PhD

Crop production on laterite and associated soils have been found
to be low due to constraints such as hardening and hard crust formation.
However, there is considerable scope for improving the productivity of
these acid soils through proper land management. It is generally agreed
that iron plays a key role in the hardening process and in crust formation
in the tropics. In the highly laterised soils of Kerala, cassava and rubber
form the major crops involving entirely different management systems.
So far no study has been conducted on the effect of different land use
systems on hardening and softening of laterites as well as on the effect
of different ameliorants in softening of already hardened laterites.
Three study sites were selected VIZ., Thiruvananthapuram,
Angadipuram and Kannur. At each location, pedons were dug in soils
under cassava, rubber and uncultivated barren lying adjacent, to a depth
of 2m and soil samples were collected from genetic horizons and various
analysis done. Apart from this, indurated laterite blocks were collected
from all the study sites and locations for detailed study of the role of
iron and aluminium oxides in hardening and softening of laterites. Hard
laterite block collected from a quarry in Thiruvananthapuram was used
for laboratory incubation experiment to study the efficacy of organic and
inorganic ameliorants in softening of already hardened laterites.

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From the detailed investigations on the physico-chemical
characteristics related to hardening and softening of laterites, it has been
observed that hardening and softening of laterites have been affected by
the land uses and great variation could be recorded for parameters like
bulk density, clay content and movement, organic matter, water dispersible
clay and iron content.
The bulk density of epipedons was significantly higher m soils
under cassava and uncultivated barren compared to that of rubber. Though
the clay content of Bt I layer of soils under all land uses was less than
1.2 times than that of the overlying layer at Thiruvananthapuram; at
Angadipuram, the clay increase was more than 1.2 times in soils under
rubber (1.36) and cassava (1.36) and at Kannur, more than 1.2 times clay
increase was observed only in soils under rubber (1.27).
Both at Thiruvananthapuram and Angadipuram soils under
uncultivated barren had a significantly lower organic matter content
(expressed as kg m-2 to a depth of 1 m) compared to that of other two
land uses at each study site. No significant difference could be noted at
Kannur. With regard to water dispersible clay content of Bt! layer, soils
under rubber had a significantly lower value at all the three study sites
indicating significant clay dispersion and more induration or hardening.
The total iron was significantly higher in soils under .uncultivated barren
than that of cassava and rubber.
Among the various soils acidity parameters studied, the
exchangeable acidity of textural-B horizon was significantly higher in

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soils under cassava than that of rubber and uncultivated barren. Same
trend was observed for extractable acidity too in this layer.
The active Iron ratio showed a decreasing trend with depth and
the value was significantly lower in hard laterite layer compared to the
corresponding overlying layers. A ratio of less than 0.1 in all pedons
indicated a high rate of ferrallitisation. The crystalline iron oxide content
was significantly higher in soils under rubber than that of cassava
indicating the increased rate of crystallinity of iron oxides in soils under
rubber compared to soils under cassava.
A positive linear relationship was observed between the strength
of materials, measured as penetration pressure, and Fed content of laterite
and a negative linear relationship was observed between strength of
materials and active iron ratio of laterite.
Studies on the efficacy of organic and inorganic ameliorants III
softening of already hardened laterite showed a significant increase III
basic cations as well as decrease in iron content due to treatments with
organic ameliorants such as cowdung slurry, 20 per cent and green leaf
extract, 20 per cent. Other treatments like calcium silicate (20 per cent),
magnesium silicate (20 per cent) and sodium silicate (20 per cent),
resulted in a significant increase in the respective basic cations as well
as a decrease in total iron.
Treatment of laterite with green leaf extract (20 per cent), calcium
silicate, cowdung slurry and sodium chloride resulted in a significantly
higher Al substitution in iron oxides over the control.





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X-ray diffraction (XRD) studies showed a loss of crystallinity of
Iron oxides especially goethite, haematite and magnetite after treatment
with calcium silicate (20 per cent), cowdung slurry (20 per cent) and
green leaf extract (20 per cent). The gibbsite content also showed an
increase in its content after the above three treatments. Relative
abundance of both goethite and haematite was more in the sample before
treatment and a least value was observed in the sample after treatment
with calcium silicate (20 per cent).
Thin section studies using a petrological microscope showed the
presence of highly crystallised kaolinite in hard laterite before treatment
seen as submicroscopic plates. The samples after treatment with calcium
silicate (20 per cent), cowdung slurry (20 per cent) and green leaf extract
(20 per cent) are found to have pisolitic forms of gibbsite pseudomorphs
and goethite pseudomorphs.
The detailed studies on thepedological and management aspects
of hardening and softening of laterites under different land use systems
have unveiled many intricate phenomena of these processes which are of
great theoretical and practical importance. Contrary to the conventional
concept that application of sodium chloride will soften hard laterites,
this study reveals the importance of amendments such as calcium silicate
and application of cowdung and green manuring with leguminous crops
such as Sesbania in softening already hardened laterites. However, it is
necessary to confirm the results of these in-vitro studies by adequate
field studies in specific laterite tracts of Kerala state using these
amendments.