Aluminium speciation and phytotoxicity in alkaline soils

Highly alkaline soils (pH > 9.0) may adversely affect agricultural crop productivity. Problems encountered include poor structure and nutrient deficiency. Research based on solution cultures suggests that aluminium (Al) phytotoxicity may occur in soils with pH > 9.0, but little research has been undertaken on actual soils under controlled conditions. The nature of the Al species responsible and the pH regime of the soils when this occurs are unknown. The charge and species of Al responsible for this toxicity was investigated using Zeta Potential measurement, Nuclear Magnetic Resonance (NMR) spectroscopy, Al precipitation characteristics and electrical conductivity as a function of pH. An anion exchange resin was used to evaluate Al availability to plants at alkaline pH. To verify Al phytotoxicity, a pot experiment was performed with plants grown at near neutral and high pH, with and without Al. The anionic aluminate species of aluminium was ubiquitous at highly alkaline pH, and was the dominant charged species at pH 9.2. Aluminium was phytotoxic at high pH, significantly reducing the stem and root development of field pea test plants over and above that caused by alkalinity alone. The effects of both alkalinity in general and aluminium in particular became noticeable at pH 9.0 and debilitating at pH > 9.2. As this corresponds to the pH where aluminate becomes dominant, it is probably responsible for the phytotoxicity.