The herbicide treatments had significant effect on percentage organic matter of the soil (P < 0.001) (Table 4 and Figure 4). Weeks of herbicide treatment and the interaction between herbicides and weeks of treatment had similar effect (P < 0.001) (Table 4). The percentage organic matter of the soils treated with herbicides reduced significantly when compared with the control. Soil samples treated with glyphosate had the lowest percentage organic matter contents of 1.25, 2.35 and 3.17 of all herbicide treated soils after the second, fourth and sixth week of treatment, respectively (Figure 3).