PhD

The chemical constraints for higher yields in laterite soils are identified as
low cation exchange capacity and organic matter content, high acidity, iron and
aluminium toxicity, high phosphorous fixation and poor nutrient status. Investigations
carried out to identify the limiting factors and their mode of action in laterite soil
revealed direct effect of excess iron absorption, their metabolic interference,
unfavourable Ca + Mg / K ratio, and deficiency of certain elements manifested as
negative effects of some others.
Management of organic manuring modified by amelioration with suitable
amendments was reported to be an ideal remedial measure for reducing the above
chemical constraints. An experiment was conducted at Agricultural Research Station,
Mannuthy during 2001-2002, to characterize and quantify the effect of organic sources
on nutrient supply, amelioration and inactivation of the deleterious factors in rice
,
culture and to study the supplementary requirements of inputs and complementary
effects on yield improvement. It was also intended to develop a technology by
integrating various inputs and their levels to ensure high resource use efficiency and
consequently higher productivity.
The plant height and tiller production was highest in the treatment green
manure + lime + ash at active t.illering, panicle initiation and 50 per cent flowering.
The total dry matter production was highest in the green manuring + lime and green
manuring + lime + ash treatment. All the growth parameters such as plant height, tiller
count and total dry matter production showed a stepwise improvement with doubling
of K, doubling of K along with addition of lime and ash and doubling of K with
addition of lime, ash and sulphur over the treatment where K was given at normal rate
and without any ameliorants.
The highest chlorophyll content both 'a' and ob' was found as resulted in
the fallow - green leaf manure treatment, however, this was not reflected in the

process of yield formation since the highest yield was observed for the treatment,
green manure + lime + ash. The favourable cell sap pH around 6.21 was observed in
the treatment green manure + lime + ash which resulted in the highest grain and straw
yield.
The highest grain and straw yield of 6.64 and 6.75 t ha-I, respectively was
observed in the treatment combination green manure + lime + ash x N as NH4S04 +
half P + lime + ash + double dose of K which indicate the importance of addition of
lime, ash and sulphur as ammonium sulphate in the fertilizer package of rice in laterite
soil.
The following conclusionsare drawn from the results of the experiment.
In the mundakan - summer - virippu rice cropping system in laterite soil
leaving the land fallow' during summer may considerably reduce the yield during
virippu if not supplemented with adequate organic manure and ameliorants like lime
and ash. If the summer season is kept fallow, green leaf manu ring could be a better
substitute for addition of organic matter to the laterite soil.
Paddy straw incorporation at 15 days before the virippu cropping in laterite
soil was found to reduce the yield of virippu rice probably due to high rate of
immobilization ofN.
The increase in acidity and consequent higher solubilization and uptake of
Fe in virippu crop was evidently due to summer fallow which favour natural oxidation
of Fe and Mn.
Ameliorants like lime and ash added both during summer green manuring
and virippu rice crop enhanced the rice growth and yield through modification of pH,
supplementation of potash and calcium.

Enhancement ofN:K ratio to 1: 1 was found to enhance rice productivity in
laterite soil. Addition of K application did not increase the K content in the plant but
reduced Fe uptake.
Changing the N source to ammomum sulphate and thereby sulphur
I
fertilization was found to increase growth and yield of rice. Ammonium sulphate
treatment helped mainly to increase plant height but not tiller production. Among yield
attributes spikelet production was very much influenced by sulphur fertilization.
The physiological and uptake efficiency of the N derived from green
manuring was higher compared to other organic sources and consequently resulted in
better agronomic efficiency. This indicates the need for a green manure - rice system
in laterite soil to enhance soil and crop productivity.
The green manured rice with ameliorants such as lime and ash, and
ammonium sulphate as N and S source resulted in the highest net return and B:C ratio
of2.25.