Drainage losses of water, N and P from micro-irrigation systems in a young, high density apple planting.

Between 2002 and 2004, passive capillary wick samplers were used to test the effects of micro-irrigation systems (drip; small radius micro-sprinklers), N fertigation rates, paper-fibre mulch and municipal compost on drainage, NO₃N and PO₄-P leaching beneath the root zone of high density apple trees. Irrigation was automated and applied twice daily based on ET estimates from an electronic atmometer. Over-supply of water resulted in deep drainage and N and P leaching in both irrigation systems and increased each year with increasing irrigation applications. In 2002, precipitation likely accounted for excess drainage for both irrigation types, indicating the need for automated rainfall collection to be included in the system. In 2003 and 2004, drainage and leaching losses, not explained by precipitation, were attributed to over-estimation of ET. Modified crop coefficient curves were derived for each irrigation system and used to derive estimates of potential reduction in drainage. Over-supply resulted in greater losses under drip than micro-sprinkler, particularly close to the emitter. The occurrence of N and P leaching was determined by drainage, but the magnitudes of N and P leaching were determined by management treatments. In all years, high leaching losses resulted from high N applications and in 2002 also from compost (applied in 2000). Similarly, mineralization of compost increased plant availability of P and inorganic P mobility and leaching in all years. NO₃N losses could be greatly reduced by low fertigation rates and restricting applications to 6 weeks after full bloom, the period of high uptake for vegetative growth.