Long term (?) effects of withholding superphosphate application on North Island hill country: a 10-year update

Abstract

Measurements of soil phosphate, pasture production and botanical composition from two superphosphate fertiliser trials in northern North Island hill country (Whatawhata and Te Kuiti) have been made over a 15-year period. The impacts of superphosphate fertiliser withholding have included a decline in Olsen P, decreases in annual pasture production of 10-17% (Whatawhata) and 22-42% (Te Kuiti), decreases in the abundance of productive and desirable species (15-20% for ryegrass and white clover), and increases in the abundance of undesirable species (browntop and other low fertility grasses). These changes have increased over time, beyond that period covered by the original experiments (4-6 years). Different responses to withholding between the two sites are still apparent, and the reasons for these are explored. It is not clear that these systems have yet reached a new equilibrium, nor how long it will take for this to happen. The implications for our view of longterm grassland studies are discussed. Keywords: fertiliser, long-term, pasture production, pasture species composition