AGRICULTURAL RESEARCH COMMUNICATION CENTRE

www.arccjournals.comAgri. Review, 35 (3): 159-171, 2014

doi:10.5958/0976-0741.2014.00903.9

PHOSPHATE SOLUBILISING MICROORGANISMS: ROLE IN PHOSPHORUSNUTRITION OF CROP PLANTS- A REVIEW

Deepshikha Thakur* , Rajesh Kaushal and Vineet Shyam

Dr. Y.S. Parmar University of Horticulture and Forestry,Nauni, Solan-173 230, India.

Received: 05-09-2013 Accepted: 25-07-2014

ABSTRACTPhosphorus is the least mobile nutrient element in plant and soil compared to other essential

macronutrients. Root development, stalk, stem strength, flower, seed formation, crop maturity, N-fixation in legumes and crop quality are the attributes associated with phosphorus (P) nutrition.Phosphorus solubilizing microorganisms play role in P nutrition by enhancing its availability to plantsthrough release from inorganic and organic soil P pools by solubilization and mineralization. Principalmechanism in soil for P solubilization is lowering of soil pH by microbial production of organic acidsand mineralization of organic P by acid phosphatases. Use of phosphorus solubilizing microorganismsalone or in combination with other beneficial bacteria and mycorrhiza as inoculants increase prospectsof direct application of rock phosphate (RP) in P uptake and crop production.

Key words: Mechanisms, Phosphorus, Phosphate solubilising microorganisms, Rock phosphate.

*Corresponding author’s e-mail: deepshikhathakur212@gmail.com

Phosphorus, a fascinating essential plantnutrient element is known to be involved in a plethoraof functions in plant growth, development andmetabolism. Being an important constituent ofnucleic acids, phytins, phospholipids, nucleotides,co-enzymes and enzymes, it is of great importancein the transformation of energy, transfer of hereditarycharacters, fat and albumin formation and cellorganization in plants (Mc Vickar et al., 1963).Phosphorus stimulates early root development andgrowth, thereby helping to establish seedlings quickly(Hayman, 1975). It gives rapid and vigorous startto plants, strengthens straw, decreases lodgingtendency and brings about early maturity of crops.Phosphorus in soils is immobilized or becomes lesssoluble either by absorption, chemical precipitationor by both processes (Tilak et al., 2005). About 98%of Indian soils are deficient in phosphorus, as theconcentration of free phosphorus, i.e. the formavailable to crop plants, even in fertile soils, isgenerally not higher than 10 µM even at pH 6.5,where it is most soluble (Narsian and Patel, 2009).In soils, of the total P (0.5 %), only 0.1 % is plantavailable phosphorus (Scheffer and Schachtschabel,1988). Large amounts of P applied as fertilizer enter

into the immobile pools through precipitationreactions with highly reactive Al3+ and Fe3+ in acidicand Ca2+ in calcareous or normal soils (Gyaneshwaret al., 2002). Efficiency of applied P fertilizerthroughout the world is around 10 - 25 % (Isherword,1998), and concentration of bioavailable P in soil isvery low reaching the level of 1.0 mg kg–1 soil(Goldstein, 1994). Currently India is ranked thirdamongst the fertilizer consuming countries in theworld (Prasad, 2002). On the other hand, applicationof P fertilizer Phosphorus to soils already havinghigher amounts of P is becoming an increasinglyuneconomical and ecologically unsound practice.The repeated and injudicious applications ofchemical P fertilizers, however, leads to the loss ofsoil fertility (Gyaneshwar et al., 2002) by disturbingmicrobial diversity, and consequently reduces yieldof crops. This demands sound, eco-friendly andeconomically feasible strategies under the currentscenario when the phosphatic fertilizers are beingreplaced by naturally occurring rock phosphate (RP).Thus the application of PSM (phosphate solubilizingmicroorganisms) in the soil becomes necessary(Shenoy and Kalagudi, 2005).

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Most of the Indian soils are deficient in P,and to circumvent its deficiency, phosphate-solubilizing microorganisms (PSM) could playimportant role in supplying phosphate to plants in amore environment friendly and sustainable manner(Khan et al., 2007). This microbial conversion ofphosphates may be operating in natural soils tocounteract the well-known process of phosphatefixation both in acidic and alkaline soil.

Microorganisms involved in the solubilizationof insoluble P include eubacteria, fungi,cyanobacteria, actinomycetes and arbuscularmycorrhizal (AM) fungi (Zaidi et al., 2003; Khan etal., 2007). Phosphate solubilizing microorganismsinclude fungi (Penici llium and Aspergillus)(Chatli and Sidhu, 2008; Whitelaw, 2000),bacteria (Pseudomonas, Bacillus, Rhizobium,Agrobacterium, Micrococcus, Enterobacter, andErwinia) (Rodriguez and Fraga, 1999; Gulati et al.,2007). These microorganisms can grow in mediumcontaining calcium phosphate complexes as the solesource of phosphorus, assimilate it and release inhigher amounts. Phosphate solubilizingmicroorganisms brings about mobilization ofinsoluble phosphates in the soil and increase plantgrowth under conditions of poor phosphorusavailability (Tripura et al, 2007).

Reports on molecular engineering ofrhizosphere microorganisms in order to improve theirphosphorus solubilising efficiency and theirconsequent use under P-deficient soil are scarce(Krishnaraj and Goldstein, 2001). However,phosphate solubilizing fungi could be geneticallymanipulated in order to enhance their ability/ hostrange to increase plant growth by cloning of genesdirectly involved in both mineral phosphatesolubilization and organic P solubilization, followedby their expression in selected plant growthpromoting rhizobacterial strains.

Occurrence of phosphate solubilizingmicroorganisms: PSM occurs in the rhizosphere,i.e. zone of soil around roots that is influenced byroot activity. The intimacy of this interface betweenplants and their environment is essential for theacquisition of water, nutrients and for other beneficialinteractions with soil organisms (Ryan et al., 2009).The region is relatively rich in nutrients, supportslarge and active microbial populations capable ofexerting beneficial, neutral or detrimental effects onplant growth (Nelson, 2004).

Evidence of naturally occurring rhizosphericphosphorus solubilizing microorganism (PSM)dates back to 1903 (Khan et al., 2007). Themicroorganisms involved in phosphorus acquisitioninclude mycorrhizal fungi and PSMs (Fankem et al.,2006). Although several phosphate-solubilizingbacteria occur in soil but their numbers are notadequate as their population is low (102 cfu g –1) tocompete with other indigenous bacterial communityalready established in the rhizosphere (Glick, 1995).The population of phosphate solubi lizingmicroorganisms is more in the rhizosphere(20-40 % of the total population) as compared tonon-rhizospheric (10-15% of the total population)region (Swaby and Sperber, 1958). The rhizosphereof legumes supports greater number of P-solubilizingmicroorganisms than non-legumes (Paul andSundara Rao, 1971).

A large number of phosphate solubilizingbacteria have been isolated by researchers all overthe world from the rhizosphere of several crops(Nautiyal et al, 2000) and these constitute about 20-40% of the culturable population of soilmicroorganisms. The most efficient phosphatesolubilising bacteria belong to genus Bacillus andPseudomonas (Krishnaraj et al., 2001). Morespecifically fluorescent Pseudomonads havereceived particular attention because of theircatabolic versatility, excellent root colonizing ability,capacity to produce a wide range of enzymes andmetabolites that favour the plant to withstand variedbiotic and abiotic stress conditions (Sarvanakumaret al, 2007). A total of 193 phosphate solubilizingbacteria were isolated from the rhizosphere ofchickpea, mustard and wheat growing in differentregions of Haryana. The PSB counts showed largevariations (3-67× 105cfu/g) among and within thecrop as well as place of sampling. (Kundu andNehra, 2009).

The presence of phosphate solubilizingmicroorganisms were also reported in cold desertsoil of Lahaul and Spiti valley of Himachal Pradesh(Chatli and Sidhu, 2008). They have noted morecounts of P- solubilizing bacteria (PSB) and fungi inrhizosphere than in non-rhizosphere soil. Similartrend of biodiversity and counts of PSB was notedfor mine site (Reyes and Valduz et al., 2006).

Forms of phosphorus and their solubilizationby PSM: Microbial involvement in the solubilization

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of inorganic phosphates was first demonstrated byStalstrom (1903) by incubating TCP with bacteriafrom milk and soil infusions. Gerretson (1948)initially demonstrated that microbial activity in therhizosphere could dissolve sparingly soluble inorganicP and increase plant growth. Subsequently,Pikovskaya (1948) suggested a medium for theisolation and screening of P-solubi lizingmicroorganisms. Subsequently, work of Sackett etal., (1948) by using the agar plate technique,provided conclusive evidence to show that soilbacteria dissolve dicalcium phosphate, TCP, andbone meal and rock phosphates. Recently, a fewother methods for the isolation and selection of P-solubilizing microbes, including P-solubilizing fungi,have been suggested (Gupta et al., 1994; Nautiyal,1999).

Several phosphate solubilizingmicroorganisms can utilize insoluble phosphaticsources such as TCP, hydroxyapatite, fluorapatite,ferric, aluminium, and magnesium phosphate, bonemeal and rock phosphates and convert them into

soluble phosphate forms (Hegde et al., 1999). It hasbeen reported that rock phosphates, aluminiumphosphates and iron phosphates are less solubilizedas compared to TCP (Banik and Dey, 1982). Daveand Patel (1999) compared the solubilization ofvarious insoluble inorganic phosphates byPseudomonas isolates and observed that the trendof solubilization of different P sources as follows:Bone meal > TCP > DCP > Iron phosphate >Senegal rock phosphate > aluminium phosphate.Dipta (2013), screened four potential P solubilizersand observed maximum P solubilisation in TCP>RP> BM by isolate MK5 (B. subtilis) followed by VG1(B. sefansis).

Kundu and Gera (2002) observed faster rateof P solubilization from TCP than Mussoorie rockphosphate by 73 P-solubilizing bacteria isolated fromrhizosphere of different agricultural crops. Kaushal,(2011) isolated nineteen phosphate solubilisingbacteria from cauliflower roots/ rhizosphere from lowand midhills of Himachal Pradesh which werecapable of hydrolyzing tri-calcium phosphate inliquid as well as in solid Pikovskayas medium.

FIG 1: Forms of phosphorus and mechanisms involved in phosphorus solubilization

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Vasquez et al., (2000) evaluated the potentialof phosphate solubilizing rhizosphere microbialcommunity, both qualitatively and quantitatively, inthe media containing TCP as sole source ofphosphorus and noted that the concentration ofsoluble phosphate in liquid culture increasedabruptly with drop in pH. A nematofungusArthrobotrytus oligospora also has the ability tosolubilise the phosphate rocks (Duponnois et al.,2006). Aziz Quereshi and Narayanasamy (1999)observed that the efficiency of phosphate solubilizersin solubilizing rock phosphate sources in the orderAspergillus awamori > Pseudomonas straita >Bacillus polymyxa.

Linu et al., (2009) isolated and screened 81potential hydroxyapatite solubilizing bacteria fortheir mineral phosphate solubilization ability onPikovskaya’s and National Botanical ResearchInstitute’s Phosphate (NBRIP) medium (Nautiyal,1999). The solubilization in liquid mediumcorresponded with decrease in pH of the medium.Two bacterial strains exhibiting high solubilizatiomof TCP in Pikovskaya liquid culture were identifiedas Gluconacetobacter sp. and Burkholderna sp. ThePseudomonad solubilising insoluble Ca3(PO4)2 wasisolated from the rhizospheres of wheat and barleyby Patrik et al., (2009).

Mechanisms involved in phosphatesolubilization: Inorganic P in acidic soils forms acomplex with Fe and Al compounds (Norrish andRosser 1983) while calcium phosphate predominatesin neutral or calcareous soils (Sample et al., 1980and Lindsay et al., 1989). A wide range of microbialP solubilization mechanisms exist in nature andmuch of the global cycling of insoluble organic andinorganic soil phosphates is attributed to bacteriaand fungi (Barik and Dey, 1982). Inorganic P in soilis solubilized by the action of organic and inorganicacids secreted by PSB in which hydroxyl andcarboxyl groups of acids chelate cations (Al, Fe, andCa) and also known to decrease the pH in basicsoils (Kpomblekou and Tabatabai, 1994).

Several theories have been proposed toexplain the mechanisms of microbial solubilizationof P. Broadly these theories have been categorizedinto three groups: (i) the organic acid theory(Cunningham and Kuiack, 1992), (ii) the sink theory(Halvorson et al.,1990), and (iii) the acidificationby pH excretion theory (Illmer and Schinner, 1992).

Of these, the organic acid theory is well recognizedand accepted by majority of the workers across theglobe. In this theory, the insoluble sources of P aresolubilized by P-solubilizing fungus either by: (i)lowering the pH, or (ii) by enhancing chelation ofthe cations bound to P.

The major mechanism of mineral phosphatesolubilisation, in general accepted, is the action oforganic acids synthesized by soil microorganisms(Halder et al., 1990). Production of organic acidsresults in acidification of the microbial cell and itssurroundings. Consequently, inorganic phosphatemay be released from a mineral phosphate by protonsubstitution for Ca2+ (Goldstein, 1994). In alkalinesoils the release of H+ to the outer surface inexchange for cation uptake or with the help of H+

translocation ATPase constitute alternative ways forsolubilization of mineral phosphates. It is assumedthat these organic acids solubilize insoluble forms ofphosphate to a usable form, such as orthophosphatethus increasing the potential availability of phosphatefor plants (Kucey et al., 1989).

The phosphate solubilizing bacteria have theability to solubilize insoluble mineral phosphate byproducing various organic acids like fumaric, lactic,citric, glycolic, malonic, tartaric and succinic acids(Vazquez et al., 2000), siderophores, mineral acids,protons, humic substances, CO2 and H2S (Ivanovaet al., 2006). Vikram et al., (2007) tested 30phosphate solubilizing bacteria isolated from thecrops grown in vertisols for the production of organicacids such as fumaric, citric, gluconic, maleic,tartaric, succinic acids and other unidentified acids.The strains PSBV-9 produced six organic acids whilePSBV-10, PSBV-25 and PSBV-16 produced onlyfive organic acids. Tri acid and di aliphaticcarboxylic acids are more effective as compared tomonobasic and aromatic acids. Aliphatic acids havebeen found to be more effective in phosphatesolubilization than phenolic acids (Pareek and Gaur,1973). Important phosphate solubi lisingmicroorganisms, their echological niches andorganic acids produced are given in Table1.

A fall in pH of the liquid culture during thesolubilization of inorganic phosphatic compoundshas been reported (Bar-Yossef et al., 1999) but nocorrelation between the quantity of phosphatesolubilized and decrease in pH was observed

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Category Organism Ecological niche Organic acids produced

Bacteria

Escherichia freundii Soi l Lactic Bacillus subtilis, Pseudomonas spp., Rhizospheric soil Lactic, malic Arthrobacter spp., Bacillus spp., Bacillus fi rmus B-7650

Wheat and cowpea rhizosphere

Lactic, citric

P. radicum rhizosphere of wheat Gluconic Enterobacter agglomerans Wheat rhizosphere Oxalic, citr ic Bacillus amyloliquefaciens, B. licheni formis, Penibacillus macerans, Xanthobacter agi li s, E. aerogenes

Mangrove ecosystem

Lactic, itaconic, i sovaleric, i sobutyric, acetic

Enterobacter intermedium Grass rhizosphere 2-ketogluconic P.fluorescens Roots, Rhizosphere of

oil palm trees Citric, malic, tartaric, gluconic

Fungi

Penici lli um rugulosum Phosphate rock Citric, gluconic

A. japonicus, A. foetidus Ind ian Rock phosphate

Oxalic, citr ic gluconic succinic, tartari c

Aspergi llus spp., Penici llium spp., Chaetomiumnigricolor

Lateritic soil Oxalic, Succinic, Citric, 2-ketogluconic

Aspergi llus flavus, A. niger, Penicillium canescens

Stored wheat grains Oxalic, citr ic, gluconic succinic

Aspergi llus niger Tropical soil Gluconic, oxalic Aspergi llus niger, Penicillium spp. Soi l Citric, glycolic, succinic

TABLE1: Important phosphate solubilising microorganisms, their echological niches and organic acids produced

(Goenadi et al., 2000). This results in acidificationof the surrounding soil or rhizosphere, thus, releasingsoluble orthophosphate ions (H2PO4

-1 and HPO3-2

and PO4-3), which can be readily taken up by plants

(Kundu and Nehra, 2009; Goldstein et al., 1999).Fankem et al., (2006) evaluated the ability ofFluorescent Pseudomonas isolates rhizospheric soilsof oil palm tree and concluded that Phosphorussolubilization from insoluble sources resulted due toproduction of organic acids (citrate, malate andtartarate) and subsequently decrease in pH.

However, Illmer and Schinner, (1995)examined the P-solubilization of inorganicphosphates (hydroxylapatite and brushite) andproved that the solubilization is not always causedby the release of organic acids. The probable reasonfor solubilisation, without acid production, wasopined to be the release of protons accompanyingrespiration or NH4

+ assimilation.

The hyphae of ectomycorrhizal fungi, alsoknown to release organic acids in soil, dissolved theinsoluble inorganic phosphorus (Tawarya et al.,2006). Aspergillus brasilense and Aspergilluslipoferum produced gluconic acid when grown insparingly soluble calcium phosphate medium(Rodriguez et al., 2004), responsible for release ofsoluble P in the medium.

Carbon dioxide produced by plant roots andmicroflora is also responsible for the solubilizationof inorganic phosphates and thus increase theavailability of phosphorus to plants by lowering thepH due to the formation of carbonic acid (Kapooret al., 1989) in the rhizosphere.

Under anaerobic conditions, hydrogensulphide (H2S) formed either by the reduction ofsulphur containing amino acids by severalheterotrophic microorganisms or from sulfate bysulphate reducing bacteria of the genus Desulfovibrio,reacts with soil minerals and thus release phosphate(Doelle, 1969). The phosphate solublizing bacteriaare also known to secrete phosphatase enzymes,which act on insoluble phosphates and convert thesame into soluble form (Ponmurugun and Gopi,2006).

Soil phosphates, mainly the apatites andmetabolites of phosphatic fertilizers, are fixed in theform of calcium phosphates under alkalineconditions. Phosphate solubilization is the result ofcombined effect of pH decrease and organic acids,production (Fankem et al., 2006). Microorganisms,through secretion of different types of organic acidse.g. carboxylic acid (Deubel and Merbach, 2005)and rhizospheric pH lowering mechanisms (He andZhu, 1988) dissociate the bound forms of phosphate

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like Ca3(PO4)2. Nevertheless, buffering capacity ofthe medium reduce the effectiveness of PSB inreleasing P from TCPs (Stephen and Jisha, 2009).Acidification of the microbial cell surroundingsreleases P from apatite by proton substitution /excretion of H+ (accompanying greater absorptionof cations than anions) or release of Ca2+ (Goldstein,1994; Villegas and Fortin, 2002). While the reverseoccurs when uptake of anions exceeds that of cations,with excretion of OH ̄/ HCO3 ̄exceeding that ofH+ (Tang and Rengel, 2003).

Solubilization of Fe and Al occurs via protonrelease by PSB by decreasing the negative chargeof adsorbing surfaces to facilitate the sorption ofnegatively charged P ions. Proton release can alsodecrease P sorption upon acidification whichincreases H2PO4

" in relation to HPO42" having higher

affinity to reactive soil surfaces (Whitelaw, 2000).Carboxylic acids mainly solubilize Al-P and Fe-P(Henri et al., 2008; Khan et al., 2007) through directdissolution of mineral phosphate as a result of anionexchange of PO4

3- by acid anion, or by chelation ofboth Fe and Al ions associated with phosphate(Omar, 1998). It is through root colonizingpseudomonads with high-affinity iron uptake systembased on the release of Fe3+ chelating molecules,i.e. siderophores (Altomare, 1999). Moreover,carboxylic anions replace phosphate from sorptioncomplexes by ligand exchange (Whitelaw, 2000) andchelate both Fe and Al ions associated withphosphate, releasing phosphate available for plantuptake after transformation.

Ability of organic acids to chelate metalcations is greatly influenced by its molecularstructure, particularly by the number of carboxyl andhydroxyl groups. Type and position of the ligand inaddition to acid strength determine its effectivenessin the solubilisation process (Kpomblekou andTabatabai, 1994). Phosphorus desorption potentialof different carboxylic anions lowers with decreasein stability constants of Fe - or Al - organic acidcomplexes (log KAl or log KFe) in the order: citrate >oxalate > malonate / malate > tartrate > lactate >gluconate > acetate > formiate (Ryan et al., 2001).

Mineralization of soil organic P (Po) playsan imperative role in phosphorus cycling of a farmingsystem. Organic P may constitute 4-90 % of the totalsoil P. Almost half of the microorganisms in soil and

plant roots possess P mineralization potential underthe action of phosphatases (Cosgrove, 1967;Tarafdar and Classen, 1988). Alkaline and acidphosphatases use organic phosphate as a substrateto convert it into inorganic form (Beech et al., 2001).Principal mechanism for mineralization of soilorganic P is the production of acid phosphatases(Hilda and Fraga, 1999). Release of organic anions,and production of siderophores and acidphosphatase by plant roots / microbes (Yadav andTarafdar, 2001) or alkaline phosphatase (Tarafdarand Classen, 1988) enzymes hydrolyze the soilorganic P or split P from organic residues. The largestportion of extracellular soil phosphatases is derivedfrom the microbial population (Dodor and Tabatabai,2003). Mixed cultures of PSMs (Bacil lus,Streptomyces, Pseudomonas etc.) are most effectivein mineralizing organic phosphate (Molla et al.,1984).

Enrichment of compost with phosphatesolubilising microorganisms and rockphosphate: Phosphate fixation in soils can beminimized for economic crop production by addingheavy doses of compost, farmyard manure (FYM),green manure or by composting/mineralization ofrock phosphate with organic wastes beforeapplication to the soil. As early as in 1918, Lipmanand Mc Lean had shown that phosphate from groundrockphophate could be made more available forcomposting with manure and sulphur. Later, manystudies revealed that phosphate availability can beincreased with feeding and mixing the insoluble Psource with composting materials (Mathur et al.,1980 and Thakur and Sharma. 1997).

Processing of agri-wastes with Plant GrowthPromoting Bacteria for preparation of composts richin nutrients provides a greater scope for developmentof bioinoculants for crop production. The effect ofinoculation with Azotobacter, phosphate solubilisingmicroorganisms and of addition of Mussorie rockphosphate during composting increased both citrateand water soluble P in compost (Tiwari et al., 1988).Vassilev and Vassileva (2003) employed freeimmobilized microorganisms in composting, andobserved that production of organic acids helped insimultaneous solubilization of rock phosphate.Humic and fulvic acids produced duringdecomposition of organic materials such as compost

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(Mishra et al., 1982) and green manures form stablecomplexes with Ca, Fe and Al, thereby releasingphosphate.

Compost was prepared from wheat strawenriched with Rajasthan rock phosphate (12.5%)and Aspergillus awamori. The resulting phospho-compost was rich in total organic carbon, nitrogen,microbial biomass, and humus content of soil washighest when farm yard manure (FYM), after itsenrichment with rock phosphate (Gaind, 2006).Nicholas et al.,(2007) supplemented the compostwith Busumbu rock phosphate (BRP) conducted toimprove the availability of phosphorus from rockphosphate through feeding, mixing and compostingmanure.

Composts were produced from rice strawenriched with rock phosphate and inoculated withAspergillus niger, Trichoderma viride and/orfarmyard manure (FYM). In pot experiments, resultsshowed that the composts inoculated wi thAspergillus niger + T. viride with or without FYM,showed maximum amount of soluble phosphorus(Gaber and Heba, 2005). Biswas andNarayanasamy (2006) reported that rock phosphateenriched compost (RP-compost) were prepared bymixing four low grade Indian rock phosphates withrice straw with and without Aspergillus awamori.RP-compost had higher total P, citrate soluble P,organic P, acid and alkaline phosphatase activities,and lower water soluble P and microbial biomass Cthan normal compost. Thus, RP enriched compostcould be an alternative and viable technology toutilize both low grade RPs and rice straw efficiently.

Effects of PSB on crop production: The use ofphosphate solubilizing bacteria as inoculantssimultaneously increases P uptake by the plant andcrop yield. Strains from the genera Pseudomonas,Bacillus and  Rhizobium are among the mostpowerful phosphate solubilizers (Hayat et al, 2010).Research reports have demonstrated a substantialincrease in plant growth following single, dual orthree member association of rhizosphericmicroorganisms (Zaidi et al., 2003; Zaidi and Khan,2007). Phosphate solubilizing bacteria (PSB) arebeing used as biofertilizer since 1950’s (Kudashev,1956). Specific strains of the Pseudomonasfluorescens, P. putida group rapidly colonize plantroots of several crops and cause statisticallysignificant yield increase (Leong, 1986).

Plant growth stimulation following inoculation ofphosphate solubilizing bacteria (PSB) to crops insoil containing low levels of phosphorus has beenreported (De Freitas et al., 1997). Phosphate-solubilizing bacterial application has promoted P-uptake as well as the yields in several crops (Tomar,1998; Khalid et al., 2004). PSMs can increase cropyields up to 70 percent (Verma, 1993). Applicationof compost enriched with PSM and rock phosphate,at the rate of 10 t/ ha significantly increased the grainyield of rice and black gram in an experimentconducted by Manna et al., (2001). Singh et al.,(2009) studied the utilization of rock phosphatethrough phosphocompost in rice-wheat system in afield experiment and found increase in the relativeyield of rice and wheat by 110 and 104 %,respectively.

Combined inoculation of arbuscularmycorrhiza and PSB give better uptake of both nativeP from the soil and P coming from the phosphaticrock (Goenadi et al., 2000 and Cabello et al., 2005).Inorganic phosphate solubilizing bacteria maybenefit crops like legumes, maize and lettuce byincreasing the availability of phosphate content,when inoculated alone or in combination (Chabotet al., 1996) with mycorrhizal fungi (Singh andKapoor, 1998). Sole application of bacteriaincreased the biological yield, while the applicationof the same bacteria along with mycorrhizaeachieved the maximum grain weight (Mehrvarz etal., 2008). Inoculation with PSB increased sugarcaneyield by 12.6 percent (Sundara et al., 2002).Mycorrhiza along with Pseudomonas putidaincreased leaf chlorophyll content in barley(Mehrvarz et al., 2008). Ashrafuzzaman et al., (2009)suggested that the use of PGPR isolates PGT3 asinoculant biofertilizers might be beneficial for ricecultivation as they enhanced growth of rice becauseof their phosphate solubilization property. Theinoculation of Penicillium oxalicum CBPS-3F-Tsa,used either alone or along with fused phosphates(FP) and rock phosphate, increased the growth, andN and P accumulation in maize (Zea mays) plantscompared to the control (Shin et al.,2006). Inaddition, a substantial increase in plant height (1.4times), plant weight (5.2–8.1 times) and root length(1.1–1.2 times) of maize following inoculation of P-solubilizing fungus Penicillium sp. PS-113 wasreported (Kang and Choi, 1999). Accordingly, several

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authors have reported a profound increase in yieldof wheat (Triticum aestivum) (Whitelaw et al., 1997),and soybean (Glycine max) and faba bean (Viciafaba) (Abd-Alla et al., 2001) through inoculation ofP-solubilizing fungi. The efficient isolates of PSM areavailable for oil palm tree (Deubel and Etoa, 2006),Cauliflower (Kaushal, 2011), Ginger (Kaundal, 2012)and Cherry (Shyam, 2010).

CONCLUSIONPhosphorus is a major crop growth-limiting

nutrient element in soil. Soil microorganisms play akey role in biogeochemical cycling P- cycle occurringin acidic or alkaline soil. Microorganisms enhancethe P availability to plants by mineralizing organic Pin soil and by solubilising precipitated phosphates.Inoculation of phosphate solubilizing bacteria maybenefit many crops like legumes, cereals and leafy

vegetable crops by increasing the availability ofphosphate content when inoculated alone or incombination with phosphate sources. So, thephosphate solubilising microorganisms can be usedfor biofertilization and making use of natural reservesof phosphate rocks for enhancing crop productivity.The use of efficient PSM (phosphate-solubilizingmicroorganisms), opens up a new horizon for bettercrop productivity besides sustaining soil health.However, the viability and sustainability of PSMtechnology largely depends on the development anddistribution of good quality inoculants to farmingcommunities. Therefore, there is a need for extensiveand consistent research efforts to identify andcharacterize more PSM without/ with minimalapplication of greater efficiency for their ultimateapplication under field conditions.

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