Digital mapping of Ghana’s soil properties and nutrients: performance of spline and weighted average approaches
Kora B. Désiré Simperegui, Anselme K. K. Kouame, Benjamin Kwesie, P.S. Bindraban, William Adzawla, Eric Asamoah, Mohamed El Gharous
Abstract
Soil fertility is a critical factor in sustaining crop productivity and meeting the food demands of the world’s growing population. Assessing the fertility status of soil allows for the adoption of targeted agricultural practices to enhance crop yields. However, national-scale soil property maps in Ghana remain scarce. This study entailed the development of 250 m resolution maps of Ghana showing 9 soil properties at a depth of 0–30 cm, including total nitrogen (TN); available phosphorus (avail. P); exchangeable potassium (exch. K); soil clay, sand, and silt content; soil organic carbon (SOC); pH; and cation exchange capacity (CEC). This involved two profile data harmonization methods—equal-area spline and thickness-weighted average—with a quantile random forest model. On average, the thickness-weighted approach (coefficient of determination [R 2 ]: 0.008–0.687, concordance correlation coefficient [CCC]: 0.103–0.793) outperformed the spline approach (R 2 : 0.180–0.567, CCC: 0.175–0.688) for eight of the 9 soil properties. Ghanaian soils are deficient in nitrogen (within or below the maize critical range of 400–1500 ppm) and phosphorus (below the maize critical threshold of 11 ppm), while potassium levels are generally within the maize critical range of 17–74 ppm, with concentrations exceeding 74 ppm in the upper northern area of the country. Ghana’s soils are predominantly sandy, and agroecological zones, including the Western Evergreen, Moist Evergreen, lower-west Deciduous Forest, and lower-west Coastal Savannah, show pH values below 5.5, indicating acidity levels that could limit crop productivity. The study shows that regional or continental soil maps do not adequately reflect the needed spatial details of soil properties, as provided in national-level maps, for effective targeting of fertilization strategies and other agronomic measures to enhance agricultural productivity.