Autumn is the season where many farmers apply maintenance and capital fertilisers. Typically, fertiliser can be one of biggest expenses incurred on-farm so it is important that soil testing is conducted before any purchase.
This is where the old adage 'if you don’t measure it, you can’t manage it' rings true. Soil testing is a small investment in relation to the clarity it adds to the management of one of your largest farm expenses. Soil testing to inform maintenance and capital fertiliser applications typically focuses on: soil acidity (pH), phosphorus (P), potassium (K) and magnesium (Mg).
Soil testing gives a picture of the nutrient status, for example, the soil’s nutrient tank and how big and full the tank is. It is important to know this, as over the farming season, product removal, for instance milk, meat and crop leaving the farm, and soil processes can deplete the soil of nutrients. If nutrients are not returned to the soil, the tank becomes empty and productivity declines.
Soil testing helps guide fertiliser management by identifying if it is just the nutrients removed during the farming season that need to be replaced (maintenance), or if the soil nutrients levels are below an optimum level and extra nutrients (capital) beyond maintenance need to be applied to lift the soil test values. Conversely, if there is already more than enough of a particular nutrient in the soil, then there may be some disadvantages associated with applying more of this nutrient.
While any single set of soil test results can help make informed fertiliser decisions; it does however need to be compared to trend lines built up by a history of soil tests to ensure the most accurate fertiliser recommendations are made. One-off soil test results do provide valuable information but it is of limited value as it is essentially a snapshot in time of a dynamic environment, which by definition is variable from place-to-place on the farm and year-to-year. Trend lines of individual nutrients (pH, P, K and Mg) ‘smooth out’ some of this variability and give a better picture of soil fertility, and how well your fertiliser strategy is suited to your farming practice.
Soil testing needs to be carried out in a consistent manner to see valid trend lines. Testing should be conducted along the same transect (avoiding hot spots such as gateways and water troughs), at the same time of year and to the same depth each time (7.5 cm for pastoral soils and 15 cm for cropping soils).
Monitoring the effectiveness of previous fertiliser applications and keeping track of soil fertility trends over time enables better-informed management decisions, optimising your fertiliser dollar. Contact your local PGG Wrightson Technical Field Representative to help you with your fertiliser programme this autumn.