A while ago I met with a group of agronomists and asked them this question: “From a fertility standpoint, please describe… what is a zone?”
Answers: “Soil types, organic matter (OM) variability, topography areas.”
"And what is the major factor driving this variability and yield?" Answer: “Water.”
Boom. Bingo. Those are the kind of answers I love to hear. So why is the industry often confused about this question? Two reasons:
1. Yield potential maps are in every toolbox and they are the Easy Button
The majority of companies are selling zones based on the software tools they have, which are based on data that is easily accessible, scalable, and cheap. Satellite imagery and yield maps provide maps of crop growth, but it is influenced by 100’s of things (insects, disease, weeds, trash, soil types, too wet/dry, seeding issues, frost, etc.). Images can be completely opposite in dry versus wet years. The fact is: yield potential is just measuring the crop response to everything that is happening in the field over the course of the growing season. These maps are unstable for this reason; no two years are the same. This is not a fertility focus.
2. Soil potential maps are not easy and buttons are in progress.
They require covering the field with sensors to get pictures of what is happening below the surface. We haven’t seen a software system (other than SWAT MAPS) that can put all the layers together to make proper zones. No government soil survey databases have accurate data because it is of such low resolution it is almost useless. Soil colour maps for OM don’t know textures or if the area is well-drained or flooded. Elevation does not know the different between water collecting or water shedding areas. Is a depression well drained, does it flood, or is it saline? Has there been erosion and where?
Nutrients in soils are related to topsoil depth and OM, and crop response to nutrients depends on how much water is in that area of the field. You cannot use a single layer (such as EC or OM alone). Building the proper layers and then turning all of the data into refined zone maps is effortful. But once they are made, the picture comes clear. Drier areas with erosion are a zone. Salty areas are a zone. Flood prone areas with deep topsoil and soil organic accumulation are a zone. They are stable. A lot of experience and proper tools are needed for maps that provide a fertility focus.
Summary
A fertility zone is a combination of yield potential and soil potential. Yield potential maps are easy, quick, and cheap to make but are unstable and do not relate directly to fertility zones. Soil potential maps are challenging and require effort to develop but they provide a stable foundation to build soil sampling, water knowledge, and a proper fertilizer response map when paired with yield potential.
Cory Willness, CCA, CAC, PAg
President
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