Soil testing informs crop nutrition decisions. Collecting representative samples is the most important step in getting useful information.
Plan your soil sampling
- Map non-uniform paddocks into zones. Base the zones on observed soil differences (or EM38 maps if available), and sample them separately.
- Use a sampling plan within zones. Grids, zig zags and randomised are the common styles. Taking more samples within each zone improves the reliability of the analysis – and fertiliser recommendations.
- Use a consistent method to take samples.
Tips for collecting soil samples:
- Sample at the same time each year, as many soil properties change with the season. Sampling each year is ideal, but you can often get away with sampling paddocks every 2-3 years. Soil test values for immobile nutrients change slowly.
- Map sampling areas. Keep GPS records of collection sites.
- Collect more than 20 cores or sub-samples from the sampling zone.
- Use a sampling tool eg. soil probe, tread sampler, auto-corer. A shovel is not a good collection tool as depth is hard to control. Tapered tools (such as a trowel) bias the sample with more soil from upper layers.
- Place samples into clean plastic bags, or a clean plastic bucket. Metal containers can contaminate the sample, especially for micronutrient tests.
- Avoid areas that may have abnormal soil nutrient levels (e.g. headlands, old fertiliser dumps, areas near trees, fence lines, roads, tracks and stock camps).
- Sample to the correct depth for the information you need:
- 0-10 cm is the most common depth for samples (P, K, S, micros)
- 10-60 cm for deep samples (N, K, S)
- Subsoil samples (>10 cm) can examine chemical constraints limiting root depth – such as pH (v. high or v. low), sodicity, salinity, boron
- Obtain more precise information on nutrient distribution by sampling depth intervals separately (e.g. 0-10, 10-20, 20-30, 30-60cm).
Look after your samples:
- Keep samples cool in an esky or fridge – important for N in moist soils.
- Thoroughly mix the cores from a sample area, breaking up clods. DON’T ASSUME THIS WILL BE DONE AT THE LAB.
- Check with the lab you are dispatching to on how to pack the sample. Some laboratories provide bags or containers. Make sure your packing containers or bags are new and clean.
- Fill out the lab’s information sheet completely and keep a copy
- Dispatch samples as soon as possible.
- Use ASPAC/Fertcare accredited laboratories. These labs use methods accepted in Australia and have demonstrated that they achieve accurate results.
This article was first published in 2017. This information is still relevant in 2018.