Aluminum rhizotoxicity in acid soils inhibits root elongation and limits crop acquisition of water and nutrients from the subsoil layers. Studies with soybean [Glycine max (L.) Merr.] in hydroponic systems have shown that µM additions of Mg²⁺ were as effective in ameliorating Al rhizotoxicity as additions of Ca²⁺ in the mM concentration range. The objective of this study was to assess the ameliorative effect of Mg on soybean root elongation into acid sub-surface layers of North Carolina Ultisols. Replicated greenhouse experiments with a soybean cultivar Plant Introdction 416937 were conducted using a split-root system. Roots extending from a limed surface soil compartment grew for 28 days into a subsurface compartment with acid subsoil samples from soil series of three NC Ultisols: Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic). The three Mg treatments consisted of the native equilibrium soil solution concentrations in each soil (50 µM for Creedmoor and Norfolk, and 100 µM for Cecil) and MgCl₂ additions to achieve 150 and 300 µM Mg in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO₃ treatment limed to achieve a soil pH value of 6. Liming of Cecil and Creedmoor subsoils increased both dry matter of soybean tops and subsoil root length at harvest relative to the lowest Mg treatment, whereas there was no response to subsoil treatments in the Norfolk soil. Although dry matter and subsoil root elongation responses to Mg treatments were less than for limed subsoils, reductions in water content of the subsurface compartment for two of the three soils suggest that increased Mg supply improved plant access to subsoil water reserves.