The primary constraint of suboptimal soils under wet tropical regions, besides their low pH, is their low soil organic matter (SOM) content. This research was aimed to determine SOM stock of suboptimal soil orders (Ultisol, Oxisol, and Inceptisol) under corn (Zea mays) cultivation. This research was conducted using survey method, and soil samples were taken from 0-20 cm soil depth from each soil order under two types of land use (LU), corn cultivation and forest, in Lima Puluh Kota Regency, West Sumatra, Indonesia. The soil samples were analyzed at the soil laboratory at Andalas University, Padang. Parameters analyzed were soil bulk density (BD), texture, SOC, total-N. Data resulted showed that the soil texture from the three soil orders was classified as clay loam to clay, with the sand size particle was Inceptisol >Oxisol>Ultisol. Then, soil BD was categorized as medium. The highest SOM stock was at Oxisol under forest LU, then Ultisol and Inceptisol. Generally, SOM stock under forest was higher than under corn LU except at Ultisol, which the SOM stock was comparable. SOM stock under corn cultivation reached 43.7%, 87.7%, and 102.8% of that under forest, respectively, for Inceptisol, Oxisol, and Ultisol. Approximately 52-89% of the SOM was in the form of particulate organic matter (POM). The percentage of POM was generally higher under corn cultivation than that under forest LU. Stock soil total-N under corn cultivation compared to under forest was 37.7%, 68.7%, and 90.9% for Inceptisol, Oxisol, and Ultisol, respectively. The C/N ratio value of the SOM was considered medium.