Soil testing in the 0-10 cm zone has traditionally been used for phosphorus-based fertiliser recommendations. Deeper tests are often used to gauge nitrogen (N) availability, especially when mineral N has leached deeper after significant rain. But there are other nutrient and chemical constraints that can be better analysed by soil testing down to 60 cm.
The roots of crop plants commonly develop down to 1.5 to 2.0m in the soil profile. Like N, Sulphur (S) tends to move deeper with rainfall. On sandy acid soils, Potassium (K) can also leach into the lower root zone. Deep, well-calibrated soil tests allow better predictions of the N, S, and K available across the life of a crop plant. Deep soil reserves do not necessarily preclude starter fertiliser applications but provide more information about the likely rates needed. For a true picture of N, K and S availability, take samples from 0-30 cm or better still, 0-60cm.
Subsoil sampling at a depth greater than 10 cm will also help monitor and diagnose subsoil constraints limiting root depth. These include boron toxicity, salinity, soil structure issues such as sodicity, and pH issues such as acidity or alkalinity. For diagnosing subsoil acidity in broadacre crops and pastures, 10-20 cm and 20-30 cm sample depths are recommended. In some circumstances, soil sampling for acidity should be in 5 cm increments to identify any acid layers.
Although taking subsoil samples may take longer, the extra information they provide is valuable. Fewer numbers of cores are required in subsoil samples as they generally display less variability. The reduced sample number is also a compromise for the extra effort required to deep sample. But it is important to use the correct sampling technique to take them, as outlined in APAL’s soil sampling instructions. The most important factor is that all the subsoil samples come from the same zone in the paddock.
More precise information on nutrient distribution will be obtained by sampling the depth intervals separately. Ideally take samples from 0-10, 10-20, 20-30 and 30-60 cm. But if this is too impractical, take 0-10 and 10-30 cm as a minimum. Also, consider sampling using a horizon approach rather than strict depths when taking deep soil tests. Nutrient distributions often change as they cross over different horizons.
When taking soil samples, a ute-mounted drill is ideal but is often not available for growers to use. An auger can work well unless the ground is very rocky. If a shovel is the only sampling tool available, check the sample depth carefully. Shovels can also shear the soil so make sure the sample is truly from the target depth or horizon.
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