MSc

Abstract
The Agro-ecological unit designated as North Central Laterites (AEU 10) represent midland laterite terrain. The soil is basically poor in humus, low in bases and silica, with dominance of kaolinitic clay and sesquoxides. Heavy rainfall and stagnant water create partially or completely reduced state in soil, alter biochemical activities and cause physical modifications. The deluge of August 2018 had triggered great variation in the soil environment with drastic changes in soil fertility posing threat to crop productivity. ‘Soil quality’ is an extensively used term to describe the functional capability of soil, which primarily depends on its dynamic properties that significantly change under environmental disturbances. The present study entitled “Assessment of soil quality in the post flood scenario of North Central Laterites (AEU 10) in Thrissur District of Kerala and mapping using GIS techniques” was undertaken to relook and evaluate the altered status of soil quality in the affected areas and to develop maps on soil characteristics and soil quality using GIS techniques.
One hundred geo-referenced surface soil samples (0-20 cm) were collected from severely affected locations comprising eleven grama panchayats of AEU 10 in Thrissur District. The samples were processed and characterised for physical, chemical and biological properties. Bulk density of soil ranged from 0.67 to 1.64 Mg m-3 and was found to be less than 1.40 Mg m-3 in eighty one per cent of the samples. Soil porosity was medium (30-50%) in ninety six per cent of samples and water holding capacity of soils ranged from 23.16 to 43.78%. Aggregate stability analysed in terms of mean weight diameter (MWD) ranged from 0.63 to 5.39 mm.
Ninety two per cent of samples were acidic in reaction of which forty seven per cent were extremely to strongly acidic (pH 3.5-5.5). Available nitrogen content was found to be low (<280 kg ha-1) in fifty three per cent of the soils, while the concentration of available phosphorus was very high in all samples. Ninety seven per cent soils were medium to high in available potassium (>116 kg ha-1) and sufficient (>300 mg kg-1) with respect to available calcium. Deficiency of available magnesium (<120 mg kg-1) was observed in sixty seven per cent samples, whereas all the soils were sufficient in available sulphur status. Due to leaching of boron as boric acid under acidic soil conditions, deficiency of available boron was predominant in seventy nine per cent of the samples.
Soil organic carbon content varied from 0.40 to 3.75 per cent and the content was high (>1.50 %) in twenty nine per cent of the samples and medium (0.75-1.5%) in fifty seven per cent of the samples. Low content of microbial biomass carbon (24.99 to 1022µg g-1) and dehydrogenase activity (1.02 to 532.00 µg TPF g-1 24h-1) was recorded in the soils, which indicated poor survival and activity of microorganisms.
Soil organic carbon content showed strong positive correlation with porosity, water holding capacity, and aggregate stability and strong negative correlation with bulk density and particle density. Available nitrogen in soil was found to have strong positive correlation with organic carbon. Exchange acidity showed significant negative correlation with soil pH and available calcium, indicating the replacement of calcium with aluminium on soil exchange sites at lower pH
Nutrient index of the flood affected areas of AEU 10 in Thrissur District was low (<1.67) with respect to available nitrogen, and high (>2.33) with respect to available phosphorus and potassium. Soil Quality Index (SQI) was formulated from the weighted scores of key indicators, selected through principal component analysis. Available calcium, soil pH, bulk density, available phosphorus, copper, boron, nitrogen and microbial biomass carbon were found to be the key indicators. A strong positive correlation (0.79**) was observed between the non-linear and linear scoring methods employed to compute the Soil Quality Index. The mean value of SQI ranged from 0.50 to 0.70 and from 0.71 to 0.87 as per non-linear and linear scoring methods respectively. Soil pH and bulk density were found to have the highest contribution to SQI whereas, the lowest contribution was by available boron, followed by microbial biomass carbon and available nitrogen.
The study also showed that the relative soil quality index (RSQI) was medium in more than 80 per cent of the sampling locations and poor in 13 locations. The mean relative soil quality index (RSQI) of all panchayats ranged from 50-70 % and was rated as medium. Shifting of key indicators from the optimal range contributed to decline in SQI. Consequently, site specific soil management practices to optimize quality indicators can improve soil fertility and enhance productivity.