Phosphorus (P) is an essential nutrient for life. In many tropical countries, P-fixing soils and very low historical P input limit uptake of P in crops and thus yields. This presents a serious obstacle for achieving the Sustainable Development Goal (SDG) target 2.3 of doubling productivity in smallholder farms. We calculated the geographic distribution of actual over potential P uptake (P limitation) and the P input required to achieve this SDG target in 2030 following the shared socioeconomic pathway (SSP2) scenario for five world regions where smallholder farms dominate. To achieve target 2.3, these regions require 39 % more P application (126 Tg). While P limitation is most widespread in Sub-Saharan Africa, it is the only region on path to achieving the doubling of productivity (increase by a factor of 1.8). Achieving the target requires a strong increase in P input, while protecting soils and waterways from excessive P runoff.