Conventional and chlorophyll meter–based nitrogen fertilization management for sweetpotato in the tropics

Sweetpotato [ Ipomoea batatas (L.) Lam.] exhibits low nitrogen (N) uptake during early growth stages, suggesting that N fertilization at planting may be unnecessary. A portable chlorophyll meter can be a useful tool for more precise determination of the timing and rate of topdressed N applications, thereby aligning fertilization with crop demand. This study evaluated whether chlorophyll‐based N management, using a N sufficiency index (NSI) threshold of 90% and an N application rate of 1.0 kg N per unit of NSI <90, enables N fertilization at planting while improving plant growth, N uptake, and storage root yield. A field experiment was conducted at three sites in Brazil (Braúna, São Manuel, and Regente Feijó) using a randomized complete block design with four replications. A laboratory incubation test assessed the availability of native soil N in each area over a period of 150 days. Treatments included control (without N), conventional fertilization (50 kg ha −1 N), reference fertilization (150 kg ha −1 N), and chlorophyll meter–based N fertilization, with 0 or 15 kg ha −1 N at planting and 1.0 kg ha −1 N as topdressing for each 0.1 unit of NSI <90. Results showed that N supply for sweetpotato, as measured with a chlorophyll meter and an NSI threshold of <90%, was influenced by native soil N. In scenarios with lower native soil N availability, the absence of starter N application either advanced the sidedressing application by 2 weeks or increased the total N rate by 57%, whereas in scenarios with higher initial NO 3 – availability, using the chlorophyll meter saved >70% of N compared with conventional management. These results indicate that it is recommended to maintain a starter N application or use stricter NSI thresholds to ensure root yield and fertilizer savings in restrictive or N‐loss environments.