Tropical regions like French Guiana need local, sustainable construction materials to meet the increasing demand for urbanisation. Earth construction, composed of local soil and additives, is gaining increasing attention due to its minimal environmental impact, local availability, and full recyclability. One of the major limitations of earth construction is the high variability of soils. This results in unpredictable mechanical strengths and prevents its widespread industrial use. Therefore, new methods for the rapid identification of suitable soils are required. This paper explores whether several properties of tropical soils could help predict the mechanical strength of associated mortars. Particle size, major oxide composition, infrared spectroscopy, methylene blue value, and soil pH measured in water or potassium chloride (pH KCl) were characterized in nine different soil types. The nine soils were composed of kaolinite clay, iron, and aluminium oxide. Results showed that the mechanical strength of tropical soil mortar was correlated with the soil iron and aluminium oxide content. Importantly, the pH KCl, an easily measurable soil property, was also highly correlated with the soil iron and aluminium oxide content and, consequently, strongly correlated with the compressive strength (R2 = 0.95). Overall, this study shows that the pH KCl can be used to predict the compressive strength of mortars made with tropical soils composed of kaolinite clay, iron, and aluminium oxide.