Improving nitrogen and phosphorus use efficiency in agriculture: a critical review of management practices

Introduction Nitrogen (N) and phosphorus (P) are major nutrients in agricultural production worldwide, but only about half of applied nutrients are recovered, contributing to financial losses, soil degradation, and nutrient pollution. This systematic review and meta-analysis quantified the effects of management practices on nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE), with particular attention to smallholder farming systems in Zimbabwe, especially granitic sandveld soils and fertility gradients between homefields and outfields. Methods The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines were used to screen 4,908 papers; 62 papers were retained, containing 1,148 paired samples for random-effects meta-analysis using log response ratios. Heterogeneity was assessed, and publication bias was evaluated using Egger’s regression test and trim-and-fill analysis. Results Compared with conventional practice comparisons, NUE increased by 20.6% (95% CI: 17.4–23.9%) and PUE by 18.9% (95% CI: 15.2–22.7%), with significant heterogeneity (I 2 = 74.3% and 71.8%, respectively). For P, Egger’s regression test indicated funnel-plot asymmetry (p = 0.041), and trim-and-fill analysis adjusted the pooled PUE estimate from 18.9% to 16.4%. The pooled NUE and PUE values combined agronomic efficiency (AE), apparent recovery efficiency (RE), partial factor productivity (PFP), and partial nutrient balance (PNB); because these metrics are not numerically interchangeable, separate analyses were performed and confirmed consistent directional effects across metrics. Practices such as microdosing (28.4%), deep placement (26.1%), integrated organic–inorganic nutrient management (24.3%), and starter-band placement of P (24.8%) were most effective, whereas conservation tillage had the lowest effect (9.7%). Efficiency increases were greater on sandy, acidic, and infertile soils characteristic of Zimbabwean communal farming systems, and efficiency declined rapidly when N application rates exceeded 120 kg ha − ¹. Soil texture, soil organic carbon content, rainfall, and application rate were more important moderators than the type of practice used. Discussion These estimates are context-dependent and should not be interpreted as a ranking of universally best practices. Nutrient use efficiency depends on soil fertility, rainfall, nutrient application rate, cost, labour, and farmers’ resource capacity.