Non‐phytotoxicity of the aluminum sulfate ion, AlSO⁺₄

Abstract

Aluminum is a phytotoxic element in many soils and occurs in a variety of chemical species. In order to determine whether AlSO + 4 is toxic, seedlings of wheat ( Triticum aestivum L. cv. Tyler) and red clover ( Trifolium pratense L. cv. Kenland) were transferred to solutions containing controlled activities of Al 3+ , AlSO + 4 , Na + and Ca 2+ . Root elongation was inhibited by Al 3+ (or mononuclear hydroxy‐Al species that are in equilibrium with Al 3+ ), but not by AlSO + 4 . We assumed a formation constant (K AlSO + 4 = {AlSO + 4 }/[{Al 3+ } {SO 2‐ 4 }]; braces indicate activities} of 10 3.2 for AlSO + 4 in the computation of ionic activities, but use of K AlSO + 4 values ranging from 10 2.8 to 10 3.6 had very little effect on the computed toxicities of Al 3+ and AlSO + 4 . Sulfate did not promote the formation of polynuclear Al complexes in our experiments. A practice in studies of Al phytotoxicity has been to attribute toxicity to mononuclear Al, but now it would seem advisable to exclude AlSO + 4 . That AlSO + 4 is non‐toxic, or is at least 10‐fold less toxic than Al 3+ , has implications for the physiology of Al toxicity and for the use of sulfate salts in experimental work and in agriculture.