**7. Integrated application of RP and compost**

As mentioned earlier, the direct application of RP has been useful only in soils with acidic pH due to poor solubility in alkaline and neutral soils [85]. However, a limited number of climatic and soil situation are available in which direct application of RP would sufficiently provide nutrients for the fast growing crops to feed the fast growing population of the world. So the scientists are on the way to find out alternate strategies to increase the solubility of directly applied RP. One such strategy is the use of bio-inoculants, i.e. the use of PSMs and PGPRs and as explained in the earlier section. The other strategy is the mixing of RP with a well rotten product of crop residues and daily waste materials through a process known as composting. Many researchers have reported increased availability of P to the plants by increasing the solubility of RP in the soil through composting [86–89]. The mechanisms behind this solubili‐ zation includes the release of organic acids during the decomposition of organic residues which helps solubilize RP by lowering the pH, and by the conversion of inorganic P in RP into organic P which might become available to the plants after mineralization in the soil [89–91].

The processing of RP through the processes like that of composting is essential before applying to the alkaline soils found in Pakistan. In Pakistan, most of the phosphatic fertilizers are imported, as most of the reserves of available RP are of poor quality, which hinders its use for the preparation of phosphatic fertilizers. So as mentioned earlier, the possible strategy to cope with this situation would be to utilize the capacity of composting in increasing the solubility of raw RP. It is one of the most efficacious strategies for the recycling of organic waste materials which helps in boosting the level of organic matter, thereby playing an important role in productivity and sustainability of the soil [92]. Many researchers have authenticated the role of composting in ameliorating the soil fertility, structure, and plant growth [84, 89, 93–95]. They have found that organic acids (humic, fulvic acids, etc.) are released during this process which helps solubilize the fixed or unavailable P present in RP, thereby increasing the availability of P to the plants.

For composting process, a variety of waste material like cow dung, rice husk, poultry waste, fruit peels, mango stones, and many others could be used with RP, which does not only improve the physicochemical properties of the soil and a good source of nutrients [26], but also helps in cleaning our polluted environment. During this process, a variety of organic acids are produced, which helps reduce the pH, and ultimately results in better solubilization of P from the RP [89]. Composting process is very old and has been used for many centuries but up until now, very few or no studies have considered this process to be utilized for increasing the availability of P from the raw source of P, the RP. In a field experiment at the Agricultural Research Farm of NWFP Agricultural University, Peshawar, Pakistan, the effect of different levels of the combined application of chemical phosphatic fertilizer (0, 30, 60, 90 kg P2O5 ha−1) and that of the compost (0, 5, 10, 15 Mg ha−1) was investigated for improving nodulation, growth, and yield of chickpea. There was no significant interaction effect of compost and phosphatic fertilizer that was suggested to be due to the P-deficiency of the experimental site. However, there was a significant effect of P on the nodulation and yield parameters studied, and 60 kg ha−1 P2O5 from the phosphatic fertilizers was found to be optimum [96].

(GMOs) which could sustain the harsh environmental conditions and thereby improved shelf

As mentioned earlier, the direct application of RP has been useful only in soils with acidic pH due to poor solubility in alkaline and neutral soils [85]. However, a limited number of climatic and soil situation are available in which direct application of RP would sufficiently provide nutrients for the fast growing crops to feed the fast growing population of the world. So the scientists are on the way to find out alternate strategies to increase the solubility of directly applied RP. One such strategy is the use of bio-inoculants, i.e. the use of PSMs and PGPRs and as explained in the earlier section. The other strategy is the mixing of RP with a well rotten product of crop residues and daily waste materials through a process known as composting. Many researchers have reported increased availability of P to the plants by increasing the solubility of RP in the soil through composting [86–89]. The mechanisms behind this solubili‐ zation includes the release of organic acids during the decomposition of organic residues which helps solubilize RP by lowering the pH, and by the conversion of inorganic P in RP into organic

P which might become available to the plants after mineralization in the soil [89–91].

The processing of RP through the processes like that of composting is essential before applying to the alkaline soils found in Pakistan. In Pakistan, most of the phosphatic fertilizers are imported, as most of the reserves of available RP are of poor quality, which hinders its use for the preparation of phosphatic fertilizers. So as mentioned earlier, the possible strategy to cope with this situation would be to utilize the capacity of composting in increasing the solubility of raw RP. It is one of the most efficacious strategies for the recycling of organic waste materials which helps in boosting the level of organic matter, thereby playing an important role in productivity and sustainability of the soil [92]. Many researchers have authenticated the role of composting in ameliorating the soil fertility, structure, and plant growth [84, 89, 93–95]. They have found that organic acids (humic, fulvic acids, etc.) are released during this process which helps solubilize the fixed or unavailable P present in RP, thereby increasing the

For composting process, a variety of waste material like cow dung, rice husk, poultry waste, fruit peels, mango stones, and many others could be used with RP, which does not only improve the physicochemical properties of the soil and a good source of nutrients [26], but also helps in cleaning our polluted environment. During this process, a variety of organic acids are produced, which helps reduce the pH, and ultimately results in better solubilization of P from the RP [89]. Composting process is very old and has been used for many centuries but up until now, very few or no studies have considered this process to be utilized for increasing the availability of P from the raw source of P, the RP. In a field experiment at the Agricultural Research Farm of NWFP Agricultural University, Peshawar, Pakistan, the effect of different levels of the combined application of chemical phosphatic fertilizer (0, 30, 60, 90 kg P2O5 ha−1) and that of the compost (0, 5, 10, 15 Mg ha−1) was investigated for improving nodulation,

**7. Integrated application of RP and compost**

118 Organic Fertilizers - From Basic Concepts to Applied Outcomes

availability of P to the plants.

life.

The quantity of available P varies with the nature of organic residues being used in the composting process and its rate of decomposition [84]. They found an increased citric acid solubility of RP by the composting of unreactive Mussoorie RP, chopped grasses, and tree leaves. Moreover, when this product was applied to the crop on equivalent total P basis, the grain and straw yield of the test crop, Guar, were similar to that of the application of SSP. Similar results were obtained using pigeon pea as test crop. The reason behind increased P availability from the phosphocompost might be due to the conversion of unavailable P in RP to water soluble P, thus increased efficiency of the dissolved P for the plant [97].

To investigate the impact of two RPs (nutriphos guano powder and Indian potash limited) with and without the composts (compost mulch and compost residues) on soil P pools and P uptake, a glasshouse experiment was conducted using wheat as test crop for 75 days in a loamy sand soil [98]. The composts were applied as a thick layer of 2.5 cm on the soil surface and nutriphos guano powder and Indian potash limited were applied at the rate of 35 and 26 mg P kg−1, respectively. There was a control with an un-amended soil and amended soil with a soluble source of P (KH2PO4) at the rate of 50 mg P kg−1 soil (inorganic P fertilizer applied soil). The results showed that there was no significant effect of the treatments on the total organic carbon concentrations and pH of the soil. However, the soil respiration was significantly higher in case of compost applied soil with and without RP compared to un-amended and inorganic P fertilizer applied soil. The plant growth was increased by 30–50% in the case of soils applied with compost alone or with RP sources. In case of the concentration of NaHCO3-P and microbial P after 75 days of the applied treatments, an increase of 30% was noted in the case of the combination of compost with either RP compared to compost or RP applied alone, which suggested that compost helps in the mobilization of P present in RP. A significant change in labile pools of P was observed in case of the combined application of compost and RP compared to RP applied alone. Moreover, there was no significant effect on the plant growth and P uptake of wheat in case of combined application of compost and RP compared to compost applied alone, which shows that compost contains sufficient amounts of nutrients for the plants.

The organic matter in the soils or soil organic matter (SOM) is a good indicator of the produc‐ tivity and quality of the soil as it mediates the physicochemical and biological properties of the soil. Many studies have authenticated the beneficial effects on the physicochemical, e.g. improves structure and water holding capacity of soils, decreases P fixation, increases CEC and buffer capacity of soils to resist pH change, and biological properties of the soil, e.g. provides energy to a variety of soil fauna and flora. For example, P sorption capacity of the soil was reduced through the addition of the SOM, which resulted in the alteration of chemical properties of the soil, e.g. complex formation of P compounds on the reaction sites [85]. The organic matter in the soil also serves as a reservoir of micro- and macronutrients.

The SOM exists as partially decomposed residual layer of plants and animals, microorganisms, and humus. The humus, a stable compound of carbon, comprises about 50–75% of the total soil carbon. It has been found that the humus content of the soil are increased by the addition of organic fertilizers, and it also enhances the microbial activities in the soil [100]. It has also been speculated that farming methods aiming at increasing the organic matter in the soil would ultimately reduce the requirement of P application [100].

As mentioned earlier, SOM also serves as a basic source of mineral nutrients in the soil, e.g. nitrogen, sulfur, and phosphorus. About 95% of the total N and S, and up to 75% of P in the surface soil is in organic forms [101, 102]. The organic P in the soil exists in various forms of which phytic acid is the most important one. It is stored in the plant seeds to accomplish early establishment of the seedlings from the germinating seeds. Other organic compounds include mono and di-esters, phospholipids, nucleotides, sugar phosphate, phosphoproteins, and phosphonates [103]. The release of P from its organic compounds is not a very simple process and depends on many factors, like the relative stability of the organic substances and their chemical composition, climatic conditions, physicochemical properties of the soil, cropping scheme, and their interaction with mineral fertilizers [104–106]. Organic fertilization through compost application is a common practice for sustainable agricultural production and P cycling [107]. However, the amount or concentration and type of P compounds depend on the source of the material being used for composting [108]. Long term application of organic fertilizers in the form of composts results in boosting the organic P in the soil [109–111]. The conversion of this organic P into inorganic P or the mineralization depends on the type of compound being mineralized, e.g. orthophosphate di-esters are quickly mineralized com‐ pared to orthophosphate monoesters [112, 113]. Mineralization helps in increasing the total available P, which has been suggested to be due to reduction in the P adsorption and increased rates of microbial enzyme activities, which boost up the biologically mediated turnover of organic P into inorganic P [114].

It has been well established that the addition of organic fertilizers not only increases the organic matter and nutrient status of the soil, but also reduces the amount of costly chemical phosphatic fertilizers to be added into the soil, thereby, promoting healthier and sustainable environment of the soil [115]. It has also been documented that sustainable production from the continuous cropping system through the application of recommended levels of chemical fertilizers may not be possible in future [116]. The integrated use of chemical and organic fertilizers would be a sustainable strategy and has achieved substantial attention throughout the world. Chemical and organic fertilizers both supplement each other's efficiency for nutritional deficiencies and would ultimately decrease the exclusive dependence on chemical fertilizers [117].

Another field experiment was conducted to study the effect of different coir dust based composts on dry matter production and nutrient uptake of maize. It was found that the C:N ratio of the soil samples taken after 120 days of crop growth was decreased, and more biomass was produced with the application of organic fertilizer [118]. Another pot experiment using four different combinations of organic wastes including horse manure and bedding, sewage sludge along with clarifier solids from pulp mill, mink farm wastes, and municipal solid waste (MSW) was conducted to compare the capacity of different organic wastes to improve the growth and yield of tomato, and to assess the phytotoxicity of these organic wastes in radish and cress (*Lepidium sativum*) seedlings. It was concluded that paper mill waste, applied alone without the application of organic fertilizer, causes toxic effect in vegetables, i.e. radish and cress [119].

soil carbon. It has been found that the humus content of the soil are increased by the addition of organic fertilizers, and it also enhances the microbial activities in the soil [100]. It has also been speculated that farming methods aiming at increasing the organic matter in the soil would

As mentioned earlier, SOM also serves as a basic source of mineral nutrients in the soil, e.g. nitrogen, sulfur, and phosphorus. About 95% of the total N and S, and up to 75% of P in the surface soil is in organic forms [101, 102]. The organic P in the soil exists in various forms of which phytic acid is the most important one. It is stored in the plant seeds to accomplish early establishment of the seedlings from the germinating seeds. Other organic compounds include mono and di-esters, phospholipids, nucleotides, sugar phosphate, phosphoproteins, and phosphonates [103]. The release of P from its organic compounds is not a very simple process and depends on many factors, like the relative stability of the organic substances and their chemical composition, climatic conditions, physicochemical properties of the soil, cropping scheme, and their interaction with mineral fertilizers [104–106]. Organic fertilization through compost application is a common practice for sustainable agricultural production and P cycling [107]. However, the amount or concentration and type of P compounds depend on the source of the material being used for composting [108]. Long term application of organic fertilizers in the form of composts results in boosting the organic P in the soil [109–111]. The conversion of this organic P into inorganic P or the mineralization depends on the type of compound being mineralized, e.g. orthophosphate di-esters are quickly mineralized com‐ pared to orthophosphate monoesters [112, 113]. Mineralization helps in increasing the total available P, which has been suggested to be due to reduction in the P adsorption and increased rates of microbial enzyme activities, which boost up the biologically mediated turnover of

It has been well established that the addition of organic fertilizers not only increases the organic matter and nutrient status of the soil, but also reduces the amount of costly chemical phosphatic fertilizers to be added into the soil, thereby, promoting healthier and sustainable environment of the soil [115]. It has also been documented that sustainable production from the continuous cropping system through the application of recommended levels of chemical fertilizers may not be possible in future [116]. The integrated use of chemical and organic fertilizers would be a sustainable strategy and has achieved substantial attention throughout the world. Chemical and organic fertilizers both supplement each other's efficiency for nutritional deficiencies and

Another field experiment was conducted to study the effect of different coir dust based composts on dry matter production and nutrient uptake of maize. It was found that the C:N ratio of the soil samples taken after 120 days of crop growth was decreased, and more biomass was produced with the application of organic fertilizer [118]. Another pot experiment using four different combinations of organic wastes including horse manure and bedding, sewage sludge along with clarifier solids from pulp mill, mink farm wastes, and municipal solid waste (MSW) was conducted to compare the capacity of different organic wastes to improve the growth and yield of tomato, and to assess the phytotoxicity of these organic wastes in radish and cress (*Lepidium sativum*) seedlings. It was concluded that paper mill waste, applied alone

would ultimately decrease the exclusive dependence on chemical fertilizers [117].

ultimately reduce the requirement of P application [100].

120 Organic Fertilizers - From Basic Concepts to Applied Outcomes

organic P into inorganic P [114].

During composting, organic acids like humic and fulvic acids are released, which decreases the pH of the material being composted [120], and increases the solubility of fixed P in the soils with high pH. These composts act like plant growth regulators when applied to the soil. The effect of these humic acids isolated from the cattle vermicompost was tested on the earliest stages of lateral root and on the plasma membrane H+ -ATPase activity in an experiment using maize as a test crop. From the results, it was clearly found that the humic acids significantly increased the overall root growth of maize seedling in conjunction with lateral root emergence. Also a significant stimulatory effect on the activity of H+ -ATPase was observed, which implies that the humic acids enhance the expression of this enzyme. Moreover, exchangeable auxin groups in the macrostructures of humic acid-containing compost, as revealed through structural analysis, shows that the hormonal activity is enhanced by the application of organic wastes containing humic acids [121]. In a pot experiment, to confirm the production and impact of organic acids on the growth and yield of crop plants during the composting process, the comparison of the impact of natural and synthetic humates like that of humic acids found in composts and exogenously applied synthetic humates, e.g. potassium humate on the growth of chicory plants and behavior of soil microbial population was studied. The results confirmed that during composting, there is production of organic acids, and these acids have a stimula‐ tory effect on the growth and yield of crop plants and also increases the microbial population of beneficial microorganisms as observed in this study [122].

The impact of commercially produced vermicomposts produced from the mixture of cattle manures and food and paper wastes was tested in a two year filed study using pepper (*Capsicum annuum* L.) as a test crop. It was suggested that increased growth and yield of pepper resulted due to the production of humic material and plant growth hormones by the increased microbial biomass present in the compost [123]. Similarly, they also found that the composted material significantly increase the soil microbial biomass and their dehydrogenase activities. Similar results were observed by Manna *et al*., [124] who tested the effect of the application of compost and chemical fertilizers on the growth and yield of chickpea and wheat. They found that the compost application not only significantly enhance water soluble, citrate soluble, and total P in the soil, which resulted in an increased growth and yield parameters of chickpea and wheat, but also increase the microbial biomass and their enzyme activities.

The impact of different rates of application of vermicompost on the physicochemical properties of the soil was investigated in a field experiment using tomato (*Lycopersicum esculentum* L.) as test crop. The compost was applied into the upper 15 cm layer of the soil at the rates of 0, 5, 10, 15 Mg ha−1. From the results, it was found that the application rate of 15 Mg ha−1 significantly enhanced the electrical conductivity, total organic carbon, total N, P, K, Ca, Zn and Mn, as compared to that of the control. Overall, it was concluded that vermicompost can significantly improve the physicochemical properties of the soil [125].

As mentioned earlier, the effect of every compost/vermicompost depends on the chemical nature of that compost. It has been found that with the application of vermicomposts, there was an improvement in the biological properties, which resulted in an increased yield of the rice, while the impact of cow dung in improving the biological properties of the soil was more pronounced as compared to that of the green forage [126]. Similar results were observed regarding the biological properties and fertility status of the soil [127, 128].

From the above discussion, it has been clear that different composts have a well renowned effect on improving the fertility status, physicochemical, and biological properties of the soil which ultimately results in improving the growth and yield of crop plants. Intensive agricul‐ ture has become essential to feed an ever-increasing population of the world. Similarly, burning of farm waste not only increases the environmental pollution due to the emission of CO2 but also causes the wastage of nutrients and very precious organic matter. So depending solely on organic fertilizers would not be a wise strategy, instead the combined use of organic and an economical source of P like that of RP would serve better compared to the use of organic or chemical fertilizers alone [50]. Moreover, the composting process helps in the recycling and stabilization of organic wastes, which reduces their contribution to the environmental pollution, and this stable product can increase the plant production [129]. In this area of research, the interest has been increased and new strategies are underway to make a valuable product. These include partially acidulating RP with natural or synthetic organic acids as through composting, decreasing the particle size [130] and through the addition of bioinoculants [131].

The properly employed process of composting converts the organic wastes into a stable and mature product of carbon, i.e. humus [132], while improperly composted organic wastes lead to the immobilization of plant nutrients and cause phytotoxicity [127, 133, 134]. During composting, heat is produced which helps in the destruction of pathogens as well [135]. The properties of composts depend not only on the chemical nature of organic wastes being composted but also the make-up of the organic material during composting [136]. In other words, stability and maturity indices are good indicators of the worth of composted material. However, it is very difficult to measure these indices and still no standards have been devised yet [137–140]. In general, the following parameters including C:N ratio, carbon contents (water soluble), CEC, humus contents, and the evolution of carbon dioxide from the finished com‐ posted material have been used to evaluate the stability and maturity of the composted material [141, 142]. For the measurement of the phytotoxicity of the matured composted material, germination index is used [143].

Many studies have proved that if we could find a natural and non-polluting way of increasing the solubilization of RP, its use could serve as a valuable substitutional source of chemical phosphatic fertilizers [144]. There have been many reports about the preparation of RPenriched compost and their ability to increase the total P in the soil compared to the straw compost alone without RP, but the quantity of water soluble P is decreased in the RP-enriched compost compared to the straw compost or RP alone due to the dilution effect of RP being mixed with a large amount of composted material reaction of soluble P with CaCO3 present in RP [64, 83]. On the other hand the RP-enriched compost has considerably more citrate soluble P compared to the straw compost, which is due to the production of organic acids like citric, oxalic, tartaric, 2-ketogluconic, acetic, malic, and succinic acids, etc., which enhance the dissolution of RP-P [64, 145]. These organic acids are in anionic form and produced during the degradation of complex organic compounds [146]. A lot of CO2 is produced during composting which results in the formation of carbonic acid, ultimately increasing the solubility of P in RP decreasing the pH [64, 147]. The RP-enriched compost has low microbial biomass carbon compared to the straw compost due to the dilution of carbon over a large biomass of compost material. The levels of organic P are also higher in case of RP-enriched compost compared to the straw compost as clear from the higher amount of alkaline phosphatases activities and acids [64]. The level of citrate soluble and organic P increases with increasing the addition of RP but up to a certain level and then decrease. So overall, during this organic matter decom‐ position, the available P and calcium contents are increased to the plants [147].

rice, while the impact of cow dung in improving the biological properties of the soil was more pronounced as compared to that of the green forage [126]. Similar results were observed

From the above discussion, it has been clear that different composts have a well renowned effect on improving the fertility status, physicochemical, and biological properties of the soil which ultimately results in improving the growth and yield of crop plants. Intensive agricul‐ ture has become essential to feed an ever-increasing population of the world. Similarly, burning of farm waste not only increases the environmental pollution due to the emission of CO2 but also causes the wastage of nutrients and very precious organic matter. So depending solely on organic fertilizers would not be a wise strategy, instead the combined use of organic and an economical source of P like that of RP would serve better compared to the use of organic or chemical fertilizers alone [50]. Moreover, the composting process helps in the recycling and stabilization of organic wastes, which reduces their contribution to the environmental pollution, and this stable product can increase the plant production [129]. In this area of research, the interest has been increased and new strategies are underway to make a valuable product. These include partially acidulating RP with natural or synthetic organic acids as through composting, decreasing the particle size [130] and through the addition of bio-

The properly employed process of composting converts the organic wastes into a stable and mature product of carbon, i.e. humus [132], while improperly composted organic wastes lead to the immobilization of plant nutrients and cause phytotoxicity [127, 133, 134]. During composting, heat is produced which helps in the destruction of pathogens as well [135]. The properties of composts depend not only on the chemical nature of organic wastes being composted but also the make-up of the organic material during composting [136]. In other words, stability and maturity indices are good indicators of the worth of composted material. However, it is very difficult to measure these indices and still no standards have been devised yet [137–140]. In general, the following parameters including C:N ratio, carbon contents (water soluble), CEC, humus contents, and the evolution of carbon dioxide from the finished com‐ posted material have been used to evaluate the stability and maturity of the composted material [141, 142]. For the measurement of the phytotoxicity of the matured composted

Many studies have proved that if we could find a natural and non-polluting way of increasing the solubilization of RP, its use could serve as a valuable substitutional source of chemical phosphatic fertilizers [144]. There have been many reports about the preparation of RPenriched compost and their ability to increase the total P in the soil compared to the straw compost alone without RP, but the quantity of water soluble P is decreased in the RP-enriched compost compared to the straw compost or RP alone due to the dilution effect of RP being mixed with a large amount of composted material reaction of soluble P with CaCO3 present in RP [64, 83]. On the other hand the RP-enriched compost has considerably more citrate soluble P compared to the straw compost, which is due to the production of organic acids like citric, oxalic, tartaric, 2-ketogluconic, acetic, malic, and succinic acids, etc., which enhance the dissolution of RP-P [64, 145]. These organic acids are in anionic form and produced during the

regarding the biological properties and fertility status of the soil [127, 128].

122 Organic Fertilizers - From Basic Concepts to Applied Outcomes

inoculants [131].

material, germination index is used [143].

An experiment was conducted to compare the impact of pill millipede (*Arthrosphaera magna*) compost and farm yard manure, using black gram (*Phaseolus mungo*) and finger millet (*Eleusine coracana*) as test crop. From the results, it was found that the former compost resulted in better growth and yield of both the crops as compared to the farm yard manure, which was suggested due to the provision of plant nutrients present in both the types of composts [25]. Similarly the combined effect of mimosa compost and phosphatic fertilizers was more pronounced com‐ pared to the application of mimosa compost and phosphatic fertilizer alone [28]. Another field experiment found that RP-enriched compost performed better in case of yield and nutrients concentration in cow pea [24].

However, the extent of P solubilization depends on many factors like that of the ratio between the RP and compost, time and rate of application to the soils, which has not been done yet. It has been reported that P from this RP-enriched compost is available even at high pH of 8.5 or more [81], so it would ultimately serve as an economical and environment friendly way to reduce the use of chemical phosphatic fertilizers.
