Coupling urea and Inhibitor-Stabilized urea with Humalite Modulates Soil Nutrient Availability

Abstract To determine how urea, urea with dual (urease + nitrification) inhibitor, Humalite (humic biostimulant), and their combinations influence nutrient dynamics in prairie soils differing in pH and soil organic matter (SOM) content. Five agricultural soils from five sites (three Black Chernozemic, two Grey Luvisolic; pH 4.8–5.8; SOM 2.3–15.5%) received the fertilizer and Humalite variants above and were incubated for 84 days. Soil pH, ammonium ( $$\:\text{N}{\text{H}}_{4}^{+}$$ ), nitrate ( $$\:\text{N}{\text{O}}_{3}^{-}$$ ), available phosphorus (P av ), potassium (K av ), and sulfate sulfur contents, and electrical conductivity were measured on days 1, 7, and 84. Nitrogen dynamics were governed initially by pH and later by SOM content. The urease inhibitor delayed NH₄⁺ release only where pH > 5.3; the delay was shortened in high-SOM soils, consistent with faster inhibitor degradation, and was minimal in more acidic soils. The nitrification inhibitor lowered $$\:\text{N}{\text{O}}_{3}^{-}$$ only when both acidity (pH < 5.5) and ample SOM sustained nitrifier activity; otherwise, effects were negligible. P av under urea increased and decreased in one Black and Grey soil, respectively, consistent with small post-fertilization pH shifts and greater calcium–phosphate precipitation. Humalite consistently slowed urea hydrolysis (greater $$\:\text{N}{\text{H}}_{4}^{+}$$ retention), and in combination with dual inhibitors, further prolonged $$\:\text{N}{\text{H}}_{4}^{+}$$ persistence, indicating synergy. K av increased and decreased in one Black and Grey soil, respectively, plausibly reflecting $$\:\text{N}{\text{H}}_{4}^{+}$$ /K + competition and clay fixation differences. Effective use of urea with inhibitors and Humalite is site-specific. Adjusting for pH, SOM, and mineralogy is essential to optimize nutrient cycling and reduce losses in contrasting prairie soils.