HIGH mechanical resistance of soils, related to naturally occurring hard pans, or to compaction by machinery and untimely cultivation1–3, often reduces crop yields through poor development of root systems1,4–6. In the laboratory, progressive increases in mechanical resistance of the growth medium cause reduced root elongation7–9, stunting and thickening of root systems3,10 and reduced shoot growth11. Loosening of subsoil in the field can reduce compaction, increase aeration and waterholding capacity of the soil and improve the distribution of available water1. Roots can then penetrate to greater depths and crop yields often increase1,12,13. White light also affects root growth, inhibiting axial extension in some species14–16. This effect can be counteracted in cress roots by 3,5-diiodo-4-hydroxybenzoic acid (DIHB)17–19. The fact that compacted soil and white light can both induce stunting and thickening of root systems suggested to us that similar growth control mechanisms might be involved. We have therefore investigated whether DIHB can improve root development in compacted soils; we have shown that this is the case.