Changes in soil fertility, biology and organic carbon under contrasting phosphorus fertiliser and sheep grazing management

Abstract

The phosphorus (P) fertiliser and sheep grazing experiment established at Ballantrae in 1975 has become an invaluable field laboratory. In-depth experimental studies have explored the long-term link between P fertiliser inputs and pasture and animal production, including the impact of withholding P fertiliser. In this paper we report on changes in soil nutrient fertility, organic matter, biology and physical status of soil across three farmlets that are receiving different rates of P fertiliser. Since 2020, soil pH has dropped by 0.5 units. Prior to 2020 pH values showed very little
change, averaging 5.3 over the previous 45 years, despite very limited use of lime. There has been a trend of declining exchangeable magnesium and potassium levels, with differences seen across slopes, while exchangeable calcium has been sustained in the two farmlets receiving P fertiliser. The measured declines in total nitrogen in soil and earthworm abundance are consistent with the decline in pasture production and sheep stocking rates. If we are to fully understand the implications of variable inputs and management for permanent pasture systems within a changing climate, we must continue monitoring and measuring such longterm field experiments.