Water absorption from air and ammonia loss from urea fertilizers applied to grassland in southeastern United States

Information on water absorption from the air by urea fertilizers and on NH₃ loss when applied to grasslands is limited. Urea application to grassland is typically broadcast (Bcast), whereas urea-ammonium nitrate (UAN) is applied either Bcast or in bands (dribble). This work was conducted to (1) evaluate water absorption from the air by Bcast granular urea, Bcast UAN, and dribble UAN under laboratory conditions, and (2) compare NH₃ losses from Bcast urea, Bcast UAN, and dribble UAN when applied to a grassland. Six field studies were conducted from 2017 to 2019. In the laboratory, Bcast UAN exposed to 100% relative humidity absorbed water from air at a faster rate than dribble UAN and Bcast urea. In the field, all three fertilizers lost similar amounts of NH₃ when applied to relatively wet soil (> −0.1 MPa). In contrast, when the fertilizers were applied to dry soil (≤ −1.2 MPa), Bcast UAN lost the most NH₃ (17.3% and 19.8%) likely because of its capacity to absorb water from the air. Also, at −1.2 MPa, dribble UAN lost more NH₃ than Bcast urea (15.3 vs. 10.7%, p < 0.05), probably because the low osmotic potential of UAN (−55 MPa) allowed it to absorb water from the soil at a faster rate than urea could absorb water from the air. In contrast, when the soil water potential was −5.7 MPa, dribble UAN lost less NH₃ than Bcast urea (4.4 vs. 17.3%, p < 0.05), likely because the low soil water potential reduced its water absorption.