ASSESSMENT OF THE NITROGEN STATUS OF FIELD-GROWN CANOLA (BRASSICA-NAPUS) BY PLANT ANALYSIS

Field trials were conducted over 2 seasons at Greenethorpe and Canowin dra in the Cowra region of New South Wales to develop and calibrate pl ant tests for assessing the nitrogen (N) status of canola (Brassica na pus). Plants were tested at 3 and 7 growth stages up to the start of f lowering at Greenethorpe and Canowindra, respectively. The petiole of the youngest mature leaf (YML) was the most suitable plant part to sam ple for tests based on nitrate-N. Suitable plant parts for tests based on total N were the YML petiole or lamina, or the whole shoot. There was good agreement between the 2 sites in the just-adequate fertiliser N rates (rates giving 90% of maximum yield) and the critical N concen trations in the plant parts tested. Critical nitrate-N concentrations in the fresh YML petiole for dry matter production at the time of samp ling the plants decreased from 1.62 to 0.14 mg nitrate-N/g fresh weigh t between the 4-5 leaf rosette stage (4-5 RS) and the start of floweri ng (SF). Critical nitrate-N concentrations in the dry YML petiole decr eased from 16.5 to 0.8 mg/g dry weight between 4-5 RS and SF. Critical total N concentrations decreased from 4.5 to 2.0, 7.2 to 5.0 and 6.2 to 2.8% dry weight, in the YML petiole, YML lamina, and whole shoot, r espectively, between 4-5 RS and SF. Critical nitrate-N and total N con centrations for assessing potential seed yield were similar to those f or dry matter production at the time of sampling for each of the growt h stages. The critical total N concentrations obtained for the YML pet iole and lamina, and the whole shoot before the start of stem elongati on are likely to be less precise than the critical nitrate-N concentra tions in the YML petiole because of the Limited response of total N co ncentrations to increasing rates of fertiliser N. However, total N in the YML petiole or lamina, or in the whole shoot may be a better indic ator of N status for plants sampled after the start of stem elongation as nitrate-N concentrations become low and more variable, and it is h arder to identify the YML. The decline in critical N concentrations mu st be taken into account when interpreting the results of plant tests for diagnosing the N status of canola, as sampling needs to correspond to the plant growth stage for which a particular critical N concentra tion has been obtained.