Fall Soil Tests After Harvesting Corn and Soybean

Key Takeaways

• Fall soil tests are recommended after corn and soybean harvests to measure residual nutrients in the soil profile.
• Consistent soil testing at the same time each year is important as nutrient levels may differ from fall to spring in some soil types.
• Post-harvest operations should begin with soil testing, before tillage and fertilizer applications.
• In the fall, nematode testing can be conducted at the same time as nutrient testing.

Q. Why should corn and soybean growers soil test in the fall?

Soil sampling in the fall measures the residual nutrients left in the field after corn and soybean harvest. Fall is often the preferred time for purchasing and planning fertilizer operations, because:

• Compared to spring, there is a shorter turnaround time for receiving test results.
• Lime delivery, spreading, and raising soil pH requires months.
• Fertilizer purchases (based on soil test recommendations) made in the fall are expected to cost less than spring purchases.
• Soil testing in the fall allows for planning the placement of lime piles and manure delivery in fields with the greatest nutrient needs.
• Field conditions are usually more cooperative for sampling in the fall than in the spring, with more time allowance for soil tests.
• When planting winter wheat, fall soil test results may indicate that phosphorus (P) and potassium (K) applications are advisable. These nutrients are recommended to be applied in full at winter wheat planting.¹

Q. How often are soil tests recommended?

Soil tests are recommended at least every two to four years in regular rotations.^1,2 Crop nutrient removal – over a four-year period – typically stays within the recommendation category given by soil test results. Large-acre operators may sample a third of the acreage each year as an alternative. In general, soil sampling should occur at the same time each year, though not after a recent fertilizer amendment.

Some situations may benefit from more frequent soil testing. Silage corn fields have greater nutrient removal rates and thus their phosphorus (P) and potassium (K) levels should be sampled every two years. For more detailed information, please read Effects of Stover Removal/Silage Harvest on Nutrients in the Following Year. Sandy and loamy sand soils should also be tested every two years due to their low buffering and cation exchange capacities (less than 6 me/100 g).^2,3 The pH and nutrient contents of sandy and loamy sand soils may have wider differences compared to other soils.

Q. How should soil samples be collected for testing?

The number of samples collected should capture the variability of the soil in the field, with soil cores taken from distinct areas, though different numbers of samples covering different levels of detail can be used depending on the grower’s needs.

• Whole-field sampling is the least expensive option, using one representative sample per field.
• Conversely, zone sampling represents geo-referenced areas related to landscape, slope, soil texture, or other yield-affecting factors and subdivides those areas into their own soil samples to produce more specific results.
• Grid sampling divides a field into equally sized areas and takes samples covering regular intervals across fields.

While zone and grid sampling are the more expensive and labor-intensive options, growers are expected to save enough on fertilizer expenses to pay for the additional tests. For example, if soil sampling is done every three years, its cost may run an average of one to two dollars per acre while lime and fertilizer costs may be over $100 per acre. Adding fertilizer to areas indicated to have P and K within maintenance levels does not usually result in a yield response. Hence, more detailed testing can provide information to allow more targeted applications, potentially reducing the amount of fertilizer applied and saving costs.^4,5

For more on zone and grid sampling methods, please read Soil Testing 101.

Q. Are nutrient levels different between fall and spring soil tests?

Yes. Potassium levels may differ between a sample taken immediately after harvest and one taken a month later or in the spring, particularly on sandy soils.² Soil pH and phosphorus tend to be lower in the fall compared to the spring.^2,3 A late-season drought could result in slightly greater soil pH, while rains could leach basic cations and lower the pH level.² High-nutrient soils are expected to show higher K in the fall soil tests compared to spring.

Q. Does fall tillage affect soil test results?

Yes, so soils should be tested prior to tillage. The recommended sampling depth for plowed fields is six inches, while the recommended depth for minimum and no-till fields is eight inches.² A sampling depth increases one to three inches after tillage because of the reduced soil density.⁶ The recommended six-inch sampling depth becomes a seven- to nine-inch sampling depth after tillage. Comparing samples from different depths can skew results even more than seasonal differences.

If tillage was already done, wait six to eight weeks before soil testing to allow root activity and organic matter to normalize.² Most root activity and organic matter is in the top six inches of soil coinciding with soil sample depths. Deep and aggressive fall tillage may mean waiting until spring for soils to return to an appropriate density that allows for accurate test results.

Q. Is fall the time to test for corn and soybean nematodes?

This answer depends on the nematode species, crop, and soil texture. Though testing soil for nematodes in the fall can help identify the species of nematode affecting the harvested crop, it does not predict the nematode numbers for the following year. Since nematode damage can look similar to pH and nutrient deficiencies, fall may be the best time to collect both nematode and nutrient samples in symptomatic, lower-yielding fields.

The methods used to sample for nematodes are species-dependent, and specific steps must often be taken to produce accurate results.

• Endoparasitic nematodes (e.g., lesion, corn vermiform): live within roots and must be induced to crawl out using an extraction procedure.^7,8
• Sting and needle nematodes: these are missed by soil probes because they are larger and able to move several feet deeper into the soil profile.⁸
• Soybean cyst nematodes: egg counts are best sampled in the fall after harvest. Roots do not need to be sampled, and samples should not be collected when the soil is frozen or saturated with water.

To determine the baseline nematode population in a field, a random pattern of soil cores can be taken from an area of no more than 40 acres.⁸ More information about nematode sampling can be found at How and When Do I Sample for Soybean Cyst Nematode (SCN)? and Parasitic Nematodes in Corn. Contact the diagnostic laboratory for directions on collecting and preserving post-harvest nematode samples.

Sources

¹Crain, B. 2025. Why test soil in the fall? Michigan State University. https://www.canr.msu.edu/news/why_soil_test_in_the_fall
²Clark, C.A. and Bandura, C. How can soil sampling affect lab results? University of Wisconsin–Madison. https://cropsandsoils.extension.wisc.edu/articles/how-can-soil-sampling-affect-lab-results/
³Warncke, D.D. 2000. Sampling soils for fertilizer and lime recommendations. Michigan State University Extension. E498. https://msu-prod.dotcmscloud.com/uploads/resources/pdfs/soil_sampling_guide_(e0498).pdf
⁴White, C. 2024. Fall is the perfect time for soil sampling. Pennsylvania State University. https://extension.psu.edu/fall-is-the-perfect-time-for-soil-sampling
⁵Lindsey, L. Matcham, E., and Culman, S. 2021. Fall soil fertility sampling. Ohio State University. C.O.R.N. Newsletter. 2021-37. https://agcrops.osu.edu/newsletter/corn-newsletter/2021-37/fall-soil-fertility-sampling
⁶2019. Growers not delaying tillage long enough to collect soil samples. A&L Great Lakes Laboratories, Inc. https://algreatlakes.com/blogs/news/growers-not-delaying-fall-tillage-long-enough-to-collect-soil-samples
⁷University of Illinois Plant Clinic. Guidelines for submitting soil nematode samples to the plant clinic. University of Illinois Urbana–Champaign. https://extension.illinois.edu/sites/default/files/nematology_submission_guidelines.pdf
⁸Jackson-Ziems, T. 2015. Corn nematode sampling. University of Nebraska–Lincoln. https://cropwatch.unl.edu/corn-nematode-sampling/
Web sources verified 9/22/26. 1213_134996