Studies on the Acid Sulphate Soils of Kuttanad of Kerala

Kuttanad, the rice bowl of Kerala is a unique agricultural tract lying 0.6 to 2.2 m below MSL. The soils of the area are highly acidic, saline and high in organic carbon content. Several parts of this delta have subsoil layers containing pyrites which on oxidation produce severe acidity. Hence the present study was envisaged to characterize the acid sulphate wet land soils of Kuttanad. For the collection of soil samples, stratified random sampling technique was followed. Surface (0-15 cm) and sub surface (15-30 cm) soil samples were collected from the identified six soil series viz. Ambalapuzha, Kallara, Purakkad, Thakazhi, Vaikom and Thuravoor. From the study it is inferred that the pH of the samples varied between 3.0 -4.0 which is typical for acid sulphate soils. Lab incubation studies also revealed that there was a drastic decline in the pH of the soil with submergence while the EC ranged between 0.1 – 8 dS/m. With regard to the enzyme assay between the locations, subsurface soil samples collected from the wetlands of Thuravoor reported to have the highest value of 76.1 ppm of urea hydrolysed g-1 of soil hr-1 for urease and surface samples of Thakazhi had the highest of 105.91 μg of p-nitrophenol released g -1 of soil hr -1 for phosphatase. In the case of respiratory activity, which is an indicator of soil microbial biomass, surface samples from Thuravoor recorded the highest followed by Vaikom.


INTRODUCTION
Kuttanad, the rice bowl of Kerala is a unique agricultural tract lying 0.6 to 2.2 m below the mean sea level on the west coast of India. It has a geographic area of 854 sq. km which represents a deltaic formation traversed by numerous water courses that drain into the Vembanad lake. The soils of the area are highly acidic, saline and high in organic carbon content. Several parts of this delta have subsoil layers containing pyrites which on oxidation produce severe acidity. Depending on the type of soil, the entire wetland area of the region can be classified into Kayal lands (13000 ha.), Karappadams (33000 ha.) and Kari lands (9000 ha.).
Kayal lands are reclaimed beds from Vembanad lake and are mainly located in Kuttanad and Kottayam taluks. Karappadams are situated along the waterways and lakes, mainly in the eastern and southern parts of Kottayam district while Kari lands are situated in the taluks of Vaikom, Cherthala and Ambalapuzha.
Acid sulphate soil is the common name for soils that contain metal sulfides. In an undisturbed and waterlogged state, these soils may pose no or low risk. However, when disturbed or exposed to oxygen, acid sulfate soils undergo chemical oxidation, produces sulphuric acid which has led to these soils being called acid sulphate soils. The pH of the soil will be neutral or slightly acid in the field. Upon drainage, the soil becomes strongly acidic, which directly affects the growth of plants as a result of aluminium and iron toxicity and indirectly decreases the availability of phosphorus and other nutrients. Kuttanad showed a declining trend in rice production for the past few decades despite the use of high yielding varieties and modern farming techniques, definitely due to loss of soil health and fall in cropped area. Another major problem is the low inflow into Kuttanad during summer months (February-May) which leads to an increase in salinity, acidity and lack of water. Hitherto several studies have been carried out to characterize the soil chemically and evaluate the health. In contrast, a handful of research has been undertaken to characterize the acid sulphate soils in terms of the biological status which is very important for nutrient cycling and microbiologically mediated transformations. This study will add to the knowledge about the fate of the microflora and agriculturally significant enzymes in severe acidic conditions, which is a serious problem. More over the characterization of enzymes in these soils would provide a database for the future researcher to probe into this field of study.

MATERIALS AND METHODS
The present study was conducted in Kari soils of Kuttand. The soil samples were collected from six representative acid sulfate soil series. The soil series are 1. Ambalapuzha 2. Thuravoor 3. Vaikom 4. Kallara 5. Purakkad 6. Thakazhi.
The sample collection was done by stratified random sampling technique from both surface (0-15 cm) and subsurface ( 15-30 cm). Ten samples each were collected from both levels.
The samples were analysed to estimate soil respiratory activity, enzymes like phosphatase and urease, pH and EC to know the biological fertility status of acid sulphate soils. The urease activity was estimated by following the method described by Broadbent et al (1964). Phosphatase activity was determined by making use of procedure described from Eivazi and Tabatabai (1977). The method proposed by Jenkinson and Powlson (1976) was used to estimate the soil respiratory activity.

RESULTS AND DISCUSSION
The present study was undertaken to investigate the dynamics of enzymes in acid sulphate Kuttanad soil and to know the soil fertility status. The study comprised of soil sample collection, incubation studies and analysis of major enzymes. Results based on statistically analysed data pertaining to the experiment conducted during investigation are presented in the following paragraphs.     Table.4 the soil respiratory activity of Kuttanad kari soils were noticed. The soil respiratory activity was measured by the evolution of carbon dioxide from the soil incubated and the evolved carbon dioxide was collected in an alkali kept in a vial. The alkali was then titrated against acid. The soil respiratory activity is an indirect measure for arriving facts about soil biological activity and microbial population. According to Santruckova (1993), it is a strong indicator of soil metabolic and ecological functions. In this study the high values for soil respiratory activity was noticed in subsurface soils of Thuravoor kari and lowest value was in the subsurface soils of Thakazhi. The urease activity also showed the same trend. It might be due to the abundance of microbes in the Thuravoor kari than the other series.  Waterlogging associated with rice cropping mighy also enhances the accumulation of organic carbon. Stolt et al. (2000) explained that hydrologic regime played a role in organic matter accumulation. The highest organic matter of 5.14 % reported in Kallara soils might be also due to enrichment of weed biomass and paddy straw in the cultivated fallows (Pillai and Subrahmanyan, 1929;Fores and Comin, 1987). Beena (2005) also observed highest organic carbon content of 5.35 % in Kallara soils.

CONCLUSION
From the study it can be inferred that there is a relation between soil enzyme activity and soil respiration as it both have relation with soil microbial population and organic matter status of the soil. Also from the study it is again stressed that the pH of the Kuttanad soil is extremely acidic which also has a relation with enzyme activity and all other chemical properties.

Acknowledgement
The author wishes to acknowledge the Kerala Agricultural University and the faculties in Soil Science and Agricultural Chemistry department of College of Agriculture, Vellayani.