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V. Plant Growth Promotion by Phosphate-Solubilizing Fungi

V. Plant Growth Promotion by Phosphate-Solubilizing Fungi

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Table IV

Plant Growth Promotion and Enhancement of Soil P Availability by Phosphate-Solubilizing Fungi

Microorganism



Soil type or growth mediuma



Aspergillus niger



Hydroponics glasshouse experiment:

nutrient solution, sand



Aspergillus awamori



Greenhouse experiment: alluvial

soil (alkaline) from Delhi, India.

Soil available P: 9 mg kgϪ1, pH

unknown



Penicillium digitatum

A. awamori (isolated

from soil, Tamil

Nadu, India)



Greenhouse experiment: red soil



134



A. awamori with

Pseudomonas striata



Field trial: IARI, New Delhi, India



A. niger van Tieghen

(isolated from

Lateritic soil, India)



Flask culture experiment: typic

Ochragual soil, Kapgari, West

Bengal, India. Soil available

P: 7 mg P kgϪ1, pH 5.4

Greenhouse experiment: sandy loam,

India. Soil available P: 3 mg

P kgϪ1, pH 5.6



A. niger [isolated from

rhizosphere of

cowpea (Vignia

unguiculata (L.)

alp., India]

A. niger with Bacillus

mobilis (N fixing

bacterium, isolated

from rhizosphere

of cotton, India)

A. niger with Bacillus

mobilis and Glomus

fasciculatus



Plant

American elm

(Ulmus americana)

harvested at 3 months

Wheat (Triticum

aestivum Kalyan

sona) harvested at

maturity

Groundnut (Arachis

hypogaea L.) harvested

at 30 days



Wheat (Sonalika and

HD2122) harvested

at maturity

No plant



Onion (Allium cepa var.

chikkaballapur red.)



P source



Yield increase (dry weight) (%)



Increase in P (and N) uptake



RP



Mussoorie

RP (219 kg

P haϪ1)



84 (grain)



83% (grain P)



Gaur (1972)



80 (grain)

ND



98% (grain P)

30%b (total plant P)



Vidhyasekaran et al. (1973)



ND



383%b (total plant P)



16 (straw)

11 (grain)



ND



Gaur et al. (1980)



ND



ϩ2 mg P kgϪ1 (soil

available P)



Banik and Dey (1981b)



24 (aboveground)

NS (root)

62 (bulb fresh weight)



24% (shoot P)

51% more VAM spores

in soil



Manjunath et al. (1981)



25 (aboveground)

16 (roots)

88 (bulb fresh weight)



25% (shoot P)

44% (shoot N)

54% more VAM spores

in soil



57 (shoot)

82 (roots)

155 (bulb fresh weight)



48% (shoot P)

71% (shoot N)

84% (root P)

125% more VAM sproes

in soil



Nil P fertilizer



Farmyard manure

(10 tons haϪ1)

44 kg P haϪ1

Mussoorie RP

and paddy straw

Nil P fertilizer



Nil P fertilizer



(aboveground)



900%b,c



Reference



600b,c



(aboveground P)



Rosendahl (1942)



Aspergillus fumigatusd

[isolated from a

Gangetic alluvial,

(Fluvaquent) soil,

West Bengal, India;

pH 7.4]

Aspergillus candidus

(isolated from a

Gangetic alluvial

soil, West Bengal,

India; pH 7.4)

A. awamori



Greenhouse experiment: partially

sterilized Gangetic alluvial soil

(Fluvaquent) silty clay, West

Bengal, India. Soil available P: 7

mg P kgϪ1, pH 7.4



Penicillium bilaii

(isolated from soil,

Canada)



Greenhouse experiment: brown

Chernozemic loamy sand from

Alberta, Canda. Soil sterilized and

allowed to reequilibrate for 6

weeks. Soil available P: 2 mg

kgϪ1, pH 7.2



Field trial



135

Penicillium bilaii

Field trial: orthic brown Chernozemic

clay loam, Lethbridge, Canada.

Soil available P: 2 mg kg Ϫ1,

pH unknown



P. bilaii plus mixed

culture of VAM

fungi



P. bilaii



Greenhouse experiment: brown

Cherozemic loamy sand, Alberta,

Canada. Soil sterilized to kill native

VAM fungi. Soil available P: 2

mg kgϪ1, pH 7.2

Greenhouse experiment: Orthic

Brown Chernozem from Canada.

Soil available P: 3 mg kgϪ1,

pH 8.0



No plant



RP (30 kg P haϪ1)

and farmyard

manure (40

kgN haϪ1)



ND



ϩ3 kg P haϪ1 (soil

available P)



No plant



RP (30 kg P haϪ1)

and farmyard

manure (40

kgN haϪ1)



ND



NS (soil available P)



Soybean harvested at

maturity

Field beans (Phaseolus

vulgaris) harvested

at maturity



Nil P fertilizer



NS (grain)



44% (grain P)



Gaur (1985)



Idaho RP (45 mg

P kgϪ1)



10.3 (aboveground)



NS (aboveground P)



Kucey (1987)



Field beans (P. vulgaris)

harvested at maturity

Wheat (Chester) harvested

at maturity

Wheat (Chester) harvested

at maturity

Wheat (Chester) harvested

at maturity



Nil P fertilizer



53 (aboveground)



31% (aboveground P)



Idaho RP (45 mg

P kgϪ1 soil)

Nil P fertilizer



21 (aboveground)



9% decrease (aboveground P)



5 (aboveground)



3% (aboveground P)



Straw and Idaho

RP (20 kg

P haϪ1)



25 (grain)

9 (straw)



31% (aboveground P)



Wheat (Chester) harvested

at maturity

Wheat (Chester) harvested

at maturity



Nil P fertilizer



27 (grain)

17 (straw)

39 (aboveground)



34% (aboveground P)



Wheat (Neepawa)

harvested at the early

heading stage



Idaho RP (20 mg

P kgϪ1 soil)



28% (aboveground)



25% (aboveground P)



Straw and Idaho

RP (45 mg

P kgϪ1 soil)



Banik and Dey (1982)



46% (aboveground P)



Asea et al. (1988)



continues



Table IV—Continued

Microorganism



Soil type or growth mediuma



P. bilaii



Greenhouse experiment: dark brown

Chernozemic clay loam soil,

Lethbridge, Alberta, Canada. Soil

available P: 4 mg kgϪ1, pH 7.7



136



Field trial: orthic dark brown

Chernozemic clay loam soil,

Lethbridge, Alberta, Canada.

Soil available P: 4 mg kgϪ1,

pH 7.7



P. bilaii



Greenhouse experiment: brown

Chernozem soil (Loamy sand),

Canada. Soil available P: 2 mg

kgϪ1, pH 7.2



P. bilaii



Penicillium sp.

(isolated from

rhizosphere of

tomato, eggplant,

or cucumber,

Baghdad, Iraq)



Greenhouse experiment: calcareous

soil (typic Torrifluvent) from Iraq.

Soil available P: 4 mg kgϪ1, pH8.2



Plant



P source



Yield increase (dry weight) (%)



Increase in P (and N) uptake



Wheat (Neepawa)

harvested at the early

heading stage

Wheat (Chester) harvested

at 8 weeks



Nil P fertilizer



35% (aboveground)



27% (aboveground P)



Idaho RP (700

mg P kgϪ1 soil)



93 (aboveground)



Wheat (Chester) harvested

at 8 weeks



Nil P fertilizer



86 (aboveground)



Wheat (Chester) harvested

at maturity



RP (20 kg P haϪ1)



11 (grain)

4 (straw)



47% (aboveground P)

ϩ2 mg kgϪ1 (soil

available P)

214% more P-solubilizing

fungi in the rhizosphere

73% (aboveground P)

ϩ2 mg kgϪ1 (soil available p)

283% more P-solubilizing

fungi in rhizosphere

8% (aboveground P)



Wheat (Chester) harvested

at maturity

Canola (Brassica napus

L. “Westar”) harvested

at maturity



Nil P fertilizer



6 (straw)

10 (grain)

NS (straw or pods)



36% (aboveground P)



MAP (20 mg

P kgϪ1)

nil P fertilizer



NS (straw or pods)



19% (aboveground P)



NS (straw or pods)



62% (aboveground P)



RP from

Akashatmine,

Iraq (45 mg

P kgϪ1)



11 (aboveground)



17% (aboveground P)

ϩ2 mg kgϪ1 (soil

available P)



TSP (45 mg

P kgϪ1)



16 (aboveground)



8% (aboveground P)

9 mg kgϪ1 (soil available P)



Wheat (Chester) harvested

at 8 weeks

Wheat (Chester) harvested

at 8 weeks

Sorghum (Sorghum bicolor

Moench) harvested at

70 days



Wheat (Chester) harvested

at 8 weeks



Florida RP (20

mg P kgϪ1)



Reference



Kucey (1988)



15% (aboveground P)

Kucey and Leggett (1989)



Salih et al. (1989)



Aspergillus foetidus

(isolated from

rhizosphere of

tomato, eggplant,

or cucumber,

Baghdad, Iraq)



A. awamori



P. bilaii



137

P. bilaii and

Rhizobium

leguminosarum

P. bilaii



P. bilaii



Greenhouse experiment: calcareous

soil (typic Torrifluvent) from Iraq.

Soil available P: 4 mg kgϪ1, pH 8.2



Field trial: alluvial loamy soil (Pura

Farm). Soil available P: 6 kg haϪ1,

pH 7.5



Field trial: moderate alkaline loamy

soil (Chakeri farm). Soil available

P: 12 kg P haϪ1, pH 8.6

Pot trial: Shellbrook orthic gray-black

Chernozemic fine sandy loam from

Porcupine Plain, Saskatchewan,

Canada. Soil available P: 4 mg

kgϪ1, pH 7.3



Field trials: 37 locations, 3 Canadian

provinces, over 3 years. Low to

medium available P soils (0–22 kg

P haϪ1).



Greenhouse experiment: brown

Chernozemic soil (loamy sand)

from Saskatchewan, Canada. Soil

available P: 13.5 mg kgϪ1, pH 7.6



Sorghum (Moench)

harvested at 70 days



RP from

Akashat mine,

Iraq (45 mg

P kgϪ1)



13 (aboveground)



19% (aboveground P)

ϩ1 mg kgϪ1 (soil

available P)



Sorghum (Moench)

harvested at 70 days



TSP (45 mg

P kgϪ1)



4 (aboveground)



Rice (Saket-4) harvested

at maturity



RP (13 kg P haϪ1)

and TSP (13 kg

P haϪ1)

RP (26 kg P haϪ1)



12 (grain)



4% (aboveground P)

ϩ 5 mg kgϪ1 (soil

available P)

ND



RP (26 kg P haϪ1)



NS (grain)



110% more rhizobium

nodules

ND



RP (26 kg P haϪ1)



14 (grain)



ND



Peas (Pisum sativum)

harvested at 43 days



Nil P fertilizer



22 (roots plus aboveground)



NS (roots plus aboveground P)



Peas harvested at 43 days



TSP (44 kg

P haϪ1)



NS (aboveground)



NS (aboveground P)



17% decrease cf. Rhizobium

alone (roots plus aboveground)

2 (grain)



ND



Chickpea (T3) harvested

at maturity

Wheat (HD-1553)

harvested at maturity

Wheat (HD-1553)

harvested at maturity



23 (rain)



Wheat harvested at

maturity



TSP (4.4 kg

P haϪ1)



Wheat harvested at

maturity

Pea (Trapper) harvested

as first flower bud

opened at 41–51 days



Nil P fertilizer



2 (grain)



ND



Nil P fertilizer



48 (aboveground)



39% (aboveground P)

55% (aboveground N)



Tiwari et al. (1989)



Downey and van Kessel

(1990)



Gleddie et al. (1991)



Gleddie (1993)



continues



Table IV—Continued

Microorganism



Soil type or growth mediuma



P. bilaii



Field trials: four P-fertilizer responsive

locations, 1989 and 1991. Thin

black and black soil zones, Western

Canada. Available soil P: “low to

medium levels”



A. awamori



138



Greenhouse experiment: Patharchatta

sandy loam (typic Hapludoll),

India. Soil available P: 27 mg kgϪ1,

pH 6.2



Plant



Field trial: moderately alkaline soil

(loam), Chakeri, Kanpur (U.P.),



Increase in P (and N) uptake



TSP (10 mg

P kgϪ1)



12 (aboveground)



18% (aboveground P)

NS (aboveground N)



Nil P fertilizer



NS (aboveground)



NS (aboveground P)

7% (aboveground N)



Pea (Trapper) harvested

at 8 weeks after

emergence

Pea (Trapper) harvested

at 8 weeks after

emergence

Soybean (Glycine max, L.

merr.) (Bragg) harvested

at maturity



TSP (4.4 kg

P haϪ1)



16 (aboveground)



18% (aboveground P)

19% (aboveground N)



TSP (8.7 kg

P haϪ1)



NS (aboveground)



7% (aboveground)

NS (aboveground N)



Nil P fertilizer



34 (straw)

44 (grain)



Mussoorie RP

(86 mg P kgϪ1)



10 (straw)

5 (grain)



Nil P fertilizer



24 (grain) compared to

inoculation by Bradyrhizobium

sp. alone



Wheat (HD 1553)

harvested at maturity



Nil P fertilizer



NS (grain)

12 (straw)



58% (grain P)

ϩ8 mg P kgϪ1 (soil

available P)

40% (N uptake)

100% (nodule dry weight)

5900% more P-solubilizing

microbes in soil

13% (grain P)

ϩ3 mg P kgϪ1 (soil

available P)

11% (N uptake)

NS (nodule dry weight)

271% more P-solubilizing

microbes in soil

35% (grain P)

NS (soil available P)

21% (grain N)

NS (nodule dry weight)

ND



Wheat (HD1553)

harvested at maturity

Wheat (HD1553)

harvested at maturity



Mussoorie RP

(26 kg P haϪ1)

Nil P fertilizer



NS (grain or straw)



ND



8 (grain)

7 (straw)



ND



A. awamori and

Bradyrhizobium sp.



Field trial: alluvial loamy soil at Pura,

India. Soil available P: 6.3 kg haϪ1,

pH 7.5



Yield increase (dry weight) (%)



Pea (Trapper) harvested

as first flower bud

opened at 41–51 days

Pea (Trapper) harvested

at 8 weeks after

emergence



Soybean (G. max, L. merr.)

(Bragg) harvested at

maturity



A. awamori



P source



Reference



Singh and Singh (1993)



Tiwari et al. (1993)



India. Soil available P: 12 kg haϪ1,

pH 8.6



Aspergillus sp.

(KAR0210)



Aspergillus sp.

(KAR0210)

Penicillium glaucum

(HE4) (isolated

from sunflower

rhizoshpere,

Bangalore, India)

Penicillium

aurantiogriseum

(isolated from

forest soil, Austria)



139

Paecilomyces

fussiporus

Aspergillus sp.



A. awamori



Greenhouse experiment: autoclaved

Sang Hyang Damar Ultisol soil.

Soil available P: 0.5 mg P kgϪ1



Greenhouse experiment: Rajamandala

Ultisols soil. Soil available P: 37 kg

haϪ1, pH 3.9

Greenhouse experiment: red soil

GKVK farm, Bangalore, India.

1% farmyard manure. Soil

available P: 7 mg kgϪ1, pH

unknown

Greenhouse experiment (no plant):

soil from Austrian alps. Soil

available P: 0.6 or 0.7 ␮g P dmϪ1,

pH 4.2 or 4.6. Soil amended with

0.2% glucose, 0.2% sucrose,

0.002% N (from NH ϩ

4)

Field trial: clayey soil, Junagadh,

India. Soil available P: 31 kg P

haϪ1, pH 7.9

Field trial: sandy loam Bangalore,

India. Soil available P: 14 kg

P haϪ1, pH 6.2



Greenhouse experiment: soil, India.

Soil available P: 1.2 kg P haϪ1,

pH 7.1



Wheat (HD1553)

harvested at maturity

No plant (15 days)



Mussoorie RP

(26 kg P haϪ1)

Nil P fertilizer



NS (grain or straw)



ND



ND



ϩ14 mg P kgϪ1 (soil

available P)



No plant (30 days)



RP (300 mg

P kgϪ1)

RP (300 mg P

kgϪ1)



ND



ϩ38 mg P kgϪ1 (soil

available P)

ϩ24 mg P kgϪ1 (soil

available P)



Sunflower (Helianthus

annuus L.) (BSH-1)



SSP (82 mg P

kgϪ1 soil)



NS (plant height, 60 days)

NS (leaf area, 45 days)

35 (seed yield at harvest)



NS (shoot P at harvest)

47% more P-solubilizing

fungi in the rhizosphere



Gururaj and

Mallikarjunaiah (1995)



No plant



Nil P fertilizer



ND



ϩ2 ng P mlϪ1 soil solution

(soil available P) (No

significant difference when

soil not amended with

glucose, sucrose, or N)



Illmer and Schinner

(1995b)



Groundnut (Arachis

hypogaea)



Nil P fertilizer



NS (pod)



Mehta et al. (1996)



Soybean (Hardee)

harvested at maturity



RP (17.5 kg

P haϪ1)



7 (seed)

7 (oil)



19% (P in kernel)

24% (P in haulm)

8% (nodule dry weight)

NS (protein)



RP (35 kg P

haϪ1)

RP (17.5 kg

P haϪ1) and

SSP (17.5 kg

P haϪ1)

Nil P fertilizer



5 (seed)

8 (oil)

12 (seed)

12 (oil)



NS (protein)



34 (aboveground)



36% (aboveground P)

206% (nodule dry weight)



SSP (20 kg

P haϪ1)



14 (aboveground)



24% (aboveground P)

22% (nodule dry weight)



No plant (14 days)



Gram (JG 315) (legume)

harvested at flowerinitiation stage

Gram (JG 315) (legume)

harvested at flowerinitiation stage



ND



Goenadi (1995)



Goenadi et al. (1995)



Thimmegowda and

Devakumar (1996)



7% (protein)



Vaishya et al. (1996)



continues



Table IV—Continued

Microorganism



Soil type or growth mediuma



A. awamori



Penicillium radicum

(isolated from

wheat rhizosphere,

Australia)



Greenhouse experiment: red earth

soil. Soil available P: 17 mg kgϪ1

pH 4.6



140



Field trial: red earth soil, Wagga

Wagga, NSW, Australia. Soil

available P: 16 mg kgϪ1, pH 4.9



Plant



P source



Yield increase (dry weight) (%)



Increase in P (and N) uptake



Gram (JG 315) (legume)

harvested at flowerinitiation stage

Gram (JG 315) (legume)

harvested at flowerinitiation stage

Wheat (Dollarbird)

harvested at maturity

at 20 weeks



SSP (40 kg

P haϪ1)



13 (aboveground)



52% (aboveground P)

22% (nodule dry weight)



RP (17 kg

P haϪ1)



NS (above ground)



38% (aboveground P)

20% (nodule dry weight)



Nil P fertilizer



26 (grain)

14 (aboveground)



23% (grain protein)

NS (grain P)



Wheat (Dollarbird)

harvested at maturity

at 20 weeks

Wheat (Dollarbird)

harvested at maturity

at 20 weeks

Wheat (Dollarbird)

harvested at maturity

at 28 weeks

Wheat (Dollarbird)

harvested at maturity

at 28 weeks

Wheat (Dollarbird)

harvested at maturity

at 28 weeks

Wheat (Dollarbird)

harvested at maturity

at 28 weeks



KH2PO4 (5 kg

P haϪ1)



10 (grain)



NS (grain P)



KH2PO4 (15 kg

P haϪ1)



15 (grain)



20% (grain P)

15% (grain protein)



Nil P fertilizer



NS



NS



SSP (5 kg

P haϪ1)



18 (grain)



18% (grain protein)



SSP (15 kg

P haϪ1)



25 (grain)



33% (grain protein)



SSP (20 kg

P haϪ1)



16 (grain)



NS



Reference



Whitelaw et al. (1997)



Note: Abbreviations used: MAP, monoammonium phosphate; ND, not determined; NS, not statistically different; RP, rock phosphate; SSP, single superphosphate [mixture of Ca(H2PO4)2 and CaSO4 produced by the

action of H2SO4 on RP]; TSP, triple superphosphate [Ca(H2PO4)2 “monocalcium phosphate” produced by action of H3PO4 on RP]; VAM, vesicular arbuscular mycorrhizal fungi.

a“Soil-available P” determined by extraction with NaHCO unless otherwise indicated.

3

bStatistical significance not given.

cCompared to sterile plants.

dHuman pathogen.



PHOSPHATE-SOLUBILIZING FUNGI



141



Growth promotion and increased P uptake by plants inoculated with P-solubilizing fungi have been reported by many investigators (Table IV). Many of the

studies reported in Table IV have investigated the ability of P-solubilizing fungi

to promote P uptake and plant growth in soil under greenhouse conditions. Under

these conditions, rooting volumes are usually restricted so that if microbial P solubilization does take place, the plant response may be higher than that in field trials (Kucey et al., 1989).

In an early sand and nutrient solution greenhouse study, inoculation of American elm with A. niger was able to increase the yield and P uptake by 600 and 900%,

respectively, but in this study inoculated plants were compared to plants in a sterile medium (Rosendahl, 1942). Illmer and Schinner (1995b) point out that nearly

all rhizosphere microorganisms, not only those which solubilize P, increase the nutrient supply of plants. This means it is advisable to use nonsterile soil to ensure

that uninoculated control plants have adequate nutrition.

In soil greenhouse trials, yield and/or P uptake has been increased by inoculation of wheat, onion, sorghum, soybean, and gram with P-solubilizing fungi (Gaur,

1972; Manjunath et al., 1981; Salih et al., 1989; Singh and Singh, 1993; Vaishya

et al., 1996; Whitelaw et al., 1999). Yield alone was also increased by inoculation

of sunflower and soybean (Gururaj and Mallikarjunaiah, 1995; Thimmegowda and

Devakumar, 1996). Penicillium bilaii (ATCC strain No. 20851) is commercially

available under the trade name Provide and has consistently increased grain yield

and P uptake by wheat grown on neutral or alkaline soils under greenhouse conditions. Penicillium bilaii has also increased the yield and P uptake by field beans

and peas and the uptake of P by canola (Kucey, 1987, 1988; Asea et al., 1988;

Kucey and Leggett, 1989; Downey and Van Kessel, 1990; Gleddie, 1993) (Table

IV).

Growth promotion of plants by P-solubilizing fungi under field conditions has

also been reported by many investigators. P uptake was increased in groundnut inoculated with Paecilomyces fussiporus (Mehta et al., 1996), whereas both wheat

yields and P uptake were increased by inoculation with P. bilaii (Kucey, 1987,

1988). Wheat yield has been increased by inoculation with P. bilaii, A. awamori,

and P. radicum (Gleddie et al., 1991; Tiwari et al., 1989, 1993; Whitelaw et al.,

1999) and yields of pea, rice, chickpea, and soybean have been increased by inoculation with P-solubilizing fungi (Gleddie et al., 1991; Gleddie, 1993; Tiwari et

al., 1989; Thimmegowda and Devakumar, 1996) (Table IV).

Some studies of P-solubilizing fungi included the effect of P-solubilizing bacteria in a mixed inoculum. A mixed inoculum of A. awamori with Pseudomonas

striata increased the yield of wheat under field conditions (Gaur et al., 1980). VAM

fungi, which are known to enhance the ability of the host plant to absorb P, have

also been included in mixed inocula. The individual growth-promoting effects of

P-solubilizing fungi, bacteria, and VAM fungi have been reported to be additive.

An inoculum consisting of a mixture of A. niger, Bacillus mobilis, and the VAM



142



M. A. WHITELAW



fungus Glomus fasciculatus increased the P content of shoots and roots and the

bulb weight of onion grown on unsterilized soil to a greater extent than the individual microbial components (Manjunath et al., 1981). Wheat plants inoculated

with both P. bilaii and VAM fungi and grown on soil which was sterilized to kill

native VAM fungi received greater benefit from rock phosphate addition than

plants receiving only one of the microorganisms (Kucey, 1987) (Table IV). Many

studies have been made on the effect of P-solubilizing fungi on legumes. Fungi

such as unknown species of Penicillium, Cephalosporium, and Alternaria and

Penicillium lilacinum, A. niger, A. flavus, and A. terreus were associated with

legume root nodules and were capable of solubilizing Ca3(PO4)2 (Subba-Rao and

Bajpai, 1965; Chhonkar and Subba-Rao, 1967) (Table I). Increased soil P availability is known to increase nodulation in legumes (Jones et al., 1977) and in some

studies inoculation of legumes with P-solubilizing fungi increased nodulation and

N uptake (Gleddie, 1993; Tiwari et al., 1989; Singh and Singh, 1993; Mehta et al.,

1996; Vaishya et al., 1996) (Table IV).

The effect of inoculation of unplanted soil on the NaHCO3-extractable P or “soil

available P” levels was investigated in several greenhouse studies. Increases in soil

available P in unplanted soil were reported by Banik and Dey (1981b) with A.

niger, Goenadi et al. (1995) with an unidentified Aspergillus sp., and Illmer and

Schinner (1995b) with P. aurantiogriseum. Soil available P was also determined

in some plant growth studies. Increases in the levels of plant available P found in

the soil after wheat, sorghum, and soybean experiments were reported by Kucey

(1988), Salih et al., (1989), and Singh and Singh (1993) (Table IV).

Inoculation of plants with P-solubilizing fungi was sometimes reported to encourage the proliferation of other P-solubilizing fungi in the rhizosphere. Kucey

(1988) found that after 8 weeks of growth, the total number of P-solubilizing fungal isolates from the rhizosphere of wheat inoculated with P. bilaii was 283% higher than in uninoculated plants. Gururaj and Mallikarjunaiah (1995) also reported

an increase (47%) in the number of P-solubilizing fungi in the rhizosphere of sunflower inoculated with Penicillium glaucum at harvest.

The relative benefits of inoculation with P-solubilizing fungi have been observed to decrease as the soil P availability increases. Downey and Van Kessel

(1990) and Gleddie (1993) found that the response to inoculation of pea with P.

bilaii under greenhouse conditions depended on whether fertilizer P had been

added to the soil. When soluble P fertilizer was added, the response to inoculation

was lower than when no P was added. Whitelaw et al. (1999) also found that the

response to inoculation of wheat with P. radicum under greenhouse conditions was

lower when fertilizer P was added. Gleddie et al. (1991) reported results of 55 separate field trials of wheat inoculated with P. bilaii and found that in soils which had

high available P levels, neither P fertilization nor inoculation induced a yield response. Salih et al. (1989) reported higher responses in yield and P uptake to inoculation of sorghum with an unidentified Penicillium sp. or with A. foetidus in



PHOSPHATE-SOLUBILIZING FUNGI



143



soil treated with rock phosphate in comparison with soil treated with triple superphosphate. They surmised that this was probably due to the presence of adequate

quantities of readily available P in the soil treated with triple superphosphate,

whereas available P in the soil treated with rock phosphate appeared to be the limiting factor for plant growth.

Many greenhouse or field experiments have investigated plant growth promotion by P-solubilizing microorganisms using only a single level of P application.

Abbott and Robson (1984), while discussing VAM fungi which are capable of increasing P availability to plants, state that there are advantages in studying a complete P response curve (i.e., a full range of applied P with a low and at least two

high P application levels resulting in a plateau defining maximum yield response

to P). The study of the complete response curve is important to demonstrate that

the effect of the inoculum can be eliminated by applying P fertilizer, thus implying that the P fertilizer has replaced the advantage gained by the P-solubilizing

effect of the inoculum (Abbott and Robson, 1984). Where less data are available

(e.g., less than a full response curve), confirmation of a negative interaction between inoculation and the P rate is evidence for a P-solubilizing effect but it is not

possible to separate alleviation of P deficiency from a combination of alleviation

of P deficiency and other plant growth-promotion mechanisms.

Three classes of response to the inoculation and P application treatments need

to be considered when interpreting the data in the previously discussed experiments. First, if the effect of inoculation on plant growth is caused by the alleviation of P deficiency as a result of fungal-induced solubilization of soil inorganic

phosphates, then application of P fertilizer at a rate sufficient to eliminate the deficiency should eliminate the response to inoculation (i.e., a negative interaction

between inoculation and P application rate would be expected). Second, if the effects of inoculation are other than the alleviation of P deficiency, then the response

should not be eliminated by P fertilizer. Therefore, a negative interaction between

inoculation and P application rate would not be expected and there would be a positive yield response to inoculation no matter how high the P application rate. Third,

if both P-solubilizing and other mechanisms are present, then a negative interaction between inoculation and P application rate could be expected but a difference

in yield would still be expected at P rates high enough to eliminate P-deficiency

effects.



VI. CONCLUSION

Phosphorus is an important plant nutrient which is in short supply in many agricultural soils. Because a large percentage of phosphatic fertilizer is fixed by soil

and thus rendered less available to plants, the long-term application of P fertiliz-



144



M. A. WHITELAW



ers has resulted in an accumulation of total soil P, most of which is poorly soluble.

Many soil fungi, predominantly of the genera Aspergillus and Penicillium, have

been shown to possess the ability to solubilize sparingly soluble phosphates in vitro by secreting inorganic or organic acids. The microorganisms and soils reported

here are very diverse and it is difficult to generalize about the success of plant

growth promotion by P-solubilizing soil fungi. The relationship between soil pH

and phosphate solubility is not a simple one, and controversy exists over the effect

of increasing or decreasing pH on P solubility in soil. However, in unbuffered liquid media studies mentioned previously, P solubilization appeared to often be associated with a lowering of pH. Some of the fungi tested appeared to solubilize P

in soils and in unbuffered liquid media. Field and greenhouse trials have demonstrated that inoculation of plants with some P-solubilizing fungi increased the concentration of “available” P in the soil and enhanced the yield and P uptake by the

plant. Continued work in this area could yield plant fungal inoculants capable of

more consistent performance over a range of soil and climatic conditions.



ACKNOWLEDGMENTS

I thank Associate Professor Terence Harden of the School of Wine and Food Sciences, Charles Sturt

University, Wagga Wagga, and Dr. Mark Conyers of N.S.W. Agriculture, Wagga, Wagga, N.S.W. Australia, for critically reviewing the manuscript.



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Agnihotri, V. P. (1970). Solubilization of insoluble phosphates by some soil fungi isolated from nursery seed beds. Can. J. Microbiol. 16, 877– 880.

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