This study aimed to improve soil suitability mapping for teak (Tectona grandis) plantation in northeast Thailand. The study areas were in Udon Thani and Nong Bua Lam Phu Provinces. We checked soil properties and classified teak growth class by field surveys, and compared the soil suitability class with the teak growth class. The soil suitability classes were revised from three classes to five classes. The results of Udon Thani Province showed that moderately suited areas accounted for 42.3% (462,086 ha) mostly in the east part of Province, and not suited area 23.6% (257,626 ha) mainly in the central part. Nong Bua Lam Phu Province showed that moderately suited and more suited areas accounted for 37.4% (151,524 ha) mainly in north and central part of Province, and unsuited areas 29.1% (118,046 ha) mostly in the south part. But the ratio of suited areas was supposed higher in Nong Bua Lam Phu Province, due to more mountain area than Udon Thani Province.
The map accuracy was investigated by comparing the revised soil suitability class from soil group with actual teak growth class, and resulted that 20 sites were accord (69 %), five sites were underestimated (17 %) and four sites were overestimated (14 %). The map accuracy remained at 69 % to be acceptable. The farmer’s management was supposed as an important factor, which affected to raise the teak growth class than corresponding to the revised soil suitability class.