ADJUSTMENTS OF SUFFICIENCY RANGES OF SELECTED ORNAMENTALS AND TURFGRASSES FOR ASSESSING NITROGEN WITH DUMAS-N DATA

With recent advances in nitrogen (N) analyzers, the Dumas method may r eplace the Kjeldahl method for the routine diagnosis of N in plants. S ince these two methods recover different N fractions and no conversion factor is available to convert Dumas N (Dn) to Kjeldahl N (Kn) data, Kn:Dn ratios were determined for selected ornamentals (anthurium, Anth urium andraeanum Linden; orchid spp. Cattleya, Dendrobium, Oncidium, P halaenopsis, and Vanda; leatherleaf fern, Rumohra adiantiformis (G. Fo rst) Ching; tree fern, Asparagus densiflorus (kunth) Jessop) and turfg rasses (creeping bentgrass, Agrostis palustris Huds. cv. Penncross; be rmudagrass, Cynodon dactylon L.). Samples were dried at 70 degrees C f or 72 hr and ground to pass a 20-mesh sieve. Kn was determined by colo rimetry after digestion of 0.4 g of tissue using a CuSO4/TiO/K2SO4 cat alyst and 10 mL of H2SO4 at 450 degrees C for 2 hr. Dn was determined using 0.2 g of sample and a LECO FP-428 N Analyzer. Over the 0.4-6.6% N range, Dn was a good predictor of Kn; Kn = 0.90 Dn + 0.09 (R(2)=0.93 , p-model<0.01, n=397 obs.). The Kn:Dn ratio was significantly (p<0.01 ) affected by plant type (Kn:Dn = 0.85, 0.92, 0.99, and 1.00 for anthu rium, turfgrasses, orchid and fern, respectively). The more practical way to use the ratios in routine interpretation was to adjust existing sufficiency ranges with the inverse of these ratios. Adjusted suffici ency ranges (in %N) were 4.9-6.6 for creeping bentgrass, 2.4-4.4 for b ermudagrass, and 1.9-3.6 for anthurium. Existing sufficiency ranges fo r orchid and fern need not be adjusted for Dumas N.