Wastewater from rubber wood furniture factories contains large amounts of boron (B) which contaminates the soil and causes unusual symptoms in plants in the area. We aim to establish the toxicity effects in tomato seedlings planted in Kho Hong (Kh) soil series irrigated with wastewater and compare them with seedlings grown in the same soil series but receiving tap-water containing B, S, Ca, or Na at the same concentrations as found in the wastewater. Then, we study B leaching from B-contaminated soil in a column by applying distilled water at different rates. We alleviate the B contamination in the Kh soil series by using water to leach out the B, then planting tomato seedlings. The results showed that in plants subjected to the wastewater or B treatments, growth was reduced and the plants showed tip and marginal chlorosis and necrosis of mature leaves at high B concentrations. The boron in B-contaminated soil was decreased to a greater degree by the application of a larger amounts of leaching water. Using 167.5 mm (field capacity) of water leached the B in the topsoil (0-15 cm) from a concentration of 11.11 mg/kg down to 2.83 mg/kg, a nontoxic level (< 3 mg/kg), and the leached B accumulated in the subsoil (15-30 and 30-50 cm). However, using 1000, 2000, or 4000 mm of water leached the B from the soil to below a toxic level with only a slight accumulation in the subsoil. This indicates that applying water to leach B from soil decreased the extractable B to below a toxic level, resulting in a decrease in plant B uptake. As a result, the tomato seedlings showed better growth in terms of height, leaf number, and plant dry weight than those of the control treatment.