Numerous chemical species of aluminum (Al) occur in acid soils and simple but accurate methods are needed to characterize phytotoxicity. Kinetic reaction studies between Al and ferron were used to separate Al into instantaneously reactive (Ala), metastable (Alb), and non‐reactive (Alc) fractions in soil solutions of two North Carolina Ultisols. Relative root growth of field‐grown corn (Zea maysL.) was used to assess rhizotoxicity of kinetically measured Al forms. Reaction of Albwith ferron conformed to first order kinetics, with rate constant values ranging from 0.53 to 9.13 minute‐1. The Alafraction correlated significantly with Al reacting in 30 seconds (r2=0.99), but the former averaged 10% less than the latter. Both Alaand Al reacting in 30 seconds correlated significantly with the Albfraction (r2≥0.89), but not with the Alcfraction (r2<0.1). Correlation exercises between ferronreactive Al fractions and various Al species predicted by the GEOCHEM‐PC speciation program revealed a strong relationship between Alaand the sum of activities of Al3+, Al(OH)2+, and A1(OH)2+, suggesting an instantaneous reaction of these species with ferron. The Alaand Abfractions correlated significantly with root growth. The Alcfraction did not correlate significantly with root growth and may be relatively less toxic. Further soil solution studies may be required to ascertain the nature of actual Al species measured in the Alaand Abfractions.