Abstract
Ammonia (NH3) volatilization is a major pathway of N loss which limits the efficiency of urea as a fertilizer when surface-applied to soils. High pH and low cation exchange capacity in soils have been identified as the principal causes of NH3 volatilization from urea. An attempt was made to establish a preferred environment within the urea-soil reaction zone (microsite) using humic substances derived from palm oil mill effluent (POME) and peat. Both POME and peat are rich in organic matter, and contain humic substances across their respective organic matrices. Humic substances have been shown to interact with ammoniacal compounds and urea. Derived humic substances were separately matrixed with urea into pelletised form and evaluated under laboratory regimes for per cent NH3 volatilization, pH change, and NH4+-N recovery. The results showed that the effects of humic substances, particularly humic acid, on reducing NH3 volatilization was more pronounced in the acid soil. However, such a reduction was not accompanied by a corresponding increase in ammonium ion (NH4+) recovery. The ability of humic substances to reduce NH3 volatilization from urea could be attributed to other mechanisms, i.e., urease inhibition, urea absorption, and NH3 fixation which possibly operated simultaneously.
