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Expect pH to vary in fields where soils aren't uniform

Crop Watch 2016: Soil sampling by soil type and zones and variable-rate application makes sense.

Tom Bechman, Editor, Indiana Prairie Farm

April 8, 2016

2 Min Read

While the field where the Crop Watch corn will grow this year contains three soil types, it is primarily one type, with smaller amounts of two others. Soil types range from silt loam to slightly heavier soil, with some natural soil drainage problems across much of the field.

It’s a perfect candidate for soil sampling by soil type, with some adjustments made for management zones, such as adjustments made on the basis of looking at yield maps over time. The field is sampled every year. All the fields on this farm are sampled every year, at least until this point. Economics could cause that to change in the future.


A crops consultant pulls samples and helps with recommendations. One factor that varies across this field is soil pH.

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The Corp Watch farmer reports that pH varies currently from 5.9 to 7.2. Purdue University recommendations call for a pH of 6.0 or higher for corn and soybeans. Availability of some micronutrients can come into play at very low or very high pH levels. Sometimes the micronutrient may be less available. Sometimes it may become so available that there could be enough to be toxic to the crop.

For example, according to Purdue’s Corn & Soybean Field Guide, 2016 edition, manganese becomes less available as the pH moves very far above 7.0. Both phosphorus and magnesium, primary nutrients, are less available as the pH moves below 6.0.


According to the farmer’s records, lime was last applied on this field three years ago. Because of the differences in pH, the application was a variable-rate lime application. The amount of lime applied varied from 0.5 to 2.5 tons per acre.

Does variable-rate application pay for lime? The farmer points to the average pH level for the field reported on his test form. The average is 6.5, nearly perfect for corn and soybeans. Hidden in the average are the 7.2 pH spots, which don’t need more lime, and the 5.9 spots, where crops yields should respond rather quickly to correct soil alkalinity.

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Even if they readjust how often they sample, expect them to continue with variable rate application of lime and nutrients. They realize that besides affecting crop growth, pH levels can also impact how certain herbicides perform in the field.   

- Seed Consultants, Inc., is a sponsor of the Crop watch project.

About the Author(s)

Tom Bechman

Editor, Indiana Prairie Farm, Indiana Prairie Farmer

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