A Rapid Nondestructive Technique to Predict Leaf Nitrogen Status of Grapefruit Tree with Various Nitrogen Fertilization Practices

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  • 1 Assistant professor, University of Florida, Institute of Food and Agricultural Sciences, Tropical Research and Education Center, Homestead, FL 33031.
  • | 2 Associate professor, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, Lake Alfred, FL 33850-2299.
  • | 3 Professor, University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, FL 34945-3138.
  • | 4 Visiting scientist, Department of Soil and Agrichemistry, Shandong Agricultural University, Taian, Shandong, China 27108.

The objective of this study was to evaluate the use of a leaf chlorophyll meter (SPAD-502) to predict nitrogen status and yield response of grapefruit trees. The study included two irrigation treatments, three sources, and four rates of fertilizer [dry soluble granular fertilizer broadcast or through fertigation with N rates either 50, 100, 150 lb/acre/yr (56, 112, or 168 kg.ha-1.yr-1)], and controlled-release fertilizer with N rates either 25, 50, 100 lb/acre. Irrigation treatments did not affect SPAD readings and leaf N concentrations. Correlations between SPAD readings and leaf N were greater for the spring flush (r2 = 0.61) than for the summer flush (r2 = 0.49). High correlations of SPAD meter readings with either fruit yields or leaf N suggests that SPAD meter reading is an acceptable index of N status and fruit production of the trees. SPAD readings can be made rapidly without destructive sampling of the leaves. This study demonstrates that the SPAD meter can be used to evaluate the tree N status and improve a N fertilization program for grapefruit trees.

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