Soluble Polymeric Hydroxy‐aluminium Ions in Acid Soils

Summary Solution concentrations of total Al and of mononuclear Al ions were determined as a function of pH for two acid soils. Polynuclear hydroxy‐Al was taken as the difference between total and mononuclear Al. Salt concentration at 1 : 5 soil: solution ratio was varied from 0·008 to 1·o equiv/l, for CaCl 2 , KCl, and NH 4 OAc at pH 4·8. Polynuclear hydroxy‐Al concentrations increased with electrolyte concentration and the valency of the desorbing cation, and rose to 0·2 mM (equivalent to 3 meq/kg soil) in unbuffered salt suspensions, and to 1·0 mM in NH 4 OAc at pH 4·8. Polynuclear hydroxy‐Al thus behaves like a normal exchangeable cation. I.0M NH 4 OAc released much more total A1 from some of the more acid samples than might have been expected from its pH of 4·8.