Thai soils are dominated by extremely weathered and developed Ultisols that are characterized by acidity, low fertility, and high leaching of basic cations, including magnesium (Mg). In this study, the distribution of Mg chemical fractions in Thai soils and major commercial crop fields was assessed using sequential extraction. Moreover, the status of available soil Mg was estimated using single leaching. Similar distribution patterns of Mg fractionation in the inorganic soils and the commercial crop cultivation soils were noticed. The mean percentage distributions of Mg fractions spread in a decreasing magnitude: the mineral Mg > the available Mg > the carbonate Mg > the organic complexed Mg; and the recorded values were 68.43, 22.81, 5.90, and 2.86 in the inorganic soils and 69.67, 23.60, 4.52, and 2.21 in the commercial crop cultivation soils, respectively. A distinct Mg fractionation pattern occurred within organic soils with a spread percentage of 71.79, 14.68, 8.91, and 4.62 for available Mg, organic complexed Mg, mineral Mg, and carbonate Mg, respectively. The available Mg concentration within organic soils was 541.21 mg/kg; while the inorganic soils per textural group produced 510.86, 26.87, and 20.89 mg/kg in fine, medium, and coarse textured soils, respectively. Among the commercial crop cultivation soils, rice and coconut were grown in soils with adequate Mg; whereas oil palm, durian, longkong, and rubber were cultivated in Mg deficient soils. Mg is an essential element in plants; therefore, medium, coarse textured soils, and the extremely weathered tropical Ultisols, regardless of their texture, require Mg replenishment for optimum plant productivity within the tropics.