This dataset supports the research article: Carbon Saturation and Deficit in Sandy Cassava Ultisols: A Multi-Method Evaluation of Mineral-Associated Organic Matter (submitted to Soil Use and Management, Wiley). Study overview: Soil samples were collected from an organic cassava field experiment at six smallholder farms in Ubon Ratchathani and Yasothon provinces, northeastern Thailand (15.2–15.8°N, 104.5–104.9°E). The experiment followed a randomised complete block design (RCBD) with seven organic fertiliser treatments (No = unfertilised control; App1/App2 = animal manure ×1/×2; Farmer1/Farmer2 = farmer practice ×1/×2; LPOF1/LPOF2 = locally produced organic fertiliser ×1/×2) across three depth increments (0–10, 10–20, 20–30 cm), yielding 126 observations. Three indirect MAOM C-saturation estimation methods were compared: the Hassink (1997) equation (Method 1), the TOC/(silt+clay) mineral surface C-loading approach (Method 2), and the Guillaume et al. (2022) equation with three MAOM proportion scenarios (Method 3). Files included: Suppinfo_1.docx — Table S1: Chemical properties and nutrient concentrations of organic fertiliser materials (n = 15 samples; chicken manure, cow manure, and LPOF). SuppInfo_2.xlsx — Table S2 (four sheets): S2a: Full raw soil dataset (n = 126) including measured and derived C-saturation indicators S2b: Summary statistics by treatment and depth S2c: Summary statistics by soil depth increment S2d: Data dictionary and variable descriptions Key variables: %Sand, %Silt, %Clay, SC (silt+clay, %), bulk density, pH, %TOC, bulk SOC (mg C g⁻¹), WEOC (mg kg⁻¹), WEOC/SOC ratio, C stock (Mg C ha⁻¹), TOC/SC (mg C g⁻¹ SC), MAOM saturation M1 and M3 (%), C deficit M3 (mg C g⁻¹), SOC:clay ratio. Funding: Agricultural Research Development Agency (ARDA), Thailand (Grant No. PRP6707031410).