Agriculture, Ecosystems and Environment 100 (2003) 201–207 The influence of source and timing of nitrogen fertilizers on yield and nitrogen use efficiency of four sweet potato cultivars R.O. Ankumah ∗ , V. Khan, K. Mwamba, K. Kpomblekou-A Department of Agricultural and Environmental Sciences, Tuskegee University, George Washington Carver Agricultural Experiment Station, 305 Millbank Hall, Tuskegee, AL 36088, USA Abstract Nitrogen (N) availability is a very important determinant of crop yield. It is also one of the most expensive inputs in crop production. Thus, the efficient management of N by farmers with limited resource is a very important part of successful soil and crop management system. A field study was conducted on a Norfolk sandy loam soil at the George Washington Carver Agricultural Experiment Station, Tuskegee, AL, to evaluate the influence of fertilizer source and timing on the yield of four sweet potato [Ipomoea batatas (L.)] cultivars. Two N sources (urea, CO(NH 2 ) 2 and ammonium nitrate, NH 4 NO 3 ), four sweet potato cultivars (‘Georgia Jet’, GA-Jet; ‘TU-82-155’, TU-155; ‘TU-1892’; and ‘Rojo Blanco’, Rojo-Bl) and three times (zero, single, and split applications) were used in a factorial designed experiment. Plants were harvested after 80 and 120 days and total and marketable yields determined. Nitrogen recovery efficiency, yield efficiency and physiological efficiency were also determined. Total and marketable yields of early maturing cultivars were significantly higher than of late maturing cultivars (P < 0.05). The single application of N resulted in significantly higher yield in storage roots than split application (P < 0.05). Physiological efficiency values were highly correlated with total marketable yields. Recovery rates increased with time. Late maturing cultivars tended to have higher N recovery and physiological efficiency than early maturing cultivars. These results suggest that cultivar maturity group should play an important part in N fertilization recommendations for sweet potato. © 2003 Elsevier B.V. All rights reserved. Keywords: Nitrogen fertilizers; Sweet potato; Nitrogen use efficiency 1. Introduction Sweet potato [Ipomoea batatas (L.)] is an impor- tant staple for many developing countries. Nitrogen (N) fertilizers have been used to increase the yield and improve its quality. The effective use of N fertil- izers to achieve maximum yields while reducing the amount of nitrate-N in the soil is very important in both developed and developing countries. ∗ Corresponding author. Tel.: +1-334-727-8400; fax: +1-334-727-8552. E-mail address: rankum@tusk.edu (R.O. Ankumah). Presently, a split application of N as ammonium nitrate, ammonium sulfate, or urea is recommended for sweet potato productions (Barber et al., 1979). However, there is uncertainty about the fertilization rates for different cultivars. Hill (1982, 1984) reported that ideal rates may range from 0 to 46 kg N ha -1 . Haynes (1970) reported that the response of sweet potato cultivars might depend on the variety used. Wilson (1977) classified sweet potato cultivars into N-responsive, N-indifferent, and N-depressive types. Mortley and Hill (1991) observed that sweet potato cultivars developed under fertilizer-independent con- ditions (NFI), outyielded those developed under N fer- tilized conditions (ND) when no fertilizer was applied. 0167-8809/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-8809(03)00196-8