The significance of algal biofertilizer lies in the fact that unlike the chemical fertilizers,
these are not directly utilized by the crop. Only the products of their activity are used.
During the crop growth cycle, the algae grow, multiply, fix atmospheric nitrogen and make
it available to the crop by way of excretion and autolysis. During unfavourable season,
they form perennating bodies which germinate with the onset of congenial conditions. Thus,
there is a possibility to build up populations of these algae in the soil, through
superimposed inoculations for 3-4 consecutive seasons. Algalization of rice crop has been
found to supplement nitrogenous fertilizers to the extent of 30-40 kg N/ha/season.
Successful
establishment of desired algae in the rice fields has been found to form a source of slow
release of nitrogen for the crop plants. They have also been found to protect a part of
the applied fertilizer nitrogen from being lost. Studies using N15 have been
shown that the nitrogen fixed by the blue green algae is actually taken up by the crop
plants.
Saline-alkali
soils are generally unsuitable for raising crops. Blue green algae have been shown to help
in reclamation of such soils. This is because of the preferential absorption or adsorption
of sodium by them. The growth of these blue green algae in saline alkaline habitats
reduces salinity by 25-30%, pH, electrical conductivity and exchangeable sodium. It also
increases aggregation, hydraulic conductivity, soil nitrogen and permeability.
However,
we still dont know the mechanism by which the blue green algae scavanges Sodium.
Attempts are to be made to investigate as to what happens to this absorbed/adsorbed
sodium. It is also essential to develop an adaptable technology which can be used to grow
and multiply the salt tolerant algal strains in such areas.
Acidic
soils pose another problem in getting good results from algalization. Majority of blue
green algae have a wide pH range of 6.5 to 8.5. However, some algal species are delicate.
Quite a few of them have been found to be true acidic forms showing optimum growth only
when the pH is below 5.0. On the other hand, there are forms, which prefer only alkaline
pH. Thus, it is possible to isolate pH specific algal forms from the natural algal flora
which can be used in acidic soils without any soil amendments.
Combined sources of nitrogen, which are commonly used in rice production, constitute an important
stress on microbial systems. The fact that blue green algae are able to revert back to
nitrogen fixing mode, when exogenous nitrogen is depleted. This indicates that a cyclic
process of fertilization by these organisms can be used as an adjacent to the linear
fertilization by the chemical fertilizers. Strains of blue green algae capable of
absorbing and growing at higher fertilizer nitrogen levels will be more useful. They
increased biomass in presence of combined nitrogen which is expected to bring about
increased addition to nitrogen through nitrogen fixation when exogenous nitrogen is
depleted. If we can develop strains with depressed nitrogenase, which can continue to
function in presence of high level of combined nitrogen, the algal supplementation effect
can be considerably increased.
Pesticides
have no adverse effect on the growth of blue green algae when pesticides are used at
recommended doses. Some pesticides have been found to accelerate the growth of blue green
algae.
The
results from extensive field trials conducted in different agroclimatic regions of our
country during the last two decades have shown that the effect of algal inoculation in
terms of increased crop yield was satisfactory. Studies with algal inoculation at
different levels of fertilizer nitrogen have shown that the supplementation effect is more
pronounced at low level of nitrogen. This effect does not change with the soil type and
rice variety. The subdued effect in acidic soils can be enhanced by the application of
appropriate quantities of lime.
In long
term experiment, there was a gradual increase in organic carbon due to algal inoculation
but the amount remained steady at end of 3 years.
An
increase of organic matter, water holding capacity and exchangeable Ca was reported to
from medium to high level. Due to algalization of saline and alkaline soils. Algalization
was also reported to increase available phosphorus in the soil, possibly because of
excretion of organic acids by the blue green algae. The blue green algae have been found
to solubilize the unavailable phosphate sources like Missouri rock phosphate.
The
process of photosynthesis provides energy and carbon skeletons for nitrogen fixation. This
is evidenced by evolution of oxygen under nitrogen fixing conditions. The nitrogen
fixation is drastically reduced in dark because of depleting supply of photosynthates. The
strains with very high rate of photosynthesis and ability to store the photosynthetes will
be capable of fixing nitrogen for a comparatively longer period in the dark.
It is
known that under certain conditions, nitrogenase can act as ATP dependent hydrogenase
systems. An understanding and enzymology of this hydrogen evolution will enable us to
train this system to evolve hydrogen from water at the expense of solar energy. This is
expected to reduce our dependence on the fuel energy especially in the manufacture of
fertilizer nitrogen.
Another
approach to achieve this can be through the development of ammonia leaking strains. Anabaena
azollae actually makes available the nitrogen fixed by it to Azolla as
ammonia. Algal mutants with the defect in the regulation of the enzyme glutamine
synthetase (GS) can be made to liberate ammonia. The ammonia excreted into the medium can
be harvested and used for various purposes.
The
efficiency of inoculum can also be increased if it contains a heavy load of the
perennating bodies like akinetes and hormocysts. Many species of Anabaena and
Nostoc form a long chains of spores and sometimes the entire trichome gets transformed
into spores. A culture containing such organisms will multiply faster when inoculated into
the field. Genetic engineering can be used to clone together, the properties of growth,
nitrogen fixation and spore formation in addition to the tolerance to fertilizer nitrogen
and pesticides.
Although,
use of algae has been restricted to mainly rice especially low level rice cultivation
successful results have been reported in sugarcane, jute, vegetables, tomatoes, fruit
trees, banana, etc.