Hydroponics Experiments Rank Forage Legume Genotypes for Physiological Aluminium Tolerance but is Physical Avoidance of Aluminium Toxic Subsoils as Important?

Southern Australia has large areas of acid, infertile soils with low pH (<5.0) and associated high levels of exchangeable aluminium (Al). Current agricultural practices such as the application of fertilisers and the use of forage legumes further acidify the soil. The use of lime to ameliorate soil pH is often cost prohibitive. Therefore to improve productivity and profitability in Australian livestock systems there is a need for forage legumes with tolerance of soil acidity and associated soil constraints.

Ornithopus compressus and O. sativus are forage legumes that are highly productive in light, sandy soil types with low pH where the traditional options, Trifolium subterraneum and Medicago sativa (alfalfa), fail to persist. There has been little research on Al tolerance in Australian forage legumes and much of this past research is now outdated or describes superseded cultivars. Therefore, we grew a number of Ornithopus compressus and O. sativus cultivars in hydroponics to determine their physiological Al tolerance by measuring the reduction in root length in response to increasing concentrations of Al. We also conducted a pot experiment with field soil to determine whether these species alter root distribution in response to an Al toxic subsoil.

We found significant variation in physiological tolerance among cultivars in hydroponics (Figure 1). Generally, O. compressus cultivars were more tolerant of Al (i.e had less reduction in root length at higher concentrations of Al) than O. sativus cultivars, which proved quite sensitive. However, in the pot experiment, when grown with an Al toxic subsoil, O. sativus cultivars had greater topsoil root exploration, better nodulation and better shoot growth than O. compressus cultivars.

Thus, physiological tolerance is only one means to aid persistence in Al toxic soils and physical avoidance may be equally or more important under field conditions.

Figure 1. Effect of increasing concentrations of aluminium (Al) in hydroponics for the sensitive reference species Medicago sativa (left) and tolerant Ornithopus compressus (right). Al concentrations are 0, 1, 2, 3, 4, 6, 8 and 10 ppm (left to right).