Background and Aims Root system development is affected by soil conditions. The effects of bulk density, water content and penetration resistance on root development processes were investigated in peach trees. Methods Peach tree rootstocks were grown in various soil conditions, combining two bulk densities (1.2 and 1.5 g soil. cm(-3)) and three water contents (0.14, 0.17 and 0.20 g.g(-1) soil). Root parameters (tip diameter, length of apical unbranched zone, branching density and diameters of main and lateral roots) and plant growth (leaves, branches, trunk, root dry mass) were measured. Root growth processes (elongation, branching) were studied using relationships between root parameters. Results The proportion of biomass allocated to each plant compartment was similar whatever the soil conditions. Variations in root development were best explained by the variation in penetration resistance, rather than other soil properties. Increased soil penetration resistance reduced the root elongation rate, especially for thick roots. In addition, the branching pattern was affected. In soil with a high penetration resistance, the root system shape differs from a typical herringbone pattern. Conclusions These results allow quantification of the root system plasticity, and improve our understanding of the interactions between root development and soil properties.