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

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 Al3+, AlSO+4, Na+ and Ca2+. Root elongation was inhibited by Al3+ (or mononuclear hydroxy‐Al species that are in equilibrium with Al3+), but not by AlSO+4. We assumed a formation constant (KAlSO+4= {AlSO+4}/[{Al3+} {SO2‐4}]; braces indicate activities} of 103.2 for AlSO+4 in the computation of ionic activities, but use of KAlSO+4 values ranging from 102.8 to 103.6 had very little effect on the computed toxicities of Al3+ 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 Al3+, has implications for the physiology of Al toxicity and for the use of sulfate salts in experimental work and in agriculture.