PhD

A research programme entitled "Nutritional constraints of rice- legume
system in laterite soils of humid tropics" was carried out from 1997-'98 to'1999-
'00 at Regional Agricultural Research Station, Pattambi. The objectives of the
programme were to study the effect of crop residue recycling, nutrient
management practices and tillage and water management practices on soil and
plant elemental status and productivity of rice- rice-legume cropping system
There were two experiments in the programme. The first experiment entitled,
"Soil- plant elemental status and productivity as influenced by crop residue
recycling and nutrient supply in the rice-legume cropping system" was taken up
to study the impact of nutrient management practices as well as crop residue
recycling on soil productivity and crop growth. The crop residues added in the
system involved harvested left overs of grain cowpea crop during summer as
well as addition of two-third portion of straw of kharif crop to rabi season crop.
Silica at 250 kg ha", higher K dose of 70 kg ha-I and soiVfoliar supply of Mg,
Zn, Cu, B and Mo were also included in various treatment combinations.
The treatments exhibited significant influence on grain and straw yield. The
treatment which did not receive any inorganic fertilizers registered the lowest
yield during all the seasons. Straw addition did not' bring about yield advantage
in kharif, but in rabi there was steady and significant increase over the seasons.
Silica application was not beneficial in initial seasons, but from fourth rice crop
onwards, significant increase was noticed. Though there was an improvement in
organic carbon of soil during some seasons, summer cowpea raising did not
bring about specific yield improvement. The absence of response to addition of
summer cowpea residues revealed that either lack of N or a lower level of
organic carbon may not be the limiting factors for low yield in these laterite
soils.
Potassium application even at the higher dose of 70 kg ha-I was "found to
have no appreciable effect on growth and yield of paddy. Foliar application of







boron and molybdenum and soil as well as foliar spray of Mg, Zn and CLl
continuously for five seasons did not bring about any significant influence on
growth and yield of paddy
The second experiment, " Effect of tillage and water management
practices on soil-plant, elemental status and productivity of rice in rice- rice
legume cropping system" was aimed to study the influence of deep ploughing
and intermittent drainage. Deep ploughing during kharif season had no
significant influence during the current season or for the following rabi crop
indicating that in puddled soil condition deep ploughing will not be of much
advantage. The two water management practices did not produce any appreciable
difference in growth or yield of the crops during the kharif seasons; 'but it
adversely affected the performance of rabi 1999-'00 crop which could be due to
, the dry atmospheric condition during the season. During rabi 1998-'99, the
desiccating effect was considerably low and hence the drainage did not affect the
growth of the crop. Hence the effect of intermittent drainage depends on season
as well as the current weather situation.
The elemental composition of the plant showed that there was no deficiency
of any of the elements analysed in the plant system and hence the general low
yield obtained from these soils could be due to nutritional constraints owing to
excesses, imbalances or metabolic unavailability of elements. The plant had high
content of non-applied elements particularly Fe. This excess absorption -of iron
might be due to the high content of available iron in the soil. The absorbed
nutrients, particularly N was not effectively utilized for grain production as
evidenced by the relatively high content of 0.9 per cent N in straw as against 1.1
per cent in grain. Management practices like addition of cowpea residues, supply
of nutrients, adoption of deep tillage or intermittent drainage did not show any
advantage. Recycling of paddy straw as well as continuous addition of silica were
effective in improving yield.