For e.g.  1) Spp. Of Rhizobium

            2) Azo-rhizobium

            3) Brady-rhizobium live in symbiosis with roots & this association leads to the formation of root nodules present in leguminous plant. Healthy nodules appear pink or reddish in colour due to the presence of leghaemoglobin pigment. The bacteria enter through root hairs and proliferates inner cortical cells. These proliferations which become nodules are richly supplied by the branches of vascular bundles derived from vascular cylinder of the roots. This association is useful to the plants as bacteria help in atmospheric nitrogen fixation. The plants provide carbohydrates to the bacteria. The electrons obtained by the oxidation of carbohydrates are used by the bacteria to reduce nitrogen to ammonium ions. Ammonia formed in the process of biological nitrogen fixation is subsequently used for the synthesis of amino acids. These amino acids are transported through phloem to other parts of the plant where they are needed. Ultimately proteins are synthesized from amine acids. The bacteria not only supply nitrogen to the plants but also leave a good amount of nitrogen in the soil which benefits the subsequent crop.

2) Azolla-Anabaena symbiosis

• Azolla is minute floating water fern which harbors a blue-green alga, Anabaena azollae, in its leaf cavities.
• The alga fixes atmospheric nitrogen & releases nitrogenous compounds in leaf cavities.
• Identification of azolla & organic manure in rice cultivation was reported first of all in North Vietnam in 1957. It increases crop yield by over 50%.

3) Loose association of nitrogen fixing bacteria

• Few bacteria, for e.g. Azospirillum lipoferum shows loose association with roots & above ground parts of cereal crops like-sorghum, maize, rice, barley & wheat etc which increases fertility of soil.
• It was reported by Brazilian scientist J. Dobrenier in 1957.

4) Diazotrophs

• Thiobacillus, Desulphovibrio, members of Rhodospirillaceae, Chromatiaceae, Chlorobiaceae, Azotobacteriaceae, Bacillaceae, Coigneybacteriaceae, Spirillaceae.
• They have free living or associative forms, fix nitrogen in the rhizosphere of a variety of crop species.
• They also grow endophytically in stem & leaves of sugarcane & rice. Azoarcus inhabits in roots of Leptochloa fusea (Kaller grass) & rice.
• They fix atmospheric nitrogen & make it available to crop plants.

5) Cyanobacteria

• Several cyanobacteria e.g. (Anabaena, Nostoc, Aulosira, Plectonema, Scytonema, Calothrix, Mastigocladus, Lyngbia, Tolypothrix, have the capacity to fix atmospheric nitrogen usually in rice field. Besides being a source of nitrogen, cyanobacteria provide the following added advantages.

1. Algal biomass accumulates as organic matter
2. Growth promoting substances are produced which stimulate growth of rice seedlings.
3. Help in reclamation of saline & alkaline soil.

6) Mycorrhiza

• A symbiotic relationship between fungal hyphae & roots of higher plants is called as mycorrhiza (The term coined by Frank in 1885).
• The fungal partner of mycorrhiza obtains food from roots of higher plants & in return, it supplies mineral elements to that plant.
• According to the relationship of the fungal organism to the host, 2 types of mycorrhiza have been reported.

1. Ectomycorrhiza
2. Endomycorrhiza

1. Ectomycorrhiza

• g. Balatus, Lactarius, Amanita etc. produce a dense sheath external to the root called mantle. Few hyphae may traverse in the intercellular spaces.
• Ectomycorrhiza are found on the roots of pine, peach &Eucalyptus.
• Besides absorbing N, P, K, Ca elements, they convert complex organic molecules into simpler available from, also protects root from pathogens & produce growth hormones, cytokinin.

1. Endomycorrhiza

• g. Phoma, Rhizoctonia, Armillaria etc. fungus live within a cells of root.
• Roots with endomycorrhiza are short branched & swollen having poor root hairs.
• They convert the decaying organic materials & minerals of the soil into simple forms accessible to the host intern make available sugar, amino acid to the fungus.
• They are reported in many orchids. Coffee, cardamom & betel vine.

Some soil fungi e.g. Entrophospora, Gigaspora Glomus, Sclerocystis, Scutellospora  & Acaulospora penetrate roots & produce vesicles & arbuscules within the cortex. These fungi are called as VAM.

V- Vesicular, A-Arbuscular, M-Mycorrhiza

The vesicles help in the storage of nutrients while arbuscules help in transference of nutrients to the plants. Glomus is the most abundant in the soil. VAM are extremely useful in agriculture & forestry.

7) Phosphate solubilizing bacteria & fungi

• Phosphorus is the most important nutrient which is required by the plants. Most of the soil phosphorus is in unavailable form.
• Many heterotrophic & autotrophic soil microbes have the capacity to solubilize in organic phosphates through their metabolic activities directly or indirectly.
• Phosphate solubilizing bacteria are Acrobacter, Nitrobacter, Eischeridia, Brevibacterium, Bacillus, Pseudomonas & Micrococcus & fungig. Aspergillus awamori, Fuscurium & Selerotium are new bio fertilizers.
• They convert non-available inorganic phosphate into soluble phosphate which can be used by crop pants.

8) Frankia

• Some higher plants like Alnus & Elder produce nodules with Frankia and are known to recuperate/recover soil nitrogen in temperate forest ecosystems.
• Nodules are also found on stems of aquatic plants e.g. Aschynomene & Sesbania rostrata which serve as a green manure in rice cultivation.
• Nitrogen fixing nodules are also found on roots & aerial portion of Casuarina spp.
• Under field conditions, nodules of Alnus & Casuarina occur in clusters attaining diameter of 5-6 cm somewhat resembling a tennis-ball, often weighing of 444 kg dry wt. to nodule per hectare.