The Effects of Soil Acidity and Aluminium on the Root Systems and Shoot Growth of Lotus pedunculatus and Lupinus polyphyllus

<i>Lotus pedunculatus</i> (lotus) and <i>Lupinus polyphyllus</i> (Russell lupin) persist in the upland grasslands of New Zealand, where soil acidity and associated aluminium (Al) toxicity impede conventional pasture legumes. This experiment investigated the response of lotus and Russell lupin to soil acidity and Al. The species were sown in 20 cm tall 1.2 L pots of acidic upland soil. A mass of 4.5 or 6.7 g lime (CaCO₃)/L was added to either the top or bottom or both soil horizons (0-9 cm and 9-18 cm), resulting in six treatments across six randomised blocks in a glasshouse. The soil pH was 4.4, 4.9, and 5.4; the exchangeable Al concentrations were 24, 2.5, and 1.5 mg/kg for 0, 4.5, and 6.7 g lime/L. At 16 weeks post-sowing, the plants were divided into shoots and roots at 0-9 cm and 9-18 cm. Root morphology, shoot and root dry matter (DM), shoot nitrogen (N), and nodulation were measured. The total plant DM and shoot-to-root DM ratio were higher, and the shoot %N was lower for the lotus plants than the Russell lupin plants for the various lime rates (13.2 vs. 2.9 g plant^-1, 5.6 vs. 1.6, and 2.4 vs. 3.3%, <i>p</i> < 0.05). No response to lime in terms of total DM or total root morphology parameters was exhibited in either species (<i>p</i> > 0.05). Root morphology adjustments in response to acidity between soil horizons were not observed. The results indicated that lotus and Russell lupin are tolerant to high soil acidity (pH 4.4-5.4) and exchangeable Al (1.5-24 mg kg^-1), highlighting their considerable adaptation to grasslands with acidic soils.