Assessment of rhizotoxic aluminum in soil solutions by computer and chromogenic speciation

Abstract

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 (Al_a), metastable (Al_b), and non‐reactive (Al_c) fractions in soil solutions of two North Carolina Ultisols. Relative root growth of field‐grown corn (Zea mays L.) was used to assess rhizotoxicity of kinetically measured Al forms. Reaction of Al_b with ferron conformed to first order kinetics, with rate constant values ranging from 0.53 to 9.13 minute^‐1. The Al_a fraction correlated significantly with Al reacting in 30 seconds (r²=0.99), but the former averaged 10% less than the latter. Both Al_a and Al reacting in 30 seconds correlated significantly with the Al_b fraction (r²≥0.89), but not with the Al_c fraction (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 Al_a and the sum of activities of Al³⁺, Al(OH)²⁺, and A1(OH)₂ ⁺, suggesting an instantaneous reaction of these species with ferron. The Al_a and A_b fractions correlated significantly with root growth. The Al_c fraction 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 Al_a and A_b fractions.