**6. Integrated application of RP and PSMs**

It is well renowned that RP has no substitute as a source of P however; minimum processing is required for its direct application to the soils especially the non-acidic soils. It has been found that RP could be an effective source of P after four years of its application in soils with pH 5.5– 6.0 [65]. Moreover, RP is not economically feasible for the soils with high adsorption capacities, low CEC, high pH, low rainfall, low organic matter content, and low microbial activity [66]. For these reasons, various strategies like that of the application of PSMs have been proposed in order to increase the solubilization of P from RP [67].

PGPR are the bacteria present in the rhizosphere (the area near plant roots up to where the effect of roots and its exudates is found), which helps directly or indirectly to the growth and yield attributes of the plants. Many species of PGPR have been identified like PSMs, 1 aminocyclopropane-1-carboxylate deaminase (ACC-deaminase) producing bacteria depend‐ ing on the mechanism of action employed [68]. As mentioned in the above section, phosphate solubilizing bacteria (PSBs) and/or plant growth-promoting rhizobacteria not only improves the physicochemical properties of the soils, but also help in the solubilization of RP, which leads to increased availability of P to the crop plants. Certain mechanisms employed by these PGPR and PSMs have been identified regarding the solubilization of RP. This includes: 1) production of certain organic acids formic, acetic, propionic, lactic, gluconic, fumaric, and succinic which results in lowering the pH of microclimate in the rhizosphere, 2) synthesis of chelating compounds which help in easy provision of nutrients to the crop plants and many more [19, 21]. Other mechanisms reported in literature include the production of phytohor‐ mones, siderophores, nutrient assimilation, protection of seeds from pathogens through antagonistic action, competition for the nutrients and space, induction of systemic resistance and emission of volatile organic compounds [69–76]. Moreover, the presence of microbiota in the soil is an indicator of good soil health. In this way, the application of these bio-inoculants not only helps in the solubilization of fixed P in RP but also reduce the amount of costly prepared chemical phosphatic fertilizers being applied to the soil, thereby providing a cheaper and sustainable source of P for the plants [77–78].

Previously, it has been found that under biotic and abiotic stress like that of nutrients; there is increased production of ethylene that is a well renowned stress hormone and causes senes‐ cence, abscission, and chlorosis in the environment where it is produced. So as we have phosphorus stress due to the low recovery efficiency of phosphatic fertilizers as mentioned earlier, we have to adopt certain strategy that can reduce the amount of ethylene in the rhizosphere of the plants. Studies aimed on finding the biosynthetic pathway of ethylene have found that ACC is the precursor of ethylene.

One strategy for the direct application of RP on andepts soils might be the application of partially acidulated RP by using 20% H2SO4. It has been reported to increase the plant response from applied RP from 0–16% to 59–77% relative agronomic efficiency (RAE) on Andepts [61]. However, the application of inorganic acid like 20% H2SO4 could have detrimental effects on the soil microbiota. Moreover, some other procedures like mixing with elemental sulfur, partial acidulation with an acid, thermal alteration, combination with chemical phosphatic fertilizers, preparation of RP enriched compost, and dry compaction with water-soluble chemical phosphatic fertilizers [62–64] have been reported to increase the efficiency of RP in increasing the availability of P for the crops. However, these procedures are labor intensive, costly, and inappropriate to be practiced at large scale. Due to these problems, there is a growing interest in manipulation certain biological procedure like the application of PSMs. Composting have

It is well renowned that RP has no substitute as a source of P however; minimum processing is required for its direct application to the soils especially the non-acidic soils. It has been found that RP could be an effective source of P after four years of its application in soils with pH 5.5– 6.0 [65]. Moreover, RP is not economically feasible for the soils with high adsorption capacities, low CEC, high pH, low rainfall, low organic matter content, and low microbial activity [66]. For these reasons, various strategies like that of the application of PSMs have been proposed

PGPR are the bacteria present in the rhizosphere (the area near plant roots up to where the effect of roots and its exudates is found), which helps directly or indirectly to the growth and yield attributes of the plants. Many species of PGPR have been identified like PSMs, 1 aminocyclopropane-1-carboxylate deaminase (ACC-deaminase) producing bacteria depend‐ ing on the mechanism of action employed [68]. As mentioned in the above section, phosphate solubilizing bacteria (PSBs) and/or plant growth-promoting rhizobacteria not only improves the physicochemical properties of the soils, but also help in the solubilization of RP, which leads to increased availability of P to the crop plants. Certain mechanisms employed by these PGPR and PSMs have been identified regarding the solubilization of RP. This includes: 1) production of certain organic acids formic, acetic, propionic, lactic, gluconic, fumaric, and succinic which results in lowering the pH of microclimate in the rhizosphere, 2) synthesis of chelating compounds which help in easy provision of nutrients to the crop plants and many more [19, 21]. Other mechanisms reported in literature include the production of phytohor‐ mones, siderophores, nutrient assimilation, protection of seeds from pathogens through antagonistic action, competition for the nutrients and space, induction of systemic resistance and emission of volatile organic compounds [69–76]. Moreover, the presence of microbiota in the soil is an indicator of good soil health. In this way, the application of these bio-inoculants not only helps in the solubilization of fixed P in RP but also reduce the amount of costly prepared chemical phosphatic fertilizers being applied to the soil, thereby providing a cheaper

been proposed, which are discussed in the next sections separately.

**6. Integrated application of RP and PSMs**

116 Organic Fertilizers - From Basic Concepts to Applied Outcomes

in order to increase the solubilization of P from RP [67].

and sustainable source of P for the plants [77–78].

One strategy could be to use the microorganisms that can use its precursor the ACC as a nutrient source and reduce the amount of ethylene produced. Scientists have isolated the socalled PGPR with ACC-deaminase activity. These rhizobacteria contain an enzyme ACCdeaminase that can convert ACC, the precursor of ethylene, into ammonia and α-ketobutyrate, thereby reducing the ethylene stress and ultimately increase the plant growth especially through the proliferation of root growth with increased surface area to explore more soil volume [80]. Many studies have confirmed the efficacy of these PGPR in increasing the root growth of the crop plants, and hence improved yield through increased absorption of nutrients [71–72, 74–76]. In literature, there are many reports about the use of RP along with PSBs as an alternative cheap source of P for costly chemical fertilizers [22–23]. Zaidi and Khan [80] have suggested a synergistic interaction between PSBs and nitrogen fixers like *Azotobacter chroococ‐ cum* that helped in the better utilization of poorly soluble RP. Organic acids produced by different PSBs, are mainly responsible for this solubilization as reported earlier [20, 82]. The degree of P-solubilization of RP by the application of PSMs and their impact on nodulation, growth, and yield of mung bean was studied on an acidic and alkaline soil. The results clearly showed an increased nodulation, growth, and yield attributes of mung bean over control. Moreover, it was suggested that optimum results regarding nodulation, growth, and yield of mung bean could be because of PSMs applied along with an initial dose of chemical phosphatic fertilizer [83].

From the above discussion, it could be imperative to use both types of microorganisms with the ability to solubilize P from the RP, and decrease the level of ethylene through the enzyme ACC-deaminase. In soil microbiology, we usually use a term co-inoculation which involves the application of microbes with more than two traits. In plant sciences, it would be the application of microbes with phosphorus solubilizing activity and ACC-deaminase activity i.e. the application of PSBs and PGPR with P-solubilizing and ACC-deaminase activity.

This strategy has been opted under pot and field conditions and many success stories have shown its effectiveness. However, this strategy has been studied for increasing the availability of P from the so-called chemical phosphatic fertilizers, which are becoming a burden for the farmers to use them due to high cost as already mentioned in the previous section, and not from the RP. Similarly, there are certain problems associated with biofertilizers like shelf life and lack/limited knowledge about their mechanism of action. Poor handling and lack of availability in remote areas are also one of the problems due to poor extension work to disseminate their beneficial effects to the crops in remote areas. These issues could be solved by employing biotechnological approaches to produce certain genetically modified organisms (GMOs) which could sustain the harsh environmental conditions and thereby improved shelf life.
