Overall sampling pre-planting

To get a representative sample, avoid places that are “not representative” such as headlands, old barn sites and stream channels. To account for variability, your sample should include a minimum of 15 to 20 cores 150 mm deep per paddock or block along a “transect” (see Figure 3).

Figure 3 Example of a Soil Sampling Transect (blue) with 20 cores sampled.

It is best to take samples from a known transect across the zone and return to the same transect when taking future samples.  Ideally you will use GPS to mark these sampling points so in following years you can take cores from the same places. Smart phone apps will let you record points with ± 5m accuracy which is adequate if not perfect (Figure 4).

Figure 3. Example of a transect mapped with phone GPS on Google Maps

It can be tricky to work out the best sampling strategy when paddock arrangements change. The ideal sampling pattern for the image on the left in Figure 5 is different to the ideal for the image on the right. One approach is to sample the whole paddock regularly, and ensure the background phosphate, potassium, sulphur and other nutrient levels are sufficient for the most demanding crop. Nitrogen should be sampled in a more targeted way, focusing on each management zone (crop type and sowing/planting).

Figure 5: These Google Earth images show the same paddock in different years: the ideal sampling strategy differs with these different arrangements