Context. Limitation in soil organic matter interferes with plant growth and productivity. Chemical fertilisers can provide limiting nutrients to plants, especially in barren soil, but they have detrimental effects on living organisms; therefore, plant growth-promoting (PGP) rhizobacteria may be an alternative to synthetic fertilisers to improve plant tolerance to nutrient stress.

Aims. This study evaluated the effects of single and co-inoculation of two PGP rhizobacterial strains with macroalgae (Cladophora glomerata) extract on barley growth. We also tested the ability of the respective strains to produce biofilm and exopolysaccharide (EPS) at different pH values under in vitro conditions.

Methods. Plants were cultivated in an organic matter-limited alkaline soil under field conditions.

Key results. All treatments increased barley growth parameters to some extent. Growth parameters increased most in Bacillus + macroalgae extract (B + MA) and Bacillus + Azotobacter + macroalgae extract (B + A + MA) treatments. These two treatments significantly enhanced shoot length by 22.9–25.7%, dry weight of shoot + spike by 118.2–154.5%, total weight of grains by 76.5–78.1%, and number of grains per spike by 94.8–112.1% in comparison with the control. The respective two treatments also increased biofilm formation and EPS production in response to alkaline pH.

Conclusions. Barley productivity may be increased by using PGP strains with greater potential to produce biofilm and EPS, and inoculating them with macroalgae extract. The B + MA treatment could be applied singly or in combination with Azotobacter.

Implications. Single or co-inoculation of such PGP bacteria with macroalgae extract may be commercialised to enhance barley productivity in organic matter-limited soils.