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Ask questions before soil sampling

Ask questions before soil sampling

Spring soil sampling is right around the corner. The soil samples that will be pulled in 2021 will be providing useful data through the 2025 growing season.

Soil sampling is a major investment and carries tremendous value. The samples will drive most of the fertility-management decisions on the farm. Planning should be a key part of the process, long before a probe enters the ground. To help ensure representative samples are collected come spring or fall sampling season, ask a few key questions of an agronomist.

What is the timeline?

Soil is always there. That means it can always be sampled. While the first statement is true the second may require an asterisk. The soil is always there, but pulling an excellent-quality soil sample isn’t always possible.

Many factors come into play that prohibit pulling a quality soil sample. Frozen ground and snow can make collection impossible. Tillage can make getting through a field difficult, and the fresh-tilled soil can make depth consistency a major problem. Thawing soil and excessive soil moisture can also make field access impossible. The best window to pull a soil sample is following harvest or after fields are fit for planting -- about three to four weeks after emergence.

Is timeline important?

Sampling timeline can be a tricky subject. On one hand there's a drive to pull samples in the fall, and have results and recommendations back in time to apply fall fertilizer and perform fall tillage. On the other hand fall can be short and wet; keeping plows moving is the goal. For that reason many growers look toward spring sampling. After all the soil is always there.

The old adage was to never switch the seasons when it comes to soil sampling. If normally sampled in fall, keep sampling in fall. If sampled in spring, stay in spring. Why introduce another variable into interpreting the results? That strategy seemed ideal before mega data sets with far more samples in those data sets.

At Rock River Laboratory there have been in-house comparisons conducted of spring versus fall, and researchers found slight differences but never enough to change a recommendation. The two analytes that are most likely to change due to season are potassium and pH. In the study the average change in potassium was about three parts per million greater while pH was about 0.1 greater in the spring.

The University of Wisconsin-Madison did a similar review of past data and found a similar trend. Carrie Laboski found that seasonal increase of soil test value to be slightly greater, but the change was still not enough to warrant a major management change.

Who will be pulling the samples?

With the role the sampling results will play for years to come, a qualified individual is an important cog in the sample-collection process. If the grower has time to pull the samples themselves, it can serve as a valuable learning opportunity. The grower knows what areas do well and what areas are frequent underperformers. The grower will also have a chance to see compaction and traffic-management problems, sheet and rill erosion that would be missed from the cab, and disease and insect pressure that may have been missed at harvest.

But a grower doesn’t always have the time or equipment to do his or her own sampling. If hiring out sampling, ask the service provider who will be pulling the samples.

Where are samples going?

When excellent-quality soil samples are pulled, they need to be submitted to a qualified laboratory. Be sure to ask where samples are going. If a producer doesn’t ask, samples may be submitted to a laboratory that's not state-approved. Different states have different requirements when it comes to soil testing. In Wisconsin a laboratory must be certified by the state and participate in the Wisconsin laboratory-proficiency program. In other Midwestern states the requirements are different.

The state may require participation in one of the nationwide proficiency programs such as Agricultural Laboratory Proficiency or North American Proficiency Testing. At its core a laboratory-proficiency program requires labs to use specific prep and analysis procedures. It also requires the laboratory to analyze known and unknown check samples, and submit them to the proficiency organization for validation.

If unsure if a laboratory is certified, the first place to check is the lab's website. Most laboratories have certifications and proficiency-program participation listed on the website.

With a bit of planning the soil-sampling process can provide even more value to the grower – for years to come. The best way to capitalize on that increased value is to ensure the best soil sample possible is being collected. Review the sampling timeline to ensure it fits into the operational timeline. When it’s time to sample be sure the samples are pulled in a professional manner and sent to an accredited laboratory.

Scott Fleming is a nutrient-management specialist and sampling director for Rock River Laboratory in Watertown, Wisconsin. Visit for more information.

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