Types of biofertilizers

Biofertilizers add nutrients through the natural processes of fixing atmospheric nitrogen, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. They can be categorised in different ways based on their nature and function.

One simple broadly disseminated classification is as follows:

Nitrogen Biofertilizers

This group fixes nitrogen symbiotically. Nitrogen biofertilizers help to correct the nitrogen levels in the soil. Nitrogen is a limiting factor for plant growth because plants need a certain amount of nitrogen in the soil to thrive. Different biofertilizers have an optimum effect for different soils, so the choice of nitrogen biofertilizer to be used depends on the cultivated crop. Rhizobia are used for legume crops, Azotobacter or Azospirillum for non-legume crops, Acetobacter for sugarcane and blue-green algae and Azolla for lowland rice paddies.

Phosphorus Biofertilizers

Just like nitrogen, phosphorus is also a limiting factor for plant growth. Phosphorus biofertilizers help the soil to reach its optimum level of phosphorus and correct the phosphorus levels in the soil. Unlike nitrogen biofertilizers, the usage of phosphorus biofertilizers is not dependent on the crops cultivated on the soil. Phosphatika is used for all crops with Rhizobium, Azotobacter, Azospirillum and Acetobacter.

Compost Biofertilizers

Biofertilizers are also used for enrichment of your compost and for enhancement of the bacterial processes that break down the compost waste. Suitable biofertilizers for compost use are cellulolytic fungal cultures and Phosphotika and Azotobacter cultures. A 100% pure eco-friendly organic fertilizer is Vermi Compost: this organic fertilizer has nitrogen, phosphorus, potassium, organic carbon, sulphur, hormones, vitamins, enzymes and antibiotics, which helps to improve the quality and quantity of yield. It is observed that, due to continuous misuse of chemical fertilizers, the soil looses its fertility and becomes saline day by day. To overcome such problems, natural farming is the only remedy and Vermi compost is the best solution.

Another eco-friendly organic fertilizer which is prepared from sugar industry waste material that is decomposed and enriched with various plants and human-friendly bacteria and fungi is Biocompost. Biocompost consists of nitrogen, phosphate-solubilizing bacteria and various beneficial fungi like the decomposing fungus Trichoderma viridae, which protects plants from various soil-borne diseases and also helps to increase the soil fertility, resulting in a good quality product for farmers.

A more detailed classification of biofertilizers is as follows:

Just to remind, biofertilizers are defined as biologically active products or microbial inoculants of bacteria, algae and fungi (separately or in combination), which may facilitate the biological nitrogen fixation for the benefit of plants. Biofertilizers also include organic fertilizers (manure, etc.), which are rendered in an available form due to the interaction of microorganisms or due to their association with plants.

Biofertilizers thus include the following:

• symbiotic nitrogen fixers, Rhizobium spp.;
• non-symbiotic, free-living nitrogen fixers (Azotobacter, Azospirillum, etc.);
• algal biofertilizers (blue-green algae or blue-green algae in association with Azolla);
• phosphate-solubilising bacteria;
• mycorrhizae;
• organic fertilizers.

The various biofertilizers are as follows:

• Nitrogen-fixing biofertilizers Nitrogen-fixing bacteria function under two types of conditions, symbiotically and as free-living (non-symbiotic) as well as associative symbiotic bacteria.

Free-Living Nitrogen-Fixing Bacteria:

They live freely in the soil and perform nitrogen fixation. Some of them are saprotrophic, living on organic remains, e.g., Azotobacter, Bacillus polymyxa, Clostridium, Beijerinckia. They are further distinguished into aerobic and anaerobic forms.

The property of nitrogen fixation is also found in photoautotrophic bacteria, e.g., Rhizobium, Rhodopseudomonas, Rhodospirillum, Chromatium. Inoculation of soil with these bacteria helps in increasing the yield and cutting down on nitrogen fertilizers. For example, Azotobacter occurring in fields of cotton, maize, jowar and rice not only increases the yield, but also cuts down on nitrogen fertilizer to about 10–25 kg/ha. Its inoculant is available under the trade name of Azotobactrin.

Rhizobia are soil bacteria which are able to colonize the legume roots and fix the atmospheric nitrogen symbiotically. The morphology and physiology of rhizobia will vary from free-living conditions to the bacteroid of nodules. They are the most efficient biofertilizer as per the quantity of fixed nitrogen. There are seven genera that are highly specific in forming nodules in legumes, referred to as a cross-inoculation group.

Azotobacter is a genus of heterotrophic free-living nitrogen-fixing bacteria present in alkaline and neutral soils. It is aerobic in nature, recommended for non-leguminous crops like paddy, millets, cotton, tomato, cabbage and other monocotyledonous crops. Azotobacter also produces growth-promoting compounds. Azotobacter performs well if the soil organic matter content is high. Response to Azotobacter has been seen in rice, maize, cotton, sugarcane, pearl millet, vegetable and some plantation crops.

Free-Living Nitrogen-Fixing Cyanobacteria:

A number of free-living cyanobacteria, or blue-green algae, have the property of nitrogen fixation, e.g., Anabaena, Nostoc, Aulosira, Totypothrix, Cylindrospermum, Stigonema. Cyanobacteria are photosynthetic microorganisms. Therefore, they add organic matter as well as extra nitrogen to the soil. These chlorophyll-containing prokaryotic organisms fix atmospheric nitrogen.

Aulosira fertilissima is considered to be the most active nitrogen fixer of rice fields. Cylindrospermum licheniforme grows in sugarcane and maize fields. Cyanobacteria are extremely low-cost biofertilisers. Phosphate, molybdenum and potassium are supplied additionally.

Loose Association of Nitrogen-Fixing Bacteria:

This bacterial group live partly within the root and partly outside. There is a fair degree of symbiosis between the host and the bacteria. Hence, they are called associative symbiotic bacteria. Azospirillum is an important bacterium in this group, recommended for millets, grass, wheat, maize, sorghum, rice etc.

Symbiotic Nitrogen-Fixing Bacteria:

They form a mutually beneficial association with the plants. The bacteria obtain food and shelter from plants. In return, they give to the plants part of their fixed nitrogen. The most important group of symbiotic nitrogen-fixing bacteria are rhizobia (Sg. rhizobium). They form nodules on the roots of legume plants. There are about a dozen Rhizobium species which form associations with the roots of different legumes, e.g. R. leguminosarum, R. lupini, R. trifolii, R. meliloti, R. phaseoli.

These bacteria, also called rhizobia, can live freely in the soil but cannot fix nitrogen except for a strain of cowpea Rhizobium. They develop the ability to fix nitrogen only when they are present inside the root nodules. In the nodule cells, bacteria (bacteroids) lie in groups surrounded by the membrane of the host cells, which is lined by a pink-red pigment called leghemoglobin. Presently cultures of Rhizobium specific for different crops are raised in the laboratory.

Frankia, a nitrogen-fixing mycelial bacterium (actinomycete), is associated symbiotically with the root nodules of several non-legume plants like Casuarina, Alnus (Alder) Myrica, Rubus etc. The leaves of a few plants (e.g., Ardisia) develop special internal cavities for pro­viding space to symbiotic nitrogen-fixing bacteria, Xanthomonas and Mycobacterium. Such leaves are a constant source of nitrogen fertilizer to the soil.

Symbiotic Nitrogen-Fixing Cyanobacteria:

Nitrogen-fixing cyanobacteria (blue-green algae) form symbiotic associations with several plants, e.g. cycad roots, liverworts, Azolla (fern), and lichenized fungi. Azolla is an aquatic floating fern, found in temperate climate suitable for paddy cultivation. The fern appears as a green mat over water, which becomes reddish due to excess anthocyanin pigmentation. The blue-green algae, cyanobacteria (Anabaena azollae), present as a symbiont with this fern in the lower cavities actually fixes atmospheric nitrogen.

Azolla pinnata is a small free-floating fresh water fern which multiplies rapidly, doubling every 5–7 days. The fern can coexist with rice plants because it does not interfere with their growth.

Anabaena azollae resides in the leaf cavities of the fern. It fixes nitrogen. A part of the fixed nitrogen is excreted in the cavities and becomes available to the fern. The decaying fern plants release this nitrogen for utilization by the rice plants. When a field is dried up at the time of harvesting, the fern functions as green manure, decomposing and enriching the field for the next crop.

Microphos Biofertilizers:

They release phosphate from bound and insoluble states, e.g., Bacillus polymyxa, Pseudomonas striata, Aspergillus species.

Mycorrhiza (Pl. Mycorrhizae, Frank, 1885):

The mycorrhiza is a mutually beneficial or symbiotic association of a fungus with the root of a higher plant. The most common fungal partners of mycorrhiza are Glomus species. Mycorrhizal roots show a sparse or dense wooly growth of fungal hyphae on their surface. Root cap and root hairs are absent.

Mycorrhiza is a potential biofertilizer which mobilizes P, Fe, Zn, B and other trace elements. It supplies moisture from far-off inches and is ideal for long duration crops. It can be stored up to 2 years and is dry powder resistant.

Depending upon the residence of the fungus, mycorrhizae are of two types— ectomycorrhiza and endomycorrhiza.

Ectomycorrhiza (= Ectotrophic Mycorrhiza):

The fungus forms a mantle on the surface of the root. Internally, it lies in the intercellular spaces of the cortex. The root cells secrete sugars and other food ingredients into the intercellular spaces that feed the fungal hyphae. The exposed fungal hyphae increase the surface of the root to several times. They perform several functions for the plant as follows:

• Absorption of water,
• Solubilisation of organic matter of the soil humus, release of inorganic nutrients, absorption and their transfer to root,
• Direct absorption of minerals from the soil over a large area and handing over the same to the root. Plants with ectomycorrhiza are known to absorb 2–3 times more of nitrogen, phosphorus, potassium and calcium,
• The fungus secretes antimicrobial substances which protect the young roots from attack of pathogens. Ectomycorrhiza occurs in trees such as Eucalyptus, oak (Quercus), peach, pine, etc. The fungus partner is generally specific. It belongs to Basidiomycetes.

Endomycorrhiza (Endotrophic Mycorrhiza):

Fewer fungal hyphae lie on the surface. The remaining live in the cortex of the root, mostly in the intercellular spaces with some hyphal tips passing inside the cortical cells, e.g., grasses, crop plants, orchids and some woody plants. At the seedling stage of orchids, the fungal hyphae also provide nourishment by forming nutrient-rich cells called pelotons. Intracellular growth occurs in order to obtain nourishment because, unlike ectomycorrhiza, the cortical cells do not secrete sugars in the intercellular spaces.

Vesicular Arbuscular Mycorrhizal (VAM) fungi possess special structures known as vesicles and arbusculars. VAM fungi are intercellular, obligate endosymbionts and, on establishment on the root system, act as an extended root system. Besides harvesting moisture from deeper and faraway nitches in the soil, they also harvest various micronutrients and provide them to the host plants. VAM facilitates the phosphorus nutrition by not only increasing its availability, but also increasing its mobility. VAM are obligate symbionts and improve the uptake of Zn, Co, P and H₂O. Its large-scale application is limited to perennial crops and transplanted crops. A single fungus may form a mycorrhizal association with a number of plants, e.g., Glomus.

The different types of biofertilizers are preparations made from natural beneficial microorganisms. They are safe for all plants, animals and human beings. Being beneficial to crops and natural nutrient cycles, they not only are environmentally friendly, but also help in saving of chemical inputs.

Main roles of biofertilizers:

• Make nutrients available.
• Make the root rhizosphere livelier.
• Growth-promoting substances are produced.
• More root proliferation.
• Better germination.
• Improve the quality and quantity of produce.
• Improve the fertilizer use efficiency.
• Higher biotic and abiotic stress tolerance.
• Improve soil health.
• Residual effect.
• Make the system more sustainable.

Liquid Biofertilizers

At present, biofertilizers are supplied to the farmers as carrier-based inoculants. As an alternative, liquid formulation technology has been developed which has more advantages than the carrier inoculants.

Benefits:

The advantages of liquid biofertilizer over conventional carrier-based biofertilizers are listed below:

1. Longer shelf-life – 12–24 months;
2. No contamination;
3. No loss of properties due to storage up to 45º C;
4. Greater potential to fight with native population;
5. Easy identification by typical fermented smell;
6. Better survival on seeds and soil;
7. Very easy to use by the farmer;
8. High commercial revenues;
9. High export potential.

Characteristics of different liquid biofertilizers

Rhizobium

Physical features of liquid Rhizobium biofertilizer:

1. Dull white in colour;
2. No bad smell;
3. No foam formation, pH 6.8–7.5

Azospirillum

Physical features of liquid Azospirillum biofertilizer:

1. The colour of the liquid may be blue or dull white.
2. Bad odour confirms improper liquid formulation and may be considered as mere broth.
3. Production of yellow gummy colour materials confirms the quality product.
4. Acidic pH always confirms that there are no Azospirillum bacteria in the liquid.

Role of liquid Azospirillum under field conditions:

1. Stimulates growth and imparts green colour which is a characteristic of a healthy plant.
2. Aids utilization of potash, phosphorous and other nutrients.
3. Enhances the plumpness and succulence of fruits and increases the protein content.

Azotobacter

Physical features of liquid Azotobacter biofertilizer:

The pigment that is produced by Azotobacter in aged culture is melanin, which is due to oxidation of tyrosine by a copper-containing enzyme, tyrosinase. The colour can be seen in liquid forms. Some of the pigmentations are described below:

1. Produces brown-black pigmentation in liquid inoculum;
2. Produces yellow-light brown pigmentation in liquid inoculum;
3. Produces green fluorescent pigmentation in liquid inoculum;
4. Produces green fluorescent pigmentation in liquid inoculum;
5. Produces, pink pigmentation in liquid inoculum;
6. Produces less, gum-less, greyish-blue pigmentation in liquid inoculum;
7. Produces green-fluorescent pigmentation in liquid inoculum.

   Acetobacter

These are sacharophillic bacteria associated with sugarcane, sweet potato and sweet sorghum plants. Acetobacter fixes 30 kg N/ha/year. This bacterium is mainly commercialized for sugarcane crops. It is known to increase the yield by 10–20 t/acre and sugar content by about 10–15 percent.

Advantages of the production technology of biofertilizers

Carrier-based	Liquid-based
Cheap	Longer shelf-life
Easier to produce	Easier to produce
Less investment	Temperature tolerant
	High cell counts
	Contamination-free
	More effective
	Product can be 100% sterile
Disadvantages
Low shelf-life	High cost
Temperature sensitive	Higher investment for production unit
Contamination prone
Low cell counts
Less effective
Automation difficult