Phosphorus and the Winchmore trials: review and lessons learnt

Abstract

Phosphorus (P) is central to the sustainability of New Zealand pastoral farming. The Winchmore long-term field experiments are the longest running grazed field trials of their type in the world. Treatments include variable inputs of both fertiliser (largely superphosphate, SSP) and irrigation. Insights from the trials include the following. After initially accumulating quickly as organic P, a steady state was reached whereby P preferentially accumulated as inorganic P in proportion to SSP applications rates. Pasture production paralleled accumulated P when SSP application stopped. Organic P decreased when lime was applied, but quickly recovered when more P was applied than necessary for plant production. P unaccounted for in a mass balance was lost in irrigation outwash. Losses related to stocking rate more than soil test P or SSP applications. The long-term field trials at Winchmore therefore represent a valuable resource to improve knowledge of agri-environmental P dynamics.