Assessment of soil quality in the post flood scenario of AEU 15 (northern high hills) in Thrissur district of Kerala and mapping using GIS techniques

Abstract

Agro-ecological unit 15 (AEU 15) represents the Northern High Hills which is characterised by long dry spells (4 months in a year), a tropical humid monsoon type climate with an average annual precipitation of 3459.5 mm and a mean annual temperature of 26.20 C. The hilly terrains have deep, well drained clayey soils rich in organic matter, strongly acidic and low in bases whereas the valleys have deep, imperfectly drained acid clayey soils. The August floods of 2018 had caused great havoc to life, property and agriculture of the state causing drastic changes in soil properties thereby affecting soil quality and fertility and thereby its productivity. The study entitled ‘Assessment of soil quality in the post flood scenario of AEU 15 (Northern High Hills) in Thrissur district of Kerala and mapping using GIS techniques’ was therefore conducted with an objective to assess soil quality in the designated AEU and prepare thematic maps using GIS. A total of one hundred and four geo-referenced soil samples were collected from five grama panchayats namely Pazhayannur, Pananchery, Puthur, Varantharappilly and Mattathur, which were affected by floods. These soils were characterized for physical, chemical and biological properties. The bulk density values ranged from 0.83 to 1.74 Mg m-3 and 80.77% of the soils had a bulk density greater than 1.20 Mg m-3. Porosity ranged between 30 to 60% in 99.04 per cent of the samples and 83.65 per cent of the samples had maximum water holding capacity in the range of 30-50 %. Among the soil samples, 53.84 per cent belonged to moderately acidic /slightly acidic/neutral category (pH ≥ 5.6). All the soils had electrical conductivity less than 1.0 dS m-1. Exchangeable acidity was greater than 1 cmol kg-1 in 81.73% of the samples. In case of organic carbon, 39.40 per cent of the samples had low (< 0.75%) and 53.80 per cent of the samples had a medium (0.75- 1.50 %) organic carbon content. Available nitrogen content was low (< 280.0 kg ha-1) in 97.12 per cent of the samples whereas available phosphorus content was high (>25.0 kg ha-1) in all the samples. In the case of available potassium, 58.65 per cent of the samples had medium (116.0 -275.0 kg ha-1) and 31.73 per cent had high (>275.0 kg ha-1) contents. Available calcium content was sufficient (>300 mg kg-1) in 99.04 per cent of the samples. Deficiency of available magnesium (<120 mg kg-1) was found in 59.62 per cent of the samples. In case of available sulphur, 58.65% of samples were deficient. All the soils were sufficient in terms of available micronutrients viz., iron, manganese, copper and zinc. But all the samples were deficient in available boron. In 42.30 per cent of the samples, dehydrogenase activity was less than 75 μg TPF g -1 24 hr-1. Nutrient indices of flood affected areas in AEU 15 in Thrissur district were low (<1.67) with respect to available nitrogen; medium (1.67- 2.33) with respect to organic carbon and available potassium and high (>2.33) with respect to available phosphorus. Using principal component analysis (PCA), seven principal components with eigen values greater than one were extracted and eight soil parameters were identified as the key indicators determining the soil quality of the area. The key indicators formed the minimum data set (MDS) viz., porosity, water holding capacity, pH , available nitrogen, potassium, magnesium, manganese and boron. Non linear scoring method was adopted to assess soil quality. The products of score and weightage factor of the MDS parameters were summed up to obtain a soil quality index (SQI) of that particular site. Soil quality indices were rated using relative soil quality index (RSQI). It was found that 79.81 per cent of the soil samples had a medium relative soil quality index . Organic carbon showed a significant positive correlation with available nitrogen, zinc, magnesium, copper, maximum water holding capacity and dehydrogenase activity. In comparison with preflood data (GoK, 2013) where all samples were found to be acidic (pH 4.50- 6.50), 8.65% of the post flood soils exhibited neutral range of pH. Organic carbon, available nitrogen, sulphur and boron became more deficient after the floods. But there was an increase in the content of nutrients like available phosphorus, potassium, calcium, magnesium, copper and zinc. Bulk density of soils also increased after the floods. The soil quality of post flood soils in the AEU have to be improved by adopting site specific and integrated nutrient management practices in a comprehensive manner including fertilizers, organic sources and biofertilizers.