Wheat and pea roots were grown through soil crumbs until they encountered cores of more finely structured earth. The cores were prepared from a loam at various bulk densities and matric suctions. Over a range commonly found in the field, the penetration and growth of the roots were controlled chiefly by the strength of the soil. Soil strength should be regarded as a property that has a general influence on root elongation, rather than as a limiting condition encountered in unusual soils.