Abstract <p>The study was conducted to assess the magnitude of gaseous ammonia losses upon surface application of conventional urea and new forms of urea-formaldehyde fertilizers under model experiment conditions. Laboratory experiments with composting were conducted in 2024 on sod-podzolic medium loamy soil and leached medium-deep heavy loamy chernozem with surface application of conventional urea and experimental forms of urea-formaldehyde fertilizers (UF 12-1, UF 13-1, and UF 14-1) differing in the content of total nitrogen and its individual fractions. Significant gaseous ammonia losses occurred on the third day of composting and reached a peak on the 28th day. Nitrogen losses were most intense on sod-podzolic soil. With the application of urea, ammonia release reached 12.06 mg of nitrogen per pot on the 28th day. On leached chernozem, urea ammonia losses were much lower amounting to only 2.19 mg/pot on the 28th day. After four weeks of composting on sod-podzolic soil, ammonia nitrogen losses came to 18.1 % of the applied amount, while on leached chernozem they were almost six times lower (3.3%). Ammonia release in the options with the application of experimental forms of UF was insignificant, not exceeding 1.2% on sod-podzolic soil and 0.7% on chernozem during 28 days of monitoring. No significant differences in the amount of ammonia released from different forms of UF were detected.</p>