Lucerne (Medicago sativa L.) dry matter, root growth and nodulation in response to lime and phosphorus in an acid, high country soil in New Zealand

Soil acidity is the main constraint for lucerne establishment and growth in high country regions of New Zealand. Root growth is suppressed in these aluminium-rich acid soils. In this experiment, lucerne was grown in PVC columns (0.8 m long, 0.15 m diameter), filled with soil collected from the high country overlaying gravel at 0.3 m, to mimic the actual soil profile. Nitrogen (rhizobium peat inoculant, 50 kg N/ha, and control), lime (0, 0.5, 1, 2 and 4 t/ha), and phosphorus (0 and 250 mg P/kg soil) responses were measured from a split-split plot experiment at Lincoln University. The soil pH (H2O) was elevated from 5.4 to 5.5, 5.6, 6.1, and 6.7 by the lime rates, respectively. Shoot dry matter (DM) was increased from 27 to 551 g/m2 by lime application and from 127 to 464 g/m2 by rhizobium inoculant. Root DM was increased by the three way interaction effect of lime, phosphorus and nitrogen. Root volume and total length were higher, with added lime, in the rhizobium treatment than with inorganic N. There was no difference between applying 50 kg N/ha and control on shoot or root DM yield. The number of nodules was increased by lime in the inoculated treatment from 11 to 150 per column. The applied phosphorus without lime, was not used by plants and was fixed by Al in soil. The application of half a tonne of lime with P reduced Al to 1.8 mg/kg, elevated the soil pH (H2O), and more P became available. Results indicate the importance of elevating soil pH for phosphorus availability and nitrogen fixation rather than applying P and N fertilizer, to overcome soil constraints