What Can a Pair of Cotton Underwear Tell You About Your Soil?

Alvaro Garcia, DVM, PhD

The idea sounds more like a social media challenge than a scientific experiment. Bury a brand-new pair of 100% cotton underwear (“Tighty Whities”) in your field, dig it up six to eight weeks later, and see what remains. If little is left, your soil is considered biologically active. If the underwear is still largely intact, something may be limiting soil life. As unusual as it sounds, there is solid science behind this demonstration.

This popular test is based on the same principle as the cotton strip assay (CSA), a scientific method that has been used for decades to evaluate soil biological activity. Cotton is composed almost entirely of cellulose, a complex carbohydrate that serves as food for many soil microorganisms. As bacteria and fungi colonize the fabric, they produce enzymes that break cellulose into progressively simpler compounds. The faster the cotton decomposes, the greater the biological activity occurring in the soil.

For nutrient management, this distinction is important. Soil fertility is often viewed primarily through a chemical lens. We routinely measure pH, organic matter, nitrogen, phosphorus, potassium, and other nutrients to determine fertilizer requirements. While these analyses remain the foundation of sound nutrient management, they tell only part of the story. Productive soils are living ecosystems where billions of microorganisms continuously decompose crop residues, mineralize organic nutrients, improve soil structure, and recycle nutrients into forms that plants can absorb. Without this biological engine, even nutrient-rich soils cannot function efficiently.

The cotton test does not measure soil fertility, microbial biomass, or nutrient availability. Instead, it provides a simple indicator of cellulose decomposition, one of the many processes driven by soil microorganisms. Greater decomposition generally reflects more active microbial communities, whereas limited decomposition may indicate constraints such as compaction, low organic matter, poor aeration, prolonged dry conditions, or limited residue inputs. It is therefore better viewed as an indicator of biological activity than as a direct measure of soil health.

Where Does the Cotton Test Fit?

The scientific version of this demonstration, the cotton strip assay, has been evaluated against laboratory methods used to measure soil biology. One study compared it with substrate-induced respiration, a technique that estimates microbial activity by measuring carbon dioxide production after adding an easily decomposable carbon source. Although the relationship between the two methods was modest, the researchers concluded that they measure different aspects of soil biology rather than the same process. The cotton strip assay reflects how actively microorganisms are decomposing cellulose under field conditions, whereas substrate-induced respiration estimates the size and metabolic potential of the microbial community.

More recently, researchers demonstrated that the cotton strip assay successfully distinguished differences among long-term tillage and crop rotation systems. Soils managed under crop rotations and reduced tillage showed greater cellulose decomposition than conventionally tilled monocultures, illustrating how management practices influence biological activity. These findings reinforce that the cotton test is sensitive enough to detect meaningful differences among production systems, even though it cannot replace quantitative laboratory analyses.

A Practical Tool for Farmers

The greatest value of the cotton test is not evaluating a single field but comparing management practices. Identical cotton garments buried at the same depth and recovered after the same period can provide a simple visual comparison between no-till and conventional tillage, cover crops and bare soil, manure-amended and fertilizer-only fields, or compacted and uncompacted areas. Although the results are qualitative, they often stimulate discussions that conventional soil test reports cannot.

The popularity of the cotton underwear test comes from its simplicity, but its real contribution is that it makes soil biology visible. It reminds us that nutrients do not cycle on their own. Every season, soil microorganisms decompose residues, release nutrients from organic matter, improve soil structure, and help sustain crop productivity. A buried pair of cotton underwear will never replace laboratory analyses or fertilizer recommendations, but it can serve as a powerful reminder that the living organisms beneath our feet are every bit as important as the nutrients we measure in the laboratory.

References

Nachimuthu, G., P. C. Dalal, J. L. Smith, et al. 2007. "Comparison of Methods for Measuring Soil Microbial Activity Using Cotton Strips and a Respirometer." Journal of Microbiological Methods 69(2):322-329.

Nachimuthu, G., A. Hundt, B. Palmer, G. D. Schwenke, and O. G. G. Knox. 2022. "Cotton Strip Assay Detects Soil Microbial Degradation Differences among Crop Rotation and Tillage Experiments on Vertisols." Journal of Microbiological Methods 200:106558. https://doi.org/10.1016/j.mimet.2022.106558

Verheyen, K., et al. 2024. "Field Evaluation of the Cotton-Strip Assay for Quantifying Decomposition Rates in Extensive Green Roof Substrates." Urban Forestry & Urban Greening 94:128292. https://doi.org/10.1016/j.ufug.2024.128292