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Response of castor cultivar "Hale" to rate and method of nitrogen fertilizer application in different environments of Zimbabwe Hikwa, D. & Mugwira, L.M. Abstract The effects of different rates and methods of N fertilizer application were assessed at five sites over two seasons (1987/88 and 1988/89). Fertilizer rates of 0, 30, 60, 90 and 120 kg N ha-1 were applied in a single dose at planting or in two splits, half applied at planting and the other half as topdressing at primary floral initiation. The sites at Makoholi, Mlezu and Matopos were on sandy soils in low rainfall areas while those at Kadoma and Panmure were in sandy clay loam soils in higher rainfall areas. There were significant interactions between N rate and season on seed yield in the sandy soils except at Makoholi. The effect of N rate on these parameters contrasted at each site between the two seasons, indicating that the amount of N fertilizer needed for castor (Ricinus communis L.) on the sandy soils depended on the amount of rainfall. Application of N in a single dose did not significantly increase seed yield when available soil N was medium to high (>24 ppm) except at Kadoma where application of 30 kg N ha-1 increased yield when soil N status was rated as medium. Yield responses to N applied at planting in 1987/88 at Matopos and Panmure and in 1988/89 at Makoholi and Mlezu indicated that 60 kg N ha-1 was the optimum application rate for castor. When compared to a single dose, splitting N application enhanced its effectiveness in increasing yield in 1987/88 at Mlezu and Matopos, and at Kadoma in two seasons. These results suggest that split N applications are beneficial to castor when seasonal rainfall is greater than 700mm. Keywords Nitrogen rate, rainfall, Ricinus communis L., soil type

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Hikwa, D. & Mugwira, L.M. Résumé Les effets de differentes proportions et methodes d'application d'engrais azote ont ete evalues dans cinq stations au cours des saisons 1987/88 et 1988/89. Les proportions du fertilisant de 0, 30, 60, 90 et 120 kg de N ha-1 ont ete utilisees. Elles ont ete appliquees soit a dose unique aux plantes et l'autre a dose dedoublee dont une partie aux plantes et l'autre a l'initiation de l'epanouissement floral primaire. Les stations de Makoholi, Mlezu et Matopos etaient situees sur des sols sablonneux dans des regions a faibles precipations. Celles de Kadoma et de Panmure etaient, par contre, situees sur des sols sablo-angileux dans des regions a fortes precipitations. Les interactions entre la teneur en N et la sason sur la production des graines se sont sevelees hautement significatives dans des stations a sols sableux sauf a Makoholi. L'effet de la teneur en N sur ces parametres variait a chaque site en fonction de la saison. Cela monre que la quantite d'engrais azote souhaitable pour la poivriere (Ricinus communis) sur des sols sablonneux est fonction de la quantite de pluies. L'application de N a dose unique n'a pas augmente de facon significative la production des graines lorsque l'azote disponible du sol etait de moyenne a eleve (>24 ppm) sauf a Kadoma. Dans ce dennier cas, l'application de 30 kg de N ha-1 augmentait la production quand le statut de l'azote du sol etait considere comme moyenne. Les reponses de la production a l'azote applique aux plantes en 1987/88 a Matopos et Pammure et en 1988/89 a Makoholi et Mlezu a montre que 60 kg de N ha-1 etait la proportion optimale pour la poivriere. Contrairement a la dose unique, le dedoublement de l'application de l'azote a accru son efficacite. La production a donc augmente a Mlezu et Matopos en 1987/88 et a Kadoma au cours des deux saisons. Ces resultats montrent bien que les applications de l'azote a dose dedoublee sont benefiques a la poivriere loisque les precipations saisonnieres sont superieures a 700mm. Mots Clés Proportion d'azote, precipitations, Ricinus communis, type de sol

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