PhD

Soil acidity is a major constraint in the wetland rice soils of the tropics, which has got aggravated in recent times with the introduction of high yielding varieties of rice and intensive use of fertilizers. Kerala State, with its unique geomorphology, climate, hydrology and other environmental factors has led to the formation of wetland soils with specific physico-chemical and mineralogical properties. These soils sustain a major part of the rice tracts in the State. Attempts to bring these lands under rice cultivation brought to light the intensity and severity of soil acidity in the natural as well as manmade wetlands of this region. Though studies have started way back in 1920 to tackle the problem of acidity and associated nutritional factors, a detailed study to unveil the intricate physico- chemical phenomena contributing to acidity and an economic and efficient methodology to alleviate the severity of acidity has not been attempted so far. Hence a study has been conducted to evaluate the acidity parameters in relation to nutrient availability in wetland rice soils of Kerala.
Twelve major wetland rice soils have been identified and classified according to Soil Taxonomy. They belong to natural as well as manmade wetlands, some of them having been indigenously classified and known by vernacular names like Kari land (acid peat), Karapadam (riverine alluvium), Kayal land (lake bed sediments), Pokkali lands (saline acid), Kole lands (flood plain), Kaipad lands (saline marsh).
Twelve profile samples representing the major wetland rice soils comprising an area of 6 lakh ha have been subjected to detailed investigations for morphological, physical, chemical, mineralogical and electro chemical parameters.
Surface samples, each at a radial distance of 5 km from the twelve locations were collected and subjected to detailed studies to evaluate the acidity parameters and acidity generating factors. Studies were also taken up to observe the kinetics of pH and nutrient availability under submergence for different intervals of time. A comparative study on the efficacy of different methods of estimating the lime requirement was also taken up.
From the detailed investigations on the evaluation of acidity parameters of wetland rice soils of Kerala, it has been observed that there exists a great extent of variation with respect to the severity of acitive acidity in the profile and surface samples. Barring Chittoor soils which were slightly alkaline, all the soils under study were acidic in reaction. The profile and surface samples of Kari soil of Thakazhi and Pokkali soil of Njarakkal recorded the highest active acidity with a dry soil pH around 3.2. The rating of these surface soils on the basis of active acidity was Kari ≥ Pokkali > Karapadom > Vellayani > Kayal > Kole > Wyanad > Pattabi > Kaipad > Karamana > Kattampally. However, with respect to the mean values of pH of profile samples the rating was Kari > Pokkali > Kole > Kayal > Karapadom > Vellayani > Wyanad > Pattambi > Karamana > Kattampally > Kaipad. All the soils having high active acidity recorded high exchange, non-exchange and potential acidity with significant correlation between them. It was observed that major part of potential acidity of wetland soils of Kerala was constituted by non-exchange acidity due to the preponderance of Kaolinite clay minerals and high content of sesquioxides. Highest values for potential acidity were recorded by 18-30 cm. of Kari and surface layers of Pokkali profiles. Based on mean potential acidity of surface samples the soils could be rated as Kari > Pokkali > Kayal > Karapadom > Kole > Pattambi > Kattampally > Vellayani > Kaipad > Karamana > Wyanad. HOWEVER, WITH RESPECT TO PROFILE SAMPLES THE SEQUENCE WAS Kari > Pokkali > Kayal > Karapadom > Kole > Wyanad > Vellayani > Kattampally > Pattambi > Kaipad > Karamana.
From the correlations worked out, it was found that aqueous pH (1:1) under dry condition was a more reliable parameter for evaluating acidity in the wetland rice soils of Kerala than pH under wet condition or in salt solution, because this parameter gave more significant correlations with acidity generating factors and available nutrients. It was also observed that available sulphur determined both active and potential acidity rather than total sulphur in sulphur rich soils. Path coefficient analysis of acidity contributing factors indicated that exchangeable aluminium was the best parameter for measurement of acidity, 87 per cent of variation in exchange acidity, 57 per cent of variation in non- exchange acidity and 63 per cent of variation in potential acidity.
Comparison of different methods for calculating lime requirement proved the superiority of exchangeable aluminium method for the wetland rice soils of Kerala. Lower rates of lime requirement were observed by this method for Kattampally, Wyanad, Karamana, Pattambi and Kaipad soils.
Studies on submergence showed that wetland rice soils of Vellayani, Karamana, Karapadom, Kole, Kaipad, Kattampally and Wyanad do not require lime to raise the Ph for rice cultivation because all these soils attain a pH value of 5.5 within two weeks of submergence. Consequently there was increased availability of nitrogen, phosphorus, potassium, calcium, magnesium, iron, manganese and silica. However, zinc, copper and sulphur were found to decrease by prolonged submergence.
The detailed investigations conducted under this study on the evaluation of acidity parameters of wetland rice soils of Kerala in relation to nutrient availability have unveiled many intricate phenomena of soil acidity which are of great theoretical and practical significance. Contrary to the conventional concept that liming is a must in all acid soils, the present study underlines the cheap and easy method of alleviating acidity of certain wetland rice soils by optimum submergence and scientific water management. Further the study has revealed the feasibility of liming in soils with severe acidity based on specific methods of estimating the lime requirement of soils. However it is necessary to confirm the results of these in- vitro studies by adequate field studies in specific wetland rice tracts of Kerala State.