| dc.description.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. |
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