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