There is always nutrient variability in any paddock, and this must be considered when taking samples for testing. Temporal variability applies to when the sample is taken.  Spatial applies to where the sample is taken. There will also be a degree of variability present from the lab testing process, but this is small compared with taking the samples and preparing soil to send for testing in the first place.

Temporal variation occurs because of seasonal nutrient cycles that change availability. Avoid confusion by sampling at the same time of year, or by only comparing soil samples from the same paddock taken at the same time of year. Many people do sampling in winter with time to plan programmes before spring fertiliser applications are started.

Spatial variation will be the largest source of variability so must be carefully considered to make sure a representative sample is collected.  Variation is caused by factors such as different soil types, different crop histories and different fertiliser application patterns.  Results reflect the sample measured and are affected by where in the paddock samples are taken.

Fertility also changes down the soil profile, so the depth of sampling is very important. In New Zealand, nutrient research and fertiliser recommendations for horticultural crops including vegetables are based on soil cores collected from the top 150 mm of the profile. The main exception is testing for available nitrogen (nitrate) which can use soil cores from the full root depth.

Electromagnetic sensors can be used to map soil patterns or variability. EM maps measure electrical soil properties that are affected by wetness, salinity, density and clay content. This means EM values can be related to soil texture, highlighting the variation in sand, silt and clay content. EM maps show variation across a field and can be used for zoning areas. These maps have been used to guide soil sampling, but also other tasks such as irrigation or variable rate planting.

An EM or EC map of a 3ha field at the LandWISE MicroFarm is shown in Figure 1. Note that although the map shows a wide range of colours, the values are actually very similar, and not sufficient to indicate different sampling areas.

Another way to view a paddock or crop is to access canopy vigour maps. These might use images captured by a UAV or use downloaded satellite data. Typical images show the ratio of red and near infrared light. This is known at the normalised difference vegetative index, or NDVI. This has been shown to give a good indication of the amount of chlorophyll present. That will vary with the amount of ground cover, and the amount of greenness of tissue, which is often related to nutrient differences.

Figure 2 is a satellite image of the MicroFarm downloaded from datafarming.com.au. The area inside the white polygon, which matches the EM map above, is a barley crop. The NDVI image indicates area B is less vigorous than area A. This suggests two separate soil tests, one from each of areas A and B, could provide useful information.