Abstract
The existence, prevalence, and severity of subsoil constraints for crop production globally are under recognized and under-reported. Subsoil constraints (acidity, acid sulphate horizons, alkalinity, compaction, deep sand layers, gravel layers, high-density horizons, pans, pathogens, salinity, sodicity, waterlogged horizons) may be natural features of soil profiles or induced by land use and management practices. The subsoil in this chapter is considered to be the layers of the root zone below the depth of sampling for soil analysis, which typically corresponds to soil below 10–25 cm depth, depending on the soil sampling conventions of the region. Tropical regions, in particular (in Africa, Asia, Northern Australia, and Latin America), contain large areas of deeply weathered profiles that commonly have hostile subsoils that constrain root growth. The main consequence of subsoil constraints is that water and nutrients contained in subsoils are not accessed or efficiently utilized, and hence crops fail to reach their yield potential. Even when best management practices are applied to the topsoil, yield of crops is depressed by subsoil constraints. Crops may acquire up to 75% of N, 85% of P, and 70% of K uptake from the subsoil if root growth is not constrained. Technologies to sense, identify, map digitally, and ameliorate subsoil constraints represent a promising frontier for soil management, with the potential to substantially lift crop productivity in many parts of the world.