PhD

Field experiments on aerobic and flooded systems of rice were conducted in
second crop season with the objectives to study the nutrient dynamics and
transformations in these systems in second crop season with rice (variety Jyothi), in
farmer’s field, at Nellikkattiri, Thirumittakode panchayat, Palakkad district. The
treatments with two doses of fertilizers (as per Package of Practices
Recommendations, KAU and based on soil test) and three doses of lime (as per POP,
as per ΔpH and as per SMP buffer method) were imposed in plots of 20m2 area in
Randomized Block Design with four replications. Under flooded condition, two field
experiments were conducted to standardize the method of sampling and analysis for
soil test based application of lime and fertilizers. One was based on sampling and soil
testing on wet basis keeping the anaerobic environment unchanged, while the other
was based on routine sampling and analysis after air drying. Better correlations with
respect to available nutrients and plant nutrient content were obtained for wet analysis
based recommendation and hence the data from this experiment were considered for
comparison of the nutrient dynamics with that of the experiment on aerobic rice.
In situ measurement of pH, electrical conductivity and redox potential was
done under both systems of rice cultivation. Redox potential was measured from
three different depths under flooded system (15, 30 and 45 cm) and from two
different depths under aerobic system (30 and 45 cm). The soil and plant samples
were collected at three stages viz. at active tillering, panicle initiation and at harvest
of the crop. The soil samples collected were analysed for pH, EC, OC available
nutrients (P, K, Ca, Mg, S, Fe, Cu, Mn Zn and B), and were also assayed to estimate
fractions of soil phosphorus, iron, zinc and boron. The plant samples were analysed
for N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu and B. At harvest straw and grain samples
were analysed separately.
The increase in pH in both systems was in proportion to the quantity of lime
applied. Higher rate of increase in pH was observed under aerobic system. Increase in
EC was in proportion to the quantity of lime and fertilizers added, and it was more in
aerobic system due to less dilution. The redox potential became negative due to
reduced environment in flooded system within two weeks of transplanting while it
was consistently positive under aerobic system. The organic carbon content was
higher under aerobic environment at active tillering and panicle initiation due to
quicker decomposition of applied organic matter especially in presence of lime while
it was lower under flooded condition initially due to slower rate of decomposition.
Available P was highest under flooded system due to release of bound P from
Fe and Mn by reduction of these elements to their respective soluble forms. Under
aerobic condition, the available P recorded at active tillering and panicle initiation
was lower than that of the initial value, due to its precipitation as tri calcium
phosphate [Ca3 (PO4)2].
The available K status was higher under aerobic condition throughout the crop
growth because of reduced rate of leaching under this environment. The rate of
increase in available K was concurrent to the quantity of fertilizer added under both
systems of rice cultivation. Highest K content in plant was recorded under aerobic
rice system.
The highest available Ca was recorded at active tillering and panicle initiation
in flooded system of rice cultivation, because of the solubilization of applied lime. At
harvest, the available Ca became precipitated as tri calcium phosphate which
decreased the availability of both Ca and P under flooded condition. The
transformation of tri calcium phosphate to mono calcium phosphate occurred only
under aerobic condition during later stages. The highest Mg in plant was recorded in
treatment where fertilizer application was done based on soil test under both systems
of rice cultivation. The available sulphur status was higher under flooded condition
during all the stages of sampling because of the increased solubility of applied
factomphos and MgSO4.
The status of available Fe was higher under flooded environment because of
the reduction of Fe3+ to soluble Fe2+, while the available Fe status was found to
decrease under aerobic condition due to oxidation of Fe2+ to insoluble Fe3+. The
available Mn status under flooded environment decreased when compared to that of
aerobic condition because of enhanced absorption by rice. The lower status of
available Zn under aerobic condition resulted from more absorption of Zn by the
crop, because of decreased competition from cations such as Fe3+ and Mn4+ under
aerobic condition. The available boron status and boron content in plant was high
under flooded condition because of the enhanced solubility of applied borax.
Ultimately, aerobic rice recorded significantly higher grain and straw yield
(6.23 t ha-1 and 6.35 t ha-1 respectively) than that under flooded system (5.12 t ha-1
and 5.52 t ha-1 respectively). The treatment with fertilizer application based on soil
test and with lime as per SMP buffer method under aerobic situation recorded
significantly higher grain yield of 6.8 t ha-1 because of balanced nutrition in this
treatment combination. Higher root CEC, root mass, shoot mass root volume and root
length were recorded under aerobic system. The decline in productive tillers during
active growth phase was observed under flooded environment. Well developed
aerenchymatic tissue in the roots was observed only under flooded environment. The
water requirement was reduced by 57 % in aerobic rice, than that in flooded rice