Yes effective microbial isolate and in favourable soil condition the biofertilizer may work  better and it may not be effective under certain soil conditions as discussed by some colleagues.

The ability of phosphate solubilizing bacteria (PSB) to solubilize P into available form may be influenced by several factors. The PSB population was shown to have poor correlation with pH, EC, soil organic matter (SOM) in some studies, while the crop species had significant effect on it in some other reports. Enhanced population of PSB is likely to show significant positive correlation with available P only through supplementation of compost and P-sources. On the other hand, available P has always highly significant relationships with pH and acidity (titratable acidity). With the abundance of microbes (108 per ml), recommendation of 2-3 kg/ha is reported. Applying more enhanced dose(s) to fetch additional benefit should be considered upon the issues mentioned, or any other issue(s) suggested by experts.

I can't comment on populations, but the choice of experimental medium to test the effect will depend on whether your bacteria recover P from organic or inorganic sources, I presume the former.  The second consideration will be how strongly the soil may bind the released P.   Choose carefully.

what is best way to determine the best population of PSB to use by farmers? 

I am searching in this field....

It is not the population of PSBs, it is the rhizo-competence of applied PSB  to do the job, but probably you need a minimum population of 105-107  cfu/g soil...

Dr. Amandeh, the 108 CFU /g viable cell count is all right  provided you have favorable condition in your soil environment. We found that between 107 and 109 CFU /g is good population to get optimum effect. But you must apply P solubilizers as seed inoculation for best effect. For that you may use Guar gum/ Carboxymethyl cellulose as sticking material and French chalk powder/lignite/charcoal as absorbent material.

I think this thing depend on soil pH ,crop species,EC,mineral availability, soil environment especially  in root zone.I use 108 CFU /g in many fruit crops experiments ,and i get a variable responses from crop to crop .

The way to get the best response needs to incorporate use fine rock phosphate and granulate it. As the P source use rock phosphate but combine using humic and fulvic acid with the microbial material and include binders such as clay guar, biochar or talc in making your granules.  The ideal would be to produce a granular which would readily disperse in the seedling operation.  The target is to use Bacillus active endosporic bacteria over 10 to the 9th power fermentation can give up 10 to the 12th power.  Endosporic bacteria assure an appropriate shelf life.  As Dr Anoop Srivastava pointed out the application should focus on the area of developing seedling roots. Since there should be no phytotoxicity it could be applied in the seed furrow with the seeding operation.  Ideally a variety of isolates 6 to 12 so the diversity allows best adaptation to the local soil environment.  Make sure you test for isolate phosphate solubility as well as positive growth response of your crop targets.

Several suggestions given by Dr.Hepperly are interesting.I think there is need for research in those lines in diverse soils and crops under field conditions.Adoption of these suggestions may increase the effectiveness of P solubilizing bacteria to solubilize P under farmer field conditions.

How do we decide these limits of microbial load to be really effective with a given soil and given crop , Dr Tarafdar , Dr Nilay , Dr Paul,  Dr Hepperly.....I personally feel , it is simply guesstimate . We deliberately keep microbial load much higher than they  originally exist in the  soil so that once such microbial formulations are inoculated  into the soil , some effective colonies are left in the inoculated soil so that the real purpose of microbial inoculation remains fulfilled..?

lot of work done on experimental field now needs to farmers fields

Tperformance of phosphate solubilizing bacteria depends more on their viability in soil  and its pH rather than the introduced population load. Moreover, the selection of  strain for developing bio-fertilizer should be based on its ability to utilize complex carbon source as refined carbon source is not available in soil that results in its population decline and so does the performance of inoculated strain for release of P. The selected strain should also has the potential to dissolve rock phosphate, iron and aluminium phosphate besides its ability to dissolve calcium phosphate.

Mostafa, your population size of selected PSB is sufficient to display the desired results , provided the selected strain  of PSB is effective enough on the merits as described by Dr Sunita. This is what i have been inquiring from our colleagues , how do you decide the a particular population density of an effective microbe  will be effective or not..?

Dear colleagues,good discussion on an important topic.In the biofertilizers including P solubilizers,certain level of bacterial population and its viability in storage and transport are important.So in India for the BIS certifiction of biofertilizers the inoculants need to meet certain criteria in terms of base,viable cell count ,contamination level,pH,total viable propagules/g of product,minimum and infectivity potential.The quality characteristics are also now brought under Fertlizer Control Order 1985.I believe that certain level of microbial population should be there in the inoculant for proper establisment and solubilization of P in soil.If soil conditions are favourable the biofertilizer may work  better and it may not be effective under certain soil conditions as discussed by some colleagues.The following reference may provide more details on the subject.

Mass production and quality control of microbial inoculants . Yadav,A.K. and Chandra,K.

Proc Indian Natn Sci Acad 80(2)2014 483-489.pdf available online

www.insa.nic.in/writereaddata/UpLoadFiles/PINSA/Vol80_2014_2_Art29.pdf

Initially, soil bacteria should be prospected with the ability to raise phosphate. After obtaining pure cultures, it is necessary to screen those bacteria with greater potential to express the in vitro solubilization capacity. Bacterial strains that are promising for solubilization are considered those with solubilization index (IS) greater than 2. IS = the ratio between the diameter of the solubilization halo and the diameter of the bacterial calony. Field testing is recommended, because a promising in vitro bacterial line does not always exhibit the same performance as field-tested.

The seed inoculation is the preferred mode of introducing cells of desired microbial inoculant. In both cases whether it is carrier based or liquid formulation, microbial load per g seed needs to be taken into account rather than the population per ml broth

i agree with you Dr de Souza , in-vitro screening should be backed up  by in-field evaluation in order to have double check on the effective microbe or microbes with respect to soil and crop as well. ...

Yes effective microbial isolate and in favourable soil condition the biofertilizer may work  better and it may not be effective under certain soil conditions as discussed by some colleagues.

The current specification of phosphate solubilizing bacteria in terms of viable count is CPU minimum 5x10 to power 7cells /g of powder,granules or carrier material or 1x10 to power 8 cells /ml of liquid.Seed size and  rate per ha may differ in different crops.It is a question as to how one can express the microbial load per gram of seed.Dr.(Ms.) Sunita is there any published work in this line?

Dr Rao, microbial load depends on the organisms you are inoculating. For example, Rhizobium strains the recommended load is 200g per 10 kg seed (pulses, soyabean, groundnut); for Azotobacter 200g per 10 kg seed ( cereals, millet, cotton, vegetables); for blue green algae 10 Kg per hectare (Rice) etc. Moreover, the rate also depends on method of application i.e. soil treatment or seed treatment.

Publication :

S. Gaind and A.C.Gaur 1991. Thermotolerant phosphate solubilizing microorganisms and their interaction with mung bean. Plant and Soil. 133:141-149

Good information Dr Tarafdar..Do we have any such information for fruit crops ...Do we have any publication comparing the evaluation of  same microbial  species  though seed inoculation versus soil inoculation..

Please find enclosed two very interesting publications . On entitled Evaluation of Rhizobial Inoculation Methods for Chickpea published in Agron. J. 94:851–859 (2002)...PDF enclosed .

Another publication is entitled as Soil inoculation method determines the strength of plantesoil interactions..Abstract mentioned below: 

Abstract : There is increasing evidence that interactions between plants and biotic components of the soil influence plant productivity and plant community composition. Many plantesoil feedback experiments start from inoculating relatively small amounts of natural soil to sterilized bulk soil. These soil inocula may include a variety of size classes of soil biota, each having a different role in the observed soil feedback effects. In order to examine what may be the effect of various size classes of soil biota we compared inoculation with natural field soil sieved through a 1 mm mesh, a soil suspension also sieved through a 1 mm mesh, and a microbial suspension sieved through a 20 mm mesh. We tested these effects for different populations of the same plant species and for different soil origins. Plant biomass was greatest in pots inoculated with the microbial suspension and smallest in pots inoculated with sieved soil, both in the first and second growth phase, and there was no significant population or soil origin effect. Plant-feeding nematodes were almost exclusively found in the sieved soil treatment. We show that processing the soil to obtain a microbial suspension reduces the strength of the soil effect in both the first and second growth phase. We also show that the results obtained with inoculating sieved soil and with a soil suspension are not comparable. In conclusion, when designing

plantesoil feedback experiments, it is crucial to consider that soil inoculum preparation can strongly influence the observed soil effect.Source : http://dx.doi.org/10.1016/j.soilbio.2012.05.020

Please find enclosed another very exhaustive review..entitled Advances in plant growth-promoting bacterial inoculant technology: formulations and practical perspectives (1998–2013) Plant Soil (2014) 378:1–33

DOI 10.1007/s11104-013-1956-x

Abstract: Background Inoculation of plants to enhance yield of crops and performance of other plants is a century old, proven technology for rhizobia and a newer venue for plant growth-promoting bacteria and other plant symbionts.

The two main aspects dominating the success of inoculation are the effectiveness of the bacterial isolate and the proper application technology.

Scope : An assessment of practical aspects of bacterial inoculants for contemporary agriculture and environmental restoration is critically evaluated from the point of view of their current technological status, current applications, and future use. This is done because there are windows of opportunity for new developments in applied research using renewable, non-contaminated natural resources and new venues for research. Special emphasis is given to formulations and polymeric carriers. This review concentrates on practical aspect of inoculation technology dating from 1998 to 2013. Earlier publications are mentioned only for clarification of a specific point.

Conclusions:  This review discusses characteristics of a carrier for inoculants, formulations of inoculants including liquid, organic, inorganic, polymeric, and encapsulated formulations. Technical aspects include inoculation techniques (soil and seed application), mass culture. production, bulk sterilization, seed coating, shelf-life, and effect of moisture. Future research venues needed are noted.PDF enclosed for further reading...PDF enclosed for further reading...

Very good attachment Dr Srivastava. The right answer is yet to come. 

Dr. Shirgure  PSB is normally use as seed inoculation where 50g of seed is inoculated with 25-35 mL of inoculant of 108 population. For soil application , you can not do broadcasting, only you may apply as line sowing. My experience is you need at least 12-15L of material per hectare depends upon soil moisture status.

Dr Tarafdar, how do you fix such population density and dose ( so much mL),  unless experimented. Is it a researchable issue or just a guess that invariably works..

We tested it under arid crops of clusterbean, moth bean, mung bean and pearl millet  fields with two PSB namely Pseudomonas striata and Bacillus polymyxa.

Its a good question

Rather than use of PSB as soil application , we should go for seed treatment as seed treatment is the best way for bioinoculation. for seed treatment , 50 ml of log phase of PSB culture should be used per acre of seed in small seeded crops and in the large seeded crop, 50ml for ten kg seed. for soil application this amount should be doubled.

In the biocontrol studies with compost tea the positive effect of the compost tea is optimized at high inoculum densities the biocontrol starts being visible at exp 6 populations and is maximized at exp 9, Failure to get the optimized effect idea has a population density or inoculum component. Mycorrhizae in my opinion is the chief for its P ability. What they need is plant provided carbohydrate and low to moderate soil P to stimulate a plant to fungus communication. In old tropical soils which are low in P the need for P can be provided by rock phosphate which solubilizes under the acid reaction and the vast majority of the phosphate will be from organic P forms. The mass of well mycorrhizal dominated soil can provide all the C which will enhance the allied rhizosphere populations which are maximized by C excreted by the plant root. The most effective nutrient application is a modest amount of starter P in the initial seed zone from vermicast and compost and the stimulated early root system will engage will mycorrhizae from the soil. Alternatively the growth of colonized mycorrhizal transplants can have an advantage that the root growth and mycorrhizal association is preestablished and will growth together. For production agriculture the ability to provide growers with precolonized plants is a niche of market which needs to be filled to reduce input dependence of more chemically refined P fertilizers and reduce their impacts.

Is it the population of PSBs that matters or the effective strain that matters . At the same time , we also need ensure favorable ecoligy of rhizosphere by midulating the optimum conditions required for better efficacy of PSBs inoculated into the soil either by considering the optimum population ( which is difficult to quantify ) or by considering the effective strains...

Population level and strains matter for inoculum the adaptation to the soil environment is critical it is for this reason that mixtures of 6 to 12 effective strains are recommended. The ability to use carriers which have characteristics of carrying more population. Importance is a very limited population can be carried on seeds especially small seeds. Great population allowing longer survivability all interact for succes. The use of Bacillus strains have the great advantage of endospore production and the ability to combine Phosphorus solubility with the ability to produce positive growth regulation and biocontrol activities. Focusing on any one of these alone might be too restricted and endangers getting the best overall results. Bacillus is certainly in the hall of fame of biologically beneficial bacteria.