Soil surrounding a growing root must be displaced to accommodate the increased root volume. To ease soil penetration, root caps produce border cells and mucilage that lubricate the root surface, decreasing friction at the root-soil interface. Rhizosphere deformations caused by roots with or without a functional root cap were compared to determine the effects of the root cap on sand displacement and penetration. Intact (KYS wild type) and decapped (agt1(dec) mutant) primary maize roots were grown in observation chambers filled with sand. Non-destructive time-lapse micro-imaging combined with particle image velocimetry was used to visualize and quantify sand displacements as small as 0.5 mu m caused by growing roots. Decapped (agt1(dec)) roots displayed typical responses of mechanically impeded roots at sand densities that did not affect intact KYS roots. Sand displacement decreased exponentially with distance from the root and extended four to eight root radii into the sand. The calculated mean sand density increase and the compressed sand area were doubled by decapping. Maximum density often occurred in front of the apex of decapped roots whereas it occurred along the sides of intact roots. Periodic variation in sand deformation was observed, probably associated with root circumnutation, which may also facilitate soil penetration. Sand particles moved alongside KYS roots more easily than they did alongside agt1(dec) roots. A functional exuding cap was therefore essential for efficient rhizosphere deformation and penetration by roots. Manipulating root tip, and specifically root cap, properties is a possible target for improving root penetration in hard soil.