Calcium- and Magnesium-Enriched Organic Fertilizer and Plant Growth-Promoting Rhizobacteria Affect Soil Nutrient Availability, Plant Nutrient Uptake, and Secondary Metabolite Production in Aloe vera (Aloe barbadensis Miller) Grown under Field Conditions

Abstract

This work investigates the effects of an organic fertilizer enriched in Ca and Mg and two bacterial inoculants, applied alone and in combination, on soil fertility, plant growth, nutrition, and production of secondary metabolites, namely, acemannan and total phenolic compounds (TPCs), by Aloe vera (Aloe barbadensis Miller), under field cultivation. The first inoculum consisted of five native bacterial strains (Pseudomonas sp., Enterobacter sp., and three strains of Pantoea sp.), characterized in vitro as putative plant growth promoters, isolated from local organic farming fields of Aloe vera. The second inoculant was a commercial product (BACTILIS-S and HUMOFERT) and consisted of three Bacillus species: B. pumilus, B. amyloliquefaciens, and B. subtilis. The organic fertilizer (HUMO-CAL M-8O) was a mixture of humic and fulvic acids, with an additional CaCO3 (40% w/w) and MgO (4% w/w). The most significant increase in the content of acemannan and TPCs was detected under single application of the organic fertilizer, which was linked to enhanced concentration of Mg and Ca in the leaf gel. The concentration of acemannan tended to be increased with the combined application of the organic fertilizer and microbial inoculants. TPCs were significantly increased in both single and combined treatments, seemingly related to Fe concentration in the leaf rinds.