Laboratory study of transformation and recovery of urea-N in three Queensland soils

Abstract

Recovery, movement and transformations of urea-15N applied to three cultivated Queensland soils were measured as the soil dried out from two initial moisture contents. The soils were a Prairie Soil, a Grey Clay and a Red Earth, providing a range of pH (6.6-8.1) and cation exchange capacity (7-42 C/g). Urea was applied by banding and mixing into the top 2.5 cm layer of soil. Ammonium sulfateJ5N was applied to a second set of soil samples. The soil was incubated at 35°C for 21 days. Recovery of urea-15N was 93-103% of the amount applied in the Prairie Soil, 72-92% in the Grey Clay, and 55-83% in the Red Earth. The larger recoveries were for banding urea into dry soil and the smaller for mixing it into moist soil. Some 15N moved into the 2.5 to 5.0 cm layer of soil, the amounts averaged 21%, 18% and 12% of the amount of 15N recovered in the Red Earth, Grey Clay and Prairie Soil respectively. Transformations of urea-N into ammonium-N approached completion after 3 days when urea was mixed into moist soil, but only 55% of completion after 21 days when it was banded into dry soil. The sequential losses of 15N and changes in the ammonium-N content in the soil demonstrated a strong affinity for ammonia by the Prairie Soil, a moderate affinity by the Grey Clay and a weak affinity by the Red Earth. Special precautions needed in the field to achieve efficient use of urea fertilizer therefore increase in importance from the Prairie Soil to the Grey Clay to the Red Earth.