The mapping of soils in Africa is at least a century old. We currently have access to various maps depicting mapping units locally and for the continent. In the past two decades, there has been a growing interest in alternatives for generating soil maps through digital soil mapping (DSM) techniques. There are, however, numerous challenges pertaining to the implementation of DSM in Africa, such as the unavailability of appropriate covariates, age and positional error in the measurements, low sampling density, and spatial clustering of the soil data used to fit and validate the models. This review aims to investigate the current state of DSM in Africa, identify challenges specific to implementing DSM in Africa and the ways it has been solved in the literature. We found that nearly half of African countries had an existing digital soil map covering either a local or national area, and that most studies were performed at a local extent. Soil carbon was the most common property under study, whereas soil hydraulic variables were seldom reported. Nearly all studies performed mapping for the topsoil up to 30 cm and calculated validation statistics using existing datasets but without collecting a post-mapping probability sample. Few studies (i.e., 11%) reported an estimate of map uncertainty. Half of the studies had in mind a downstream application (e.g., soil fertility assessment) in the map generation. We further correlated the area of study and sampling density and found a strong negative relationship. About 30% of the studies relied on legacy soil datasets and had a lack of sufficient spatial coverage of their area of study. From this review, we highlight some research opportunities and suggest improvements in the current methodologies. Future research should focus on capacity building in DSM, new data collection, and legacy data rescue. New initiatives, that should be initiated and led from within the continent, could support the long-term monitoring of soils and updating of soil information systems while ensuring their contextualised usability. This pairs with better delivery of existing DSM studies to stakeholders and the generation of a value-added proposition to governmental institutions.

中文翻译：

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数字土壤制图在非洲的应用和挑战


非洲的土壤测绘至少有一个世纪的历史。我们目前可以访问描绘本地和大陆地图单位的各种地图。在过去的二十年里，人们对通过数字土壤测绘 （DSM） 技术生成土壤地图的替代方案越来越感兴趣。然而，在非洲实施 DSM 存在许多挑战，例如无法获得适当的协变量、测量中的年龄和位置误差、采样密度低以及用于拟合和验证模型的土壤数据的空间聚类。本综述旨在调查 DSM 在非洲的现状，确定在非洲实施 DSM 的具体挑战以及文献中解决它的方法。我们发现，近一半的非洲国家拥有覆盖局部或国家区域的现有数字土壤地图，并且大多数研究都是在局部范围内进行的。土壤碳是研究中最常见的特性，而土壤水力变量很少被报道。几乎所有研究都对 30 厘米以下的表土进行了测绘，并使用现有数据集计算了验证统计数据，但没有收集测绘后概率样本。很少有研究 （即 11%） 报告了对地图不确定性的估计。一半的研究考虑了地图生成中的下游应用（例如，土壤肥力评估）。我们进一步将研究领域和采样密度相关联，发现存在很强的负相关。大约 30% 的研究依赖于遗留的土壤数据集，并且缺乏对其研究领域的足够空间覆盖。从这篇综述中，我们强调了一些研究机会，并提出了对当前方法的改进建议。 未来的研究应侧重于 DSM 的能力建设、新数据收集和遗留数据拯救。应该从非洲大陆内部发起和领导的新举措可以支持土壤的长期监测和土壤信息系统的更新，同时确保其情境化的可用性。这与更好地向利益相关者提供现有的 DSM 研究以及为政府机构产生增值主张相结合。