EVALUATION OF WATER POTENTIAL, FERTILIZER PLACEMENT AND INCUBATION-TIME ON VOLATILIZATION LOSSES OF UREA IN 2 NORTHERN IDAHO SOILS

The increased use of urea as a nitrogen (N) fertilizer source in north ern Idaho may require precautions to prevent potential losses via vola tilization. The purpose of this study was to determine the effects of urea placement, water potential and time on volatilization losses of N in two northern Idaho soils under laboratory conditions maintained at 25 degrees C. A completely randomized block factorial design with rep eated measures [2 soils x 2 urea placement treatments x 4 soil water p otentials x 3 replications-measured at 0, 2, 4, 8, and 16 d] was utili zed in this study. The two soils, a DeVoignes silt loam (fine silty, m ixed nonacid, frigid Histic Humaquept) and a Santa silt loam (coarse s ilty, mixed, frigid Ochreptic Fragixeralf), were maintained at water p otentials of -0.8, -2.0, -3.0 or -6.0 bars. Urea was either surface ap plied or incorporated into the soils at the rate of 200 kg/ha. Data we re analyzed by SAS-GLM and Omega squared (omega(2)) values were used t o identify the impact of each main effect and interaction. The 4-facto r interaction of urea placement (NIT) x soil (SL) x water potential (W P) x time of incubation (IT), 4-3 factor interactions, 6-2 factor inte ractions and 4 main effects were found to be significant in this study . Due to the number of significant sources of variation, omega(2) valu es were used to assess their relative importance. The urea placement t reatment accounted for 31.8% of the variation in this study as N losse s after 16 d of incubation from the surface urea applications were 28. 35% compared to 6.52% where urea was incorporated into the soil. The i ncubation time x urea placement interaction was the second most import ant factor and accounted for 21.98% of the observed variability. Volat ilization losses were much greater at all incubation times in the surf ace-applied urea treatment when compared to incorporated urea. The dif ferences in N lost by volatilization increased with time. Soil water p otential also affected N volatilization as losses increased with incre asing soil moisture levels. When the soil water content was not limiti ng, substantial volatilization occurred. This may be due to an increas ed urea hydrolysis rate as the moisture content increases. This study demonstrated that volatilization losses may be significant for surface applications of urea on some Idaho soils. To counter these losses inc orporation of urea into the soil should be considered.