The need for effective biofertilizers is increasing due to the spread of environmentally conscious agricultural work practices. For better usage of biofertilizers, we need further research into the mechanisms of plant growth promoters and their social interactions in commercial products. In this work, we have compiled a procedure for the screening of potential bacterial plant growth promoters (PGPB), belonging to the genus Bacillus. The process consists of the identification of isolates and the characterization of traits, associated with plant growth promotion (PGP). The procedure was successfully used to narrow down a collection of 269 isolates to 46 potential PGPBs. These were separated into “kin” and “non-kin” strains, based on their kin differentiation (KD) phenotype with the commercial PGPB strain B. velezensis SQR9. We composed experimental “kin" and "non-kin" mixtures of biofertilizers, which gave us an insight into the influence of KD on the PGP ability of the biofertilizers, and tested those biofertilizers on cucumber plants, compared to the negative control (NK) and the positive control SQR9 (PK). Plants, inoculated with experimental biofertilizers, flowered faster and more abundantly compared to NK and PK. We expected a better performance of "kin" biofertilizer, due to the absence of antagonism between the strains in the mixture. Although we did not observe a significant increase in plant growth, we discovered a connection between the biofertilizer and the root system. The mixture of "kin" strains significantly stimulated the lateral growth of roots and thus the increase in overall size of the root system on cucumbers, compared to the other inoculums.
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