“Root casts” are calcified structures in the shape of roots often found exposed on aeolian sand-dunes. A centralchannel of the cast may be void or filled with soft carbonate. Transpiration drives water flux towards the rootsresulting in ‘mass-flow’ of dissolved nutrients towards the root. Itwas hypothesised that root casts formin cationrichsubstrates as a consequence of transpiration-drivenmass-flowof cations to roots in excess of that required bythe plant, with subsequent cementation of sediments around the root mould through formation of carbonates.The elemental compositions of both ancient root casts (6870±60 BP) of an unknownplant fromStill bay (WesternCape, South Africa) and concreted sediments associated with cluster roots of a presently occurring dune plant,Morella cordifolia (L.) Killick (Myricaceae), were found to have similar elemental compositions and werepositively correlated. Both root casts and cluster root sediments had higher [Ca] and other cations and also higherinorganic [C] than that of dune sand. Fromisotope analysis, themost likely sources of C for formation of carbonatesin the cluster root sediment (δ13C=−0.8±0.29‰) and in the root casts (δ13C=−5.3±0.6 to−7.6±0.3‰)wasa varying combination of the bulk atmosphere (predicted δ13C of concretion=1.3‰) and plant respiration derivedCO2 (predicted δ13C of concretion=−11.5‰). Despite extremely low [P] (0.05 μmol g−1 sand) in dune sand, theleaves of M. cordifolia accumulated amounts of P comparable to other cluster-rooted plants in the adjacent,somewhat nutrient-richer Fynbos. It was concluded that in the generally nutrient-poor but cation-rich sand,transpiration-driven water flux would deliver scarce nutrients to the cluster root rhizosphere of M. cordifolia. Inthe process of thismass-flow, Ca is delivered in excess of plant demand and accumulates inboth the root tissue andthe rhizosphere. This Ca is chemically converted to CaCO3 over time, resulting in the formation of persistentroot casts.