Ph.D

Phosphorus transformation in acid sulphate rice soils of Kerala
Acid sulphate soils are lowland soils situated below the mean sea level along
the coastal tracts of Kottayam, Alappuzha, Thrissur and Malappuram districts of
Kerala, spread in two agro-ecological units namely Kuttanad (AEU 4) and Kole lands
(AEU 6). These soils are characterised by high levels of organic matter, low pH and
toxicity of iron (Fe) and aluminium (Al) with wide variability in available phosphorus
(P) content. Acid sulphate soils generally contain substantial quantities of Fe sulphide
minerals or the oxidation reaction products of these sulphidic minerals and the
protonated/non-protonated surfaces of Al/Fe hydroxides and oxides, resulting in P
sorption and formation of the complexes, thus decreasing P availability in soil.
However, the recent fertility evaluations across the state showed high available P status
due to solubilization of the accumulated applied P. To develop an economically viable
phosphorus nutrient management strategy, a pioneer study on P transformations in acid
sulphate soils is highly required. The study aimed to understand seasonal variation of
nutrients and transformations of phosphorus in acid sulphate soils.
The investigation was conducted at Radiotracer laboratory, College of
Agriculture, Vellanikkara, with four experiments viz., collection and characterization
of soil samples from the acid sulphate soils of Kerala in two seasons, fractionation of
phosphorus, adsorption study and an incubation study. In the first experiment, 125
representative soil samples from Kuttanad (AEU 4) (15 each from Upper Kuttanad,
Lower Kuttanad, Vechur Kari, Purakkad Kari and Kayal lands) and Kole lands (25 each
from Thrissur and Ponnani Kole) in pre-monsoon (April) and post-monsoon
(November) were collected and characterised. The experiments on the fractionation and
adsorption of phosphorus were done in thirty-five samples ie., five each from the seven
selected locations with varying available P status. Phosphorus fractionation was carried
out to quantify different fractions of phosphorus and their contribution to the available
pool. The inorganic P fractions includes saloid bound P, Fe-P, Al-P, reductant soluble
P and Ca-P. Adsorption study was carried out to work out the quantity/ intensity (Q/I)
relationship of phosphorus and the pattern of adsorption in acid sulphate soils. Finally,
an incubation experiment of 90 days duration was conducted to understand the effect
of phosphatic fertilisers, lime and farmyard manure at different levels in low and high
available P soils collected from Kuttanad and Kole lands. The treatment details of the
incubation study were three levels of phosphatic fertilisers (as SSP - 0, 35 and 70 kg
ha-1), two levels of lime (as CaO - without lime and lime as per POP recommendations)
and two levels of farmyard manure (0 t ha-1 and 5 t ha-1). The observations were made
on 30th, 60th and 90th days of incubation.
Seasonal characterisation of soil samples during the two seasons revealed that
among the collected samples, 50 per cent was sandy clay loam and 40 per cent was
sandy loam in texture. The acidity characterisation of soil samples reaffirmed the
extreme acidity condition of acid sulphate soils with 40 per cent of samples coming
under extremely acidic class and 20 per cent under very strongly acidic. Among the
collected soil samples, 37.60 per cent was low in available P, 23.20 per cent and 39.20
per cent under medium and high P respectively. The soil sample from Vechur Kari
region showed the highest value of organic carbon (11.46 %). The available
micronutrients viz., Fe and Mn showed very high concentrations. Among the acidity
fractions, potential acidity was dominant followed by pH-dependent acidity in both
seasons.
Seasonal variation in electrochemical properties like pH and EC was noticed
between two seasons. Ultra acidic soil reaction with high electrical conductivity was
noticed during pre-monsoon period which reaffirm the salt water intrusion in these
locations. Oxidation of pyrite mineral resulted in high amount of available sulphur as
well as potential acidity during pre-monsoon. Reduced condition prevailing in the post
monsoon resulted in high concentration of available iron.
The fractionation study of phosphorus indicated that the per cent distribution of
different phosphorus fractions followed the order, Fe- P > organic P > reductant soluble
P > calcium P > aluminium P > saloid bound P. The contribution of dominant fraction
(Fe-P) to the available P is mainly through saloid bound P. The phosphorus fixing
capacity was significantly and positively correlated with clay, organic matter content,
and negatively correlated with the available P.
In the adsorption study, high buffer power indicated the ability of acid sulphate
soils to replenish the depleted available P. Freundlich adsorption isotherm was found
to be the best to explain P adsorption followed by Langmuir and Temkin adsorption
isotherms. The Freundlich adsorption constant, KF was found to be correlated with
organic matter which confirmed the fixation of phosphorus by organic matter in acid
sulphate soils. In most of the soils, Langmuir adsorption constant (KL) increased with
rise in temperature, indicates chemisorption behaviour of P adsorption.
The incubation study affirmed that in soils with low available P, the addition of
SSP at doses of 70 kg ha-1 and 35 kg ha-1 along with lime and FYM showed a significant
increase in pH. In these soils, the addition of SSP (70 kg ha-1) along with lime and FYM
registered the highest available P of 51.63 and 73.33 kg ha-1 respectively in Kuttanad
and Kole soils at 60th day of incubation. In the same treatment, Fe bound P fraction was
found to be highest with a decreasing trend towards 60th day of incubation, which
increased after that. In the case of soils with high available P, the treatment combination
with the addition of the highest dose of SSP (70 kg ha-1) along with lime and FYM
showed the highest pH with an increasing trend towards 60th day. On the contrary to
low P soils, in high P soils, the addition of SSP at 35 kg ha-1 along with lime and FYM
registered the highest available P of 309.44 and 126.94 kg ha-1 respectively in Kuttanad
and Kole at 30th day of incubation followed by a reduction towards 60th day of
incubation.
Phosphorus, a widely varying nutrient in acid sulphate soils is prone to high
adsorption and fixation in soil depending upon the type of clay, organic matter content
and amount of oxides and hydroxides of Fe and Al. There was no significant seasonal
variation of P in soil samples from AEU 4 and AEU 6. In acid sulphate soils, the most
dominant fraction, Fe-P contributed to available P mainly through saloid bound P,
which is the water-soluble and loosely bound P fraction. The reduction of available P
with the increase in P fixing capacity necessitates its estimation at least for the grouping
of soils for the efficient management of phosphorus. In soils with high available P,
addition of lower dose of SSP (35 kg ha-1) along with organic manure (FYM @ 5 t ha-
1) and lime (POP recommendations) is sufficient to enhance the P availability. Where
as, in soils with low available P, addition of higher dose of SSP (70 kg ha-1) along with
organic manure (FYM @ 5 t ha-1) and lime (POP recommendations) enhanced the P
availability than its sole application.
In this regard, field experiments in acid sulphate soils have to be conducted to
confirm the results of incubation study in rice under natural system and more
investigation is needed to know the interaction between organic matter and organic P
fraction. As the organic matter plays the dual function in the environment- as a link and
as a bottleneck for phosphorus availability, detailed study should be undertaken to know
the complex formed by P fixation and its degree of crystallinity for developing better P
management strategies in acid sulphate soils.