The application of plant beneficial microorganisms has been widely accepted as an efficient alternative to chemical fertilizers and pesticides. Isolation and selection of efficient microorganisms, their characterization and testing in soil-plant systems are well studied. However, the production stage and formulation of the final products are not in the focus of the research, which affects the achievement of stable and consistent results in the field. Recent analysis of the field of plant beneficial microorganisms suggests a more integrated view on soil inoculants with a special emphasis on the inoculant production process, including fermentation, formulation, processes, and additives. This mini-review describes the different groups of fermentation processes and their characteristics, bearing in mind different factors, both nutritional and operational, which affect the biomass/spores yield and microbial metabolite activity. The characteristics of the final products of fermentation process optimization strategies determine further steps of development of the microbial inoculants. Submerged liquid and solid-state fermentation processes, fed-batch operations, immobilized cell systems, and production of arbuscular mycorrhiza are presented and their advantages and disadvantages are discussed. Recommendations for further development of the fermentation strategies for biofertilizer production are also considered.

Microbial-based products (either as biopesticide or biofertilizers) have a long history of application, though their use is still limited, mainly due to a perceived low and inconsistent efficacy under field conditions. However, their efficacy has always been compared to chemical products, which have a completely different mechanism of action and production process, following the chemical paradigm of agricultural production. This paradigm has also been applied to regulatory processes, particularly for biopesticides, making the marketing of microbial-based formulations difficult. Increased knowledge about bioinocula behavior after application to the soil and their impact on soil microbiome should foster better exploitation of microbial-based products in a complex environment such as the soil. Moreover, the multifunctional capacity of microbial strains with regard to plant growth promotion and protection should also be considered in this respect. Therefore, the methods utilized for these studies are key to improving the knowledge and understanding of microbial-based product activity and improving their efficacy, which, from farmers’ point of view, is the parameter to assess the usefulness of a treatment. In this review, we are thus addressing aspects related to the production and formulation process, highlighting the methods that can be used to evaluate the functioning and impact of microbial-based products on soil microbiome, as tools supporting their use and marketing.

Study on potential mechanisms influencing the growth of raspberry and strawberry plants showed that the most active was Bacillus sp. strain AF75BC producing IAA and siderophores, and having the ability to release phosphorus. The latter feature was also present in the strains Sp115AD (B. subtilis) and SP116AC (Paenibacillus polymyxa). Two of the tested strains: SP116AC and JaFGU (Lysobacter sp.) showed the ability to fix atmospheric nitrogen, while the AF75AB2 (Bacillus sp.) produced siderophores and IAA. All strains showed an antagonism toward the most important pathogens of strawberry and raspberry, i.e. Verticillium dahliae, Botrytis cinerea, Phytophthora cactorum and Colletotrichum acutatum, limiting their growth to a different extent on the PDA medium. Inoculation of raspberry roots with the tested bacteria resulted in an increase of some growth parameters of their above-ground part in cv. Poemat. In the case of cv. Polana, a significant increase was found only in the chlorophyll content in the leaves. All the inoculants caused an increase in dry mass of roots in cv. Polana, and in cv. Poemat similar effect was observed after applying Inoculants 1 and 3. The treatments of strawberry roots with any of the inoculants resulted in a significant increase in the total leaf surface area in cv. Rumba, but they had no effect on the chlorophyll content in the leaves of either cultivar. All the inoculants significantly increased the total length of roots and their total surface area in cv. Rumba. This parameter also increased in cv. Elsanta, and the number of root tips also significantly increased in this cultivar. Our study showed that the tested inocula is a promising alternative as a bio-fertilizer for small fruit production in sustainable and organic agricultural systems.

To increase biodiversity in tomato cultivation, two herbal aromatic plants, thyme (Thymus vulgaris) and basil (Ocimum basilicum L.), were introduced as companion plants. Their role was to improve crop plant growth and stress resistance. Moreover, the effect of the soil application of Trichoderma microbial preparations on tomato growth parameters and yield, in combination with companion plants, was studied. Ligno-cellulose multi-layer microcapsules with Trichoderma atroviride TRS14 spores (MIC14) and the commercial preparation Trianum G (TG) were used as microbial preparations. This experiment was carried out in a certified organic field. Tomato plants were intercropped with thyme or basil in the arrangement of two tomato rows alternating with one herbal row. In all intercropping arrangements and in the control (tomato plants grown without herbs), subplots were sectioned. The soil in the subplots was amended with the MIC14 and TG preparations used at a concentration of 10⁴ spores g⁻¹ of the soil and planted with tomato transplants. No control measures were applied during tomato growing, and the plants were naturally infected with late blight. Tomato plant growth parameters and yield were assessed, and late blight severity was monitored. The degree of soil colonization by Trichoderma fungi and the effect of these applications on soil microbial activity and biodiversity (dehydrogenases activity, EcoPlates AWCD, and Shannon index) were evaluated. The results clearly showed a significant influence of thyme and basil on tomato growth and yield in organic production. The cultivation of thyme adjacent to tomatoes had a beneficial effect on the development of the root system and the number of flowers and fruits on the crop plants. Basil, on the other hand, clearly decreased tomato yield and adversely affected the effect of Trichoderma applications by reducing root system development. Moreover, basil as a companion plant increased late blight symptoms. Both Trichoderma strains colonized soil, but they had no significant effect on the microbial activity or metabolic potential measured on the EcoPlates with the use of the BIOLOG system. However, a decrease in dehydrogenases activity was noted. In organic cultivation, the Trichoderma preparations used had no significant effect on tomato yield, opposite to its increase in integrated tomato production.

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