TOTAL NITROGEN DETERMINATION IN PLANT-MATERIAL BY PERSULFATE DIGESTION

Determination of total plant N concentration is important in understan ding crop nutritional value, nutritional requirements, and physiologic al responses to the environment. Nitrogen analysis by either the Kjeld ahl or Dumas method is expensive and requires specialized equipment. A n alternative procedure, developed primarily for use on water samples, was optimized for digestion of vegetative soybean [Glycine max (L.) M err.] tissue and compared with the Dumas method. Finely ground plant m aterial (0.425 mm) was digested in alkaline potassium persulfate (K2S2 O8) at 120 degrees C (0.1 MPa) for 1.5 h in an autoclave, converting o rganic N to NO3-. Nitrate in the digest was reduced to NO2- and quanti fied by the sulfanilamide-alpha-naphthyl-ethylenediamine dihydrochlori de procedure. Persulfate digests of soybean material agreed closely wi th determinations made by the Dumas method (r = 0.99), Digestion of NI ST standard tomato (Lycopersicon esculentum Mill.) leaves (4.8 g N 100 g(-1)) indicated complete N recovery (4.9 +/- 0.1 g N 100 g(-1)). Per sulfate digestion of total plant N was accurate and precise, and requi red little initial investment.