Efficiency of nitrogen fertilizer in the sugarcane vertisol system in Guadeloupe according to growth and ratoon age of the cane

Sugarcane is one of the main economic resources of Guadeloupe (France). Cane grown on the island's vertisols shows nitrogen deficiency which is accentuated with each successive ratoon. This deficiency could partially explain the observed decrease in yield. The present study, based on the isotopic N method applied to different ratoons in the field, was aimed at : (i) diagnosing the problem in the crop environment itself; and (ii) quantifying the fertilizer-N balance. The results indicated that decrease in yield and N absorption by the cane was related to ratoon number. The real utilization coefficient for the fertilizer (RUC%) ranged from 6 and 34%, and a high proportion (30-40%) of fertilizer-N was immobilized in the soi] (NiS%) after the annual crop cycle. The N absorbed by the cane was essentially derived from the soil. Rainfail at the beginning of (re)growth determined crop development and N supply to the crop. When the water requirements of the crop are satisfied, nitrogen supply and cane yield can bc improved in two ways : (i) by increasing the efficiency of the applied N fertilizer (RUC% and NiS%); and (ii) by maintaining the soil's capacity to supply cane with N. This implies maintaining and, if necessary, upgrading the structural state of the vertisols. (Résumé d'auteur