The efficacy of glyphosate (N-phosphonomethyl glicine) was measured using different concentrations of soil into the spray solution. Wheat plants, at 4th leaf stage, were sprayed with water containing 0, 5000, 10000 and 15000 ppm of Chicureo soil (30.6% clay), with two glyphosate rates (0.48 and 0.72 kg ai·ha-1), at two spray volumes (200 and 400 L·ha-1). Twenty-one days after application, foliar damage percentage, mortality and fresh weight of treated plants were measured. Results showed that when using both the lower rate and spray volume, glyphosate efficacy decreased progressively when increasing the soil concentration of the spray solution. By contrast, at the higher spray volume, glyphosate activity decreased notoriously with the presence of soil, independently of glyphosate rate. Hence, glyphosate efficacy depends on soil concentration of the spray solution, spray volume and glyphosate rate.