Development of critical shoot and root N curves for diagnosing sweetpotato shoot and root N status

Abstract No critical nitrogen (N) curve, used for diagnosing the in-season N status of crop plants, has been constructed for sweetpotato. Moreover, the critical root N curve has rarely been studied. This study determined three new critical N curves based on shoot (SDM), root (RDM), and plant (PDM) dry matter to assess N nutrition in sweetpotato in China. Three field experiments were conducted with varying N application rates (0–180 kg N ha–1) using two sweetpotato cultivars, Xinxiang and Shang 19. The relationship between dry matter and N concentration was analysed, and a model of critical N concentration for sweetpotato was constructed. Critical shoot N for sweetpotato based on SDM (Ns) is described by the equation Ns = 5.14 SDM–0.661 (SDM ≥ 1.7 t ha–1); for SDM