Information about site conditions is essential to select teak (Tectona grandis) plantation sites in Northeast Thailand. We examined the effects of topography (slope steepness, elevation, and slope aspect), soil organic matter, and chemical factors on tree growth (site index) in 7- to 30-year-old teak plantations (n = 87) in Northeast Thailand. Soil organic matter and chemistry data were collected from the topsoil (0–20 cm) and subsoil (40–60 cm). The linear mixed model indicated that teak growth was positively affected by slope steepness (P < 0.05) and Ca content and negatively by Na content. Mg showed a weak correlation (P < 0.1) due to its association with Ca. We suggest that slope steepness alters the balance of exchangeable cations, and that accumulation of soil organic matter due to surface drainage plays an important role. In conclusion, suitable sites for growing teak in the study area in Northeast Thailand are those on slopes with topsoil rich in Ca.