The use of total nitrogen in youngest fully expanded blades for assessing the nitrogen status of onion (Allium cepa L.) crops

Abstract

This paper describes the derivation of critical total nitrogen (N) concentrations in youngest fully expanded blades (YFEB), to assess the N status of onion crops. Five rates of N, from 0 to 390 kg/ha, were applied in randomised block experiments to cv. Cream Gold at 2 sites in 1988 and 1989. YFEB were sampled 5 times between the 3- and 9-leaf stages (41-108 days after the 'hook' stage). Marketable yield significantly (P<0.01) increased as the rate of N increased at both sites. Nitrogen rates required for 95% of maximum yield were 230 and 210 kg/ha at sites 1 and 2, respectively. Sensitivity of YFEB total N concentrations to variation in N supply depended on sampling time. For example, at site 2, the increases in total N concentrations due to application of N ranged from 6.3% at 42 days to 36.3% at 93 days. Because of low sensitivity, sampling at the 3-leaf stage may be too early to detect N deficiency reliably. Mean � s.e. increases in total N concentration were 30.6 � 6.1 and 20.3 � 4.9% at sites 1 and 2, respectively. At early sampling times the relationships between relative marketable yield and total N concentration showed a marked Piper-Steenbjerg effect. Critical total N concentrations derived from these relationships decreased from 3.7-40% at 41-42 days to 1.8-1.9% at 106-108 days. This decrease highlights the importance of carefully defining sampling time to ensure correct interpretation of plant test data.