Modelling the Effect of Soil Compaction on the Growth of Maize and Cowpea in Humid Tropics
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 and its impact on plant growth is seldom studied, especially in the humid tropics. Firstly, this study performed laboratory experiments to investigate the growth response of Zea mays and Phaseolus vulgaris planted on loam-clay and loamy sand soils compacted at different levels, and secondly, numerical simulation was performed using PLAXIS 2D (soil modelling software) to determine the stress propagation in the soil due to vehicular loading. The soils were compacted at the following levels: 0 (control), 5, 10 and 15 blows using the modified standard proctor hammer (4.54 kg). Following soil compaction, there was up to 70% reduction in the permeability of the loam-clay soil from cm s-1 (control) to cm s-1 (15 blows). The plant height, biomass and root density all reduced due to soil compaction. There was a significant difference between the root weight due to the different compaction levels and soil type (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 result from the numerical simulation show that the stress distribution in the soil due to vehicular loads increases with increase in the void ratio of the soil. 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 107.53 kPa, 101.67 kPa, 87.49 kPa, 79.19 kPa, 72.31 kPa 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, penetration resistance and decreasing the total porosity of the soil, with negative consequences to crop growth and development.