Adjustments of sufficiency ranges of selected ornamentals and turfgrasses for assessing nitrogen status with Dumas‐N data

Abstract

With recent advances in nitrogen (N) analyzers, the Dumas method may replace the Kjeldahl method for the routine diagnosis of N in plants. Since 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, Anthurium andraeanum Linden; orchid spp. Cattleya, Dendrobium, Oncidium, Phalaenopsis, and Vanda; leatherleaf fern, Rumohra adiantiformis (G. Forst) Ching; tree fern, Asparagus densiflorus (Kunth) Jessop) and turfgrasses (creeping bentgrass, Agrostis palustris Huds. cv. Penncross; bermudagrass, Cynodon dactylon L.). Samples were dried at 70°C for 72 hr and ground to pass a 20‐mesh sieve. Kn was determined by colorimetry after digestion of 0.4 g of tissue using a CuSO₄/TiO/K₂SO₄ catalyst and 10 mL of H₂SO₄ at 450°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 (R²=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 anthurium, 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 sufficiency ranges (in %N) were 4.9–6.6 for creeping bentgrass, 2.4–4.4 for bermudagrass, and 1.9–3.6 for anthurium. Existing sufficiency ranges for orchid and fern need not be adjusted for Dumas N.