Published online by Cambridge University Press: 27 March 2009
Urea can be an inefficient nitrogen source, compared with ammonium nitrate, on calcareous soils (Ernst & Massey, 1960) and under dry conditions (Murphy, 1978) as a result of ammonia volatilization. However, the interaction between factors which can affect the performance of urea is not clearly understood. The risk of ammonia volatilization would appear to be higher on light soils with a low cation exchange capacity than on heavier soils (Fenn & Hossner, 1985). Field trials in the Netherlands indicated that a minimum of 5 mm of rain must fall within 2 days of application to give good dry-matter yields with urea (Van Burg, Dilz & Prins, 1982). In addition ammonia volatilization has been affected by initial soil moisture content in some studies (Fenn & Escarzaga, 1977) but not in others (Gasser, 1964).