Abstract The effect of variation in the amount, composition, form, origin and distribution of organic matter on the reaction of soil to compactive loading under both laboratory and field conditions is reviewed. For experimental purposes, soils having different organic matter contents may be obtained using samples from a number of sites or from a single site at which different organic matter contents have developed as a result of differences in crop management practices, or by the incorporation of organic material in laboratory samples or in field soils. Compactibility tests made with uniaxial compression and impact loading equipment may not give rise to similar conclusions concerning the role of organic matter. Living roots, and to a lesser extent dead roots, provide a filamentous network which, like geotextiles, resists compactive loads. Fungal hyphae have a similar action, particularly within aggregates. Highly humified material increases the stability and strength of aggregates, and hence decreases compactibility. Soil management practices influence the form, amount and distribution of organic matter in soils. The incorporation of straw or other bulky organic materials results in local concentrations of undecomposed material. Undecomposed organic litter may accumulate at the soil surface or within the top few centimetres after zero tillage. Under forest crops, a heavy cover of woody material acts as a protective mat during the passage of loaded wheels. Increases in organic matter may reduce compactibility by increasing resistance to deformation and/or by increasing elasticity (rebound effects). Compactibility is sensitive to even quite small changes in the amount of organic matter. These observations have important implications in the improvement of soil management to avoid over-compaction problems in crop production.