Nutrient requirements for irrigated lucerne in Central Otago

Abstract

Abstract Two field trials were commenced in September 2000 on newly sown irrigated lucerne near Tarras in Central Otago. The first trial measured the effects of phosphorus (P) (0, 30, 60, 120 kg P/ha) and potassium (K) (0, 50, 100, 200 kg K/ha) fertiliser applications while the second trial determined nitrogen (N) requirements for lucerne establishment. Initial soil test levels (0-75 mm) were pH 5.6, Olsen P 15 μg/ml; quicktest (QT) K 6 and sodium tetra-phenol-boron extractable K (TBK) 3.1. Lucerne production averaged 9.5 t dry matter (DM)/ha in the establishment year, 15.6 t DM/ ha in Year 2 and 14.8 t DM/ha in Year 3. There were no annual DM responses to any of the three fertilisers applied regardless of the rate of application. Despite the control mean annual herbage P concentrations being within the optimum range of 0.20-0.25% the application of P fertiliser significantly increased the P concentrations (P<0.001) in Years 1 and 2. The herbage K concentrations were above required concentrations (1.5-1.8%) for all harvests, and were only affected by K fertiliser application in Year 3. The lucerne crop removed between 300 and 600 kg N/ha/year, 230 to 300 kg K/ha/year and 25 to 40 kg P/ha/year. Biological N fixation appears to be replacing the N removed as the lucerne did not respond to applied N and the soil N levels were maintained throughout the 3 years. The removal of nutrients such as P and K, to a greater level than fertiliser applied, under a hay regime has implications for the longevity of the stand. This is highlighted for P on the 0P plots with the soil Olsen P levels dropping to <8 μg/ml for the 0-75 mm soil layer and <5 μg/ml for the 0-150 mm soil layer by the third year. Potassium reserves in the soil also declined, highlighting slow depletion under hay cropping. It would appear that 0-75 mm Olsen P levels of at least 12-15 μg/ml and 0-150 mm Olsen P levels of 10-12 μg/ml are required for optimum production (95% of maximum) at this site. Regular monitoring of soil and plant nutrient levels is recommended to ensure fertiliser inputs are sufficient to maintain maximum production. Key words: lucerne, irrigation, nutrient requirements, phosphorus fertiliser, potassium fertiliser, nitrogen fertiliser.