Effect of Plant Growth-Promoting Rhizobacteria Inoculation on Sweet Potato Yield and Nutritional Quality in Northeast Thailand

Excessive fertilizer use poses environmental risks in the long term. Thus, plant growth-promoting rhizobacteria (PGPR) have been proposed as a complementary approach to reduce fertilizer use and prevent nutrient stress. Moreover, PGPR’s efficacy in sandy soil for sweet potato production in Northeast Thailand has not yet been reported. This study tested the hypothesis that PGPR inoculation could reduce fertilizer dependency while maintaining yield in nutrient-poor sandy soils. In this research, a field study was conducted from 2023 to 2024 in Chonnabot District, Khon Kaen, Thailand, to evaluate the effects of Azospirillum brasilense, Azotobacter vinelandii, Beijerinckia mobilis, and inorganic fertilizer on the Okinawan Orange and Carrot native sweet potato varieties. Treatments followed a factorial randomized complete block design with two factors, PGPR inoculation and fertilizer level (0%, 25%, 50%, 75%, and 100%), and were replicated three times. The results showed that PGPR dipping had no statistically detectable effect on sweet potato growth and yield (p > 0.05). However, a notable finding was that PGPR significantly increased the protein and fiber content of tubers (p < 0.01) while reducing carbohydrate content, which may have implications for the taste and the nutritional quality of sweet potatoes. In addition, the application of inorganic fertilizer had a significant effect on yield. The Carrot native variety achieved the highest yield (13,481.00 kg ha−1) with 75% fertilizer, while the Okinawan Orange variety attained the highest yield (8866.00 kg ha−1) with 100% fertilizer. These data could be used to assist farmers in determining their fertilizer usage.