By Wilks Wood, Ph.D., Field Agronomist
Soil sampling technology has never been better.
The days of spending all your valuable time and money to evaluate thousands of acres of soil are long over. With Zone Management Sampling, any farmer can decrease soil sampling costs while increasing the quality of information collected to make profit-driven decisions for the next growing season.
However, if you aren’t familiar with Zone Management Sampling, or you’ve been using the once-popular method of grid sampling, you’re probably looking for more information about this technology.
In this guide, I’ll explain how soil sampling management techniques have evolved over the years, the challenges of grid sampling, and the features of switching to zone sampling methods.
Early Soil Sampling Management Methods
Management techniques have evolved and changed significantly since soil sampling recommendations were initially developed during the 1960s and 70s. Farmers traditionally separated sampling areas based on differences in previous management, soil map units, and sometimes topography.
Determining the best way to manage land can be incredibly challenging, as soil type, topography, fertilization history, and many other factors fluctuate even between fields.
The amount and pattern of nutrient variability can vary significantly in the following ways:
- Soil chemical and physical properties directly influence nutrient availability by affecting:
- The total amount of a nutrient in the soil
- Its availability for plant growth
- Possible losses due to movement of nutrients or soil
- Soil type can affect nutrient availability indirectly by affecting yield potential and nutrient removal.
- Crop and soil management practices also have a marked influence on nutrient supply and changes in availability due to:
- Crop, tillage, nutrient removal in harvested products
- The rate and application method of fertilizers or manure
Technology Brings Progress
In the 1980s, precision agriculture technologies (mainly GPS) allowed for recording soil-test values and geographical coordinates of sampling points. This new technology could also process other field information into layers that could be easily visualized in maps.
These new developments led to a new appreciation of the high nutrient and yield variability occurring in most fields. It became apparent that uniform map units in commonly used soil survey maps had very high nutrient variability, especially P, K, and pH. Additionally, farmers realized that many fields with long histories of fertilization variability patterns did not follow patterns of soil map units.
As technology allowed farmers to collect data, they quickly adopted intensive grid sampling methods in the 1990s. However, these methods ignore landscape and soil mapping units.
The Challenges of Grid Sampling
Grid sampling subdivides a field into a systematic arrangement of small areas or cells. One composite sample usually made up of 6 to 12 cores, is collected from each cell.
Soil-test values collected by grid-point sampling may be used in one of two ways. They may be directly mapped to represent each cell area or used to create interpolated maps using one of several methods available in standard computer software.
Farmers realized that cell sizes of even 4 acres failed to describe the high variability occurring in the fields (mainly of available P, K, and pH). As a result, a cell size of about 2.5 acres is now more commonly used, but dramatically increases the cost. Small and infrequent yield advantages for the intensive soil sampling and variable-rate fertilization package clearly show that soil testing costs have to remain low, so the technology becomes economically effective.
The New Frontier: Zone Management Sampling
Management zone sampling improves traditional sampling (by soil map unit method) by using information that can be collected via precision agriculture technologies.
Zone sampling can reduce the number of samples and sampling costs while maintaining the quality of information about nutrient variability within a field. Soils are sampled not only for P and K but also for other nutrients and purposes other than fertilization.
Sampling by zone assumes that sampling units can be identified based on areas with different soil or crop characteristics across a field. It also assumes that patterns are likely to remain stable. Specific criteria used to delineate management zones, other than soil survey maps, will vary depending on the tools available to producers, cost, and how they adapt to the particular conditions of the region or field.
Yield maps can be used to define different soil productivity areas within a field. This is is a useful criterion to delineate management because different yield levels likely have different nutrient requirements, influence nutrient removal, and nutrient trends over time, and may influence economically optimal fertilization rates.
It’s important to note that stable, within-field yield patterns over time are observed in some fields but not in others. Multi-year yield maps are needed, and this may or may not result in an effective criterion to establish management zones relevant for nutrient soil sampling.
Topography (which includes elevation and slope) and aerial images of both bare soil and crop canopy can be used to identify management zones because they tend to reflect different soil properties and can be inexpensive.
Soil electrical conductivity (EC) is commonly measured in two ways, via electromagnetic induction using EM devices (TSM, EM-38, etc.) or resistivity devices, such as Veris. It’s useful to describe the spatial variation of topsoil depth and other physical and chemical soil properties. EC maps are often correlated with crop yield over a field.
While progress continues to be made every year in the ag tech industry, farmers will always need to know the details about what’s going on in their soil from year to year. Management zone sampling makes sense economically and is flexible enough to accommodate different layers of information and sampling objectives.
Do you need help building a plan for zone sampling on your farm? Contact Growers at 984.500.3797.
Mallarino, A.P., and D. Wittry. 2001. Management Zones Soil Sampling: A Better Alternative to Grid and Soil Type Sampling? In The Integrated Crop Management Conf. Proceedings. Dec. 5-6. Iowa State Univ. Extension. Ames.