Application of a Portable Chlorophyll Meter to Assess the Nitrogen Sufficiency Index and Nitrogen Requirements in Sweet Potatoes

Abstract

Balanced nitrogen (N) supply is essential for high root yield in sweet potatoes (Ipomoea potatoes [L.] Lam.). A portable chlorophyll meter can support N fertilization management. Here, we determined the appropriate N sufficiency index (NSI) for sweet potato leaves to achieve the best leaf N status, plant growth, N uptake and removal, and storage root yield and quality. Experiments were conducted at three sites (Braúna, São Manuel, and Regente Feijó) in São Paulo, Brazil, using a randomized block design with four replicates. Treatments included a control (without N application), conventional N fertilization (50 kg ha−1), reference N fertilization (150 kg ha−1), and NSI-based N fertilization (NSI: 90% or 95%, based on the chlorophyll meter readings). Plant response to N fertilization was low, with no N deficiency observed in the conventional and chlorophyll meter-managed treatments. NSI < 90% was better than NSI < 95% for N top-dressing management, reducing N application rates by 44–66%, depending on the site. In contrast, NSI < 95% increased the N application rate without any yield benefit. Thus, monitoring N fertilization using a portable chlorophyll meter with 90% NSI can reduce N fertilization rates without negatively impacting the sweet potato root yield.