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Biofertilizers
(1) Green Manures:
Green manures are prepared by cultivating quick growing crops and then
ploughing them back under the soil.
Green manuring insitu – when green manure crops grown in the field
itself pure or intercrop and buried in the same field e.g. Cowpea.
Types
Green leaf manuring – it refers to turning into the soil green leaves and
tender green twigs collected from shrubs and trees grown on wasteland
and nearby forest area.
Other manures:
Farm yard: It consists of mixture of cattle dung and crop residues like remnants
of straw and plant stalks fed to cattle.
Composted manure: It consists of rotted vegetables and animal refuse.
(2) Reasons for preference of biofertilizers over chemical fertilizers:
Biofertilizers contains living micro-organism. On
- Usage, the microbes interfer with plants and promote growth and supply
primary nutrients to the host plant.
- Give soil enrichment.
- Helps in nitrogen fixation for the benefit of crops.
- Accelerate the metabolic activity of the cell.
Since chemical fertilizers are made up of only few minerals, they impede the uptake
of other mineral and imbalance the whole minerals pattern of the plant body.
(3) Biofertilizers: Biofertilizers are living organisms that mobilize the availability of
nutrients by their biological activity.
- Helps mainly to fix nitrogen rather to depend on chemicals.
- Main source are bacteria, cyanobacteria and fungi.
Symbiotic
Bacterial biofertilizers
Non-symbiotic (free living)
Rhizobium occurs in the roots of legumes like pea, soyabean and groundnut.
Prominent characteristics is by swelling of roots nodules. They thrive into the cell
of module and fix nitrogen and gets nutrients from it.
(4) Importance of Leghaemoglobin pigment:
The root modules contains a pigment called leghaemoglobin. It gives a pinkish
colour as it is closely related to the pigment haemoglobin present in RBCs. These
combines with O2, leghaemoglobin protects the enzymes nitrogenase which
functions only under anaerobic condition.
Nitrogenase: Only enzymes that can split nitrogen molecule for nitrogen fixation.
Enzyme is molybdenum – iron protein and catalyses the conversion of atmospheric
N2 to NH3.
Note: Microbes are aerobes under free living during N fixation becomes anaerobic
(protecting the nitrogenase enzyme)
(5) Nodule formation:
(i)
Root hair of a leguminous plant comes in contact with Rhizobium bacteria,
that divide near it and upon successful infection of root hair, cause it to curl.
(ii)
Curling leads to deformation of root hair.
(iii) Bacteria develops enlarges to rod shaped.
(iv)
Plant responds to infection and formation threads inside the cell (plasma
membrane) by apparent bacteriod.
(v)
Bacteriods penetrates cortical and pericycle cell to divide.
(vi)
Cylokinis (hormone) (from bacteria) + Auxins (hormone) (from plant cell) –
leads to nodule formation.
(6) Non-Symbiotic: Azotobacter and Bacillus Polymexa can fix atmospheric nitrogen
in the soil irrespective of type of crop. E.g. Vegetable, fruits or cereals or millets.
Some bacteria like Azospirillum show loose association with certain crops. These
are isolated from grasses sorghum and maize. Simple seed inoculated with this
bacteria increases the dry weight of several cereals. When added along with the
phosphate fertilizer yields more.
(7) Role of Cyanobacteria as Biofertilizers:
Azolla, Nostoc, Anataena are cyanobacteria contain chlorophyll known as
photosynthetic nitrogen fixers. They may be symbiotic or free living. The symbiotic
cyanobacteria lives in small floating ferns. It fixes nitrogen and excretes the
nitrogenous compounds into the leaf cavity.
Azolla frequently grown along with the rice crop as it is fast growing and spreads
quickly. As it decays during summers, it enriches the soil with nitrogen. Nostoc
also the same.
(8) Biological Nitrogen Fixation:
There are very few organisms that can utilize the N in the form of the N2 which is
available abundantly in air. Only bacteria, cyanobacteria are capable for fixing
nitrogen in the form of ammonia that can be taken up by the plants. This fixation
of nitrogen into ammonia by living organisms is called biological nitrogen fixation.
(9) Nitrogen Cycle:
Ammonic
Plant & Animal
Protiers
Nitrrobacter
(Nitrifying Bacteria)
NITRIFICATION
Atmospheric N (78%)
(lightning non-enzymatic reaction)
Soil nitrates
NITRATE ASSIMILATION
Stages of Nitrogen cycle:
(1) Nitrification
(2) Nitrate assimilation
(3) Ammonification
(4) Dentrification
(5) Nitrogen fixation