Assessment of the nitrogen status of field-grown canola (Brassica napus) by plant analysis

Field trials were conducted over 2 seasons at Greenethorpe and Canowindra in the Cowra region of New South Wales to develop and calibrate plant tests for assessing the nitrogen (N) status of canola (Brassica napus). Plants were tested at 3 and 7 growth stages up to the start of flowering at Greenethorpe and Canowindra, respectively. The petiole of the youngest mature leaf (YML) was the most suitable plant part to sample 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 concentrations in the plant parts tested. Critical nitrate-N concentrations in the fresh YML petiole for dry matter production at the time of sampling the plants decreased from 1.62 to 0.14 mg nitrate-N/g fresh weight between the 4-5 leaf rosette stage (4-5 RS) and the start of flowering (SF). Critical nitrate-N concentrations in the dry YML petiole decreased 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, respectively, between 4-5 RS and SF. Critical nitrate-N and total N concentrations for assessing potential seed yield were similar to those for dry matter production at the time of sampling for each of the growth stages. The critical total N concentrations obtained for the YML petiole and lamina, and the whole shoot before the start of stem elongation are likely to be less precise than the critical nitrate-N concentrations in the YML petiole because of the Limited response of total N concentrations to increasing rates of fertiliser N. However, total N in the YML petiole or lamina, or in the whole shoot may be a better indicator of N status for plants sampled after the start of stem elongation as nitrate-N concentrations become low and more variable, and it is harder to identify the YML. The decline in critical N concentrations must 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 concentration has been obtained.