Gridding and Soil pH Variability

As the number of fields being grid sampled has increased, so have questions about how much nutrient levels vary within fields. At first everyone was astonished at how much nitrogen, phosphorus and potassium levels varied within fields. The variability of zinc levels has also surprised some experts. Soil pH variability, however, has raised the most eyebrows for dealers and growers and is a hot topic in some areas.

Soil is not a homogeneous or uniform medium for plant growth. There are many factors that can cause soil pH to vary considerable with short distances in fields. The first and foremost of these is the parent material in which the soil formed. Glaciers, flooding rivers, weathering rocks and erosion are just a few of the mighty forces that are responsible for the initial variability in soil pH. Some man-made factors, such as drainage, leveling of fields, applying fertilizer and manure, etc. have created even more variability in soil pH.

The variability in soil pH found in many fields is real and should be expected. Data compiled for 1997 from AGVISE Laboratories in Benson, shows that 40% of the fields grid sampled had soil pHs ranging over 2.0 pH units. 18% of these gridded fields had pH values ranging over 2.5 pH units (i.e. from pH 5.5 to 8.0). In the glacial till areas of North Dakota, where the average field pH is >8.0, it is not unusual to find areas in each field with a pH of 6.0 or lower.

Soil pH Range Within Grid Sampled Fields
AGVISE Laboratories, Benson MN
0.5 pH Unit 1.0 pH Unit 1.5 pH Unit 2.0 pH Units 2.5 pH Units
1995 2.3% 12.6% 20.9% 36.0% 27.2%
1996 3.9% 14.8% 16.8% 35.6% 18.9%
1997 4.8% 14.5% 22.4% 40.1% 18.2%
Example: A field with a pH range of 2.5 pH units could have soil grid pH values ranging from a low pH of 5.5 up to a high pH of 8.0 in the same field.

What should be done about these pH differences in the field? In some cases doing nothing is the best answer. In northern areas, even when the topsoil pH is less (<7.0) acidic, it is very likely that the subsoil has a high pH (>7.0)(basic), due to the calcareous nature of the soils in these drier regions. In areas like this, lime applications may not be beneficial or economical due to the high pH of the subsoil.

In southern Minnesota, Eastern South Dakota, Nebraska, Iowa and other areas with low soil pH, crop production will increase when lime is applied to areas within fields with low soil pH in the topsoil. Soils in these areas usually have acidic sub-soil. Variable rate lime application is growing in many of the areas due to the effectiveness and cost saving of liming only those areas of the field that need it.

This fall, when grid soil testing fields that may be candidates for variable rate lime application, remember to request a Buffer pH on the grid field information sheet. This test provides more information for fine tuning your lime application guidelines in low pH areas of the field. If you are grid testing a field for the first time this fall, don’t be surprised if you find soil pH values varying a lot more than you would have thought.