6Follett R E Keeney D R. Managing Nitrogen for Ground Water Quality and Farm Profitability. Ine Madison, Wisconsin, USA: Soil Science Society of America, 1991, pp 1-7,259-283.
7Tsuno Y, Fujise K. Studies on the dry matter production of sweet potato. Crop Sci Soc Jpn, 1963, 32: 297-300, 301-305, 306-310, 315-318.
8Kim J Y, Acock M C, Garner J O, Singletary C C, Salin M L. Ef- fects of nitrogen fertilization on alcohol soluble carbohydrate content in sweet potato roots. Hort Sci, 1985, 20:434-435.
9Larry K, Conrad H M, William H S, Christel H. Influence of N source, N rate and K rate on the yield and mineral concentration of sweet potato. JAm Soc Hort Sci, 1984, 109:294-298.
10Constantin R J, Jones L G, Hammett H L, Hernandez T P, Kahich C G. The response of three sweet potato cultivars to varying levels of nitrogen. JAm Soc Hort Sci, 1984, 109:610-614.
