**1. Introduction**

Agriculture plays a crucial role in the economic development and poverty alleviation in developing countries. Also, agricultural sustainability is vital for a sustainable agriculture. There must be a positive link between its supply and demand. Unfortunately, this link has been disturbed by many factors from which the deficiency of the nutrients especially that of phosphorus (P) has been a major one [1]. Moreover, its application to the soil in developing countries has been hindered due to alarming increase in its price.

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Adequate amount of phosphorus (P) is critical for normal plant growth and development as it is a vital component of plant energy system and genetic material in the form of adenosine triphosphate-ATP and deoxyribonucleic acid-DNA, respectively [2]. Moreover, it also is involved in many plant processes like photosynthesis, carbon metabolism, membrane formation, energy generation, nucleic acid synthesis, glycolysis, respiration, activation and inactivation of enzymes, and nitrogen fixation [3]. Root architecture, seed development, and crop maturity is also affected by P deficient conditions [4].

Leguminous crops engaged in symbiotic nitrogen (N2) fixation generally require high amount of P due to high energy requirement which is mainly contributed through ATP [5]. Under Psufficient conditions, nodules have a higher P concentration (up to 1.5% of the total plant P) as compared to that of the shoots and roots [6]. It is also needed for signal transduction, membrane biosynthesis, and nodule development and function [7]. Many studies concluded that P is the most limiting nutrient in many soil types for the production of crops especially the nitrogen fixing leguminous crops [8–10]. The direct and positive role of P in nodulation of red clover [11], peas [12], white clover [13], *Medicago truncatula* L. [14], and soybeans [15] has been reported. Under P deficient conditions, reduced root growth and photosynthetic carbohydrate supply to the nodules occurs [6, 16], which results in reduced nodule growth and function and ultimately reduced symbiotic N2 fixation [17]. It has been found that N and P control the nodule growth and modulate the symbiotic processes of the legume and *Rhizobium* [11]. Phosphorus has its basic function in plant energy system and reduction of N2 to NH3 requires 16 ATP molecules. The availability of soil P is optimum at pH ranging between 6.5 and 7.0 for plant absorption, hence creating a soil environment that is more favorable for N2-fixers like *Rhizobium*.

Rock phosphate (RP) is an economical source of P to the crop plants and 200–300 billion tons of RP are available throughout the world. For example in Pakistan, 20–30 million tons of different grade phosphate rock has been documented. The major issue with RP is having a low available P. Research work is needed to find out a way to enable farmers to utilize this RP as P-fertilizers to meet fertilizer demand and cope with the prices. A huge share of foreign exchange is utilized in importing phosphatic fertilizers which is not feasible for countries, especially the developing ones. Another constraint with chemical phosphatic fertilizers is that these are prepared from high quality RP which may be depleted by the year 2050 [7]. These problems have increased the necessity to find other measures and approaches so as to exploit indigenous RP resources in bioavailable form without compromising on yield.

From direct application of RP as a P source to the soils, it has been clearly found that this approach is feasible for the acidic soils having low pH and direct application of RP to the alkaline soils [18]. Another approach which could help solubilize the fixed P in RP could be to use the bio-inoculants, which through the release of organic acids (acetate, lactate, oxalate, tartarate, succinate, citrate, gluconate, ketogluconate, glycolate, etc.) reduce the pH of the micro-environment prevailing around these microbes [1, 19–21]. The plant growth promoting rhizobacteria (PGPR) include all the bacteria found in the rhizosphere which directly or indirectly enhance plant growth. Phosphate solubilizing microorganisms (PSMs) are the PGPR and can be utilized for enhancing the availability of P which has a direct effect on nodulation. There are many reports about the use of RP along with PSMs as an alternative cheaper source of P [22, 23].

Adequate amount of phosphorus (P) is critical for normal plant growth and development as it is a vital component of plant energy system and genetic material in the form of adenosine triphosphate-ATP and deoxyribonucleic acid-DNA, respectively [2]. Moreover, it also is involved in many plant processes like photosynthesis, carbon metabolism, membrane formation, energy generation, nucleic acid synthesis, glycolysis, respiration, activation and inactivation of enzymes, and nitrogen fixation [3]. Root architecture, seed development, and

Leguminous crops engaged in symbiotic nitrogen (N2) fixation generally require high amount of P due to high energy requirement which is mainly contributed through ATP [5]. Under Psufficient conditions, nodules have a higher P concentration (up to 1.5% of the total plant P) as compared to that of the shoots and roots [6]. It is also needed for signal transduction, membrane biosynthesis, and nodule development and function [7]. Many studies concluded that P is the most limiting nutrient in many soil types for the production of crops especially the nitrogen fixing leguminous crops [8–10]. The direct and positive role of P in nodulation of red clover [11], peas [12], white clover [13], *Medicago truncatula* L. [14], and soybeans [15] has been reported. Under P deficient conditions, reduced root growth and photosynthetic carbohydrate supply to the nodules occurs [6, 16], which results in reduced nodule growth and function and ultimately reduced symbiotic N2 fixation [17]. It has been found that N and P control the nodule growth and modulate the symbiotic processes of the legume and *Rhizobium* [11]. Phosphorus has its basic function in plant energy system and reduction of N2 to NH3 requires 16 ATP molecules. The availability of soil P is optimum at pH ranging between 6.5 and 7.0 for plant absorption, hence creating a soil environment that is more favorable for

Rock phosphate (RP) is an economical source of P to the crop plants and 200–300 billion tons of RP are available throughout the world. For example in Pakistan, 20–30 million tons of different grade phosphate rock has been documented. The major issue with RP is having a low available P. Research work is needed to find out a way to enable farmers to utilize this RP as P-fertilizers to meet fertilizer demand and cope with the prices. A huge share of foreign exchange is utilized in importing phosphatic fertilizers which is not feasible for countries, especially the developing ones. Another constraint with chemical phosphatic fertilizers is that these are prepared from high quality RP which may be depleted by the year 2050 [7]. These problems have increased the necessity to find other measures and approaches so as to exploit

From direct application of RP as a P source to the soils, it has been clearly found that this approach is feasible for the acidic soils having low pH and direct application of RP to the alkaline soils [18]. Another approach which could help solubilize the fixed P in RP could be to use the bio-inoculants, which through the release of organic acids (acetate, lactate, oxalate, tartarate, succinate, citrate, gluconate, ketogluconate, glycolate, etc.) reduce the pH of the micro-environment prevailing around these microbes [1, 19–21]. The plant growth promoting rhizobacteria (PGPR) include all the bacteria found in the rhizosphere which directly or indirectly enhance plant growth. Phosphate solubilizing microorganisms (PSMs) are the PGPR and can be utilized for enhancing the availability of P which has a direct effect on nodulation.

indigenous RP resources in bioavailable form without compromising on yield.

crop maturity is also affected by P deficient conditions [4].

110 Organic Fertilizers - From Basic Concepts to Applied Outcomes

N2-fixers like *Rhizobium*.

Scientists around the world have documented positive effects of organic fertilizers in improv‐ ing the physical properties of soils, thereby increasing the availability of nutrients especially the least mobile ones like P [24–29]. The combined application of RP-EC with PSMs could be helpful in improving the nodulation, growth, and yield of crop plants. There are few reports about their combined application. Shahzad and his co-workers [30] found that combined application of rhizobacteria and P-enriched compost resulted in an increased growth and yield parameters compared to uninoculated control without compost. So PGPB along with SSPenriched compost was found highly effective in improving growth, yield, and nodulation of chickpea as compared to their application alone. Saleem *et al*. [29] conducted a series of field and pot experiments to check the effectiveness of RP, compost, and PSMs in increasing the growth and yield of wheat. RP-enriched compost was used based on 25 and 50% P of the crop requirement and the balance amount of P was applied through chemical phosphatic fertilizers. From the results, it was found that application of 50% P from RP-EC with PSMs and 50% from the chemical fertilizers maximally increased most of the growth and yield parameters com‐ pared to the controlled (100% P from chemical Fertilizers).

Recently, we have conducted a series of pot and field experiments in comparing P nutrition of legumes through bio-organo-phos (a mixture of PSMs, RP, and compost) and found a signif‐ icant improvement in growth, nodulation, and yield compared to the recommended chemical fertilizers [31]. This chapter will give an overall research progress in finding out the economical and sustainable source of P for the crops especially for the legumes and their future perspec‐ tives. It also addresses the pros and cons with their future perspectives and research needs.
