COMPARATIVE-EVALUATION OF NITROGEN RELEASE PATTERNS FROM CONTROLLED-RELEASE FERTILIZERS BY NITROGEN LEACHING ANALYSIS

Seven nursery grade (8-9 month duration), polymer-coated, controlled-r elease fertilizers (CRF) were topdressed or incorporated into a 2 peat : 1 vermiculite : 1 sand (by volume) medium to yield the same amount of N per container, The pots (0.5 L) were uniformly irrigated with DI water every week to produce a target leaching fraction of 25%. Leachat e N contents (ammonium plus nitrate), employed as indicators of N rele ase, allowed for comparison of CRF performance as a function of temper ature changes over a season, Two distinct N leaching (i.e., release) p atterns mere observed over the 180-day experimental period, The fertil izers Osmocote 18-6-12FS (Fast Start: OSM-FS), Prokote Plus 20-3-10 (P ROK), Osmocote 24-4-8HN (High N: OSM-HN) and Polyon 25-4-12 (POLY) exh ibited a N leaching pattern that closely followed changes in average d aily ambient temperatures (T-avg) over the season. This relationship w as curvilinear, with N leaching rates per pot (NLR) being highly respo nsive to T-avg changes between 20 and 25 degrees C, Temperatures above 25 degrees C produced an average maximum NLR of 1.27 mg.d(-1) for the se fertilizers, OSM-FS, PROK, and OSM-HN had the highest cumulative N losses over the experimental period,In contrast, the CRF group formed by Nutricote 18-6-8 (270: NUTR), Woodace 20-4-12 (WDC), and Osmocote 1 8-6-12 (OSM) showed a more stable N leaching pattern over a wider rang e of temperatures, with rates about 30% to 40% lower than those in the temperature-responsive CRF, and averaging a maximum NLR of 0.79 mg.d( -1) for T-avg >25 degrees C, NUTR and WDC had the lowest cumulative N losses over the season, Soluble salt readings paralleled N leaching fo r each CRF, indicating similar leaching patterns for other nutrients, Incorporation produced significantly higher cumulative N losses than t opdressing, but without effect on the actual N leaching pattern over t he season, Regardless of the N formulation in the CRF, over 85% of the N recovered in the leachates was in the nitrate form.