This agronomy brief covers three main methods for soil sampling, including composite sampling, zone sampling, and grid sampling, and the pros and cons of each.

Before we discuss soil sampling, we first need to describe the two main methods for fertilization: flat rate and variable rate (VRT). 

Flat rate is when fertilizer is applied at the same rate throughout an entire field, regardless of in-field circumstances. 

VRT is when fertilizer or other inputs are applied at different rates based on the objectives for different spots within the field. 

These objectives may include variable fertility rates, reducing input costs, and managing for different yield goals. A major benefit of VRT fertilization is the ability to maximize your fertilizer investment within each field. On average, the total amount of fertilizer applied to a field differs very little between flat rate and VRT methods. 

The main difference is that some sections of the field will get more fertilizer, and others will get less depending on the objectives and soil tests, resulting in a more efficient use of that fertilizer investment.

Samples are based on compass directions and an acre size. One bag of soil (comprising 15 to 20 cores) is taken in several different spots throughout a field. An example would be sending in two separate composite samples for a field: one for the north half and a second for the south half.

 This method is relatively simple and does not require specialized training or GPS-enabled sampling equipment. Because of this, it is cheap enough ($2 to 4/A.) that it can be conducted annually and does not require modern VRT-enabled equipment for fertilizer application.


 It is a very blunt instrument that cannot measure or manage in-field variability well, leading to inefficient fertilizer applications.

This is a method of georeferenced soil sampling where the lab results are interpolated by computer software to estimate fertility between points where samples were collected. Typically, the most common method is to collect one bag of soil (containing 6 to 8 cores) for every 2.5 acres. It could also be 1, 2.5, 5, or 10 acres.

 It provides a relatively precise map of variability in soil fertility within a field. These maps provide the ability to make variable-rate fertilizer applications within a field. The training required is relatively simple because software programs do most of the work of designating sampling locations and creating the maps after the lab processes the soil data. It is the best-known method for measuring variable pH levels and providing the most efficient VRT lime applications.

 Because of the large numbers of soil samples taken per field, grid sampling is the most expensive of the three sampling methods at $8 to $12/A. Because of the cost, most fields are only sampled once every four years, which is ineffective for mobile nutrients like nitrogen and sulfur that require annual sampling results to create accurate recommendations. Lastly, the results can be highly variable depending on the type of interpolation software being used and the high variability of individual soil samples taken within the same GPS coordinate.

Zone sampling is a method of georeferenced soil sampling in which a field is divided into management zones (typically 3 to 8 per field), and one bag of soil (15 to 20 cores) is taken per zone. The most labor-intensive part of zone sampling is mapping accurate zones that achieve the objective set for that field. 

One of the most common objectives is to manage a field based on variations in productivity levels. The goal is to identify these areas, draw boundaries around them, collect soil samples, and then manage them according to their individual results and productivity levels. 

Several data sources are utilized to identify the level of variability, including yield maps, NDVI satellite imagery, topography maps, etc. The use of cheap satellite imagery has made this process very simple and cost-effective. By taking several years worth of satellite images that accurately reflect a field’s variability we can find the “average” variability and create boundaries around the zones.

Once this zone boundary creation process is finished, it will not need to be completed again unless something major changes within the field.

 It is relatively cheap ($3 to $5/A.) compared to grid sampling and, therefore, can be done more frequently (annually or semi-annually). The higher number of soil cores per bag of soil drastically reduces variability in the soil samples, making the results highly repeatable. The zones created can be used for other VRT applications like variable rate seeding.

 It takes a high level of training and specialized software to initially create these zones accurately. Zone sampling is not as effective as grid sampling for identifying variable pH levels used for VRT lime applications.

The best option for soil sampling on your farm will depend on the different aspects of your operation. Things to consider are farm size, the presence of in-field variability, and equipment capabilities. 

If you do not have the ability to VRT fertilize then you will not be able to get the full benefit from zone and grid sampling. If your field has very little variability (less than a 10% yield difference within the field), then variable rate fertilizing may not be needed. Therefore, composite sampling will be adequate. 

If you have acidity issues and need to apply lime, grid sampling will most likely provide the best return. But if you want to VRT fertilize and do not have low pH issues, zone sampling could be a good option.