Comparing Grid and Zone Sampling for Site-Specific Nutrient Management

In modern agriculture, maximizing crop yield while minimizing input costs takes precedence.  Soil sampling techniques play a crucial role in achieving this balance by providing valuable insights into soil fertility and nutrient levels.  There are two primary sampling strategies when setting up a field: grid and zone sampling.  Each method has its benefits and drawbacks, and understanding the difference is essential for farmers to make informed decisions about their soil management practices.

Variability is a key factor in determining which sampling strategy to utilize.  It influences most in-field decisions including what, how and when to sample.  To understand what types of variables are present they can first be placed in two different categories. 

Temporal variability in agriculture fields refers to the fluctuations and changes that occur over time in various aspects such as crop yields, soil conditions, pest and disease outbreaks, and weather patterns.  These variations can be influenced by seasonal changes, climate variability, and agricultural practices.  Some in-season temporal changes could include nitrification or volatilization of applications, or plant nutrient availability due to seasonal changes.  A perfect example of this is fluctuating potassium levels in the soil profile.  Levels in corn acres are typically higher in spring samples, compared to fall, due to crop uptake.  The plant must wilt then decompose and return these nutrients to the soil causing fluctuations. 

Spatial variability refers to the differences in one location to the next.  These differences include soil properties, nutrient levels, moisture content and slope typically within the same field.  This variability can result from factors like soil type, topography, drainage patterns, and management practices.  Many times, these variations can be seen as surface features such as a low, waterlogged area to a high, sandy hill.  Other times they can be hidden within, or below the soil surface such as a previous lime pile site.  This variability would create a false representation of the grid area, or zone, by having increased calcium and/or magnesium levels.

Variability is unavoidable.  How we as farmers, applicators and agronomists understand/utilize this variability can have lasting impacts.  With modern technology, variability has never been more documented.  Often referred to as “big data”, it can be broken down into simple data sets.  These are commonly called data layers.  A data layer can be added, recorded, noted or submitted through an incredible number of avenues.  A few examples are planter ride quality, seeding population, hybrid placement, aerial imagery, nutrient placement, yield maps, even applicator name and contact information. 

The soil sample and nutrient maps are the most crucial data layers to sift through when making management decisions.  They have the most return on investment, when utilized properly, and can have lasting effects for years to come.  To fully encapsulate variability in a particular field, sampling strategy plays a large role.  The two textbook practices mentioned before are grid and zone sampling.

                Grid sampling is the most straightforward strategy in the soil realm. This type of sampling assumes the nutrient levels throughout the field are random.  Historically grid sampled fields have been fertilized in build up programs and masked the natural nutrient discrepancies across the working acreage.  Imagine a transparent checkerboard lying on top of a field.  These square, or grid, sizes typically range from 1-5 acres.  Within each square are, preferably, georeferenced sample points.  As mentioned above, variability is everywhere and repetitive sampling, in the same relative area, is the only way to get a respectable data layer. 

Grid sampling can offer these precise values to an operation through accurate representation and repetitive sampling but has its setbacks as well.  It is usually more time and labor intensive.  More samples will be taken from a field set up as grids.  This means it will also take much more time to do this job and that creates more costs.  However, in uniform fields the zones are much too large to get an accurate representation of an entire area and grid sizes are simply made larger.

Zone sampling is utilized in more stable environments.  Several of these areas are naturally occurring, and the spatial/temporal variability has been present for many years.  These sampling zones are created from these variabilities through many ways.  Yield monitors are a great indicator layer, when calibrated correctly, but only offer one piece to the puzzle.  To create proper zones, one must take all factors into consideration.  The natural variations of a field are recorded, and documented, using the yield monitor, aerial imagery, and soil tests (to name a few) to determine where zones differentiate.  These zones include waterlogged areas, slopes, and soil types.  Once zones are established, they can be used for much more than soil sampling.  Seeding/fertilizer rates, hybrid/variety type and even tillage/seeding depth can be adjusted according to zones.

All these factors can be isolated through other forms of data layers like scouting, soil sampling, aerial imagery etc. and made into its own zone.  This reduces the sampling costs by having larger sample areas but may have a higher start up cost due to the technology required and subscriptions used. 

Georeferenced sampling uses GPS, global positioning system, to mark a specific area.  This is important when sampling in a grid or making zones.  A common mistake is to place a sample location in an area where ag lime was piled before application.  If it is a referenced area, it is simple to find this location and see why certain outliers were present in a soil analysis report.  Another important reason to have georeferenced sampling is variability, as discussed above, is inevitable.  Nutrient variability is highly common, hence why frequent and repetitive sampling is required.

Grid sampling is still frequently used even though variability may move some points within a grid section.  To overcome this, some agronomist practice kriging.  Kriging is mostly used in grid soil sampling to predict values of unsampled locations based on spatial variability, such as slopes and hills, and nearby sampled locations.  If variable rate technology is not being used, it is wise to move points away from areas that will change application rates drastically. 

When deciding which sampling technique to use, all these factors must be taken into consideration.  If the field is a new addition to the farm, a grid style sampling technique is highly recommended, with a small grid size, to determine where variability exists.  If the results show drastic nutrient levels, an even smaller grid size is required at the next sampling date to determine what is needed, or not needed, and where. 

While grid sampling provides precise data for targeted nutrient applications, it can require more time and costs.  On the other hand, zone sampling offers a more practical and cost-effective solution for large scale operations but may sacrifice precision.  The choice between grid and zone sampling should be based on factors such as field size, budget, goals, and the level of detail required for effective soil management practices.  By selecting the most appropriate soil sampling method, farmers can optimize nutrient use efficiency, enhance crop productivity, and sustainably manage their soils.

Your ALGL sales agronomist can help determine with method is right for you.