Classification of Biofertilizers

Biofertilizers contain microorganisms that are necessary for plant growth. The microorganisms help in absorbing nutrients from the soil. A plant or crop requirement is identified, and a suitable fertiliser is employed. It is necessary to classify biofertilizers as every individual species of crop requires certain amounts of specific nutrients. The appropriate bio fertilizer will help in improving the yield and quality of the crop. Classification is done on the basis of the type of microorganism present or the basis of the function of the biofertiliser.

Classification on the Basis of Microorganisms

The classification on the basis of microorganisms is done by identifying which type of microorganisms, such as algae, fungi or bacteria, is present in the biofertiliser. For example, a bacterial bio fertilizer contains Rhizobium, and a fungal biofertilizer contains Mycorrhiza. The different types of biofertilizers with examples are explained below.

Bacterial biofertilizer 

This classification of biofertilizers contains bacteria. They are further classified on the basis of the function of bacteria:

• Nitrogen Fixer

 As the name suggests, this classification of biofertilizers contains bacteria that can fix nitrogen. The bacteria associate themselves with the nodules present in the roots of the leguminous plants and perform nitrogen fixation in the soil. On the other hand, free-living bacteria exist as free microorganisms and fix the nitrogen of the atmosphere. These types of biofertilizers with examples are given below:

• Symbiotic

Bradyrhizobium, mesorhizobium, azorhizobium, sinorhizobium, allorhizobium, rhizobium, etc.

• Associative

Herbaspirillum, azospirillum, etc.

• Non-symbiotic

Azotobacter, derxia, rhodospirillum, rhodopseudomonas, chromatium, beijerinckia, acetobacter, etc.

• Phosphate Solubilized 

This classification of biofertilizers contains bacteria that fix and help absorb another important nutrient, phosphorus. The fixed phosphorus present in the soil is converted into a soluble form by organic acids and enzymes. Generally, this classification of biofertilizers is called phosphorus solubilising microorganisms (PSM).

• Non symbiotic

Pseudomonas striata, bacillus pseudomonas, bacillus circulans, etc.

Fungal Biofertilizer

This classification of biofertilizers contains fungi. They are further classified on the basis of the function of the fungi:

• Phosphate solubilizer

This classification of biofertilizers contains fungi. The rest of the mechanism is the same as the phosphate solubilizer of bacterial biofertilizer.

• Non-symbiotic 

Species of penicillium, species of aspergillus, species of Trichoderma, etc.

• Nutrient Mobilised 

This classification of biofertilizers helps transfer nutrients such as phosphorus from the soil to cortical cells of the roots of a crop. Therefore, they act as a carrier of nutrients and must not be confused with solubilizers. The fungi present in the bio fertilizer help absorb nutrients into the root system. An example of this type of biofertiliser is given below:

• Symbiotic 

Vesicular Arbuscular Mycorrhiza or VAM.

Algal Biofertilizer 

This classification of biofertilizers contains algae. They are further classified on the basis of the function of the algae:

• Nitrogen fixer

As the name suggests, this classification of biofertilizers contains algae that can fix nitrogen. The algal biofertilizers have the same mechanism of action as that of bacterial biofertilizers.

• Examples of these types of biofertilizers are given below:
• Symbiotic: Blue-green algae or cyanobacteria
• Non-symbiotic: Azolla

Classification on the Basis of Function

Fertilisers are classified based on the function of micro-organisms and their effect on plant growth. For example, a bio fertilizer that promotes the growth of a plant is classified under plant growth-promoting rhizobacteria or PGPR. The different types of biofertilizers with examples are explained below.

Nitrogen-fixing (NBF)

As discussed before, this classification of biofertilizers contains microorganisms that can fix nitrogen. The microorganisms either live in a symbiotic manner or freely in a non-symbiotic way. The microorganisms associate themselves with the nodules present in the roots of the leguminous plants and perform nitrogen fixation in the soil. They are further classified as:

• Nitrogen fixer for legumes: This type of fertiliser is employed for fixing nitrogen in legumes; some examples are discussed below
• Rhizobium: The protein requirements of leguminous plants can only be fulfilled with the help of this bacteria
• Cyanobacteria: This is also known as blue-green algae, which helps in increasing the growth of a plant 
• Nitrogen fixer for non-legumes: This type of biofertilizer is employed for fixing nitrogen in non-legumes; some examples are discussed below:
• Azotobacter: Substances released from this bacteria promote the growth of roots and strengthen them by inhibiting root pathogens.
• Azolla: It excretes organic nitrogen in the water.

Phosphate Mobilising (PMBF)

This classification of biofertilizers is concerned with the transportation and absorption of phosphorus by the roots of the crop. They are further differentiated on the basis of their solubilising or absorbing nature.

• Phosphate solubilizer: 
• This classification of biofertilizers contains bacteria that fix and help absorb phosphorus
• The fixed phosphorus present in the soil is converted into a soluble form by organic acids and enzymes
• Some examples are pseudomonas, nitrobacter, escherichia, bacillus, serratia, and others
• Phosphate Absorbers or Mobilisers: 
• This classification of biofertilizers helps in transferring phosphorus from the soil to cortical cells of the roots of a crop
• They act as a carrier of nutrients rather than a solubilizer and, therefore, must not be confused with solubilizers
• The microorganism present in the bio fertilizer helps in absorbing nutrients into the root system
• Some examples are aspergillus awamori, aspergillus niger, penicillium digitatum, and others

Organic Matter Decomposers (OMD)

This classification of biofertilizers is concerned with the decomposition of organic matter, as the name suggests. They are further classified on the type of organism, such as:

• Cellulolytic organisms: An example of this category is Cellulomonas
• Lignolytic organisms: An example of this category is Arthobacter

Conclusion

Biofertilisers play an important role in agriculture. The rhizosphere of crop soil is affected by biofertilizers and helps in promoting the growth of the crop. Biofertilizers are different from usual fertilisers because they do not contain harmful chemicals that may be toxic for crops and consumption. Biofertilisers work on the principle of natural interconnection between flora and fauna. With their help, humans utilise and improve naturally occurring processes such as nitrogen fixation by microorganisms, mobilising and solubilising nutrients with the help of microorganisms, intaking micronutrients and macronutrients, or stimulating natural growth hormones of plants or phytohormones. Therefore, it is important to understand the classification to identify how many types of biofertilisers are there.