Corn Residue Could Keep a Widely Used Herbicide Out of Groundwater

Every growing season, farmers around the world spray billions of liters of S-metolachlor, one of the most widely relied-upon pre-emergence grass herbicides in modern agriculture. The compound has become especially critical as weed biotypes in the grass family, the Poaceae, evolve resistance to other modes of action, leaving growers with fewer effective options. But the very traits that make S-metolachlor useful in the field—high water solubility and moderate mobility—also make it a potential threat to water resources. A new laboratory study from Brazil now suggests that something as ordinary as the corn stalks left behind after harvest could dramatically change where this herbicide ends up, trapping it in the surface layers of soil and reducing its journey toward groundwater.

Researchers affiliated with the Federal Technological University of Paraná, the University of São Paulo, the Federal University of Viçosa, the Federal Rural University of the Semi-Arid, and other Brazilian institutions set out to quantify how maize stover, the shredded residue left on fields under no-till farming, alters the environmental behavior of S-metolachlor. Their work, published in the journal Discover Soil, focused on two chemically contrasting soils collected from the state of Paraná: a clayey Oxisol from Pato Branco and a sandy Ultisol from Paranavaí. These two soil orders dominate much of Brazil’s arable land, and they differ sharply in texture, organic matter, and water retention, making them an ideal natural experiment for testing how residue management interacts with soil type.

The team amended both soils with maize stover at rates ranging from zero to one percent by weight, equivalent to field applications of zero to 20 tonnes per hectare. They then ran a battery of standardized tests. Sorption and desorption were measured using the batch equilibrium method recommended by the Organisation for Economic Co-operation and Development, in which soil samples are shaken with herbicide solutions and the amount of chemical remaining in the liquid phase is quantified. Leaching was assessed in 30-centimeter PVC columns packed with soil and covered with stover, then subjected to 60 millimeters of simulated rainfall over two hours, a scenario based on average January rainfall at the collection sites. In every case, S-metolachlor concentrations were measured by high-performance liquid chromatography, with the analytical methods validated for selectivity, linearity, detection limits, and recovery following Brazilian regulatory guidelines.

The results showed that the stover did far more than sit on the surface. Adding it measurably changed the chemistry of both soils, raising pH, increasing organic carbon, and boosting levels of phosphorus and potassium. In the Oxisol, organic carbon climbed from 2.38 percent in the unamended soil by 16 to 36 percent depending on the stover rate, while potassium rose by as much as 61 percent. The researchers attribute these shifts to the nutrient content of maize residue itself, which is known to contain appreciable amounts of nitrogen, phosphorus, potassium, sulfur, and calcium. The residue also appears to exert a mild liming effect, with its basic cations reacting with exchangeable acidity and nudging pH upward by 11 to 18 percent across the two soils.

Those chemical changes translated directly into stronger herbicide retention. Freundlich sorption coefficients for S-metolachlor ranged from 3.24 to 5.98 in the Oxisol and from 1.48 to 2.61 in the Ultisol, with the highest values generally occurring at the greatest stover rates. At the one percent amendment rate, roughly 64 percent of the applied herbicide was sorbed in the Oxisol and about 37 percent in the Ultisol. The clayey soil consistently outperformed the sandy one, a pattern the authors attribute to its 88 percent clay content and higher organic carbon, both of which provide abundant sorption sites. Desorption, meanwhile, declined as stover rates increased, meaning that once the herbicide bound to the amended soil, it was less inclined to let go. The hysteresis coefficients all exceeded one, indicating negative hysteresis and a degree of reversibility in sorption that the researchers link to the herbicide’s moderately hydrophobic character interacting with nonpolar functional groups in the plant residue.

The leaching experiments revealed an equally striking story. Without any stover, more than 70 percent of the applied S-metolachlor stayed within the top five centimeters of soil in both soils, indicating limited vertical mobility under the tested rainfall. In the Oxisol, adding stover at rates of 5 to 20 tonnes per hectare trapped 9 to 30 percent of the herbicide in the residue layer itself, with the remainder concentrated near the surface; no herbicide was detected in the leachate at all. The sandy Ultisol behaved differently. There, the herbicide appeared in the leachate at stover rates of 5, 10, and 15 tonnes per hectare, with roughly 50 to 63 percent of the applied chemical recovered from the collected drainage water, though it vanished from the leachate at the highest, 20-tonne rate and in the unamended controls.

That soil-dependent behavior is one of the study’s most important takeaways. The authors caution that the simulated rainfall of 30 millimeters per hour represented a high-intensity event that may have promoted rapid water movement and preferential flow, particularly through the sandy Ultisol, so the leaching patterns they observed should be interpreted within that specific scenario. They also note that lipophilic constituents of maize residue can alter sorption-desorption equilibria in ways that sometimes enhance vertical movement, echoing earlier findings that wheat residue increased the mobility of atrazine. In other words, crop residue is not a simple shield; it is an active chemical participant whose effects depend on the soil it covers.

For weed management, the implications cut in a promising direction. Herbicide held in the stover layer and near the soil surface may be released gradually into the soil solution, potentially extending the window of activity against emerging weeds during early crop establishment. The residue itself adds a physical barrier that delays weed emergence, reducing the need for repeat applications. At the same time, stronger sorption and reduced desorption lower the risk that S-metolachlor will migrate into aquifers, addressing a long-standing environmental concern for a compound whose sorption coefficient normalized to organic carbon typically falls between 100 and 200 milliliters per gram and whose soil half-life ranges from 15 to 50 days. The researchers emphasize that soils with higher organic carbon and intrinsic sorption capacity are better equipped to retain the herbicide safely.

The authors are careful to frame their conclusions as laboratory findings that still require validation under field conditions, where factors such as residue decomposition, microbial activity, and variable rainfall will complicate the picture. Even so, the study makes a compelling case that the residue, the herbicide, and the soil must be considered as a single interacting system rather than in isolation. Adjusting stover rates to match soil-specific conditions, they suggest, could simultaneously improve herbicide performance, sustain weed control, and shrink the environmental footprint of one of agriculture’s most indispensable chemicals. In an era when both water quality and herbicide resistance demand smarter solutions, the humble corn stalk may turn out to be one of the cheapest tools available.

Subject of Research: The effect of maize stover amendment on the sorption, desorption, and leaching of S-metolachlor herbicide in Brazilian Oxisol and Ultisol soils

Article Title: Sorption, desorption, and leaching of S-metolachlor in two soils amended with maize stover

Article References: da Silva Borges, M. P., Trezzi, M. M., da Costa Lima, A., Araújo, G. R., Junior, V. B. P., Paiva, A. C. P., Silva, D. V., da Silva Brochado, M. G., & Mendes, K. F. (2026). Sorption, desorption, and leaching of S-metolachlor in two soils amended with maize stover. Discover Soil, 3(1), Article 178. https://doi.org/10.1007/s44378-026-00333-7

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00333-7

Keywords: S-metolachlor, maize stover, herbicide sorption, leaching, Oxisol, Ultisol, soil organic carbon, no-till farming, Freundlich isotherm, water contamination, weed management, soil chemistry