Soil Sampling: Types / Tips

The goal of soil testing is to determine levels of plant available nutrients in a particular field in order to create accurate fertilizer recommendations.

Using the GPS technology we have today there are many types of soil sampling that we can do to measure the soil nutrients in your field. Knowing when to use each type can get a bit confusing, so we have created the chart below showing the type of soil sampling (based on size of area sampled) and when to use each type. The “Use” column applies to both grid or zone sampling.

Type of Sampling	Use
1 acre sampling	Very accurate / detailed way to     identify field variations. Every field should be sampled at 1 acre increments at least once.
2.5 acre sampling	Most economical way to identify field variation. Best grid / zone size to sample at when planning to create variable rate prescriptions from the results.
5 acre sampling	Better than bulk sampling, but not very accurate for variable rate application as it can lead to over or under application. A good sampling choice for rented ground or a 1 year trade where you want to get a baseline for straight rate fertilizer applications.
Bulk sampling (grid or zones 10 acres or greater)	Bulk sampling is not great at        identifying field variability. Bulk sampling is best used for general compliance requirements only. For example: nutrient management plan updates.

Soil Sampling Tips

For zone sampling to be accurate the data used to create the zones must be accurate. If you don’t have good data to make zones it is better to grid sample than to create zones that do not accurately represent the field. Types of data that are good for making zones include: high resolution bare ground imagery, yield data (multiple years is best), elevation data to show the field topography.

The smaller the area sampled, the more you can identity in-field nutrient variation. For example look at the soil pH map from one field sampled at 3 different grid sizes. The accuracy increases significantly as the sampling grid size decreases.

Map source: AgPhD soils course

The greater accuracy you have from sampling the greater the potential to save on fertilizer both today and in the future (since soil test results are used to create fertilizer recommendations for 3 to 4 years). Greater sampling accuracy reduces the risk of overapplying fertilizer in areas where it is not needed.

Choose the correct type of sampling and sample consistently.  If you don’t, it could cost you a lot from mis-application of fertilizer.