Zone Soil Testing–How Many Zones?

Dividing fields up into several management zones for soil testing is now a regular practice in many areas. It just makes sense to split fields into parts for soil testing based on different levels of production to see if soil fertility is the reason. Fields are divided into management zones from several sources of information (i.e. vegetative vigor, yield maps, salinity maps, topography, etc.) As a laboratory we do not know what information layers were used to create these management zones, but we do know the nutrient test levels of each zone. Customers often ask us how many zone samples should be taken from each field to get the best nutrient information. Common sense tells us that splitting fields into more zones will provide more detailed nutrient information.

With soil test data from thousands of zone tested fields in our database, we decided to see if the range of soil nutrients (high testing zone minus low testing zone) could tell us something about how many zone samples fields should be split into for soil testing. The table below shows the average difference from high testing zone to low testing zone for over 1000 fields. The number of samples per field ranges from 3 zone samples per field up to 8 zone samples per field. You can see, as the number of zone samples per field increases, the difference between the high testing zone and the low testing zone gets larger.

Number of Zone
Samples per Field
Average Difference
of Nitrogen
Average Difference
of Phosphorus
(high zone – low zone) (high zone – low zone)
3 28 lb/a 7 ppm
4 32 lb/a 14 ppm
5 43 lb/a 18 ppm
6 57 lb/a 17 ppm
7 47 lb/a 20 ppm
8 65 lb/a 21 ppm

*Nitrogen = 0-24″ nitrate nitrogen   *Phosphorus = Olsen P test

This data reminds us that the more zone samples tested in each field, the more we learn about the nutrient status of each field. The general nature of the soils in your area will dictate the number of zone samples you need to take per field. If you have variable soil types, rolling topography and salinity issues, you may have to take more zone samples per field (5-7) to see most of the nutrient differences in the field and take full advantage of zone sampling. If your area has fewer soil types, relatively flat terrain and no salinity issues, you can probably take fewer zone samples per field (3-5).

Once the fall soil testing season is done, we will put together a more complete analysis of the soil test data from all zone tested fields. We hope to be able to divide the data into different areas such as glacial till soils, old lake bed soils, etc.