Summer is an ideal season to run a soil sampling program. Soil sampling provides valuable insights for planning the next season and helps track trends in soil results over time. Testing at the same time each year ensures results are comparable, building a clear picture of how soil parameters change over time.
Rotational sampling
Sampling 25% of a farm each year creates a four-year rotation, meaning every paddock is assessed once every four years. As a general rule, a three-to-four-year timeframe is adequate to detect changes in pH and allow adjustments in liming practices. This timeframe is also typically suitable for key soil characteristics like phosphorus, potassium, salt, and organic carbon which are reasonably constant with consistent management.
Managing soil variability
It is common for several different soil types to occur across a single large paddock. Where soil variability in a paddock is obvious and there is an opportunity to manage these zones differently in-season, it is beneficial to sample each area separately. In most cases, samples taken at stratified depths of 0-10, 10-20, and 20-30 cm will give a good guide to the soil pH and nutrient concentration through the upper profile.
Collecting representative samples
It is important to collect representative soil samples to allow for meaningful results. Areas to avoid when sampling include fence lines, headlands, gateways, tramlines, water troughs, shelterbelts, stock camps and zones of double cultivation.
Defining the purpose of sampling
Determining the purpose of the soil sampling program will help guide the design of the plan. Typically, the purpose will fit into one of four categories:
- Predictive – informing application requirements and management decisions.
- Monitoring – tracking changes over time.
- Diagnostic – identifying constraints, deficiencies, toxicities.
- Compliance – benchmarking against critical thresholds.
Practical on-farm applications
Soil test results can inform a wide range of management decisions, including:
- Comparing areas of good performance to areas of poor performance to diagnose constraints,
- Assessing pH and nutrient concentration variability to inform variable rate lime and fertiliser applications,
- Informing suitable crop or pasture variety selection,
- Assessing soil profile characteristics in considering the suitability of an amelioration program such as deep ripping, and
- Assessing for soilborne disease, such as Predicta B analysis.
Additional resources
For more information, check out the following resources:
- Soil sampling – Soil Quality Knowledge Base
- iLime – lime decision tool – DPIRD
- Making sense of chemical indicators (factsheet) – Soil Quality
- A guide for ‘fit for purpose’ soil sampling – FertCare
Any questions? Email Bridget at Bridget.Watkins@murdoch.edu.au
Bridget Watkins is the South-West WA Regional Soil Coordinator (RSC). The RSC project provides opportunities for researchers, primary producers and community groups to collaborate and share knowledge related to soil health and management. The project is supported by the Australian Government’s Climate-Smart Agriculture Program under the Natural Heritage Trust, and in South-West WA is led by the SW WA Hub in partnership with SoilsWest at Murdoch University.
