IMPROVEMENT OF THE N FERTILIZER EFFICIENCY WITH DICYANDIAMIDE (DCD) IN CITRUS TREES

Nitrification inhibitors such a dicyandiamide (DCD) help to reduce lea ching losses by retaining applied N in the ammoniacal form. Research o bjectives were to evaluate dicyandiamide added to ammonium sulphate-ni trate (ASN) as a nitrification inhibitor in cultivated soils (Xeropsam ments) and its effect on N uptake by citrus (Citrus sinensis (L.) Osbe ck). Under field conditions, fertilization of adult trees with ASN (60 0 g N tree(-1)) either with or without DCD (2% DCD-N) was compared (AS N+DCD and ASN, respectively). The NH4+-N concentrations in plots ferti lized with ASN+DCD were significantly higher than ASN plot in the 0-15 cm layer during 5-105 day period. Nitrification started immediately a fter N application in both treatments (ASN and ASN+DCD). In all three soil layers analyzed, NO3--N concentrations were higher in the ASN plo ts than in the ASN+DCD during the first 20 days. This indicates that n itrification of NH4+ from ASN was more rapid in the absence of DCD. On the other hand, fertilization with ASN+DCD kept higher levels of NO3- -N in soils than ASN during the rest of experience period (40-160 days ). Addition of DCD to ASN showed a higher N concentration in the sprin g-flush leaves with respect to the trees fertilized with ASN, during t he growth cycle. These results suggest that the use of a nitrification inhibitor permitted a more efficient utilization of fertilizer N by c itrus trees. The plants treated with DCD added to ASN showed a higher yield in number of units per tree and a better fruit colour index than those treated with ASN alone.