A WATER AND NITROGEN BUDGET FOR 'HORT16A' KIWIFRUIT VINES

Because of its short production history, limited knowledge exists of the water and nitrogen requirements for 'Hort16A' (ZESPRI™ GOLD). Our research aims to address this knowledge gap using a measurement and modelling approach. A three-year field trial has been established on a mature orchard near Te Puke, New Zealand. Two fertiliser regimes (120 and 250 kg N ha^-1) have been set up to apply nitrogen fertiliser (calcium ammonium nitrate) as a single dressing in the spring time, and a third treatment receives zero fertiliser. Whole shoots are destructively sampled every month to determine the dry-matter allocation and nitrogen content of shoots, leaves and fruit of the new season's growth. Soil water content is measured using time domain reflectometry (TDR). Soil samples to 2.4 m depth are taken every month to follow changes in the mineral nitrogen content (i.e., nitrate and ammonium) of the root-zone soil. These data are providing parameters for a computer model (SPASMO – Soil Plant Atmosphere System Model) of the water, carbon and nitrogen budget of the orchard. Results from the first year of the trial suggest that 90–110 kg ha^-1 of nitrogen is accumulated in the fruit. Some 60–80 kg ha^-1 of nitrogen is accumulated in shoots and leaves of the current season’s growth, although this is largely returned to the soil as leaf litter and winter prunings. Preliminary findings suggest the high nitrogen treatment (250 kg N ha^-1) may delay fruit maturity and reduce fruit quality plus encourage excessive nitrate leaching. Measurements in coming seasons will investigate the physiological cost to the vine of the zero fertiliser treatment. The ultimate goal of this research is to develop new modelling tools that will help to optimise the amount and timing of fertiliser to achieve maximum vine performance with minimum environmental impact.