Please see following links

https://link.springer.com/article/10.1007/s00253-013-5228-8

https://www.researchgate.net/publication/303833733_The_Production_and_Potential_of_Biofertilizers_to_Improve_Crop_Yields

Chapter The Production and Potential of Biofertilizers to Improve Crop Yields

Very good response Dr Malhotra. It is a very vast topic , difficult to summarise the role of biofertilizers in agriculture..

Biofertilizers are known to play a number of vital roles in soil fertility, crop productivity and production in agriculture as they are eco friendly and can not at any cost replace chemical fertilizers that are indispensable for getting maximum crop yields. Some of the important functions or roles of Biofertilizers in agriculture are:

* They supplement chemical fertilizers for meeting the integrated nutrient demand of the crops.

* They can add 20-200 kg N/ha year (eg. Rhizobium sp 50-100 kg N/ha year ; Azospirillum , Azotobacter : 20-40 kg N/ha /yr; Azolla : 40-80 kg N/ha; BGA :20-30 kg N/ha) under optimum soil conditions and thereby increases 15-25 percent of total crop yield.

* They can at best minimize the use of chemical fertilizers not exceeding 40-50 kg N/ha under ideal agronomic and pest-free conditions.

* Application of Biofertilizers results in increased mineral and water uptake, root development, vegetative growth and nitrogen fixation.

* Some Biofertilizers (eg, Rhizobium BGA, Azotobacter sp) stimulate production of growth promoting substance like vitamin-B complex, Indole acetic acid (IAA) and Gibberellic acids etc.

*  Phosphate mobilizing or phosphorus solubilizing Biofertilizers / microorganisms (bacteria, fungi, mycorrhiza etc.) converts insoluble soil phosphate into soluble forms by secreting several organic acids and under optimum conditions they can solubilize / mobilize about 30-50 kg P2O5/ha due to which crop yield may increase by 10 to 20%.

* Mycorrhiza or VA-mycorrhiza (VAM fungi) when used as Biofertilizers enhance uptake of P, Zn, S and water, leading to uniform crop growth and increased yield and also enhance resistance to root diseases and improve hardiness of transplant stock.

* They liberate growth promoting substances and vitamins and help to maintain soil fertility.

* They act as antagonists and suppress the incidence of soil borne plant pathogens and thus, help in the bio-control of diseases.

* Nitrogen fixing, phosphate mobilizing and cellulolytic microorganisms in bio-fertilizer enhance the availability of plant nutrients in the soil and thus, sustain the agricultural production and farming system.

* They are cheaper, pollution free and renewable energy sources.

*  They improve physical properties of soil, soil tilth and soil health in general.They improve soil fertility and soil productivity.

* Blue green algae like Nostoc, Anabaena, and Scytonema are often employed in the reclamation of alkaline soils.

* Bio-inoculants containing cellulolytic and lignolytic microorganisms enhance the degradation/ decomposition of organic matter in soil, as well as enhance the rate of decomposition in compost pit.

* BGA plays a vital role in the nitrogen economy of rice fields in tropical regions.

Azotobacter inoculants when applied to many non-leguminous crop plants, promote seed germination and initial vigor of plants by producing growth promoting substances.

*  Azolla-Anabaena grows profusely as a floating plant in the flooded rice fields and can fix 100-150 kg N/ha /year in approximately 40-60 tones of biomass produced,

Plays important role in the recycling of plant nutrients.

 Role of Biofertilizers in Conservation Agriculture

Abstract : In the present time, chemical fertilizers are more in practice for crop production which affected the soil and environment quality. The higher amount of chemical inputs in agricultural production system affected the sustainability of the agricultural crop production systems, increased cost of cultivation, and caused partial factor productivity decline, and maintaining the global food security and environmental quality became a daunting challenge. Indiscriminate and imbalanced use of fertilizers, mostly urea, and the poor application of organic matter to cropland have led to considerable reduction in soil health. Nowadays our agriculture has shifted to old-age practice like conservation agriculture. It is using old tool and techniques with incorporation of modern science and scientific principles. In general, biofertilizer is organic in nature containing an effective particular microorganism in a concentrated form which originated either from the plant root nodule or from the soil of the rhizosphere. Biofertilizers have emerged as potential environment-friendly inputs that are benefited for agricultural crop production system. They hold vast prospective in fulfilling the plant nutrient requirements, which are reducing the chemical fertilizer application and minimizing environmental pollution. The bioinoculants are used as a seed treatment or soil treatment, improving plant nutrient availability and finally crop growth and yield. These contain living cells of diverse types of microorganisms and have the potential to solubilize and mobilize plant nutrient elements from insoluble form through biological process and also fix atmospheric nitrogen. The adequate use of biofertilizers helps in maintaining soil quality and thus provides a low-cost approach to manage crop yield along with protecting the environment.Conservation Agriculture pp 113-134

Dr. Kheloufi, there are many biofertilizers which are found effective to different crops so far, they are: Azotobacter, Azospirillum, Rhizobium, Phosphate solubilising bacteria (PSB), Phosphatase and phytase producing fungi (PPF), Blue Green Algae (BGA), Mycorrhizal fungi, FYM, Vermicompost etc. But the efficiency of any biofertilizers depends on the crops, soils, native competition, environmental condition, quality and population of the inoculum, soil pH, EC and organic matter content, moisture level, humidity and temperature etc. So, it is very difficult to generalise the thing.

Dear Kheloufi, please find enclosed an excellent review for further reading ...

Soil-Plant-Microbe Interactions in Stressed Agriculture Management: A Review

ABSTRACT

The expected rise in temperature and decreased precipitation owing to climate change and unabated anthropogenic activities add complexity and uncertainty to agro-industry. The impact of soil nutrient imbalance, mismanaged use of chemicals, high temperature, flood or drought, soil salinity, and heavy metal pollutions, with regard to food security, is increasingly being explored worldwide. This review describes the role of soil-plant-microbe interactions along with organic manure in solving stressed agriculture problems. Beneficial microbes associated with plants are known to stimulate plant growth and enhance plant resistance to biotic (diseases) and abiotic (salinity, drought, pollutions, etc.) stresses. The plant growth-promoting rhizobacteria (PGPR) and mycorrhizae, a key component of soil microbiota, could play vital roles in the maintenance of plant fitness and soil health under stressed environments. The application of organic manure as a soil conditioner to stressed soils along with suitable microbial strains could further enhance the plant-microbe associations and increase the crop yield. A combination of plant, stress-tolerant microbe, and organic amendment represents the tripartite association to offer a favourable environment to the proliferation of beneficial rhizosphere microbes that in turn enhance the plant growth performance in disturbed agro-ecosystem. Agriculture land use patterns with the proper exploitation of plant-microbe associations, with compatible beneficial microbial agents, could be one of the most effective strategies in the management of the concerned agriculture lands owing to climate change resilience. However, the association of such microbes with plants for stressed agriculture management still needs to be explored in greater depth.Source ; Pedosphere 27(2): 177–192, 2017

doi:10.1016/S1002-0160(17)60309-6

please follow the link below:

Since ancient times, "guano" has been used as a biofertilizer in the Inca lands in South America.You can read more about this here:

https://en.wikipedia.org/wiki/Guano

Today, bat guano even if it can be expensive, its long-lasting positive effects deliver a healthy investment.

A prized source of N-P-K, guanos — the excrement from birds and bats — are among nature's best biofertilizers! A typical fertilizer analysis shows 10-16% nitrogen, 8-12% phosphorus and 2-3% potassium. All completely natural! Guano high dicalcic phosphate is fundamentally different from both synthetic superphosphate and rock inorganic phosphate and provides an excellent mix of fast available and slow release phosphate. Particularly high in nitrogen and phosphorus, guano biofertilizers are excellent for use around fruiting and flowering plants. In agriculture and gardening guano has a number of uses, including as: soil builder, lawn treatments, fungicide (when fed to plants through the leaves), nematicide, and as composting activator (nutrients and microbes speed up decomposition).

Depending on the source, guano also often contains usable levels of micronutrients due to the wide variety of diets of the birds and other animals that contribute to guano formation. Some types of guano come from carnivores that eat insects and small fish, and some types come from herbivores that eat algae and plankton.

Unlike other natural and synthetic sources of nutrients, guano does not contain high levels of salt, which can potentially burn plants if over-applied.

Guano even has natural pest-control properties, meaning it can be used as a fungicide when fed to plants through a foliar application, and as a nematicide, as its decomposing microbes help control dangerous nematodes. This same microbial activity makes guano an excellent composting activator as well as excellent bioremediation capabilities. This means bat guano can aid in cleansing toxic soils. The microbes can also increase water-holding capacity and air space by loosening the soil.

Guano has phosphorus in the right form to suit phosphate sensitive mycorrhizal fungi. These are fungi that grow in association with plant roots making phosphorus and other mineral uptake more efficient. Plants with good mycorrhizal fungal levels are characteristically more resilient to dry conditions and root diseases.

https://en.wikipedia.org/wiki/Guano

There is a greater  need to improve the efficacy of biofertilizers suiting crop , soil and agroclimate ..

As Dr.Tarafdar answer the nature and the effectiveness of bio bio fertilizers application depend on many factors include( beside what Tarafdar mentioned  )the source of applied bio fertilizers if it was native or imported from another environment ,the population of micro organisms in rhizosphere ,crop age ,and the growth cycle of the crop(Annual,Biennial,or perennial).

Thank you for your contributions 

Thank you all of you

@ Abdenour Kheloufi

I think among the three types of bio-fertilizers (i.e. nitrogenous, phosphatic and cellulolytic), nitrogenous bio-fertilizers have been used so far with great success.

Thanks!

Nitro-fertilizers

I think we need the three types of biofertilizer and operating conditions for an agricultre with high performance.

Bio-fertilizer has been identified as an alternative for increasing soil fertility and crop production in sustainable farming. The exploitation of beneficial microbes as bio-fertilizers has become of paramount importance in agricultural sector due to their potential role in food safety and sustainable crop production.

Thank you all of you for your consideration and answers

very interesting question

Biotechnology of Biofertilizers: this book is very important for your question