Syed Wali JalalzaiID. NO. PALB 2098

Managing residual and fixed

phosphorus in soil

Seminar on

2A single arbuscule

PSB

Outline of seminar

Introduction

Research papers

Summary

Introduction:

Phosphorus is an important natural resource which is finite

It has great relevance to crop productivity in different parts of the globe

Phosphate deficiency in soil is indeed widespread and has been observed in several agricultural soil

Phosphorus application has almost invariably produced comparatively richer harvests of grain and biomass, in different soil types of each continent

Obviously, the inherent soil process that cause p fixation, as well as relevant mechanisms to overcome such an enormous soil fertility problem is felt strongly by agriculturists

Agricultural production needs to increase which is related to increased use of inorganic inputs such as phosphorus

Importance of Phosphorus to Plants

important in cell division and development of new tissue

Phosphorus is also associated with complex energy

transformations in plants

Adding phosphorus to soil low in available phosphorus promotes root growth and

winter hardiness, stimulates tillering, and often hastens

maturity

Phosphorus is a component of complex nucleic acid structure

of plants, which regulates protein synthesis

In soils P may exist in many different forms. In practical terms, however, P in soils can be thought of existing in 3 "pools":

Forms of Phosphorus in Soils

Residual P: When the amount of fertilizer P added to soil exceeds removal by cropping the fertilizer P residues gradually increase, with corresponding rise in P concentration in the soil solution

Also

• Very small and will usually contain only a fraction of P per acre.Solution P

pool

• P on the solid phase which is relatively easily released to the soil solutionActive P pool

• Made of inorganic phosphate compounds that are very insoluble and organic compounds that are resistant to mineralization by microorganisms in the soilFixed P pool

P dynamics in the soil/rhizosphere plant continuum. Jainbo Shen et al. 2011

Fate of Phosphorus

Tisdale et al. 1993

P fractions

P fractions in soil

Inorganic

Saliod P:

Al P

Ca P

Fe P

Occluded P

Organic: (50%).

Inositol phosphate 10-50%

Phospholipids 1-5%

Nucleic acids0.2-2.5%

Responsible for P fixation and P build up

Factors influencing P build up

Factors influencing P

build up Nature and

amount of soil minerals

Soil pH

P fixation

Fertilizer P management

Organic matter

Reaction time and

temperature

Anion effects

Cation effects

Let me explain

2

3

45

6

7

81

1) Soil pH

Lowell et al 2009

vice versa.

                         

Figure. Relationship between P adsorbed by soil and P in solution

Lowell et al 2009

2) Soil p adsorption and release

P build up - consequences

Phosphorus buildup is caused by use of inorganic fertilizer which undergo fixation reaction in soil

Buildup of phosphorus in croplands can cause poor plant growth and thus low productivity

Excessive soil phosphorus reduces the plant’s ability to take up required micronutrients, particularly iron and zinc, even when soil tests show there are adequate amounts of those nutrients in the soil

High soil phosphorus levels threaten streams, rivers, lakes and oceans due to possible pollution of the water bodies

Management of fixed and residual P

Acidifying and Liming

Application of Biofertilizers

Agronomic practices: Deep plowing,, P holiday and others

Plant breeding and Genetics: To develop cultivars that will grow well

by using native or built up P

Root geometry change

Acidic rhizosphere

Managing Organic acids in soil

1. Root modification (Proteoid roots)

Proteoid roots produced by white lupin with (A) and

without (B) phosphate.

There is an enormousincrease in the root

surface area helping to forage more p

P uptake by cluster roots

The ability of the cluster roots of white lupin to excrete organic acid is associated with several physiological mechanisms (Neumann and Martinoia, 2002)

H release by white lupin roots during a 3-week cultivation period

WHEAT TOMATO CHICKPEA WHITE LUPIN | + P - P | +P -P | + P - P | + P - P - P | non-proteoid proteoid Carboxylate concentration [µmol g-1 FW] | 1.18 5.38 | 8.48 12.03 | 8.50 17.11 | 8.70 15.98 28.12 | PEP-carboxylase activity [nmol NADH min-1 mg protein-1] | 426 703 | 90 375 | 144 302 | 120 210 330 |

P deficiency-induced changes in PEP carboxylase activity as related to organic acid accumulation in the root tissue

I. Citrate synthase enzyme involved in synthesis of citrate

II. Is also synthesized in higher quantitiesIII.The excess citrate is released into the

rhizosphere, which solubilizes bound P

PEP: phosphoenolpyruvate

A. Low molecular weight, non amino organic acid anions such as citrate, malate, oxalate, fumarate, and malonate

B. Organic acid concentrations in the cytosol are relatively stable whereas, in the vacuole vary in response to nutrient availability and metabolic activity

C. Exudation is usually localized to specific regions of the root system

Organic acids in soil

Regulation of cellular pH and osmotic

potential

Maintenance of elecro neutrality during

excess nutrient cation uptake

Involved in assimilation of

nutrients

Supply of energy to symbiotic

bacteria(malate, malonate, oxalate)

Functions of organic acids in soil

In general research efforts towards alleviating problems related to soil p chemistry have hovered around three major topics namely :

1. Minimizing reaction of P fertilizer with unfavorable soil components

2. Modifying soil chemical environment to suit better P acquisition by plant

3. Selecting proper P source and timing of its application

RESEARCH PAPERS

Evaluation of residual and cumulative phosphorus effects in contrasted Moroccan calcareous

soils

Amrani et al. 1999

1st paper

To evaluate the residual and cumulative P effects on crop growth and P uptake in contrasting calcareous soils from the arid and semiarid zones of Morocco

Objective

Materials and methods

Thirteen calcareous soils were collected from three regions of arid and semi-arid zones of Morocco. The soils were air dried, crushed and sieved through a 2-mm screen

Available P was extracted by 0.5 M NaHCO3 solution and analyzed by the ascorbic acid

method

Five kg of each soil was placed in polyethylene plastic pots. Three successive crops were grown as follows

Greenhouse experiment

Third crop: WheatFirst crop: Wheat Second crop: Corn

First crop: Wheat

Supplemental fertilization added to each pot consisted of:1) 100 mg N kg-1 from NH4NO3, one half at sowing and the other at

tillering2) 50 mg K kg-1 from K2SO4

3) 3.5 mg Zn kg-1 from ZnSO4.7H2O

4) 8 mg Fe kg-1 from Fe - Chelate (10%) 5) 4.5 mg Cu kg-1 from CuSO4.5H2O

6) 8 mg kg-1 from MgSO4.7H2O

Fertilizer P solutions were prepared using reagent grade monocalcium phosphate and mixed with soil at rates :

a. 0, 3.4, 6.7 and 13.4 mg P kg-1 soil b. Four replicationsc. Nineteen wheat seeds (cv. Merchouch) were placed at a depth

of 2 to 3 cm in each pot d. Thinned to 9 plants, 12 days after sowing

Pots were watered regularly (every 3 days) with enough deionized water to bring them to 90% of field capacity The greenhouse was maintained at 24 ◦C day and 15 ◦C at night

Soil Soil type Clay Silt Sand pH water

CECcmol(+)/kg

Organic matter%

Lime (CaCO3)%

Olsen-Pmg kg-1

MBC

——-%———

1 Palexerolic Chromoxerets

56 20 23 7.9 56 1.6 7 3 364

2 Vertic Calcixerollic 42 8 48 8.1 39 2.3 1 8 1313 Calcic Argixerolls 51 28 22 8.2 50 1.9 15 10 3634 Typic Chromoxererts 48 8 39 7.9 39 2.2 6 6 1845 Typic Rendolls 56 14 27 7.7 26 4.6 4 7 3326 Xerochrepts 47 23 27 8.0 28 1.6 21 8 3517 Xerochrepts 49 9 39 8.0 28 2.3 37 23 3678 Argiustolls 26 8 64 7.7 10 2.8 1 14 999 Typic Rendolls 28 15 67 8.2 27 3.7 14 8 10110 Aridic SG of Ustolls 27 7 66 8.1 13 2.5 1 9 10211 Chromoxererts 45 10 43 7.6 43 1.4 1 9 17412 Xerochrepts &

Ustochrepts53 8 37 7.5 28 2.6 1 26 404

13 Arguistolls 10 2 87 7.9 1 0.6 1 9 35

Table 1: Selected physical and chemical characteristics of surface (0–20 cm) soils used in study

MBC = maximum buffering capacity

The greenhouse experiments were conducted as completely randomized factorial designs with:

a) Two factors for the first crop (wheat)

b) Two factors for the second crop (corn)

c) Three factors for the third crop (wheat)

An analysis of variance and fitted model were performed on measured and calculated variables using the SAS package (SAS Institute, 1985)

Result and discussion

Variable Source DF CropsWheat Corn Wheat(1st crop) (2nd crop) (3rd crop)Direct P RP CP RP—————————-P—————————-

Grain yield Soils 12 0.01 – 0.01 0.01P rates 3 0.01 – 0.01 0.01Soils×P rates 35 0.01 – 0.01 0.01

Dry-matter Soils 12 0.01 0.01 0.01 0.03P rates 3 0.01 0.01 0.01 0.01Soils×P rates 35 0.01 0.01 0.13 0.01

Total Puptake Soils 12 0.01 0.01 0.01 0.01P rates 3 0.01 0.01 0.01 0.01Soils×P rates 35 0.01 0.01 0.02 0.04

Table 2: Analysis of variance of the direct, cumulative, and residual effect of P on succeeding crops grown under greenhouse conditions

- only corn dry matter was harvested. P is probability. RP=residual P. CP=cumulative P

Fig. Relative grain yield of wheat as a function of initial NaHCO3 – P mg kg -1

i. The effect of residual P on succeeding crops was evident in this study

ii.The response of both corn (second crop) and wheat (third crop) to previously applied P supported the observation that much of the P applied to a crop as fertilizer may not be used by that crop

iii.Response of corn to residual P was significant for soils with initial NaHCO3 – P test level less than 9 mg kg-1 and no response above 14mg kg-1

Conclusion

Crop yields and phosphorus fertilizer transformations after 25 years of applications to a subtropical soil under groundnut-based cropping

systems

Research Paper - 2

Aulakh et al 2003Objective

To determine the status of different soil P pools in long term fertilized soils To assess and make use of accumulated fertilizer P for sustainable crop production and environmental safety

A field study was conducted for 25 consecutive years (1975/1976–1999/2000) on a semiarid, irrigated.

Material and Methods

Location: Tolewal sandy loam soil (Typic Ustochrepts) at Punjab Agricultural University Research Farm, Ludhiana, India.

Temperature 6- 20Cο in January and 27-41Cο in June

Annual rainfall 600 – 1200mm

pH 8.7

EC 0.21 dSm-1

OC 3.8g kg-1

Olsen P 11.6 kg ha-1

The experimental design was a completely randomized block (CRBD)

12 treatments (four P rates x three P frequencies) were randomized within three blocks

Table 1 : Groundnut based cropping sequences, crop cultivars (cv.), and rates and frequencies of fertilizer P followed during 25 years (1975–2000)

Results and Discussion

Table 2: Pod yield of groundnut from plots receiving different rates and frequencies of fertilizer P in three groundnut based cropping sequences

a LSD005: rate = 130; year = 250; frequency, rate x frequency, year x frequency, year x rate, and year x rate x frequency = non significant. b LSD0.05: year = 200; rate, frequency, rate x frequency, year x frequency, year x rate, and year x rate x frequency = non significant. c LSD005: rate = 300; year = 380; rate x frequency, year x frequency, year x rate, and year x rate x frequency = non significant.

Table 3: Grain yield of winter-grown wheat and seed yield of mustard and rapeseed crops from plots receiving different rates and frequencies of fertilizer P in the groundnut-based cropping sequences

a LSD0 05: rate = 350, frequency = 220, year = 210, rate x frequency = 335; year x frequency, year x rate, and year x rate x frequency = non-significant. bLSD0.05: rate = 67, frequency = 49, year =110, rate x frequency = 110; year x frequency, year x rate, and year x rate x frequency = non-significant. c LSD0 05: rate = 140, frequency = 120, year = 210, rate x frequency = 210; year x frequency, year x rate, and year x rate x frequency = non-significant.

Table 4: Transformations of residual fertilizer P in different P fractions in soils after 25 years of fertilizer P applications in groundnut-based cropping systems (values are means of three fertilizer P rates)a

Pi: inorganic P; Po: organic P

Number of experimental years

Cont….

Fig. . Dynamics of Olsen-P in soil during 25 years in no-P control and with fertilizer P applications to summer-grown groundnut, winter crop and both crops each year. P1 P2, and P3 refer to 30, 60 and 90kgP2O5 ha-1 applied in groundnut-wheat; 20, 40 and 60 kg P2O5 ha-1 in groundnut-mustard; 20, 30 and 40kgP2O5 ha-1

in groundnut-rapeseed sequence, respectively.

Conclusion:

i. P-build up to a high fertility range would increase plant available P, albeit at a diminishing rate, continuous high application rates of fertilizer P may also result in P induced micronutrient deficiencies.

ii. Results indicated a marked difference in P fraction changes between fertilized and non fertilized control plots.

iii. The conversion of fertilizer P to non labile P forms is significantly enhanced by increasing rates and frequency of applied P.

Research Paper - 3

To evaluate both the initial year’s response to P fertilization of corn in south eastern Turkey as well as its residual effect over a number of years.

Objective

Material and methods

Field location

1. Multiple years field corn experiments as a second rotation crop were conducted at the Research Station of Cukurova University in Adana, Turkey.

2. The soil was a loamy, smectitic and calcareous

3. Selected physical and chemical properties of soil that might influence P use and crop growth were:

• pH 7.6 (1:5 soil:water), • organic matter content (0.74%),• High CaCO3 (29%),• cation exchange capacity (29 Cmol kg-1) • Marginal levels of plant available P, i.e., 10 mg kg-1 NaHCO3- extractable P

(Olsen).

Corn genotype (Hybrid XL72AA) was planted manually in late June to early July, based on the weather conditions of that specific year

Treatments:

In the first year (1998), the main plots were fertilized with 0,33, 66 and 99 kg P ha- (as triple superphosphate) prior to the experiment in order to establish a range of soil test values for the following years.

During the second and following years, the main plots were divided into subplots, each receiving 0, 9, 18, 27 and 36 kg P ha-1

The first year experiment was arranged in a randomized complete block design, and then became a split-plot design having:

a) 0, 33, 66 and 99 kg P ha-1 rates as main plots

b) 0, 9, 18, 27 and 36 kg P ha-1 rates as subplots

c) Four replications in 1999, 2000, 2001 and 2002

RESULTS AND DISCUSSION

Table 1. Initial (main plot) and currently applied (sub plot) fertilizer P in relation to yield and increment.

Table 2. Initial (main plot) and currently applied (sub plot) fertilizer P in relation to grain P uptake, total plant P uptake and P recovery.

Main plots P rates kg P ha Relative yield

1999 2000 2001 2002

0 119 121 128 135

33 123 124 106 132

66 126 124 107 133

99 112 126 106 136

Average 120 124 112 134

Table 3. Relative corn yield to currently applied P for 4 years after the initial P applicationa.

a Mean response of the 9,18,27,36 kg P ha a -1 rates relative to the control for each initial level of P.

Figure . Extractable P status in 2002 after 5 years of P fertilization. Initial main-plots received 0, 33, 66 and 99 kg P ha-1 in 1998 and sub-plots 0, 9, 18, 27, 36 kg P ha-1 annually from 1999-2002

CONCLUSION

i. Residual P from fertilizer application does accumulate rapidly for the benefit of succeeding crop

ii. The phenomenon occurs in calcareous Mediterranean soils which ‘‘fix’’ applied P fertilizer

iii.Based on research in the Mediterranean region, and data from that study was, the fertilization value of P fertilizer residues can easily assessed and monitored by soil testing

Rescheduling of phosphorus application to FCV tobacco grown on Phosphorus build up soils of KLS(Karnataka light soil)

region

Vasuki et al. (2002)

objective

To ascertain the effect of reducing the level as well as omitting the application of P on yield and quality of FCV tobacco

4th Paper

Material and Methods

1. Location: Agricultural Research Station Navile, Shimoga. In plot where tobacco continuously grown.

2. Available P was very high (130 kg P2O5 ha)

3. Design was RBD.

4. Long term field trial on P application was conducted with reduced level of P 30 and 60kg h in combination of with different schedules of application

1. Every year2. Once in two year3. Once in three year

Results and Discussion

S/No treatments Nicotine (%) Reducing sugar(%)

X L X L

1 No P2O5 1.63 1.70 15.2 14.9

2 30 kg P2O5 (E. yr.) 1..63 1.85 16.1 15.6

3 30 kg P2O5 ( once in 2 E. yr.) 1.68 1.83 16.5 14.1

4 30 kg P2O5 ( once in 3 E. yr.) 1.67 1.73 16.4 15.3

5 60 kg P2O5 (E. yr.) 1.66 1.81 15.2 15.1

6 60 kg P2O5 ( once in 2 E. yr.) 1.59 1.77 15.6 15.2

7 60 kg P2O5 ( once in 3 E. yr.) 1.61 1.74 16.5 15.2

S.Em. +/- 0.10 0.08 0.68 1.17

C.D. 0.05 Ns Ns Ns Ns

CV.% 11.4 9.7 13.6 15.8

Table 1: Nicotine and reducing sugar levels of FCV tobacco as influenced by P application (Average of six year)

FCV: Flue Cured Virginia

Table 2: depletion of available P at the end of sixth year

S/No treatments P2O5 kg ha-1 Extent of depletion kg ha-1

% of depletion1994 initial 1999

1 No P2O5 171.6 97.5 74.3 43

2 30 kg P2O5 (E. yr.) 194.0 131.0 63.0 32

3 30 kg P2O5 ( once in 2 E. yr.) 195.9 139.0 56.8 29

4 30 kg P2O5 ( once in 3 E. yr.) 170.4 117.8 52.5 31

5 60 kg P2O5 (E. yr.) 192.8 142.0 51.0 26

6 60 kg P2O5 ( once in 2 E. yr.) 187.1 139.0 48.0 25

7 60 kg P2O5 ( once in 3 E. yr.) 194.1 141.0 53.5 27

S.Em. +/- 11.6 9.3 11.2

C.D. 0.05 Ns 27.5 Ns

CV.% 9.6 24.3 39

S/No treatments Av. Yield kg ha-1 B: C ratio*

1 No P2O5 1102 1:18.36

2 30 kg P2O5 (E. yr.) 1075 1:14.8.

3 30 kg P2O5 ( once in 2 E. yr.) 1107 1:16.42

4 30 kg P2O5 ( once in 3 E. yr.) 1134 1:17.31

5 60 kg P2O5 (E. yr.) 1162 1:13.71

* Input – output costs arrived based on the returns of corresponding years and input multiplied by the costs of respective years.

Table 3: Economic performance of rescheduling of P to FCV tobacco.

Conclusion

The depletion of available soil P in long run in plots where no P applied and with a view to sustain the P supply to the crop at the optimum level over the years, it is advisable to go for 30kg ha application once in three years

Organic acids exuded from roots in phosphorus uptake and aluminum

tolerance of plants in acid soils

Hocking 2001

objective

I. To understand how organic acid exudation from roots benefits the P nutrition of plants.

II. To determine the role of organic acid in mobilizing fixed P for plant nutrition

5th paper

Fig 1: Outline of the main processes affecting P availability in the rhizosphere and P supply to plant roots. Root take up inorganic P (Pi) from the soil solution. Which is replenished by Pi from the solid phase and by hydrolysis of organic P (Po) from the soil solution and solid phase. Root exudates particularly organic acids increase rates of solubilization, desorption. and mineralization either directly or indirectly via the activity of microorganisms. Mycorrhizal fungi increase the absorbing area of the root system and the volume of soil exploited.

Table 1: Organic acids exuded from roots of selected crop species grown under P stress

Plant species

Growth conditions

Organic aced exuded from roots (whole root system; μ mol g-1 dry wt h-1

References

citric Malic Malonic Others

White lupine

P-deficient solution

11000 8000 - Traces of aconitic, fumaric ,oxalic and succinic

Johnson et al. 1996

Rice Low - P soil 2300 - - Traces of oxalic, malic, lactic and fumaric

Kirk et al. 1999

Maize P-deficient soil

1300 6000 - Jones and Darrah 1995

Alfalfa P-deficient sand

650 279 Succinic Lipton et al. 1987

Pigeonpea

P-deficient solution

2.3 trace 752 Oxalic, Picidic and tartaric

Otani el al. 1996

Chick pea

P-deficient solution

209 74 41 aconitic, fumaric andsuccinic

Ohwaki and Hirata 1992

Fig 2: Effect of adding various amounts of citrate to an Oxisol on the quantities of P subsequently extracted. Levels of 50 to 100 μ mol citrate g-1 soil are similar to those found in rhizospheric soil of white lupine proteoid roots. Note: citrate increase the extraction of both fixed soil P and recently added P.

Fig. 4: defferences in the capacity of crop species to access poorly availabe soil P as assessed by a 32P isotope dilution (L- value) techinque. The L- value is a measure of the total quantity of palnt avaible P: the higher the value . The greater the plant access to soil P. the plants were grown in pot culture for 30 days in a P-fixing Oxisol high in total P but low n available P. histograms with the same letter do not differ at p=0.05.

I. Based on current consumption of P fertilizer. It is estimated that world reserves of rock phosphates will be exhausted within the next 60-90 years.

II. Currently many developing countries in the tropics have to contend with very low soil P reserves and soils fixing high P

III. Consequently there is increasing pressure to manage the use of P fertilizer in food and livestock production systems in acid soil to minimize actual and potential adverse environmental effects caused by P

Conclusion:

Objective:

To investigate the effects of PSB and AMF and their interactions on, biological properties and inorganic phosphorus fractions of different soil types and crop

Research Paper - 6

Materials and Methods:

Soils: Four soil samples (0-10 cm), low in phosphorus, were used for the pot experiments (Table 1). Subsamples were used to determine chemical properties and soil texture.

Experimental Design: The experimental treatments were arranged in split plot factorial based on a complete randomized block design including:

1) Four soil types (clay, clay loam, loam and sandy loam)2) Three phosphorus fertilizer levels (0, 20 and 40 mg kg_1P)3) Four levels of phosphate solubilizing microorganisms (PSM)

A. Mixture of three phosphate solubilizing bacteria (PSM) including: Azotobacter chrocooccum strain 5, Pseudomonas fluorescens 187 and Pseudomonas fluorescens 36,

B. Mixture of arbuscular mycorrizal fungi (AMF) including: Glomus mossea and Glomus intraradices,

C. Mixture of PSB and AMF D. ControlE. The experiment was replicated thriceF. Total numbers of plots were 144.

Table 1: Some physical and chemical properties of soils

Soil type Ca10-P O-P Fe-P Al-P Ca8-P Ca2-P Total P

Clay 432 22 20 18 152 7.6 952

Clay loam 392 16 17 19 142 4.9 822

Loam 401 17 16 10 130 6.2 789

Sandy loam 255 11 14 15 113 4.6 510

Table 2: The concentration of inorganic phosphorus fraction (mg kg -1) of different soil types

Variable df Shoot dry weight

Root dry weight

Plantheight

Spikelength

Grainspike number

Grainyield

Colonpercent

Sporenumber

PSBnumber

Replication ® 2 0.8 0.3 2.7 19.8 5.2 0.4 5.3 10.8 1.1×109

Soil type (S) 3 5.3** 0.1 25.7 21.4 5.1 0.5 51.5 429.3** 7.4×109**

Phosphorus fertilizer (P)

2 7.8** 0.3 0.9 18.9 10.9 0.1 7671.0** 1300.5** 10.0×109**

S × P 6 0.2 0.2 21.4 19.6 7.3 0.1 136.0 26.4 1.2×109

Biological fertilizer (B)

3 14.7** 0.7** 1.0 21.7 45.2** 16.9** 9089.0** 662.3** 3.4×1010**

S× B 9 1.3* 0.2 23.0 21.1 7.3 1.6** 27.7 97.9** 8.1×108

P× B 6 0.9 0.4 14.0 21.7 9.1 3.6** 2245.0** 429.4** 3.9×108

S× P ×B 18 1.1 0.1 24.2 20.9 2.9 0.5 12.8 15.4 8.1×108

Error 93 0.6 0.1 18.6 0.4 17.8 0.6 94.9 17.8 8.2×108

CV - 20.4 19.4 8.0 9.3 23.6 8.9 47.7 23.6 32.3

*Significant at P < 0.05, ** Significant at P < 0.01

Table 3: Analysis of variance of measured parameters of crop performance and biological properties

Table 4: Mean comparisons of the main effects on Wheat growth properties.

Fig. 1: Interaction effects Soil kind and Biological fertilizer on shoot dry weight (a), grain yield (b), colonization(c) and ∆P-Olsen (d). (PSB: phosphate solubilizing bacteria, AMF: arbuscular mycorrhizal fungi)

Fig. 2: Interaction effects of Soil kinds and phosphorus level (P0=0, P20=20 and P40=40 mg kg 1P) on ∆Ca2-P percentage (PSB: phosphate solubilizing bacteria, AMF: arbuscular mycorrhizal fungi)

a. Application of biological fertilizers reduced%∆Ca2-P and% ∆Ca8-P and increased% ∆ P-Olsen.

b. AMF no effect on% ∆P-Olsen and PSB number whereas increased colonization percentage and spore number

c. Application phosphorus fertilizer increased% ∆Ca2-P and% ∆Ca8-P and no effect on% ∆P-Olsen whereas reduced colonization percentage, spore and PSB number

d. The present study demonstrated the benefits of arbuscular mycorrhizal fungi (AMF) and phosphate solubilizing bacteria (PSB) for enhancing the growth of wheat

CONCLUSION

Summary

I. Temporally fixed P also called residual P becomes available with time but at slow rate.

II. The availability of residual P in the soil is related to the rate of P application and quantity of P taken by plant.

III. Residual P is governed by soil characteristics which include soil physicochemical and biological properties.

IV. AMF no effect on % ∆P-Olsen and PSB number whereas increased colonization percentage and spore number

V. Based on current consumption of P fertilizer. It is estimated that world reserves of rock phosphates will be exhausted within the next 60-90 years.

STRATEGIES FOR IMPROVING P EFFICIENCY IN THE SOIL/RHIZOSPHERE-PLANT CONTINUUM

The effective strategies for P management may involve a series of multiple level approaches in association with soil, rhizosphere, and plant processes.

P input into farmland can be optimized based on the balance of inputs/outputs of P.

Soil based P management requires a long-term management strategy to maintain the soil available P supply at an appropriate level through monitoring soil P fertility because of the relative stability of P within soils.

Some soil and rhizosphere microorganisms such as AMF and plant growth promoting rhizobacteria also contribute to plant P acquisition (Richardson et al., 2009).

successful P management can be achieved by breeding crop cultivars or genotypes more efficient for P acquisition and use.

I. wheat (Triticum aestivum) variety Xiaoyan54 that secreted more carboxylates (e.g. malate and citrate) into the rhizosphere than P-inefficient genotypes (Li et al., 1995).

II. soybean (Glycine max) ‘BX10’ with superior root traits that enable better adaptation to low P soils

In addition, the ability to use insoluble P compounds in soils can be enhanced by engineering crops to exude more phytase, which results from over expression of a fungal phytase gene (George et al., 2005b).

The integration of genetically improved P efficient crops with advanced P management in the soil plant system is important for improving nutrient use efficiency and sustainable crop production.