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In most West African countries, particularly in Benin, maize (Zea mays L.) is emerging as a staple food for food security. It is one the major cereal crops that undergoes more than a hundred different modes of processing (Adjadi et al., 2015). In terms of production, high nutritional value has been attributed to its grains. The grains have been reported to be a notable source of protein, lipids, fiber and sugar (Ignjatovic-Micic et al., 2015). Maize is the most traded cereal on the domestic and subregional market (Gandonou et al., 2019). Despite the importance of this speculation and its increasing demand, its productivity faces many constraints, including the constant decline in the fertility of cultivated soils due to their degradation (Igué et al., 2013). The land is subjected to severe degradation as a result of poor farming practices that destroy the flora, organic matter and soil fauna and microfauna. Cultivated land is being depleted at an accelerating rate, and crop yields are continually declining, thereby dangerously compromising the productivity and sustainability of the entire agricultural system (Alamri et al., 2016). In modern agricultural systems, thousands of millions of synthetic agrochemicals are used to achieve high crop yields. After application, these synthetic chemicals are not entirely used by plants, but persist in the soil in different forms. In addition, excessive use of synthetic agrochemicals, declining soil nutrients, and water-use issues, amongst others, are threats to the ecosystem (Omomowo and Babalola, 2019). These chemicals seep into the soil, and thus disrupt the diversity and performance of the rhizosphere (Ai et al., 2012) and human health via the food chain (Ayala and Rao, 2002). The use of synthetic fertilizers is therefore not considered as good practice because of the high costs and acute environmental risks (López-Bellido et al., 2013). In order to reduce the use of toxic chemicals, one of the safe management options is the use of environmentally friendly solutions (Adesemoye et al., 2009). These alternatives include microbial biostimulants. Biostimulants are substances or microorganisms applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content (du Jardin, 2015). Those containing microorganism's sus as those containing Plant Growth Promoting Rhizobacteria (PGPR) can directly stimulate plant growth through the production of phytohormones (Kang et al., 2019; Ahmed et al., 2020), biological nitrogen fixation (Glick, 2014; Kumar et al., 2020), production of siderophores (Glick, 2014) and phosphate solubilization (Alori et al., 2017; Agbodjato et al., 2018). As biocontrol agents, PGPRs suppress plant pathogens (Bajracharya, 2019). Some rhizobacteria play an important role in improving soil fertility and plant growth by providing various unavailable nutrients. Rhizobacteria secrete organic acids that reduce the pH of the rhizosphere and thus freely produce phosphate available to plants (Kashyap et al., 2020). Alori and Babalola (2018) mentioned that the use of microbial inoculants is a reliable alternative to the use of chemical inputs because these microbial inoculants can act as biofertilizers, bioherbicides, biopesticides and biocontrol agents. The development of plant biostimulants has become the focus of much research interest. Plant biostimulants are diverse substances and microorganisms used to enhance plant growth. Plant biostimulants also designate commercial products containing mixtures of such substances and/or microorganisms (du Jardin, 2015). In recent years, there has been increasing use of biostimulants (Schisler et al., 2004; Viswanathan and Samiyappan, 2008; Gu et al., 2014). In Benin, several studies have been carried out on microbial biostimulants based on native PGPRs from rhizospheric soils (Adjanohoun et al., 2012; Noumavo et al., 2013; Agbodjato et al., 2015; Amogou et al., 2019; Adoko et al., 2020). Most of this work was carried out with PGPR-based biostimulant suspensions. The work carried out with solid biostimulants formulated based on different binders in Benin has proved the effectiveness of the biostimulant clay + P. putida in greenhouse conditions on ferralitic and ferruginous soil. The aim of the study was to evaluate the efficacy of solid biostimulants formulated from the rhizobacteria Pseudomonas putida and different binders on maize cultivation in the farming environment of South Benin.

The biostimulants are substances and or microorganisms that contain living microorganisms and have no toxic effects on the soil. Their use would be inexpensive compared to mineral fertilizers (Amutha et al., 2014). The application of mineral fertilizers in combination with biostimulants could be an effective strategy to improve soil health and nutrient availability for crops. The aim of the study is to test solid biostimulants formulated with the rhizobacteria P. putida and various binders in a farming environment in southern Benin. The trials were set up in three different areas in southern Benin on ferrallitic soils.

Analysis of the initial chemical properties of the test soils shows that the soils at the three sites are slightly acidic. The C/N ratio (12.46–15.61) was high in the topsoil. The level of assimilable phosphorus was lower. In general, in the soils of the study areas, the sum of exchangeable bases and the cation exchange capacity are low, which reflects their low fertility (Adjanohoun et al., 2011). In the soils of the three study zones, potassium was globally deficient in relation to calcium and magnesium. Better still, imbalances between calcium, magnesium and phosphorus were noted. These results, which were in line with those reported by Igué et al. (2013), showed that it was necessary to provide nutrients to the soil because these quantities are insufficient to meet maize's nutritional requirements (Yallou et al., 2010b). Regarding the growth parameters of maize plants on ferrallitic soil, the formulated biofertilizers were expressed differently.

In all three zones, the best height and stem diameter were obtained with biostimulants formulated with clay and peat supports. Significant differences were recorded between these applications and the farming practice on these growth parameters. The same observations were made by different authors in Benin when they combined liquid microbial biostimulants with mineral fertilizers (Agbodjato et al., 2015; Amogou et al., 2019; Adoko et al., 2020). These increases can be explained by the growth stimulating effect of the rhizobacteria P. putida (Noumavo et al., 2015) under study, on the one hand, and, on the other hand, the effect of conservation binders (clay and peat) which maintain the bacterial concentration for a long time and which would better protect the PGPR strains against abiotic factors (Brar et al., 2012).

In the same locality, there was no significant difference between treatments for the leaf area of the maize. However, from one area to another, it was highly significant, with the Zouzouvou area leading the way. The best leaf area was obtained with the application of biostimulants T1: peat-clay + P. putida + ½ NPK and Urea. This could be explained by the lack of variability in the chemical composition of the soils of the different producers that hosted the trials in the same area. In the same way, the farming practices of a locality remain similar. But from one area to another, the soils do not have exactly the same chemical properties. Similar findings were made by Adoko et al. (2020) in their studies of the liquid biostimulants P. putida in a farming environment in Benin.

Maize grain yields obtained in this study from all treatments and in all areas were similar. The results of the statistical analysis did not reveal any significant differences between the various treatments and between the different zones. The formulated biostimulants combined with the half dose of NPK and Urea had comparable effects with the full dose of NPK and Urea in all study zones on maize grain yield. The same findings have been made by several authors who have applied liquid PGPR biostimulants in research stations (Noumavo et al., 2013; Agbodjato et al., 2015; Amogou et al., 2019) and then in farmers' fields (Adoko et al., 2020) in Benin and other countries (Amutha et al., 2014; Sagay et al., 2020). The rhizobacteria P. putida contained in these formulated solid biostimulants was thus able to provide the plants with nutrients from the environment to increase their yield (Kashyap et al., 2020). This rhizobacteria was able to provide maize plants with the maximum nitrogen (N), phosphorus (P) and potassium (K) supplied or available in the soil, necessary for plant growth and yield (Ahmed et al., 2020). Ojuederie et al. (2019) also mentioned that multifaceted PGPRs are potential candidates for biofertilizer production to lessen the detrimental effects of drought stress on crops cultivated in arid regions.

In the present study, the correlation between growth and yield parameters showed that the biostimulant T1: clay + P. putida + ½ NPK and Urea expressed itself better than all the others. This biostimulant clay + P. putida was, therefore, the best in the farming environment. This result can be explained in part by the capacity of the preservative binder used for the bioformulation to maintain a good bacterial concentration in the rhizosphere for a long time. According to the work of Brar et al. (2012), the clay binder makes it possible to maintain a high population of PGPR for several months, which is favorable to the promotion of plant growth and yield. Earlier work by Noumavo et al. (2015) stated that P. putida used in this study is capable of producing growth phytohormones and solubilizing phosphate. The best growth and yield parameters recorded during this study can be explained by the combined effects of P. putida and the binder clay. Some strains of rhizobacteria of the genus pseudomonas are capable of producing ammonia, indole acetic acid (IAA), HCN, siderophores, solubilizing potassium (Verma and Pal, 2020), phosphate, zinc and increasing the bioavailability of nutrients for good plant development (Marra et al., 2012; Verma et al., 2015; Shahid et al., 2017; Singh and Jha, 2017; Ullah and Yusuf, 2019; Zaheer et al., 2019). The rhizobacteria P. fluorescens have also been reported to colonize the rhizosphere of wheat and sugarcane and stimulate plant growth (Verma et al., 2015). Oteino et al. (2015) attributed the efficacy of P. fluorescens on onion yield to its ability to produce indole acetic acid. Similarly, the biocontrol properties of this genus are well documented (Reetha et al., 2014; Khanghahi et al., 2018). PGPRs also secrete several growth phytohormones such as auxins, cytokinins, gibberellins and ethylene which improve both root growth and whole plant growth (Lugtenberg and Kamilova, 2009; Dodd et al., 2010; Wani et al., 2013). Furthermore, work carried out in Senegal by Diagne et al. (2020) has also shown that inoculation with PGPR and/or Arbuscular Mycorrhizal Fungi (AMF) can improve the salinity resistance of Casuarina obesa plants by increasing their growth parameters. The use of biologically active natural products and microbial extracts could be an important means of increasing soil nutritional status, absorption and improving the efficiency of nutrient use (NPK) by plants (De Pascale et al., 2017). Phosphorus, potassium and magnesium have been reported to improve root growth, resulting in improved water supply and drought tolerance. Cassán and Diaz-Zorita (2016) showed that the increase in crop yield was due to the ability of Azospirillum sp to provide the plant with nutrients. According to Zeffa et al. (2019) inoculation of maize seed with Azospirillum brasilense intensified plant growth and yield by improving nitrogen use in the event of nitrogen deficiency. It is in this same context that Fadiji and Babalola (2020) mentioned that the major benefit of embracing the beneficial microorganisms in the field of agriculture is to bring about a reduction in the use of different agrochemicals such as pesticides, chemical fertilizers, other artificial chemicals and this would make agriculture more productive and sustainable.

The results of the experiment show that Pseudomonas putida-based biostimulants combined with the half dose of NPK and Urea recommended (100 kg/ha NPK and 50 kg/ha) for maize cultivation in Benin gave the best performance both in terms of growth parameters and maize grain yield. The effects of these microbial biostimulants vary from region to region and according to the type of binder. The application of biostimulants formulated on the basis of clay or peat in combination with the half-dose of NPK and Urea in the different study areas is more favorable to corn plants than the recommended full dose (100% NPK and Urea). The Pseudomonas putida strain could be used as biofertilizers for environmentally friendly sustainable agriculture. It would be interesting to continue this study by repeating the trial on a larger area to assess the performance of this rhizobacteria to improve maize growth through the formulations made.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

NA, MA, OA, and FB carried out the experimental work and analysis. NA, MA, PN, AA, OB, and LB-M contributed to the design, supervision, and interpretation of the results. NA, MA, and OB revised the final draft. OB reviewed the final draft. All authors participated equally in the work and approved the final submission.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.