1. Institute of Crop Science, Univ. of Kassel; Steinstr. 19 D-37213 Witzenhausen Germany
2. International Fertilizer Development Center (IFDC); International Crops Research Institute for the Semi-Arid Tropics (ICRISAT); Sahelian Center; B. P. 12404 Niamey Niger
3. IFDC-Africa; B.P. 4483 Lome´ Togo