Site-Specific NPK Optimization Balances Yield and Starch Content in Sweet Potato: Implications for Sustainable Nutrient Management Under Contrasting Soil Nutrient Backgrounds

Sustainable nutrient management in sweet potato requires matching fertilizer inputs with site-specific soil nutrient supply while maintaining productivity and processing quality. This study characterized site-specific NPK responses and identified fertilization regimes coordinating yield and starch content under contrasting soil nutrient backgrounds. To address this objective, a three-factor, five-level quadratic orthogonal rotatable composite design comprising 23 N–P–K experimental runs was conducted independently at two sites in Beibei and Youyang, Chongqing, China, using the starch-type sweet potato cultivar ‘Yushu 17’. The two sites differed markedly in initial soil nutrient status. Photosynthetic characteristics, nutrient accumulation, dry matter production and partitioning, fresh storage-root yield, and quality were measured. Quadratic regression models combined with a desirability function were used to characterize site-specific nutrient responses and optimize yield and starch content. The effects of N, P, and K differed markedly between sites. In Beibei, fresh storage-root yield was mainly affected by the linear effect of N, while P and K also had positive effects, and the P × K interaction was significant. In Youyang, yield was primarily regulated by the linear effect of K and showed a significant negative quadratic response to P. Photosynthetic performance; N, P, and K accumulation; total dry matter accumulation (TDMA); and root-to-top ratio (R/T) responded mainly to N in Beibei but were more sensitive to K in Youyang. Fresh storage-root yield was positively correlated with net photosynthetic rate, stomatal conductance, transpiration rate, chlorophyll content, leaf area index, nutrient accumulation, R/T, and TDMA, but negatively correlated with starch content, indicating a yield–starch trade-off. When yield was prioritized while starch content was maintained, the optimal N–P2O5–K2O rates were 155.02–116.42–300.00 kg·ha−1 in Beibei, predicting 42,800 kg·ha−1 yield and 20.09% starch, and 153.34–75.00–282.90 kg·ha−1 in Youyang, predicting 35,850 kg·ha−1 yield and 24.92% starch. These site-specific optima provide a quantitative basis for more targeted fertilizer allocation while coordinating yield and starch content, thereby supporting sustainable nutrient management of starch-type sweet potato.