Effects of long-term differences in fertiliser history on hill country: seasonal pasture production, legume growth and soil phosphorus status

Abstract

The effects of 10 years of nil or maintenance phosphorus (P) fertiliser (250 kg/ha/yr superphosphate) on pasture production and composition, and on soil P status, were measured in a farmlet trial on hill country near Te Kuiti. Withholding P resulted in 29-35% less annual pasture production and 54-72% less legume (predominantly white clover) production. There was little difference in seasonality of production between the two farmlets. The loss in production in the nil P farmlet was predictable based on the decline in soil Olsen P test to 6-8 ppm and the results from past mowing trials. Additional soil measurements revealed that soil organic P levels were increasing at the site and this will have contributed to the decline in plantavailable P in soil. Re-application of P at 23 kg/ha markedly increased legume growth (by 180-330%) in the nil P farmlet, which was proportionally twice as high on steep slopes as on easy slopes. This response in legume production was most evident in summer at a time when improved feed quality is valuable for finishing lambs. Indeed, legume growth in the P re-application plots exceeded that in the maintenance P farmlet by 23-28%. This reflected reduced plant-available soil N status due to less inputs from N, fixation in the previous 10 years, and thereby increased the competitive advantage of legumes over grasses where P deficiency was diminished by fertiliser re-application. Keywords: fertiliser history, hill country, legume production, pasture production, phosphorus, seasonality, soil phosphorus