Earth has a renewed interest in eco-friendly building materials. Starch is a promising additive, but its strengthening origin needs understanding to promote better solutions according to soil variability. This multiscale study examined natural clays and starches interactions. Clays and starches were evaluated based on mechanical and rheological properties. Kaolinite was found to be better reinforced by starches. At microscopic scale, starches modified the arrangement of grains and clays, but the role of botanical origin is unclear. At molecular scale, the ratio between amylose and amylopectin led the interactions with kaolinite. These results help to understand how bio-additives affect earth-based building materials.