Assessing critical plant sulfur concentration and nitrogen to sulfur ratio in spring canola production

Abstract Aims Plant S concentration (Sc) and nitrogen (N) to S ratio (N:S) in canola are important indicators for diagnosing S deficiency. A field study was conducted on sandy and clay loam soils at two sites in eastern Canada for three growing seasons to determine plant critical Sc and N:S ratios across various growth stages (GSs). Methods For each site year, factorial experiments consisting of combinations of four N levels (0, 80, 160, and 240 kg N ha –1 ) as urea (46–0–0) and four S levels (0, 20, 30, and 40 kg S ha –1 ) as potassium sulfate (0–0–50–18) were classified orthogonally to form 16 combinations. Muriate of potash (0–0–60) was also applied as needed to ensure all plots receiving the equal amount of potassium (K). Results Our study found that S fertilization increased plant Sc at each GS but reduced the whole‐plant N:S ratio, and the effect was stronger during the flowering stage. Both whole‐plant Sc and N:S ratios varied with GS, environment, and soil type and declined with plant aging. Our results showed that whole‐plant S analysis combined with visual signs of plant S deficiency (purple‐edged and cup‐shaped leaves with less green; stunted growth, slow blooming, and fewer flowers) appeared to be the best way to identify S problems in the field, and whole‐plant Sc and N:S ratios could confirm S deficiencies. Conclusion Our findings indicated that if the whole‐plant Sc in the rosette stage is <0.35% and the N:S ratio is >9.6, the plant may be S‐deficient, and timely supplementary S fertilization measures may be beneficial to canola crop yield.