Purpose <p>Soil ammonium (NH₄⁺) loss via leaching and ammonia (NH₃) volatilization threatens agricultural sustainability and environmental quality. This study explored the effects of humic acid-modified bentonite (HAMB) on soil NH₄⁺ retention and nitrogen loss mitigation, to support soil fertility maintenance and sustainable agricultural nitrogen management.</p> Methods <p>Three treatments [the control (CK), HAMB applied in 0 − 10&#xa0;cm layer (HAMB1), and HAMB applied in 10 − 20&#xa0;cm layer (HAMB2)] were set in a wheat field during 2020 − 2023. Soil samples were collected to measure NH₄⁺ adsorption and desorption, potential NH₄⁺ leaching, and potential NH₃ volatilization. Langmuir adsorption isotherm model was used to determine soil NH₄⁺ adsorption parameters (<i>Q</i>_<i>max</i>, <i>k</i>, and <i>MNBC</i>).</p> Results <p>HAMB application enhanced NH₄⁺ adsorption, suppressed NH₄⁺ desorption (effects lasting 6 − 7&#xa0;months), increased <i>Q</i>_<i>max</i>, <i>MNBC</i> (effects lasting 5 − 6&#xa0;months), and <i>k</i> (effects lasting 1&#xa0;month), and reduced potential NH₄⁺ leaching by 5.2% − 31.0% and NH₃ volatilization by 5.7% − 58.3% (effects lasting ~ 7&#xa0;months). These effects extended vertically to the subsoil layers (10 − 20&#xa0;cm) below the HAMB-applied horizon. <i>Q</i>_<i>max</i> and <i>MNBC</i> were long-acting key parameters for nitrogen loss regulation.</p> Conclusions <p>This study highlights the effects of HAMB application on enhancing soil NH₄⁺ retention and underscores the necessity to optimize its preparation methods to further improve NH₄⁺ adsorption capacity and reduce nitrogen loss. These findings provide substantial support for soil fertility maintenance, environmental pollution mitigation, and sustainable agricultural nitrogen management.</p>