Veris maps, soil surveys and grid sampling:
Improving precision is in the details.
What about soil surveys?
"Close, but not close enough..."
Inclusions... Depending on the survey scale, soil surveys allow 2-5 acre inclusions. These are areas of another soil type within a larger soil type. With site-specific management, a several acre inclusion can represent a significant discrepancy.
Errors... Many of the errors in soil surveys were a result of not having GPS technology available when the surveys were done. Other sources of errors stem from the small number of sites investigated on a given field.
Transitions... Soils don’t change on a line—but are a continuum. A soil survey indicates this change as an abrupt line, when in reality the transition zone between the two soil types is often its own soil type.
Surveys Have Value... Many surveys are able to provide important supporting information about the field. Their lines may not be accurate, but these surveys can help supply the “what” information, while the soil EC map provides the “where”.
The NRCS website issues this warning for field-scale soil surveys:
What about grid sampling?
"Soil doesn’t change on a straight line..."
Grid soil sampling ignores the natural variability of a field. Gridding can waste money oversampling some areas, and miss what’s important in others. With soil EC you define the soil texture variability first--then make sure each sample is in the spot to do the most good.
Here’s what researchers have found when they ground-truthed grids:
Soil Science Society Journal 2005: (Indiana study)
“Data points from large grids (greater than or equal to 2 ½ acre) were too far apart to provide much information about the nature of pH or lime requirement change between adjacent sampling locations.”
Agronomy Journal 2002: (Iowa study)
“Soil pH varied from 5.4 to 8.0 over distances of about 150 m (480 ft) in most transects. In some sections soil pH varied about 2 pH units over a 12 m (40 ft) distance…”
Agronomy Journal 2005: (Ontario study)
“It was concluded that a grid spacing of 30 m (96 ft) or less would be required to adequately assess the spatial variation of STP, STK, (phosphorous and potassium) and soil pH. Sampling at this intensity would require approximately 11 times as many soil samples as the commonly used 100-m (2 ½ acre) grid.”
What about soil EC?
Soil EC maps are maps of soil texture variability: the finer the texture—the higher the conductivity. Clays conduct better than silt, and silt better than sand. Saline areas are highly conductive, but in most agricultural fields, an EC map is a texture map. Soil texture is critical as it impacts water-holding capacity, CEC, nutrient-holding capacity, and many other factors.
Soil EC maps are detailed, showing precisely where soil changes. This is done by covering the field thoroughly on 50-75’ transects. What areas you choose to manage is up to you—but at least you won’t be misled by what you can’t see without this level of detail.
Soil EC maps are a one-time investment, unless major soil-moving is done. The raw EC values will change with soil temperature and soil moisture, but the soil zones are very consistent.
Soil EC looks deep into the soil. It isn’t affected by crops or crop residue on the surface. And Veris dual depth arrays can find surprising differences within the soil profile.
Because soil texture affects productivity, EC and yield maps often show similar patterns.
Soil EC has been widely studied and is well-supported in research and extension literature.
“Soil EC is an important characteristic that can be used to map the spatial variability of soil within a production field. Better Crops (IPNI) 2009 #2
“Bulk soil electrical conductivity (EC) has quickly become a favorite tool in mapping soil variability within fields.” Agronomy Journal May-June 2003
“Soil electrical conductivity (EC) mapping is a simple, inexpensive tool that precision farmers can use to quickly and accurately characterize soil differences within crop production fields.” Site-Specific Management Guideline #30
What about on-the go soil pH mapping?
High-definition pH measurements are now feasible in-field, with the Veris Mobile Sensor Platform and pH Detector. These measurements are highly correlated with lab-analyzed samples and provide the detail needed to improve pH and lime maps.
This level of detail shows just how widely pH varies—on many fields there is as much variability within each 2 ½ acre grid as there is in the entire field!