PhD

Silicon (Si) is the second most abundant element in soil. The amount of silicon in soil
depends on parent material, soil type, pedogenic process and landscape. In soil solution, Si is
present as monosilicic acid which is the only form that the plant can absorb from soil. The
productivity of rice is comparatively low in soils of Kerala. As a ̳Si – accumulator‘, rice can
benefit from Si nutrition. The application of Si can enhance growth and yield of rice. With
this background, studies were conducted to categorize major rice growing soils of Kerala
according to plant available silicon and to evaluate the efficacy of different sources of silicon
including rice straw in wetland rice. The release of silicon from different soils added with
various silicon sources under different water regimes was also monitored.
Soil samples were collected from five different locations representing major rice
growing regions of Kerala viz., Kuttanad, Kole land, Pokkali, sandy and lateritic to categorize
them according to plant available silicon. The available Si ranged from 7.70 mg kg -1 (sandy
soil) to 34.91 mg kg -1 (Kole land soil) in the order Kole land > Pokkali > lateritic > Kuttanad
> sandy soil. All the soils under study were categorized as low in available Si. The available
Si had positive correlation with organic carbon, available N, Ca, Mg, Fe, Mn, Zn,
exchangeable K, Ca, Mg and CEC and negative correlation with available boron, AEC and
silica-sesquioxide ratio. These soils were subjected to fractionation of silicon. The major
fractions of Si were mobile, adsorbed, organic, occluded, amorphous and residual Si. The
percentage distribution of fractions of Si in these soils were in the order; residual Si >
amorphous Si > occluded Si > organic Si > mobile Si > adsorbed Si.
Quantity – intensity relationship of five major rice growing soils at two temperatures
viz. 25 0 C and 40 0 C were studied. The highest buffer power was indicated by Kuttanad soil
followed by Pokkali and sandy soils at 25 0 C. It clearly indicated that these soils have a higher
power to retain Si on solid phase and replenish its concentration in soil solution as and when
it is depleted through plant uptake or leaching. The equilibrium Si concentration and the
amount of Si adsorbed by each soil were used to test the fitness of data to the adsorption
isotherms viz., Langmuir, Freundlich and Temkin. The data obtained from the adsorption
experiments fitted into Freundlich and Temkin equations, but not to Langmuir equation at
25 0 C. At 40 0 C no adsorption equations were obtained for any soil.An incubation study was conducted to know the extent of release of Si on addition of
different sources of silicon such as rice husk ash, biodecomposed rice husk, calcium silicate
and sodium silicate in five rice growing soils under submerged water regime (SWR) and field
capacity water regime (FCWR). Addition of Si significantly increased the release of available
Si in all soils except Kole land soil after a month. Kole land soil showed higher release of
available Si after two months. The highest release of available Si was at SWR in case of Kole
land and Kuttanad soil, where as Pokkali, sandy and lateritic soils showed more release of
available Si at FCWR. Irrespective of soils, treatment with sodium silicate showed higher
release of available Si. Total Si showed a decreasing trend over the period of incubation for
three months in all the soils.
A field experiment was conducted at Agronomic Research Station, Chalakudy to
evaluate the efficacy of different sources of silicon including rice straw in wetland rice. Rice
husk ash, biodecomposed rice husk, calcium silicate and sodium silicate were used as source
of Si along with fertilizers as per package of practice recommendation (NPK alone). The
maximum number of panicles per hill, number of spikelets per panicle, thousand grain
weights and minimum number of unfilled grains per panicle were recorded in treatment with
calcium silicate application. The maximum grain yield of 6.90 t ha -1 was recorded in
treatment T 5 (T 2 + Calcium silicate) and significantly superior (fig.54) over all other
treatments. This increase in yield may be due to the effect of application of Si on soil fertility,
nutrient uptake, and plant growth. The direct effect of Si fertilization on increased number
panicle per hill, number of spikelets per panicle, and thousand grain weight and decreased
number of unfilled grains per panicle might be the reason for increased grain and straw yield
in treatment with calcium silicate. The treatment with POP + sodium silicate showed the
highest uptake of Si by grain and straw of rice. The sources of Si had no residual effect on
grain and straw yield of succeeding rice crop. In general, sandy soil low in available Si had a
high response to applied Si in achieving higher grain yield.