Evaluating Release Rates of Controlled and Slow Release Nitrogen Fertilizer.

Controlled and slow release fertilizers can increase N use efficiency (NUE) and reduce environmental contamination, but accurate prediction of release rates is needed. Urea, sulfur coated urea (SCU), polymer-sulfur coated urea (PSCU), and four polymer coated urea (PCU) fertilizers were added to moist soil. The PCU fertilizers (Duration®; DR45, DR75, DR120, and DR180) applied ranged in expected release timings from 45 to 180 days. The experiment was duplicated indoors with insulated temperatures and outdoors with fluctuating temperatures, and either incorporated at one inch or applied to the surface of a loam soil. Fertilizer prills were analyzed weekly for N release until 80% of N was released or until the expected release time expired. Urea, PSCU, and SCU fertilizers released N rapidly regardless of temperature and prill position, with 80-100% N released within 7 days. Significant differences between temperatures were not seen until after 21 days regardless of placement. After 21 days, surface applied PCU resulted in significantly greater N release at fluctuating temperatures over insulated temperatures. Comparably, incorporated fertilizer exposed to fluctuating temperatures released significantly more N for 21 of 40 remaining measured instances, with only one instance of significantly less N. Surface applied PCU at fluctuating temperatures released 80% N within 35 days regardless of expected release time. Conversely, incorporated PCU at fluctuating temperatures did not reach similar levels of N release until 56, 84, 98, and 112 days for the DR45, DR75, DR120, and DR180 day fertilizers, respectively. These results demonstrate that PCU exposed to fluctuating temperatures released N earlier than those exposed to insulated temperatures. Further studies on PCU fertilizers are needed to predict N release as a function of diurnal temperature ranges to improve N management. The PSCU and SCU products used in this study are not improved fertilizers compared to urea.