Roots provide basic functions to plants such as water/nutrient uptake and anchoring in soil. The growth and development of root systems contribute to colonizing the surrounding soil and optimizing the access to resources. It is usually admitted that the variability of plant root architecture results from the combination of genetic, physiological and environmental factors, in particular soil mechanical impedance. However this last factor has never been investigated at the soil grain scale. In this paper, we are interested in the effect of the disordered texture of granular soils on the evolution of external forces experienced by root cap. We introduce a numerical model in which the root is modeled as a flexible self-elongating tube that probes a soil composed of solid particles. By means of extensive simulations, we show that the forces exerted on the root cap during its growth reflect interparticle force chains. Our extensive simulations also show that the mean force declines exponentially with root flexibility, the highest force corresponding to the soil hardness. Furthermore, we find that this functional dependence is characterized by a single parameter that combines the granular structure and root bending stiffness. This finding will be useful to further address the biological issues of mechanosensing and thigmomorphogenesis in plant roots.