One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Types and Importance of Biofertilizers – The Future of Food Security and Food Safety
The active group of microorganisms belonging to classes like bacteria, fungi, and algae contributing fertility to the soil either individually or in a collective manner promoting the growth of the plants are called biofertilizers.
A biofertilizer (also bio-fertilizer) is a substance that contains living microorganisms that, when applied to seed, plant surfaces, or soil, colonizes the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. Bio-fertilizers add nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances.
Bio-fertilizers can be expected to reduce the use of chemical fertilizers and pesticides. The microorganisms in bio-fertilizers, restore the soil’s natural nutrient cycle and build soil organic matter. Through the use of bio-fertilizers, healthy plants can be grown, while enhancing the sustainability and the health of the soil. Since they play several roles, a preferred scientific term for such beneficial bacteria is “plant-growth promoting rhizobacteria” (PGPR).
Therefore, they are extremely advantageous in enriching soil fertility and fulfilling plant nutrient requirements by sup¬plying the organic nutrients through microorganisms and their byproducts. Hence, bio-fertilizers do not contain any chemicals which are harmful to the living soil. Biofertilizers provide eco-friendly organic agro-input and are more cost-effective than chemical fertilizers. Bio-fertilizers such as Rhizobium, Azotobacter, Azospirillum, and blue-green algae (BGA) have been in use for a long time. Rhizobium inoculant is used for leguminous crops. Azotobacter can be used with crops like wheat, maize, mustard, cotton, potato, and other vegetable crops.
Azospirillum inoculations are recommended mainly for sorghum, millet, maize, sugarcane, and wheat. Blue-green algae belonging to a general cyanobacteria genus, Nostoc or Anabaena Tolypothrix or Aulosira, fix atmospheric nitrogen and are used as inoculations for paddy crops grown both under upland and low-land conditions. Anabaena in association with water fern Azolla contributes nitrogen up to 60 kg/ha/season and also enriches soil with organic matter. The use of chemical fertilizers increases the fertility of soil considerably, but there are many disadvantages of using these chemical substances on the environment and organisms besides being expensive. Therefore, the use of fertilizers of biological origin is an alternative for overcoming the harmful effects of these chemical fertilizers. These fertilizers of biological origin are called biofertilizers. Biofertilizers are some microorganisms that use the fertility of the soil without any negative effect on the environment.
The components derived from coal, petrol, or natural gases that give fertility to soil are known as chemical fertilizers. Though the use of chemical fertilizers may increase the fertility of soil, there are some disadvantages to using these fertilizers:
- The production of chemical fertilizer is expensive.
- The production of chemical fertilizer may lead to the exhaustion of our natural resources as natural gases and minerals are used for their production.
- Several polluting agents are released during the production of chemical fertilizers.
- The washed-out fertilizers may pollute the waterbodies nearby.
- Chemical fertilizers being absorbed into the soil may pollute the underground reserve of water that may also become salty.
Fertilizers of Biological Origin
Organic matter or living organisms when used in improving the fertility of soil, are known as fertilizers of biological origin. They are broadly classified into two types: Manure and Biofertilizer.
The fertilizers formed from the wastes of animals and birds, decaying organic matter, dried leaves, etc., through their decomposition, are known as manures. This type of manure contains all the necessary minerals and nutrients for the growth of plants. With the use of this manure not only the fertility of soil is increased, but also the water and air-holding capacity of the soil is increased. Manure may be of three types, namely Farm-yard manure, Compost manure, and Green manure.
(a) Farm-yard Manure
This type of manure is produced by the decomposed mixture of dry leaves, tree branches, left-out remains of the farmyard, and faecal materials of the animals in the farmyard. Initially, all the matters are kept in aggregate at a certain region in the farmyard and they are allowed to decompose until they are converted into darkish granules. The manure contains decomposed cellulose and lignin as well as humus. This also contains all the growth-promoting minerals for plants. This type of fertilizer may increase the water and air-holding capacity of soil. Besides because of its high nutrient contents this gives better results in the crop yield. The refusal from the Gobar gas plant is an example of farmyard manure.
(b) Compost Manure
This is formed of decomposed vegetables, animal refusals, and chemical fertilizers. Initially, the decomposed organic matter is collected at a place, and then it thin layer of chemical fertilizer is added from time to time with further addition of the vegetable parts and animal refusals. To prepare compost manure it requires 3-4 months. Compost manure is good for increasing soil fertility.
(c) Green Manure
This type of manure is formed by mixing some leguminous plants in the soil after their cultivation in the field. The young plants get decomposed in the soil and provide nutrition to the crop plant. The special advantage in this is that the nodule grown in the roots harbour the nitrogen-fixing bacteria and thereby mixing these plants with soil gives special addition of nitrogen to the soil. A list of commonly cultivated leguminous plants is given in the following table.
Leguminous Plants Producing Green Manure:
|Leguminous Plant||Scientific Name|
|1. Sun hemp||Crotalaria juncea|
|2. Dhaincha||Sesbania aculeate|
|3. Cluster bean||Cyamopsis tetragonoloba|
|4. Senji||Melilotus parviflora|
|5. Berseem||Trifolium atexandrinum|
|6. Lentil (Masur)||Lens esculenta|
|7. Horse gram||Macrotyloma uniflorum|
|8. Cowpea||Vigna sinensis|
Green manure is more productive than farmyard manure and yield may be increased by about 30-40%. It also reduces soil erosion.
The bacteria and fungus or microorganisms which provide nutrition to the plants are regarded as biofertilizers. For the active growth of plants, the microorganisms individually or collectively may help in providing nutrition and therefore, they are designated as fertilizers. Therefore, biofertilizers are a combination of microorganisms and they may be applied in the held as fertilizer.
Classification of Biofertilizer
Biofertilizers may be classified into three types: Bacterial biofertilizer, Cyanobacterial biofertilizer, and Fungal biofertilizer.
1. Bacterial Biofertilizer:
There are many bacteria that may increase the fertility of the soil and promote rapid growth of the plants. Some of these bacteria reside in the soil as free-living forms and fix nitrogen there, while others form nodules before fixing nitrogen for plants. These types of bacteria increase the fertility of the soil.
(a) Nitrogen Fixing Bacteria as Free Living Organisms:
The bacterial forms such as Azotobacter, Bacillus polymyxa, Beijerinckia, Clostridium, etc., live in the soil as detritus organisms. Azotobacter, Azomonas, and Derxia are found in temperate climates and live in neutral or alkaline water and soil. However, Beijerinckia lives in the soil of the tropical climate. All of these bacteria are free-living nitrogen-fixing forms and promote the enhancement of soil fertility. An increase in the yield of cotton, maize, rice, jowar, etc., could be possible with the introduction of Azotobacter in soil and it has been found that with the use of this biofertilizer 10-25 kg of chemical fertilizer/hectare may be saved. Besides Rhodopseudomonas, Rhodospirillum and chromatium are free-living autotrophs that help in nitrogen fixation in the soil. The anaerobic sulphate reducers like Desulfovibrio and Desulfotomaculum at pH 7 – 8 may fix nitrogen in soil and clay. Some methylotrophic bacteria in the presence of methane or methanol may fix nitrogen in the soil.
(b) Nitrogen Fixing Bacteria in Loose Association with Plant:
In some higher plants, several bacterial forms exhibit an association with roots. This is known as the Rhizospheric association. Bacteria in such association may fix nitrogen for plants and in return, they get carbohydrates from plants. This sort of co-existence is called associative mutualism. As an example, Azospirillum lipoferum remains in rhizospheric association in maize and grasses and fix nitrogen for them.
(c) Nitrogen Fixing Symbiotic Bacteria:
There are some bacteria that live symbiotically in the plant roots by producing nodules. But they give nitrogen to the plant and in return they collect carbohydrates from the plant. The bacteria belonging to the genus, Rhizobium is of this category. Besides Rhizobium, the bacteria belonging to the genus Bradyrhizobium reside in the root of leguminous plants fixing nitrogen for them. There are about a dozen species of Rhizobium, which fix nitrogen in the roots of leguminous plants.
Several representative species is Rhizobium leguminosorum (lives in the root nodule of pea plants), Rhizobium phaseoli (lives in the root nodule of bean plants), Bradyrhizobium japonicum (lives in the root nodule of soybean plants), Rhizobium meliloti (lives in the root nodules of Melilotus), Rhizobium trifolii (lives in the root nodules of Trifolium), etc.
During the sowing of seeds of the leguminous plants in the fields, Rhizobium is also inoculated in the soil. As a result of this as the seedlings come out the Rhizobium takes shelter in their roots. At this time if phosphorus is mixed with the soil, nitrogen fixation becomes accelerated. When the plants grow up a little, a substantial amount of nitrogen may be fixed in the soil. At this time the leguminous plants are excised from the base of the trunk and thereby the roots remain under the soil. With the application of water in the field the roots are allowed to decompose following which crop plants are developed in the field. With this treatment, there is no need to apply nitrogen fertilizer in the field. Symbiosis with nonleguminous plants has 3 types:
- Root symbiosis: There is one type of bacteria Frankia that resides in the roots of plants like Casuarina, Alnus, Myrica, Rubus, etc., as a symbiont and fixes nitrogen.
- Leaf symbiosis: The bacteria named Xanthomonas and Mycobacterium which live in the leaves of some plants, fix nitrogen. The leaves of such plants may be used in the field as a supplement to nitrogen fertilizer. As an example, such nitrogen fixation occurs in the leaves of Ardisia sp.
- Stem symbiosis: Symbiotic bacteria Azorhizobium caulinodans live in the stem nodules of Sesbania.
(d) Phosphorus Solubilizing Microbes:
Phosphorus is an essential element for plants. There are some microorganisms that can solubilize the rock phosphate, the cheaper sources of phosphorus.
Bacteria like Pseudomonas striata and Bacillus megaterium are important phosphate-solubilizing soil microorganisms. Many fungi like Aspergillus and Penicillium are potential solubilizers of bound phosphates. They solubilize the bound phosphorus and make it available to the plants resulting in improved growth and yield of crops. Soil microorganisms secrete organic acid and make soil phosphate available to plants. They may also release soluble inorganic phosphate into the soil through the decomposition of phosphate-rich organic compounds. These microbial inoculants can substitute almost 20-25% of the phosphorus requirement of plants.
2. Cyanobacterial Biofertilizer:
The cyanobacteria are the chlorophyll-containing unicellular autotrophic prokaryotes. They are commonly known as blue-green algae. The cyanobacteria may fix nitrogen either in a free-living manner or in a symbiotic state. For this reason, they are also used as biofertilizers.
(a) Cyanobacteria as Free Living Biofertilizer:
There are several free-living cyanobacteria, such as Anabaena, Nostoc, Aulosira, Tolypothrix, Cylindrospermum, Stigonema, etc., which can fix nitrogen. Aulosira fertilissima is known to be an active nitrogen fixer in the paddy field. Cylindrospermum licheniforme grows in the field of maize and sugarcane fixing nitrogen in the soil. They can release 32-40% of their fixed nitrogen directly in the field. For this reason, if cyanobacteria grow in the crop field, soil fertility is increased. Besides if the cyanobacterial body gets mixed with the soil after death, the soil fertility is also increased. It has been observed with the growth of cyanobacteria in the field during cultivation, about 7 to 8 lakh tonnes may be obtained and this amount of nitrogen is equivalent to about 15 to 17 lakh tonnes urea. Therefore in lieu of nitrogen fertilizer, if cyanobacteria are grown in the field it may meet the need for nitrogen fertilizer in the field.
(b) Nitrogen Fixing Cyanobacteria as Symbionts:
In some plants, cyanobacteria exhibit symbiotic association and promote nitrogen fixation.
Azolla, Anabaena association is such an association in the case of agricultural cultivation. Azolla pinnata is one type of freshwater fern. They grow in the paddy field but do not interfere with the growth of paddy plants. The fern grows so rapidly that their population may be increased by two times within five to seven days. Anabaena, the cyanobacteria grow on their leaves and symbiotically fix nitrogen. On the decomposition of these ferns in the fields, their fertility may be increased substantially.
Examples of Nitrogen Fixing Microbes
|1. Cyanobacteria blue-green algae||Anabaena, Nostoc.|
B. Symbionts and their Host Plants:
|Host Plant||Rhizobium like bacteria|
|1. Soyabean||Sinorhizobium fredii|
|2. Medicago sativa||Sinorhizobium meliloti|
|3. Pisum sativum||Rhizobium leguminosarum bv. Viciae|
|4. Parasponia (non-legume)||Bradyrhizobium|
|5. Aeschenomene||Photorhizobium sp.|
3. Fungal biofertilizer:
There are some fungi that live in close associations with the roots of some plants and live symbiotically. Such symbiotic association of fungi and plant roots is known as Mycorrhiza. The fungi in Mycorrhiza get nutrition and shelter on the plants, they may help the plants in obtaining nutrition from the soil.
Types of Mycorrhizal Association
Two types of Mycorrhizal association may be observed, namely Ectomycorrhiza and Endomycorrhiza.
In this case, the fungal mycelium remains outside the root and covers it forming a mantle. From the mantle, the fungal hyphae extend both into the cortex of the root and soil. As a result of this, the absorption surface of the plant is increased allowing absorption of more water and minerals. Besides these, the fungal hyphae may promote the solubilization of insoluble organic matter in the soil and thereby these matters may be acceptable to the plants. On the whole, the soil becomes more fertile. The symbiotic association is noticed between roots of plants like Pinus, Eucalyptus, Abies, etc., and fungi like Boletus edulis, Amanita muscaria, etc.
In this case, the fungi do not form any mantle on the root surface, rather they remain distributed like a loose network. As the mycelia of the fungi enter the root cortex, so also in the soil. However, the fungal hyphae along with their growth in the inter-cellular space may also invade the inner-cellular region forming a vesicular structure called Arbuscule. This type of close association is called VAM or Vesicular Arbuscular Mycorrhizae. VAM is significantly related to phosphate uptake by plants. Endomycorrhiza is available in Orchids and woody plants.
It is a well-documented fact that in all types of mycorrhizal association, plants may get more nutrients from soil, and due to it dependence on chemical fertilizers for growth of the plants is significantly reduced. In this consideration, fungi may also be taken as an alternative to chemical fertilizer.
Mycorrhiza forming Fungi:
|1. Ectomycorrhiza||Amanita, Boletus, Phallus, Tricholoma|
|2. Endomycorrhiza||Glomus, Aspergillus, Acaulospora|
|3. Ecto-endomycorrhiza||Clavaria, Endogone, Arbutus|
Importance of Mycorrhiza in Agriculture and Forestry:
- Mycorrhiza helps in the elongation of roots. Mycorrhiza forms a hyphal organization around the root for the purpose of water absorption.
- Mycorrhizal hypha firmly attaches to the soil and resists soil erosion.
- In the case of the legume plant, mycorrhiza forms a nodule with the help of Rhizobium.
- In the case of Pinus, root mycorrhiza secretes lignolytic and cellulosic enzymes which degrade the dead plant organs and form humus.
- If the mycorrhiza is not formed in the orchid root then its normal growth is stopped.
Difference between Endomycorrhiza and Ectomycorrhiza:
|1. The fungal mycelium remains outside the root and covers it.||1. They are distributed like loose networks in the cortex.|
|2. The mycelium forms a mantle on the surface of the root.||2. The mycelium does not form any mantle.|
|3. It is mainly seen in the roots of eucalyptus, pine, oak, etc.||3. It is found in many species of orchids.|
|4. Ectomycorrhiza consists of a hyphal sheath, covering the root tip and a hartig net of hyphae surrounding the plant cells within the root cortex.||4. An endomycorrhiza arbuscular mycorrhiza is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plant.|
Differences between Manure and Biofertilizer:
|1. It is partially decomposed organic wastes or remnants increasing soil fertility.||1. It represents biological species that actively provide nutrition to plants by collecting minerals or other necessary growth-promoting components.|
|2. This may increase the water and air-holding capacity of the soil.||2. It may or may not contribute to the soil architecture.|
|3. Plants may get different types of nutrients from manure. But to obtain a satisfactory yield it also needs a chemical fertilizer supplement.||3. Biofertilizer may give one or few nutrients to the plants and it may reduce the need for chemical fertilizer.|
Utility of Biofertilizers
- It is less expensive, the technique is easy, and small and marginal farmers can use it.
- The technique does not result in pollution, fertility is sustained for future agricultural growth.
- Enhancement of yield in rice ranges from 10-45% when algal biofertilizers are applied. Residual nitrogen is left in the soil for the subsequent crops.
- Application of rhizobial biofertilizer results in fixation of about 50-150 kg N/ha/year.
- Cyanobacteria secrete growth-promoting substances like IAA, IBA, NAA, amino acids, proteins, vitamins, etc.
- Azotobacter and Azospirillum, fix nitrogen to soil as well as secrete antibiotics which act as pesticides.
- Microorganisms increase the physico-chemical properties of soils such as soil structure, texture, water holding capacity, cation exchange capacity, and pH by providing several nutrients and sufficient organic matter.
- The mycorrhizal biofertilizers make the host plant available with certain elements, increase longevity and surface area of roots, reduce plant response to soil stresses, and increase resistance in plants.
- A biofertilizer is technically living, it can symbiotically associate with plant roots. Involved microorganisms could readily and safely convert complex organic material into simple compounds that plants can easily take up.
- Microorganisms function for a long duration, causing improvement of soil fertility. It maintains the natural habitat of the soil. It increases crop yield by 20-30%, replaces chemical nitrogen and phosphorus by 25%, and stimulates plant growth. It can also provide protection against drought and some soil-borne diseases.
- Biofertilizers are cost-effective relative to chemical fertilizers. They have lower manufacturing costs, especially regarding nitrogen and phosphorus use.
Some microbes and the crops that received benefits are mentioned below:
|Main Microbes||Name of the crops that receive the benefit|
|1. Rhizobium sp. root nodule symbiotic.||All types of legume crops; some oil-producing legumes (soybean, nut); some fooder type legumes (clover), etc.|
|2. Nostoc, Anabaena, and other blue-green algae (e.g., Aulosira, Totypothrix).||Paddy.|
|3. Anabaena-Azolla. In Azolla, aquatic forms live as symbionts.||Paddy.|
|4. Azotobacterchroococcum free-living bacteria.||Paddy, Maize, Cotton, etc.|
|5. Frankia sp. of Actinomycetes group. Live as a symbiont in the root nodule of the non-leguminous plant.||Casuarina, Alnus etc.|
|6. Azospirillum lipoferum associated symbiont.||Maize, Bajra, Jowar, Ragi, etc.|
|7. Bacillus polymyxa, Clostridium sp., Rhodospiriilum free-living nitrogen-fixing bacteria.||Pests of higher plants.|
|8. Mycorrhiza.||Oaks, Eucalyptus etc.|