Alternatives to Soluble Phosphorus Fertilizers in Indian Context

Abstract

Phosphorus is one of the primary nutrients required in crop production. Rock phosphate is the raw material required for the manufacturing of soluble phosphorus fertilizers, which is nonrenewable in nature and expected to last for 50–400 years. The restriction of resources to few geographical locations makes its supply more vulnerable. In India, 90% of the rock phosphate for fertilizer manufacturing is imported. However, the low quality of rock phosphate deposits available in India can be utilized with certain modifications in the form of addition of phosphate-solubilizing bacteria, addition of gypsum, and in the form of phospho-enriched compost. Agriculture, livestock, urban and industrial waste can also prove to be a source of phosphorus through crystallization of struvite. There are encouraging results of struvite compared with soluble phosphorus fertilizers. This will reduce the import dependency in India as well as encourage the Atmanirbhar initiative in phosphorus fertilizer.

1. Introduction

The term phosphorus derived from the Greek word “Phos” meaning light, and “phorus” means bearer. Elemental form of phosphorus was discovered by German Alchemist, Henning Brandt in 1669 [1, 2]. Phosphorus evolved from seventeenth century as philosopher‘s stone to medicinal phosphorus, flammable phosphorus, essential nutrient in crop production, element of war, cause of eutrophication to its emerging scarcity in recent twenty-first century [2]. Earlier it had been established that adding ground bone increases the crop yield and subsequently Lawes (1842) patented the process of phosphate solublization [3]. With the proposition of Criteria of Essentiality (1939) by Arnon and Stout, it had been established that the roles and functions of each essential nutrient are irreplaceable in plant system. Phosphorus is one of the essential nutrients for plant, which extends to animals also [2]. Around 80% of the world phosphorus is utilized in agriculture [1]. Rock phosphate is one of the basic raw materials for the synthesis of phosphatic fertilizers [4]; however, its nonrenewable nature increases the vulnerability in the long term.

Phosphorus in Indian soils occurs primarily in inorganic form contributing 54–84% of the total phosphorus and organic contributing 16–46% [5]. More than 90% districts in India are classified under low-to-moderate phosphorus availability [5, 6]. The phosphorus status of soil doesn’t necessarily reflects its availability to the crop plants, which is governed by the presence of calcium, iron, and aluminum phosphates; thus, only 30% of the soil phosphorus is utilized by the crop and rest remains in the soil [7]. Therefore, it demands the external application of phosphorus through fertilizers. India imports the high-grade rock phosphate for the synthesis of soluble phosphorus fertilizers. This import cost along with the decontrol of the phosphorus and potassium fertilizers in 1992 has led to the fertilizer application in favor of urea. The turmoil in the Former Soviet region in present times has also increased the vulnerability of the rock phosphate import in near future. Therefore, it is important to increase self-reliance in the field of phosphorus fertilizer application. It has been reported that indigenous rock phosphate if suitable is treated with solubilizing microorganisms or acidulates; there can be increased solublization and availability of phosphorus to the plants and consequent yields [8, 9]. Precipitation of struvite from the agriculture and livestock waste can prove another efficient alternative [10]. The major objectives of the following discussion in chapter are to find out in ways and means to enhance the dependability of phosphorus fertilizer on indigenous resources in general and on farm resources in particular.

2. Importance of phosphorus in crop production

Phosphorus constitutes 0.2% (0.1–0.5%) in the plant system. Most of the phosphorus is absorbed in the form of primary orthophosphate ions; however, it is also absorbed in the form of secondary orthophosphate. Phosphorus is not reduced like nitrates and sulfates and rather exists as inorganic phosphate or esterifies to carbon chain through hydroxyl group or attaches to another phosphate group through energy-rich pyrophosphate bonds [11].

The major functions of phosphorus are as structural element of nucleic acids; phospholipids of biomembrane forming bridge between triglyceride and other molecules; energy-rich phosphates and phosphate esters in metabolism; acts as regulator in glycolysis, photosynthesis, respiration, nitrogen assimilation, starch synthesis in chloroplast; detoxification of heavy metals by binding with phytates [11, 12]. Phosphorus control over photosynthesis involves ratio of Pi to triose phosphate; light activation of ribulose bisphosphate carboxylase; activation of fructose-1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase; ATP/ADP ratio; decreased regeneration of RuBP, and low sink strength under P-deficient conditions leading to reduced photosynthesis [12]. The stored inorganic phosphate in the plant varies according to its availability; however, phosphorus in metabolism remains stable with former acting as buffer [13]. Phosphorus supply is more critical in the early season as observed in various annual crops; however, later stage supplementation improves yield though plant is also able to remobilize the stored phosphates to the grains [13].

The similar role can be played by higher seed phosphorus content where it will supply early seedling growth P requirement leading to better root development and thus giving access to growth limiting water and mineral nutrients [14]. Phosphorus is required more for the nodule growth and nitrogenase activity in the N-fixing plants than for the whole plant growth [15, 16]. Phosphorus supply increases the root diameter and dry weight; however, increased root shoot diameter, root hair length and density, root branching, root hair are observed under P-deficient conditions to increase P acquisition [17, 18].

3. Need for the alternatives in Indian context

Before the utilization of phosphate rocks for commercial fertilizer production, bones, corpolites (fossilized animal dung), and guano were the main sources of phosphorus supplementation [3]. It is evident that various sources of phosphorus had been utilized including crop residues; human, animal, fish, and bird waste from Middle East to Japan [2].

The process of super phosphate production by solubilizing bones in sulfuric acid was separately patented by J B Lawes and James Murray in 1842 leading to the development of superphosphate and mixed fertilizer industry [3, 19]. Phosphorus consumption in the post-World War II era was not that much intensified; however, with introduction of Green Revolution after mid-twentieth century, there was increase in use of phosphorus fertilizer along with nitrogen and potassium owing to introduction of the fertilizer-responsive varieties. Both annual phosphate rock extraction and per capita production have seen consistent growth of 3–4% and 1.4%, respectively, with an increase of more than 300% phosphorus fertilizer consumption between 1961 and 2013, however, characterized by a decline post 1989 for a considerable period owing to disintegration of Soviet Union and decreased fertilizer demand in Western Europe and North America [19, 20]. More than 70% of the world phosphorus reserves are located in South Africa, Morocco and Western Sahara and United States [21] and Brazil and Peru in South America; China, Iraq, Israel, and Jordan in Asia; Australia in Oceania; Former Soviet Union in Europe are some major countries continent-wise [22]. The mineral resource extraction follows a mountain/ U-shaped curve where there is initial increase followed by the plateau and then decline in production with time [22]. Phosphorus reserve exploitation has been correlated with time, and it is considered to have “peak P” analogous to the “peak oil” by 2035, after which demand will outstrip supply [3, 23]. With current utilization rate, the phosphate rock reserves are expected to last for 50–400 years [24, 25].

In India, 90% of the rock phosphate required for soluble P fertilizer manufacturing is imported, out of which 80% is imported from Jordan, Morocco, and Egypt [26]. The nonrenewable nature along with its restricted availability to few countries in the world increases the vulnerability of its supplies in case of any untoward incident happens in these nations. The present fertilizer use is also characterized by the imbalance with N:P₂O₅: K₂O use ratio for 2018–19 as 7.1:2.7:1 [27] in place of recommended ratio of 4:2:1. It can be partly attributed to the higher cost of import of phosphorus and potassic fertilizers. Fertilizer subsidy has seen increase of more than 200% since 2010–11 when it reached 1.34 lakh crores in 2020–21 [28]. This becomes more relevant in present times with the turmoil arising in Former Soviet Union region, which is dwindling the resources supply, and the rising protectionism approach of the nations. Therefore, it is important to seek alternate sources of P fertilization in India, which are more indigenous in nature. In addition to reducing the cost of farming, the concept will also support the Atmanirbhar Bharat and self-reliance as it will be more depending on the indigenous minerals and on-farm waste generated in the farmers’ field.

4. Various alternatives of phosphatic fertilizers

4.1 Indigenous rock phosphates

India has substantial reserves of low-grade phosphorus [29, 30], which can be utilized as nutrient source. Total resource of rock phosphate in India is estimated at 312.67 mT of which only 45.80 mT constitutes reserve [31]. Of the total resources, only 8% constitutes fertilizer grade and around 37% is classified under low-grade reserve [31]. However, direct application of rock phosphate is only suited for acidic soils, not for the neutral to alkaline soils where pH is more than 5.5–6.0, and is less reactive, of low grade, and has poor agronomic efficiency [32, 33, 34, 35, 36].

The acidulation of these rock phosphates either with the sulfur-based minerals or sulfur or organic matter holds promise to increase its availability in the soil. Addition of rock phosphate to compost or straw can be useful in increasing the phosphorus availability to the crop. The composting process leads to the mobilization of phosphorus in the rock phosphate, in addition to supplying secondary and micronutrients [37].

Partial acidulation of rock phosphate with locally sourced material such as farm residues, manure, and compost is cost-effective technique to make phosphorus available by the release of chelating action forming complexes and humic acid [38].

4.1.1 Addition of phosphate-solubilizing bacteria (PSBs)

An important aspect of Rock phosphate solublization is addition of phosphate-solubilizing microorganisms where they release organic acids, chelation, and increase in phosphatase enzyme activity and ion exchange reactions and thus, increase the availability [35, 36, 38]. There are various species of phosphate-solubilizing bacteria including Serratia, Burkholderia, Azotobacter, Rhodococcus, Xanthomonas, Enterobacter, and Pantoea [39]. Rock phosphate when incubated with PSB has shown encouraging results (Table 1).

Sr. No.	Crop	Modification of rock phosphate	Observations	References
1.	Potato-soybean	Rice straw enriched with Rock phosphate, mica and Aspergillus awamori (along with 50 and 75% of RDF)	Significantly higher yield over RDF and control up to 43.3 and 21.5%, respectively in potato and 27.6–46.9% increase in soybean grain yield.	[37]
2.	Maize- Wheat	Rock phosphate treated with Pantoea cypripedii (PSB-3) and Pseudomonas plecoglossicida (PSB-5)	Significantly higher yield for both inoculants in maize compared with DAP application and for wheat though highest with RP + inoculants, however, at par with DAP	[35]
3.	Maize-Wheat	Rock phosphate treated with Penicillium oxalicum	Significantly higher yield for both maize and wheat compared with control and rock phosphate alone.	[33]
4.	Soybean- Wheat	Half of recommended P by rock phosphate with Pseudomonas striata and Glomus fasciculatum	Significantly higher yield compared to control, highest in soybean (3.4% higher than when 100% is through SSP); and almost similar in wheat.	[40]
5.	Rice-rapeseed-mungbean	Mussorie rock phosphate (MRP) inoculated with Pseudomonas striata	In 3 year experiment, MRP @ 17.5 kg P ha^−1 was found significantly superior to control in all 3 years and at par with DAP @ 17.5 kg P ha^−1 in third year in terms of rice equivalent yields.	[29]
6.	Stevia rebaudiana	Mussorie rock phsophate (MRP) treated with PSB (Burkholderia gladioli; Enterobacter aerogenes and Serratia marcescens)	Increase of 136% total biomass compared with absolute control and stevioside and rebaudioside-A recorded 291 and 575% increase, respectively.	[41]

Table 1.

Research evidences showing utilization of rock phosphate with PSB strain in different crops. The comparison for the crops/cropping system is done with control and commercial phosphorus fertilizer, and summarized findings are mentioned.

4.1.2 Phosphate-enriched compost/manure

Composting is common practice in India; however, it is characterized with lower nutrient concentration. Around 500 mT of crop residues is generated in India of which three-fourth is contributed by the cereals. Enrichment of compost with rock phosphate has twin benefits with enriched nutrient content and comparatively more solublization of phosphate mineral. Addition of rock phosphate is found to not only increase the nutrient content of rice straw compost but also reduce C:N ratio when added in combination of waste mica [42].

Addition of rock phosphate is reported to reduce total C content in composting mass due to dilution effect, increase in total nitrogen content due to net loss of dry matter, and significant increase in phosphate content with higher share of citrate soluble form than water-soluble as it contributed by the rock phosphate and favorable maturity index such as C/N ratio < 20 except for tree leaf compost due to higher cellulose, hemicelluloses, and lignin content; water-soluble carbon to organic nitrogen ratio < 0.5; nitrification index> 0.16 [43, 44, 45].

Higher reduction in C:N and C:P ratio and increase in water-soluble P were recorded in rock-phosphorus-enriched manure when inoculated with phosphate-solubilizing microorganisms [34] with citric, malic, oxalic, and formic acids with citric acid having maximum P-solubilizing efficiency [46]. Higher rate of nitrogen and sulfur mineralization has been observed with rock-phosphate-enriched compost along with 50% NPK after 120 days and significant improvement in available P with progress of incubation period indicating its availability for longer period of time compared with 100% NPK [45, 47].

In cropping systems, residual effect of enriched compost is found to significantly increase the total phosphorus, enhance grain and stover yield and P uptake and its use efficiency compared with similar P dose from phosphorus fertilizer [45, 48]. Rock-phosphate-enriched compost reported significantly highest grain yield and P uptake in cowpea when compared with other treatments including P fertilizer as sole source [49].

Application of rock-phosphate-enriched compost along with RDF sharing equal to the phosphorus dose in rice recorded significantly higher labile P and grain yield, only at par with treatment having total phosphorus dose from RDF [49]. Phosphorus-enriched manure has significant effect on the number of nodules, their fresh and dry weight in legume crop and yields at par with soluble fertilizer when these are combined with soluble fertilizer in equal proportion [50, 51]. Among all the phosphorus-enriched manures, highest pod and stover yield of mung bean has been reported with that inoculated with Penicillium oxalicum [34]. Phosphate-rich organic manure (PROM) and rock phosphate mixed with urea have shown the comparative results with soluble phosphates and even higher yield under residual effect showing the long-term availability of phosphorus [52]. The effect of phospho-enriched compost on various crops is presented in Table 2.

Sr. No.	Crop	Particular of PEC	Observations	References
1.	Blackgram	PEC prepared with rock phosphate, mica, maize straw and FYM along with PSB inoculation.	Significant seed yield recorded upto 4 t ha^−1 with 15.21% increase over control and at par with 6 t ha^−1.	[53]
2.	Cowpea	Rock phosphate was added to biowaste according to P2O5 requirement of cowpea	83% yield gain over control and 55% gain over soluble P fertilizer	[54]
3.	Chickpea and lentil	Rock phosphate was added to composting material consisting of fruit peels and vegetable waste	Nodules plant^−1, fresh and dry weight of nodules was higher and 15% higher yields compared to control (P fertilizer).	[51]
4.	Wheat-Greengram rotation	Rock phosphate was added to rice residue, mustard leaf residue and tree leaf separately	No significant difference with different composts on grain yields for both crops. Enriched compost of rice residue reported highest yields and residual effect on successive greengram.	[44]
5.	Rice	Rock phosphate was added to rice straw and aerobic composting was done.	Enriched compost along with chemical fertilizer (50:50) reported numerically higher yield compared when P was supplied through fertilizer alone.	[49]

Table 2.

Effect of phospho-enriched compost on various crops/cropping system. Source of organic matter in compost is indicated and yields as well as nodule growth are mentioned in comparison to control/soluble phosphatic fertilizers.

4.1.3 Acidulation with gypsum

Rock phosphate is made to react with sulfuric acid for production of single super phosphate. Similarly, gypsum can also serve as source of partial acidulation of rock phosphate. Total reserve of gypsum in India stands at 36.6 mT with 80% of the total reserve under fertilizer grade and Rajasthan, Jammu & Kashmir, Tamil Nadu, and Gujarat as the major states [31]. Low-grade gypsum is utilized as soil amendment for the sodic soils.

It can also be utilized as an acidulate for the rock phosphate treatment. In addition to the extraction of phosphorus from rock phosphates, gypsum serves as source of calcium and sulfur to the crop plants. Among the various acidulates tested with rock phosphate in various Kharif and Rabi crops, gypsum acidulated rock phosphate has higher yields in most of crops with significant results in Indian mustard [55].

Acidulation of rock phosphate with gypsum in wheat crop reported highest though nonsignificant among all acidulates and more than the treatment having soluble P fertilizer source [56]. Gypsum along with PSB had significantly higher yield sweet pepper yield when applied with rock phosphate compared with its use without gypsum [57].

4.2 Precipitation of struvite mineral

Struvite, a crystalline mineral having formula MgNH₄PO₄.6H₂O, has equimolar concentration of phosphate, ammonium, and magnesium ions [58, 59] and thus can prove as an alternate source of phosphorus. Struvite can be crystallized from the wastewaters of agriculture, sewage effluents, industrial streams, animal waste, and urine [58]. It has various properties, which make it suitable for use in agriculture (Table 3).

Sr. No.	Parameter	Struvite characteristics
1.	Nutrient content	Rich in nitrogen, phosphorus, and magnesium
2.	Pattern of release	Slow release
3.	Influence of pH	Not soluble in alkaline soil and effective in neutral and acidic soils
4.	Suitability	Suitable for crop requiring high Mg and P doses
5.	Comparative advantages	Significantly higher dissolution rate compared with other P minerals such as fluorapatite and variscite due to weak H bonds; N leaching rates are significantly lower when compared with other N fertilizers

Table 3.

The major characteristics of struvite as fertilizer in relation to crop production [60].

Around 85% of the mined phosphorus finds way in the water bodies through soil erosion, agriculture, and livestock waste, which promotes eutrophication; therefore, struvite precipitation can be useful to reduce this pollution [61]. Two major conditions for struvite crystallization are pH between 8.5 and 9.5 with maximum at 9.5, which reduces further, and the concentration of three ions in equimolar concentration and above struvite solubility limit (>0.2 gl⁻¹) [58, 59, 60, 61, 62]. P content of the struvite remains in the range of 11–26% depending on the source, of which 1–2% is water-soluble and slow release in nature [60]. According to an estimate, taking stock of cow urine generated in India, 12000 tonnes of struvite can be produced daily and enlarged scope when considered for all the livestock population [61]. Similar potential exists for the industrial and domestic waste water in India. Globally, full-scale struvite recovery plants are functional in countries of Europe, North America, and Japan [61].

The extraction of struvite has been from the various sources. It is now utilized as the fertilizers or mixed with other fertilizers for value addition having good market and used in crops such as paddy, vegetables, and flowers and even reported to increase quality of paddy [63]. Performance of crops when applied with struvite when compared with chemical fertilizers is presented in Table 4.

Sr. No.	Source	Control fertilizer	Crop	Remarks	Reference
1.	Distillery waste water	DAP	Brassica alba	15% increase in dry weight, 3.2% increase in P uptake, 49% increase in chlorophyll content.	[64]
2.	Commercial	Mineral (KPO4H2)	Phaseolus vulgaris	Application of struvite more than 5 g plant^−1 leads to higher yield than mineral fertilizer.	[59]
3.	Human urine	DAP	Arachis hypogea	Seed yield and other yield attributing characteristics were significantly higher with DAP inoculated with Nitrogen fixing bacteria, however, similar inoculation of struvite reported significantly higher parameters than DAP alone.	[62]
4.	Livestock wastewater	Urea and Magnesium sulfate	Vegetable crops in pot	It was superior to control fertilizer for all the vegetables. The increase in dosage was less inhibitory than control fertilizer.	[65]
5.	Manure	Mono Ammonium Phosphate (MAP)	Canola	At lower rates, the mean biomass yields were similar to control which decreased at higher rates; Quadratic response for P uptake for struvite with increase in dose whereas it was linear with MAP.	[66]
6.	Commercial	Struvite and MAP were mixed in different ratios in gradation.	Maize-Soybean	Upto 50% struvite blending reported statistically similar biomass which decreased further and lowest with 100% struvite in maize and upto 25% for soybean, however, it was 50% for only root biomass for it.	[67]
7.	Cow urine	DAP	Vigna radiata	At higher rates struvite application reported significant increase in the leaf area, stem and root dry weight and total chlorophyll content.	[58]

Table 4.

Performance of struvite as phosphorus source in various crops. Source of extraction is mentioned and commercial fertilizers are used as control. The effect of struvite on chlorophyll is also mentioned as it is source of magnesium.

4.3 Steel slag

Steel slag is the by-product of steel industry. In total, 150–180 kg steel slag is generated for 1 ton of steel production in India [68] and has been estimated at 39 mT for 2017–18 [69]. Per capita steel consumption is expected to increase more than double between 2018 and 2030–31. Thus, there exists a huge potential of generation of steel slag. In countries such as United States, Japan, and European countries, more than 80% of steel slag is recycled, whereas for countries such as China and India, it is less than 30% [70]. While slag is rich source of calcium, silicon, iron, it also reported to contain up to 4% phosphorus [71]. Application of slag is reported to increase the dry weight, yield, and phosphorus uptake in maize [72]. In addition to this, application of steel slag is advantageous in increasing the quality of the produce, source of silica to the crops, promotes immobilization of heavy metals, reduction of disease incidence in crops, and promoting carbon sequestration and reducing methane emission and can act as potential liming material [73, 74, 75, 76, 77, 78].

5. Conclusion

Phosphorus is a valuable element in relation to agriculture; however, its depleting reserve presents a potential challenge to nations across the world. India is at much vulnerable position as there is huge import dependency for its raw material. Keeping the situation in view, Government of India has introduced the action plan to make country Atmanirbhar in phosphorus fertilizer production by utilizing the existing rock phosphate indigenous resources in country. It can be concluded that with certain modifications, indigenous low-quality rock phosphate can also be enriched and the phosphorus can be made available to the crops. The utilization of struvite is still need to be explored in India. Though there is commercial exploitation of this as phosphorus source in countries across the world, there is need to have extensive work on its extraction and develop low cost and customized approach. This will not only reduce the import cost but will also promote the sustainability at village level and concept of circular economy, which is expected to bring benefit of 40 lakh crores in 2050.