|
Liquid Biofertilizers: Advantages Over Carrier
Based Biofertilizers
for Sustainable Crop Production
By: M. Verma*, S. Sharma*, R. Prasad*
As
the disadvantages of the use of chemicals in agriculture have been
noticed in recent years, a search for alternatives for chemical
fertilizers and pesticides has started throughout the world. The use
of chemical fertilizers as well as pesticides is much more in
developing countries as compared to developed countries. Organic
farming is the only alternative to get rid of chemical fertilizers
and synthetic pesticides. There is increasing awareness about organic
agriculture practices in the world. Organic agriculture is the
growing food without pesticides and fertilizers. Organic production
is a system of farming that restores, maintains and enhances
ecological balance.
What are the
alternatives for chemical farming?
Biofertilizers have
been identified as alternative to chemical fertilizers to increase
soil fertility and crop production in sustainable farming. There are
abundant microorganisms thriving in soil, especially in the
rhizosphere of plants. It is well known that a considerable number of
bacterial and fungal species possess a functional relationship and
constitute a holistic system with plants. They are able to exert
beneficial effects on plant growth. Application of beneficial
microbes in agricultural practices started about 50 years ago and
there is now anincreasing evidence that these beneficial microbial
populations can also enhance plant resistance to adverse
environmental stresses, e.g. water and nutrient deficiency and heavy
metal contamination.
What are
biofertilizers?
Bio-fertilizers are
natural fertilizers which are microbial inoculants of bacteria,
algae, fungi alone, or in combination, and they augment the
availability of nutrients to the plants. The use of bio-fertilizers,
in preference to chemical fertilizers, offers economic and ecological
benefits by way of soil health and fertility to farmers. These are
products containing living cells of different types of
microorganisms, which have the ability to convert nutritionally
important elements from unavailable to available form through
biological processes. They are involved in symbiotic and associative
microbial activities with higher plants. These are natural
mini-fertilizer factories that are economical and safer source of
plant nutrition for increasing the agricultural production and
improving soil fertility. The microorganisms colonize roots of rice,
wheat, maize, sugarcane and form root nodules in leguminous plants.
Different biofertilizers have shown nitrogen fixing, phosphorus
solubilizing and phytohormone producing abilities that are used for
increasing the agricultural productivity, e.g. (Bradyrhizobium
for
legumes (grain, fodders), plant growth promoting rhizobacteria (PGPR)
for cereals (wheat, rice, grasses etc.), Azolla for
rice
ecosystem, and actinomycetes (Frankia spp.) for
forest trees.
These microorganisms convert atmospheric nitrogen to plant usable
form and can provide up to 200 kg N/ha/crop. Next to nitrogen,
phosphorus is essential for crop production. Although our soils have
sufficient phosphorus but this phosphorus is not available for plants
and most (>90%) of our soils are phosphorus deficient. Usually
the
pH of our soils is more than 7.5 and at this pH, very low amount
(3-10 mg/ kg) of phosphorus is in available form. In such soils, when
phosphatic fertilizers are added only a part of it is utilized by the
plants and remaining part is precipitated due to the presence of
calcium and with time it is converted into highly insoluble forms of
calcium. In recent years, biofertilizers have emerged as an important
component of the integrated nutrient supply system and hold a great
promise to improve crop yields through environmentally better
nutrient supplies. However, the application of microbial fertilizers
in practice, somehow, has not achieved consistent results.
A group of bacteria
referred to as plant growth-promoting rhizobacteria (PGPR), which
participate in many key ecosystem processes such as those involved in
the biological control of plant pathogens, nutrient cycling and
seedling establishment, and, therefore, deserve particular attention
for agricultural or forestry purposes. PGPR may colonize the
rhizosphere, the surface of the root, or even superficial
intercellular spaces of plants. It has been revealed that the effect
of nitrogen fixation induced by nitrogen fixers is not only
significant for legumes, but also non-legumes. Moreover, some strains
have multiple functions for plant growth. Phosphate (P) – and
potassium (K)-solubilizing bacteria may enhance mineral uptake by
plants through solubilizing insoluble P and releasing K from silicate
in soil. Soil microorganisms are important components in the natural
soil sub ecosystem because not only they contribute to nutrient
availability in the soil, but also bind soil particles into stable
aggregates, which improve soil structure and reduce erosion
potential.
Advantages of
biofertilizers over chemical fertilizers
The utilization of
microbial products has several advantages over conventional chemicals
for agricultural purposes. Microbial products are considered safer
than many of the chemicals now in use, neither toxic substances nor
microbes themselves will be accumulated in the food chain,
self-replication of microbes circumvents the need for repeated
application, target organisms seldom develop resistance as is the
case when chemical agents are used to eliminate the pests harmful to
plant growth and properly developed biocontrol agents are not
considered harmful to ecological processes or the environment.
Azotobacter
Azotobacter is a
free-living, gram negative, aerobic, nitrogen-fixing bacterium and is
therefore being used as biofertilizer to replace chemical
fertilizers. It grows from 28 – 30 0C and a pH range 7.0
to 7.5. It uses sugars, alcohols and salts of organic acid for
growth. Generally it fixes non-symbiotically about 10 mg of
atmospheric nitrogen/gm of carbohydrates (usually glucose) consumed.
It is non-spore forming but can form cyst in adverse conditions and
in older cultures grown with sugar as the carbon source. Cyst has a
characteristic structure a central body surrounded by a cyst coat,
consisting of an exocystorium and an exine. Cysts accumulate poly B-
hydroxyl butyric acid (PHB). With the onset of favourable conditions
they give rise to vegetative cells. On nitrogen free agar medium with
sugar as carbon source colonies appear within 48 hr at 30 0C.
The colonies are smooth, opaque, low convex and viscid.
Azotobacter
biofertilizers
Azotobacter
biofertilizers contain very high number of live Azotobacter
bacteria. It can be used in any non-legume crop of short, medium and
long duration. Besides fixing nitrogen these bacteria secrete certain
growth promoting hormones such as indole acetic acid, gibberellic
acid and cytokinins, which promote vegetative growth and root
development. Azotobacter cultures used as
inoculants have been
reported to produce gibberellic acid, indole 3-acetic acid and
cytokinin, which may promote seedling development and plant growth.
The potential use of Azotobacter spp. was reviewed
by Brown
(1982), who concluded that inoculation with A. chroococcum
occasionally promoted yields, probably by
mechanisms other than
biological N fixation.
Carrier-based
biofertilizers
Carrier-based
biofertilizers are prepared with the help of activated charcoal,
which act as a carrier for microbial inoculants. Biofertilizer
consumption is not very satisfactory due to certain disadvantages
associated with carrier-based biofertilizers like low shelf life (3-4
months), storage condition (stored in cool temperature) as it is
temperature sensitive, bulky to transport, therefore, high transport
cost, less scope for export, more chances of contamination, problem
of proper packing, poor cell protection, poor moisture retention
capacity and restriction on use of charcoal as a measure of
conservation.
Liquid
Biofertilizers: solution to carrier based biofertilzers
The strength of
biofertilizers is determined by two basic parameters 1. Number of
cells 2. Efficiency of the microorganisms to fix nitrogen or
solubilize phosphates. Liquid biofertilizers are liquid formulation
containing the dormant form of desired microorganisms and their
nutrients along with the substances that encourage formation of
resting spores or cysts for longer shelf life and tolerance to
adverse conditions. The dormant form on reaching the soil, germinate
to produce fresh batch of active cells. These cells grow and multiply
by utilizing the carbon source in the soil or from root exudates.
The advantages of
liquid biofertilizers over conventional carrier based biofertilizers
are: longer shelf life (12- 24 months), no effect of high temperature
and no contamination, no loss of properties due to storage at high
temp. up to 450 C, high populations can be
maintained more
then 109 cells/ ml up to 12 to 24 months, easy
to use by
the farmers, high export potential, dosages are 10 times less then
carrier-based, quality control protocols are easy and quick. Lot of
work has been done on carrier-based biofertilizers in the context of
organic food production. In view of the advantages of liquid
biofertilizers over carrier based formulations, research has now been
started on the production and testing of liquid biofertilizers.
Center
for Rural Development and Technology, Indian Institute of Technology
Delhi, Hauz Khas, New Delhi – 110016, [email protected]
|
