Long-term changes in soil fertility in hill country

It has been more than 10 years since the last comprehensive soil sampling of the long term phosphorus (P) fertiliser and sheep grazing farmlet study established at the AgResearch Ballantrae Hill Country Research Station in 1975. This paper reports the findings from a sampling in October 2014 of the soils in the farmlets that have had no fertiliser since 1980 (LFNF), received 125 kg/ha/year of superphosphate since 1980 (LFLF) or 375 kg/ha/year of superphosphate since 1980 (HFHF). Increases in total P levels in the soil reflect the differences in P inputs between the LFNF, LFLF and HFHF farmlets over the last 40 years. In sharp contrast total sulphur (S) levels in soils have showed little change, despite the large amounts of sulphur applied in superphosphate each year. Exchangeable calcium (Ca) levels have increased on the farmlets receiving fertiliser, reflecting the Ca inputs in superphosphate, while magnesium (Mg) levels are lower in the HFHF farmlet. Potassium has shown little change, with the exception of increases on low slope areas in the HFHF farmlet. Olsen P levels have not changed in the topsoil (0-75 mm) in the HFHF since the 2003 sampling, despite annual P inputs in excess of maintenance. The absence of any change might be explained by the finding that P is accumulating in large amounts in the 75-150 mm soil depths on low slopes in the HFHF farmlet. This finding was unexpected serving to highlight the insights these long-term experimental studies provide to both science and industry. Keywords: Long term fertiliser study, P fertiliser, Olsen P, soil fertility