AbstractThe projected population growth implies an increased food demand with a severe impact on Earth’s natural resources. Improving crops yield without compromising the environment will be one of the main challenges of the next decades. Among the practices that can promote sustainable agriculture, organic fertilizers, and seaweeds in particular, are receiving growing attention. The aim of the current study was to test two macroalgae (Chaetomorphasp. andCystoseirasp.), one microalga (ChlorellaCH2) and Bioguano (a mixture of macroalgae, spirulina, and guano) as organic fertilizers for the growth of barley seedlings. Remarkable plant growth was obtained on Bioguano, with seedlings showing a comparable total biomass yield with respect to the positive control plants on mineral fertilizer. Plants growth on macroalgae was reduced, even if inChaetomorphasp. not significantly compared to the positive control. Plants growth onChlorellaCH2 was close to that of the negative control (only water). Plants on Bioguano attained high uptake efficiencies of N (60%) and K (41%). Plants on macroalgae showed high N, and especially P (about 30%) uptake efficiency. Our results suggest a high potential for Bioguano and, to a lesser extent, for macroalgae without any synthetic fertilizer application to be used as growing substrates for crops.