Ph.D

The study entitled “Amelioration of surface and subsoil acidity in the laterite soils (Ultisols) of central and southern Kerala” was undertaken with the objectives of characterization and mapping of spatial variation of surface and subsoil acidity in the laterite soils (Ultisols) of central and southern Kerala - AEU 8, 9 and 10, evaluation of inorganic and organic amendments for amelioration of surface and subsoil acidity and the analysis of their effect on crop production using fodder sorghum as test crop. A survey was carried out and geo-referenced surface and subsurface soil samples were collected from 30 locations in AEU 8 (Southern laterites), 9 (South central laterites) and 10 (North central laterites) of Kerala from five depths viz. 0 20 cm, 20-40 cm, 40-60 cm, 60-80 cm and 80-100 cm. The samples were characterized for soil acidity parameters, EC, organic carbon and available nutrients. Soil acidity attributes except pH were classified using natural breaks or Jenks optimization method and graduated symbol maps were generated using ArcGIS pro 3.3.1. Soil pH maps were generated based on the existing classification. The results showed that soils of Vellayani and Balaramapuram in AEU 8, Kalluvathukal, Kottarakara and Kozhuvanal in AEU 9 and Madakkathara and Chalavara in AEU 10 displayed higher levels of surface and sub soil acidity. Among the sampling locations, soils of Vellayani which registered the lowest pH at 0-20 and 40-60 cm depths, highest exchangeable H+ at all depths and highest exchangeable Al3+ at 20-100 cm depth was selected for column incubation study and field validation. Soil samples were collected up to a depth of 90 cm from CoA, Vellayani for setting up the column incubation experiment for a period of 16 weeks. The soil columns were maintained at field capacity and treatments consisting of liming materials @ lime requirement (LR), liming materials @ lime requirement (LR) + co composted biochar (CCB) (5t/ha), and CCB (5t/ha) alone were applied to the top 15 cm. The soil columns were subjected to periodic leaching and destructive sampling through 0-15, 15-30, 30-60 and 60-100 cm depths at 4, 8, 12 and 16 weeks of incubation. The leached soil and leachate samples were characterized to elucidate Al replacement, nutrient retention and downward movement of Ca and Mg. The CCB-liming material combinations demonstrated better alleviation of soil acidity especially in the subsoil (15-90 cm) throughout the incubation experiment. Among the CCB- liming material combinations, combinations of burnt lime (BL) or dolomite (DL) with phosphogypsum (PG) alone or PG + magnesium sulphate (MS) showcased superior performance with lower exchangeable H+, Al3+ exchangeable, potential and pH dependent acidities, extractable, free and total Al and higher pH and Ca fractions at different depths. These treatments also displayed higher concentrations of Ca, Mg and Al in leachate indicating swift downward movement of Ca and Mg and removal of toxic levels of Al from the columns. The treatments ½ LR as BL + ½ as PG + CCB (5t/ha), ½ LR as DL + ½ as PG + CCB (5t/ha) and ½ LR as BL + ½ as PG & MS + CCB (5t/ha) showing higher potential for ameliorating soil acidity at different depths were selected for field validation. The field experiment was carried out using fodder sorghum as the test crop with the best three treatments selected from the soil column incubation experiment, each at 100 per cent LR and 75 per cent LR. The analysis of soil from different depths at the end of the field experiment showed that the application of ½ LR as BL + ½ LR as PG + CCB (5t/ha) @ 100 per cent LR displayed highest pH of 5.74 at 0-15 cm, 5.53 at 15-30 cm, 5.45 at 30-60 cm and 5.28 at 60-90 cm respectively. The pH values at different depths for this treatment was significantly higher than the other treatments except at 0-15 cm (5.74) where it was on par with ½ LR as BL + ½ as PG & Magnesium sulphate (MS) + CCB (5t/ha) @ 75 per cent LR (5.62). The treatment ½ LR as BL + ½ LR as PG + CCB (5t/ha) @ 100 per cent LR also recorded the lowest exchangeable H+ of 0.123 meq 100 g-1 at 0-15 cm, 0.213 meq 100 g-1 at 15-30 cm, 0.433 meq 100 g-1 at 30-60 cm and 0.490 meq 100 g-1 at 60-90 cm, respectively which was comparable with 1/2 LR as BL + 1/2 as PG & MS (3:2) + CCB (5t/ha) @ 100 per cent LR (0.133 meq 100 g-1) at 0-15 cm and with 1/2 LR as DL + 1/2 as PG + CCB (5t/ha) @ 100 per cent LR (0.437 meq 100 g-1), 1/2 LR as BL + 1/2 as PG & MS (3:2) + CCB (5t/ha) @ 100 per cent LR (0.443 meq 100 g-1) and ½ LR as BL + ½ as PG & Magnesium sulphate (MS) + CCB (5t/ha) @ 75 per cent LR (0.457 meq 100 g-1) at 30-60 cm. The treatment ½ LR as BL + ½ as PG & MS (3:2) + CCB (5t/ha) @ 100 per cent LR itself registered the lowest exchangeable Al3+ values of 0.073 meq 100g-1, 0.440 meq 100g-1 and 0.563 meq 100g-1, respectively at 0-15, 30-60 and 60-90 cm depths. At 0-15 cm depth, this was comparable with ½ LR as BL + ½ LR as PG + CCB (5t/ha) @ 100 per cent LR (0.083 meq 100g-1 ) and ½ LR as BL + ½ as PG & magnesium sulphate (MS) + CCB (5t/ha) @ 75 per cent LR (0.113 meq 100g-1), respectively whereas at 30-60 cm depth it was on par with 1/2 LR as DL + 1/2 as PG + CCB (5t/ha) @ 100 per cent LR (0.440 meq 100g-1 ), ½ LR as BL + ½ LR as PG + CCB (5t/ha) @ 100 per cent LR (0.44 meq 100g-1 ) and ½ LR as BL + ½ as PG & magnesium sulphate (MS) + CCB (5t/ha) @ 75 per cent LR (0.46 meq 100g 1), respectively. At 60-90 cm depth, ½ LR as BL + ½ as PG & MS (3:2) + CCB (5t/ha) @ 100 per cent LR was comparable with ½ LR as BL + ½ as PG + CCB (5t/ha) @ 100 per cent LR (0.567 meq 100g-1 ), 1/2 LR as DL + 1/2 as PG + CCB (5t/ha) @ 100 per cent LR (0.587 meq 100g-1 ) and ½ LR as BL + ½ as PG & Magnesium sulphate (MS) + CCB (5t/ha) @ 75 per cent LR (0.630 meq 100g-1), respectively. At 15-30 cm, ½ LR as BL + ½ as PG + CCB (5t/ha) @ 100 per cent LR displayed the lowest exchangeable Al3+ (0.163 meq 100g-1) which exhibited remarkable variation from other treatments. Similar results were observed for exchangeable, potential and pH dependent acidities, Al saturation and base saturation. The highest green fodder yield (86.7 t ha-1), dry matter yield (66.2 t ha-1), crude protein (9.14%), plant height (3.50m), root length (60.1 cm) and Ca uptake (188 kg ha-1) was also observed for ½ LR as BL + ½ as PG + CCB (5t/ha) @ 100 per cent LR. The study showed that an integrated approach utilizing organic and inorganic amendments is the most feasible option for the simultaneous alleviation of surface and subsoil acidity. The results of the incubation and field experiments suggests that the application of ½ LR as BL + ½ as PG + CCB (5t/ha) (546.6 kg BL per ha + 1161.8 kg PG per ha + CCB (5t ha-1)) has remarkable potential in alleviating surface and subsoil acidity, lowering Al saturation and improving soil reaction, base saturation and nutrient availability in the laterite soils of Kerala while ensuring higher crop productivity.