Agronomic Evaluation of New Ureaforms for Flooded Rice

Rice ( Oryza sativa L.) seldom utilizes more than 20 to 40% of the applied urea. Inefficient N utilization is largely attributed to N loss through ammonia (NH 3 ) volatilization and denitrification. Increased N efficiency can be obtained with a slow‐release fertilizer such as ureaform (UF), which is the condensation product of urea and formaldehyde. A recent development in the production process of UF utilizes paraformaldehyde (PFA) addition to urea, which produces a modified urea fertilizer containing unreacted urea and methylene urea polymers. The purpose of this study was to evaluate N uptake and yield response of flooded rice and to determine NH 3 volatilization losses from applications of 15 N urea, UF, and methylenediurea. The UF fertilizers evaluated had PFA additions to urea ranging from 0% (urea) to 15% (UF‐15). In a greenhouse study with flooded rice, N uptake was measured and apparent N losses were estimated from an 15 N balance. Direct NH 3 volatilization losses were measured in a forced‐draft volatilization system. There was an increase in N uptake and a reduction in apparent and measured N losses from the modified urea fertilizers as the proportion of PFA increased. Plant 15 N uptake at maturity can be expressed by the quadriatic equation: % recovery in total plant = 18.53 + 1.10%PFA − 0.02%PFA 2 . Total 15 N recovered ranged from 44% as urea to 64% as UF‐15. In the forced‐draft NH 3 volatilization study, total NH 3 losses were reduced from 55% of the applied N as urea to 34% as MDU, and as the proportion of PFA addition to urea increased, NH 3 losses decreased: % of applied N volatilized = 54.74 − 1.25%PFA. Ammoniacal N concentrations in the floodwater peaked on the 1st d after fertilizer application with urea and UF but peaked on the 7th d with MDU.