MSc

Three locations from 7 agro ecological units dominated by rice crop
were identified namely, Onattukkara sandy soil(AEU 3), Kuttanad(AEU 4),
Pokkali(AEU 5), Kole(AEU 6), north central laterite(AEU 10), Palakkad
central plains (AEU 22) and Palakkad eastern plains (Black soils )(AEU 23).
Geo-referenced soil samples were collected at 3 stages: before cropping
season, at active tillering and visual panicle initiation. Plant samples were
also collected during the above stages and analyzed for nutrient contents.
Initial characterization was done with air dried samples while samples during
crop growth period were collected by maintaining the wet anaerobic
conditions and analyzed as such as well as after drying. Submergence
resulted increase in pH both under wet and dry analysis. The pH on the basis
of wet analysis was higher than that by dry analysis. EC decreased during
flooding and dry analysis gave higher values than by wet analysis. The (C: N)
1 ratio (based on total carbon and total nitrogen) varied from 9.32 :1 in
Onattukarasandy soil to 18:1 in Kuttanad on the basis of wet analysis.
Analysis after drying recorded a (C: N) 1 ratio ranging from 10.84 inPalakkad central plains to 22 in Kuttanad. Comparison of wet and dry
analysis of other available nutrients indicated that higher values were
recorded for P, K, Ca, Mg and Fe in wet analysis while the values for
availableS, Mn, Zn and B were higher in dry analysis. Data on analysis after
drying, recorded significant negative correlation of pH with organic carbon (0.36**)

and available S (-0.37**) due to accumulation of organic acids and
SO
4
2-
ions under aerobic condition. Antagonistic interaction of available P
with available Ca was attributed to significant negative correlation obtained
between them in dry analysis. Wet analysis gave significant positive
correlation of pH with available Ca (0.35**) and significant negative
correlation with available S (-0.28*). All the C: N ratios computed on the basis of, total carbon and total nitrogen (C: N) 1, total carbon and available
nitrogen (C: N) 2, organic carbon and total nitrogen (C: N) 3 and organic
carbon and available nitrogen(C: N) 4 were significantly correlated with total
and available nitrogen at different stages both under wet and dry analysis.
Four soil types namely, Onattukara sandy, Kuttanad, north central laterites
and Palakkad eastern plains were used for an incubation study to unravel the
pattern of decomposition of added organic matter and to identify the C: N
ratio at equilibrium. Changes in pH and redox potential during submergence
indicated slight increase in pH after 7 days of submergence and redox
potential was constantly decreasing with increasing period of submergence.
The (C: N) 1 ratio was found to stabilize at 9.6:1 after 3 months of incubation
with organic matter while it was stabilized at 7.6:1 without organic matter in
Onattukara soil. In Kuttanad soils it was 18.3:1 and 17:3 respectively. In
Chittor soils it stabilized at 10:1. The (C: N) 3 also showed similar trends. However, available nutrient did not give any significant correlationwith the corresponding plant content of the respective nutrient. This focusesto the importance of future studies with more number of samples from eachAEU’s as each of the fertility parameters are highly varying in these units.