Yield and phosphorus use efficiency of irrigated lucerne grown in a low Olsen P soil

Abstract

Phosphorus (P) is a key nutrient recommended for optimal nitrogen fixation by legumes in New Zealand pastoral systems. However, few field studies have examined the response of legume monocultures to soil P levels. This research aimed to quantify how phosphorus affects lucerne growth and development in an irrigated cut and carry system with a starting Olsen P of 9 mg/L. A randomized complete block design with five superphosphate treatments (0, 50, 100, 200, 400 kg/ha/y) and four replicates was established within a stand of ‘Kaituna’ lucerne. All herbage was cut-and-carried to ensure no nutrient transfer or return from livestock. Lucerne stem height and annual yields (12000–20000 kg DM/ha) were not different among treatments over four years even when the Olsen P in the Control declined to ~3 mg/L in the top 150 mm. Herbage phosphorus use efficiency was consistently highest in the 50 kg superphosphate/ha treatment with eight times the quantity of applied phosphorus extracted from the soil profile. The 400 kg superphosphate/ha treatment was the only P application rate that was sufficient to fully replace P removed in herbage. These findings suggest lucerne growth was not limited at Olsen P levels below current recommendations, which suggests lucerne was acquiring P below the measured depth and/or accessing P not measured by the Olsen P test.