This study aimed to apply statistical response surface models (RSMs) to evaluate morphological parameters of plant root system changes in response to humic acid (HA). The study was carried out in a growth chamber. Experimental treatments consisted of HA extracted using IHSS methodology at increasing doses: 0, 20, 40, 60, 80, 110, and 150 mg HA L⁻¹. Seedlings were collected at 24, 48, 72, 96, 120 and 144 h, and the rice root system was analyzed with WinRHIZO software. Different parameters of the root systems were analyzed. The stimulation pattern of increasing doses of HA on the root morphology of rice seedlings showed a quadratic response surface, presenting with high statistical significance (p < 0.001). The rice root system during the early growth stage was highly stimulated by HA, especially the superfine and thick root diameter classes. The RSMs proved to be robust and useful in studying the bioactivity of HA, as they integrated dose and time variables, which are usually addressed separately, resulting in confusing interpretations. The response surface modeling approach showed that the effect of HA is cumulative during the initial period of formation of rice seedlings, which may result in more vigorous seedlings for crop planting.