduction in the loamy clay. The compaction of the soil led to a reduction in the plant height, biomass, and root density. There was a significant difference between the root weight based on the different compaction levels and soil type with p-values < 0.001. Generally, on both the loamy clay and loamy sand soils there were apparent signs of compaction stresses on the leaves of the Zea mays and Phaseolus vulgaris, making the plant leaves pale in colour and pancake-like. The from the numerical simulation show that stress distribution in the soil due to vehicular loads increase in the void ratio and increase in depth directly below the axles but increases with depth between the axles. There was a higher concentration of the effective stress at the midpoint (i.e., directly under the wheel) of the tyres compared to the stresses in between the axles; with a maximum of 107.53kPa, 101.67kPa, 87.49kPa, 79.19kPa, 72.31kPa for 0.1, 0.3, 0.5, 0.7 and 0.9 soil void ratios, respectively for the front axle. Rear tyres transmit more stress to the soil compared to the front tyres. It is concluded that soil compaction changes soil structure by increasing bulk density and penetration resistance and decreasing the total porosity of the soil, with negative consequences to crop growth and development.