When we take a soil sample before side-dressing, typically with nitrogen fertilisers, we need to be representative of the paddock as in Lesson 2, but we also need to consider where any earlier fertiliser has been applied, and the root depth of the particular crop.

Is the crop developing uniformly?

Because the crop is developing, a satellite NDVI image might show us variations in growth which may guide our sampling.  Figure 5 is an image of a beetroot crop after a long, wet spring. How might you sample this crop?

Figure 5 Free NDVI map of a beetroot crop shows differences in crop canopy cover and vigour (courtesy of Datafarming.com.au)

It might be useful to split the paddock in half and measure the “orange” end and the “blue” end separately.  Diagonal transects though each half would give a reasonable average for each.

Where have previous applications been made?

For some crops, growers apply base dressings across the whole paddock or bed width and cultivate it in before planting. In this case the added nutrients should be reasonably evenly spread.  But in many row crops, fertiliser is banded, either at planting or afterwards and there are distinct areas of high and lower fertility as a result. The plants’ roots will explore the whole volume and extract what they need, but when we sample, we need to get a fair representation.

Figure 6 shows an example of three beds of broccoli, two rows planted in each bed. This grower bands fertiliser alongside the rows several times during the season. Before applying another dose of nitrogen, they sample the soil to determine how much N is still available, adding only enough to keep the crop growing well.

Figure 6 Schematic drawing of broccoli beds showing where bands of fertiliser have been applied

To account for the previously applied nitrogen, their sampling pattern takes about 1 in 5 cores in the fertiliser band, and 4 in 5 away from the band (Figure 7). This gives a combined sample that is as fair as possible showing the average amount of N in the beds.

Figure 7 Sampling pattern for testing row crops with previous fertiliser applied in bands or strips alongside the plants

How deep should we sample?

Another consideration when sampling for nitrogen is choosing the depth of soil taken. Nitrate is very mobile and can easily move down the profile after rain or irrigation. So long as it remains in the crop’s rootzone, it is available to the plants. So, while a standard soil test for cropping takes 15 cm deep cores, for nitrogen we can consider the whole root depth (Figure 8).

Figure 8 Standard crop soil testing is based on sampling the top 150 mm. Roots extend much further and for nitrate availability, a deeper depth can be sampled

This varies by crop and even by stage of growth. For a broccoli crop, roots can easily explore 45 cm of the profile, so we can test to that depth (Figure 9).