Fertiliser Responses in Amaranthus blitum subsp. oleraceus and African Marigold (Tagetes erecta L.): A Comparative Critical Review with Evidence from Related Vegetable Amaranths

Abstract

Fertiliser management in leafy amaranths and African marigold is frequently discussed as if greater nutrient supply produces a simple, transferable increase in crop performance. The evidence is more conditional. This critical narrative review compares mineral, organic, integrated and biological nutrient strategies relevant to the accepted taxon Amaranthus blitum subsp. oleraceus (L.) Costea and Tagetes erecta L., while using studies of related vegetable amaranths only as explicitly indirect evidence where direct taxon-specific data are unavailable. Literature published from 2000 to 20 June 2026 was prioritised, with earlier mechanistic studies retained when necessary. Across both crop groups, adequate nitrogen and balanced macronutrient supply commonly increase biomass or reproductive output, but the agronomic optimum is not necessarily the quality, safety, economic or environmental optimum. In leafy amaranths, higher nitrogen can increase shoot growth while also increasing nitrate accumulation or diluting selected mineral and compositional quality traits. In T. erecta, fertiliser combinations that maximise flower yield may differ from those that maximise flower size, xanthophyll concentration, shelf life or profitability. Poultry manure, vermicompost and other organic inputs can support productivity and soil quality, yet their nutrient release, salinity and contaminant profiles are intrinsically variable. Integrated nutrient management often performs well in short-term field experiments, but many treatments combine several components, making causal attribution difficult. Plant-growth-promoting bacteria, mycorrhizal inoculation and recovered nutrient sources show potential for reducing mineral fertiliser dependence, although responses are strongly context dependent and evidence for long-term field reliability remains limited. The most important limitation is taxonomic: direct fertiliser trials on accepted A. blitum subsp. oleraceus are scarce, whereas much of the searchable agronomic literature concerns A. tricolor, A. hybridus, A. cruentus or other congeners. Consequently, no universal cross-crop fertiliser prescription is defensible. Future research should use verified plant material, soil-test-based nutrient balances, component-resolving factorial designs and simultaneous measurements of yield, product quality, nutrient-use efficiency, soil change and environmental losses.