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Managing a Fertilizer Program in Variable Soils

Growers recognize the effect of in-field variability on yield performance. Often pressed for time, most farmers manage fields uniformly from one end to the other. Some sources suggest that uniform rates of fertilizer applications over entire fields can actually be more costly and may not maximize the return on investment (ROI). An alternative approach to flat-rate applications is to use variable-rate input applications, known as zone-based management, to manage inputs on a smaller scale. This approach allows producers to address a field’s variations precisely (source: https://extension.sdstate.edu/precision-agriculture-and-zone-management).

“There are a few different ways farmers can identify variability in their fields,” explains Matt Wiebers, agronomist and AFREC researcher. “One way is to use a historical bare soil satellite image. Tillage operations provide a great opportunity to highlight color differences in the soil. Light areas indicate low organic matter, whereas darker areas indicate higher water and organic matter. Much of this variability is related to topography, or slope. On hilly ground, topsoil tends to move down the hill over the years. Yields are often lower in these areas as a result. In the low areas, poor drainage can be one of the biggest factors affecting whether an area is high- or low-performing.”

One of the first steps toward zone-based fertility management is to identify areas within fields that consistently differ from the field average. The second step is to work with your agronomist or advisor to figure out what is causing the difference.

The best fields for zone management tends to be situations in which the yields look to be consistent from one year to the next. By contrast, “If an area is high yielding one year and low yielding the next, you probably wouldn’t want to implement a zone management fertilizer program in that area until you’ve studied it for several years,” says Wiebers.

Eroded areas of fields could be the first areas to focus, as they are easy to identify. Low-lying areas of fields might be harder to manage with zones until the drainage can be improved. Fix the drainage problems first.

There are several ways to create management zones that may be used for site-specific fertilization methods, either individually or as a combination:

  • Soil testing based on soil types
  • Previous yield monitor data, particularly when multiple years can be combined
  • Satellite or aerial imagery showing NDVI or other crop-health indexes
  • Soil Electrical Conductivity (EC) mapping services, which help identify soil texture and water holding capacity
  • SSURGO Digital soil maps

Once the variability has been identified, often it makes sense to group them into three to five zones. The number of zones can change based on variability and the field size (source: https://extension.sdstate.edu/precision-agriculture-and-zone-management).

The final step is adjusting fertilizer applications to each zone to maximize plant growth and crop yield. While the AFREC 2020 on-farm research suggests that low-yielding areas may benefit from higher rates of fertilization, this was a single-year study. Growers should conduct at least two or three years of research before making changes within their own fields.

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