As well as being used to monitor long term trends in soil fertility, soil testing can be used to inform crop nutrition decisions. Collecting representative samples is the most important step in getting useful results from soil analysis.
Plan your sampling by:
- mapping paddocks into zones if they are not uniform, based on observed soil differences or management history, and sample separately
- using a sampling plan within zones. Grids, zig zags and randomised are the common styles.
- taking more samples within each zone helps improve the reliability of the analysis, and the management recommendations
- using a consistent method when taking samples.
Tips for collecting soil samples for nutrition planning
- Sample at about the same time each year as many soil properties change with season. Sampling each year is ideal, but you can often get away with sampling paddocks each 2-3 years as soil test values for immobile nutrients do not change very quickly.
- Assign permanent identification numbers for paddocks and zones and keep a map of sample areas. Keep GPS records of collection sites.
- Collect more than 20 cores or sub-samples over the sampling zone or paddock to give a composite sample for that area.
- Use a sampling tool eg. soil probe, tread sampler, auto-corer. A shovel is not a good collection tool as depth is not easily controlled. Tapered tools (such as a trowel) biases the sample with more soil from upper layers.
- Place samples directly into clean plastic bags, or a clean plastic bucket. Metal containers can contaminate the sample, especially for micronutrient tests.
- Sample to the correct depth for what you are going to analyse for:
- 0-10 cm is the most common depth for samples (P, K, S, micros)
- 0-60 cm for deep samples (N, S). Depth is critical as immobile nutrients (like P, K and micros) are largely in the top layers.
- Subsoil samples (>10 cm) can be used to evaluate properties that may limit root depth – such as pH (v. high or v. low), sodicity, salinity, boron.
- Leave out any areas that may have abnormal soil nutrient levels such as headlands, old fertiliser dumps, areas near trees, fence lines, roads and tracks and stock camps(though not really relevant for cropping)
Getting your samples ready for the lab
- Keep samples cool in an esky or fridge. Especially important for N in moist soils.
- Thoroughly mix the cores from a sample area. Clods should be broken with the fingers or a tool like a trowel or old (but clean) screwdriver while mixing is being done. DON’T ASSUME THIS WILL BE DONE AT THE LAB.
- Check with the lab you are sending to what to pack the sample in. 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, keep a copy, and dispatch samples as soon as possible.
- Use ASPAC/Fertcare accredited laboratories as they use the methods accepted in Australia, and have demonstrated that they achieve accurate results.
Article by Rob Norton, IPNI with technical input from Abigail Jenkins, NSW DPI & James Easton, CSBP.
Photo courtesy of IPNI.