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PFR Report: The Benefits of Soil Sampling

Published on Wednesday, November 6, 2019

As hard as it was, a challenging year like 2019 can still yield applicable insights. The important thing to do is carefully weigh all the factors that can impact your results and examine the data in light of those factors. This past spring, I asked an intern to work a plot and as he began to work the ground I found myself thinking “that may be the worst decision I have made in my 35-year ag career”. 

This season will also impact how we think about future decisions as it relates to soil tests and soil testing. I asked my friend Jamie Bultemeier, an agronomist with A&L Great Lakes Laboratories, to share his thoughts on the impact of 2019.

Schwartz: Jamie, how will the weather this year impact our understanding of soil testing?

Bultemeier: The weather patterns of 2019 have the potential to impact soil test levels this fall, but the impacts may vary by field. Like trend line yields, repeated soil sampling allows for the trending of soil test values over time. There was a wide range of weather and management variables during 2019 that may shift soil sample values above or below the trend line of the past few sampling cycles. 

Schwartz: Can you give me a few examples?

Bultemeier: Let’s start with prevent plant/unplanted acres. If fertilizer was applied in the fall of 2018 or in the spring of 2019, but no crop was planted to utilize the nutrients, the fall 2019/spring 2020 soil test levels may be higher than trend. If fertilizer was not applied in the fall of 2018 or the spring of 2019 due to wet soils, the soil test levels should be near trend.

Schwartz: So how do I manage those prevent plant/unplanted acres given what you just said?

Bultemeier: Fields that have been repeatedly tilled through the summer months to control weeds may prove challenging to maintain a constant sampling depth unless the soil can settle for four to eight weeks before sampling with a few rainfall events. Soils that have been tilled frequently, allowing the topsoil to dry out too much, can mimic droughty soil conditions which may result in lower than trend soil pH and K.

In addition, the chemical termination of plants can lead to a quick release of nutrients. Chemical termination just prior to soil sampling may lead to temporary increases in soil test values. 

Finally, mowing of fields would simulate plant uptake while maintaining a slow release of nutrients from plant residues back to the soil. Mowing may have the least impact on soil test values in relation to trend line soil test values.

Schwartz: If cover crops were planted on many of those acres, how will that impact soil sampling?

Terminated cereal rye in a PFR plot.

Bultemeier:  If a cover crop was planted in June or July in soil that was fertilized and allowed to grow through sampling time, the cover crop will take up a relative portion of what the intended crop would have. While the results will vary by the planting date and species of cover crop, this should reduce the chance of the 2019 fall soil test level to be higher than trend in fields where fertilizer was applied.

Fall seeded cover crops may not have much growth before sampling and thus will have little impact soil test results.

If the cover crops were terminated before soil sampling, nutrients like potassium that quickly release from crop residues may be captured in the soil sample.

Aerial drone image of the CIL PFR site after heavy April showers.

Schwartz:  Jamie, this year many of us had to get in fields before they were ready. What impact will that have?

Bultemeier: Planting into less than ideal conditions have led to a wide range of impacts from field traffic. Many fields exhibit row to row variability due to the negative impacts of driving across wet soils in the push to plant the 2019 crop. These patterns may persist into 2020 as well. The variability in plant growth will lead to variability in plant yield and can potentially increase sampling error.

Once 2019 harvest is complete, fall tillage and fertilization will have to be done carefully to keep from making the impact of tillage and traffic compaction worse if soils are wet.

Schwartz: Like every growing season, different parts of our marketing area experienced different growing conditions. How might that impact soil testing and fertilizer decisions?

Bultemeier: In some areas, it was dry enough to bias soil pH and potassium soil test levels slightly lower. Of course, the dry conditions may have also reduced crop nutrient uptake with actual yields being lower than predicted.

Schwartz: Any final thoughts?

Bultemeier: Our interpretation of 2019 data will lead to valuable knowledge as part of an overall soil fertility management program. Every growing season has factors that can potently influence soil fertility results, however, 2019 will have a greater potential to influence your soil test results than most. The key to success will be field notes about crop growth, yield, and management of the field as part of a total fertility management plan. Fieldnotes with a bit of soil fertility knowledge will be very valuable in the coming years. The higher seasonal variability has led many wise fertility managers to sample more frequently (once per crop rotation) to identify and address variability soil test data more effectively.

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Jim Schwartz

Jim Schwartz

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