Soil aluminium toxicity in New Zealand pastoral farming — A review

As most New Zealand pastoral soils are acidic, aluminium (Al) can be present at high concentrations and restrict plant root growth and shoot yield. In field trials, Al toxicity in white clover has been associated with CaCl₂-extractable soil Al levels of 3-5 ppm or exchangeable soil KCl-extractable levels of 1-2 me/100g, when soil pH levels were below 5.5-5.7 in the top 75 mm. Lucerne is less tolerant of Al toxicity than white clover and ryegrass, which in turn are less tolerant than Lotus spp., arrow leaf, subterranean, Caucasian, Persian and gland clovers, and naturalised adventive annuals such as cluster, haresfoot, striated and suckling clovers. Soil Al toxicity generally increases with soil depth. Soil pH is a reliable indicator of soil Al and, on average, can be increased by 0.1 units/tonne/ha of applied lime to reduce soil Al to below the toxic range. Lime application is the most effective strategy where it can be ground-applied. A key limitation of ground-applied lime to reduce Al toxicity is that its movement down the soil only occurs slowly except in high rainfall areas. Soil Al and pH levels and legume content in hill soils varies according to slope and aspect and there is an opportunity to differentially apply lime by air to areas with low soil pH and more legume, for the best economic return