Characterization of Low pH Soils of the coastal and eastern dry zones of Karnataka: Implications for soil acidity and nutrient management

Introduction

Soil reaction (pH) is a fundamental property governing soil chemical behaviour, nutrient availability, microbial activity, and overall crop productivity. Acidic soils, generally defined by a pH below 5.5, impose serious limitations on agricultural production due to aluminum and iron toxicity, low base saturation, fixation of phosphorus, and deficiencies of essential nutrients such as calcium, magnesium, and molybdenum (Brady and Weil, 2017; Fageria and Baligar, 2008; Rengel, 2011) [5,8,27] . These constraints are particularly pronounced in tropical and subtropical regions, where intense weathering and leaching processes dominate soil formation (Von Uexküll and Mutert, 1995; Kochian et al., 2015) [15,34] . Globally, acidic soils account for approximately 30-40% of the world's arable land, with major occurrences in humid tropics and subtropics (Von Uexküll and Mutert, 1995; Guo et al., 2010) [10, 34] . In India, acid soils cover nearly 49 million hectares and are predominantly distributed in eastern, northeastern, southern coastal, and hilly regions characterized by high rainfall and highly weathered parent materials (Sarkar, 2002; Mandal et al., 2019) [17,28] . Continuous leaching of base-forming cations, long-term application of acid-forming fertilizers, and organic matter mineralization have further accelerated soil acidification across these regions (Gupta et al., 2017; Panda et al., 2020) [11,20] . In Karnataka, soil acidity is a major soil-related constraint affecting crop productivity and soil health, particularly in the Coastal Zone and parts of the Eastern Dry Zone. Karnataka exhibits diverse agro-ecological conditions ranging from humid coastal plains to semi-arid inland regions, resulting in wide spatial variability in soil properties (Sehgal et al., 1995; ICAR-NBSS&LUP, 2020) [13,29] . The Coastal Zone of Karnataka is characterized by a humid tropical climate, annual rainfall exceeding 3000 mm, and soils derived largely from lateritic parent material. These conditions promote intense leaching of calcium and magnesium and accumulation of exchangeable aluminum and iron, leading to strongly to extremely acidic soil reactions (Hegde and Bhat, 2014; Prakash et al., 2016; Bhat et al., 2015) [2,12,23] . Several studies conducted in coastal districts such as Dakshina Kannada, Udupi, and Uttara Kannada have reported low soil pH, low base saturation, high phosphorus fixation, and excess availability of iron and manganese, all of which adversely affect nutrient uptake and crop growth (Nayak et al., 2017; Shivakumar et al., 2018; Prasad et al., 2019) [19,24,31] . Although these soils often contain moderate to high organic carbon due to dense vegetation and high biomass turnover, nutrient availability remains constrained because of acidity-induced fixation and metal toxicity (Rao et al., 2016) [25] . The study was conducted in two contrasting agro-climatic zones of Karnataka, namely the Coastal Zone and the Eastern Dry Zone, which differ markedly in climate, parent material, and soil-forming processes. The Coastal Zone, located along the western coast of the state, is characterized by humid tropical climate and high rainfall, while the Eastern Dry Zone represents a semi-arid region with comparatively lower rainfall. The selection of these two zones enabled assessment of the influence of contrasting pedo-climatic conditions on soil acidity and nutrient dynamics. The Eastern Dry Zone of Karnataka, despite receiving comparatively lower rainfall (600-900 mm), also exhibits occurrences of acidic soils. Prolonged cultivation, continuous nutrient removal by crops, depletion of exchangeable bases, limited organic matter inputs, and imbalanced fertilizer use have contributed to gradual soil acidification in this region (Patil et al., 2016; Ramesh et al., 2018; Anil Kumar et al., 2020) [1,21,26] . Soils derived from granitic gneiss parent material typically possess low buffering capacity, making them particularly susceptible to pH decline under intensive cropping systems (Srinivas et al., 2021; Shankar et al., 2017) [30,32] . Although several studies have documented soil acidity in different parts of Karnataka, most investigations have focused on isolated locations or individual soil properties. Comprehensive characterization of low pH soils across contrasting agro-climatic zones, with emphasis on soil reaction, exchangeable acidity, base saturation, and macro-and micronutrient status, remains limited (ICAR-NBSS&LUP, 2020; Panda et al., 2020) [13,20] . Such characterization is essential for understanding zone-specific soil constraints and for formulating effective soil acidity amelioration strategies, including liming and integrated nutrient management. Therefore, the present study was undertaken to characterize the low pH soils of the Coastal and Eastern Dry Zones of Karnataka with respect to their physico-chemical properties and fertility status, with the objective of supporting sustainable soil and crop management in these regions.

Materials and Methods

Study Area

The present investigation was carried out in two contrasting agro-climatic zones of Karnataka, namely the Coastal Zone and the Eastern Dry Zone, representing humid and semi-arid soil environments, respectively. The Coastal Zone is characterized by a humid tropical climate with mean annual rainfall exceeding 3000 mm and soils predominantly derived from lateritic parent material. In contrast, the Eastern Dry Zone experiences semiarid climatic conditions with annual rainfall ranging from 600 to 900 mm and soils developed mainly from granitic gneiss parent material. These zones were selected due to the widespread occurrence of low pH soils and their contrasting pedo-climatic conditions (Sehgal et al., 1995; ICAR-NBSS&LUP, 2020) [13,29] .

Soil Sampling

Representative soil sampling sites were selected from cultivated agricultural fields in both zones following a stratified random sampling approach. At each location, composite soil samples were collected from the surface (0-15 cm). The samples were air-dried under shade, gently crushed, and passed through a 2mm sieve before laboratory analysis (Jackson, 1973) [14] . Analytical methods used for characterization of soil samples are tabulated in Table 1 2.3 Chemical Analysis Soil pH was measured in a 1:2.5 soil-water suspension using a calibrated digital pH meter (Jackson, 1973) [14] . Electrical conductivity (EC) was determined in the same extract using a conductivity bridge and expressed as dS m⁻¹ at 25°C. Soil organic carbon content was estimated by the Walkley and Black wet oxidation method (Walkley and Black, 1934) [35] . Cation exchange capacity (CEC) was determined by the neutral normal ammonium acetate (1 N NH₄OAc, pH 7.0) method, followed by steam distillation of adsorbed ammonium (Chapman, 1965) [7] . [14] 2

Electrical conductivity (EC) Conductivity meter (1:2.5 soil-water extract) Jackson (1973) [14] 3 Organic carbon Walkley and Black wet oxidation method Walkley and Black (1934) [35] 4

Soil texture International pipette method Piper (1966) [22] 5

Cation exchange capacity (CEC) Neutral normal ammonium acetate (1 N NH₄OAc, pH 7.0) Chapman (1965) [7] 6

Exchangeable acidity (H⁺ + Al³⁺) 1 N KCl extraction followed by titration McLean (1982) [18] 7

Available nitrogen (N) Alkaline potassium permanganate method Subbiah and Asija (1956) [33] 8

Available phosphorus (P) Bray-I extraction method Bray and Kurtz (1945) [6] 9

Available potassium (K) Neutral normal ammonium acetate extraction; flame photometry Jackson (1973) [14] [3] 2.4 Exchangeable Acidity Exchangeable acidity (H⁺ + Al³⁺) was extracted using 1 N KCl and determined by titration with standard NaOH solution (McLean, 1982) [18] . Exchangeable aluminum was estimated separately by back titration after fluoride complexation. (Jackson, 1973) [14] .

Available Macronutrients

Available nitrogen was estimated by the alkaline potassium permanganate method as described by Subbiah and Asija (1956) [33] . Available phosphorus was extracted using the Bray-I method, suitable for acidic soils, and quantified colorimetrically using a spectrophotometer (Bray and Kurtz, 1945) [6] . Available potassium was extracted with neutral normal ammonium acetate and determined using a flame photometer (Jackson, 1973) [14] .

Available Micronutrients

DTPA-extractable micronutrients (Fe, Mn, Zn, and Cu) were extracted using a solution containing 0.005 M DTPA, 0.1 M triethanolamine, and 0.01 M CaCl₂ (pH 7.3) and analyzed using an atomic absorption spectrophotometer (Lindsay and Norvell, 1978) [16] .

The sampling and analytical procedures adopted in the present study are based on standard and widely accepted protocols in soil science research. The use of composite sampling, coupled with established laboratory methods for physical, chemical, and fertility analyses, ensures the reliability and reproducibility of the generated data. Selection of appropriate extractants and analytical techniques for acidic soils enabled accurate assessment of soil reaction, exchangeable acidity, base saturation, and macro-and micronutrient availability. The methodological framework employed in this study provides a robust basis for comparative characterization of low pH soils across contrasting agro-climatic zones and supports meaningful interpretation of results for developing site-specific soil acidity management strategies.

Results and Discussion

The physico-chemical and chemical properties of soils from the Coastal and Eastern Dry Zones of Karnataka are presented in Tables 2 and3, and their comparative trends are illustrated in Figures 1 and2. The results reveal pronounced zone-wise differences in soil reaction, nutrient availability, and exchangeable cation composition, reflecting the influence of agro-climatic conditions, parent material, and land-use history.

Soil Reaction and Exchangeable Aluminium

Soil pH varied significantly between the two agro-climatic zones Figure 1. Coastal Zone soils exhibited moderately to strongly acidic reaction with a mean pH of 5.12, whereas Eastern Dry Zone soils recorded comparatively higher pH values with a mean of 5.79, indicating slightly acidic conditions. The lower pH of coastal soils is attributed to intense leaching of baseforming cations under high rainfall and advanced weathering of lateritic parent materials (Von Uexküll and Mutert, 1995; Hegde and Bhat, 2014) [2,12,34] . Exchangeable aluminium content showed an inverse relationship with soil pH Figure 1 Coastal Zone soils recorded substantially higher exchangeable Al (mean 1.46 cmol(p⁺) kg⁻¹) compared to Eastern Dry Zone soils (mean 0.81 cmol(p⁺) kg⁻¹). Elevated aluminium levels under acidic conditions increase the risk of aluminium toxicity, which restricts root growth and impairs nutrient uptake (Fageria and Baligar, 2008; Kochian et al., 2015) [8, 15] .

Organic Carbon and Nitrogen Status

Organic carbon content differed markedly between zones (Table 2; Figure 1). Coastal Zone soils recorded higher organic carbon (mean 1.30%) owing to dense vegetation cover, higher biomass addition, and slower decomposition under humid acidic conditions. In contrast, Eastern Dry Zone soils exhibited low organic carbon content (mean 0.50%), reflecting limited organic inputs, rapid oxidation of organic matter under a semi-arid climate, and continuous cultivation (Shivakumar et al., 2018; Ramesh et al., 2018) [26,31] . Available nitrogen followed trends similar to organic carbon, with Coastal Zone soils showing slightly higher mean values (303 kg ha⁻¹) than Eastern Dry Zone soils (293 kg ha⁻¹) (Table 2). This indicates that organic matter serves as a key reservoir of mineralizable nitrogen in both zones.

Phosphorus and Potassium Availability

Available phosphorus content was consistently low in Coastal Zone soils (mean 15.8 kg ha⁻¹) compared to Eastern Dry Zone soils (mean 33.4 kg ha⁻¹) (Table 2; Figure 2). Low phosphorus availability in coastal soils is primarily due to strong fixation by iron and aluminium oxides under acidic conditions (Bray and Kurtz, 1945; Prakash et al., 2016) [6,23] . The relatively higher pH of Eastern Dry Zone soils reduces fixation intensity, resulting in improved phosphorus availability. Available potassium levels were comparable between zones, with mean values of 101 kg ha⁻¹ in the Coastal Zone and 102 kg ha⁻¹ in the Eastern Dry Zone (Table 2). However, leaching losses in coastal soils and continuous crop removal in Eastern Dry Zone soils influence district-wise variability.

Exchangeable Bases and Secondary Nutrients

Exchangeable calcium and magnesium were significantly lower in Coastal Zone soils (mean Ca 1.36 and Mg 0.37 cmol(p⁺) kg⁻¹) compared to Eastern Dry Zone soils (mean Ca 2.28 and Mg 0.44 cmol(p⁺) kg⁻¹) (Table 3; Figure 2). Depletion of base cations in coastal soils reflects prolonged leaching and their replacement by acidic cations, whereas higher base status in Eastern Dry Zone soils indicates better buffering capacity against soil acidity. Available sulphur content was also lower in Coastal Zone soils (mean 10.88 mg kg⁻¹) than in Eastern Dry Zone soils (mean 18.10 mg kg⁻¹) (Table 3), likely due to leaching losses under humid conditions.

Micronutrient Distribution

DTPA-extractable iron and manganese were markedly higher in Coastal Zone soils (mean Fe 13.8 mg kg⁻¹ and Mn 3.41 mg kg⁻¹) compared to Eastern Dry Zone soils (mean Fe 4.9 mg kg⁻¹ and Mn 1.98 mg kg⁻¹) (Table 3). Elevated availability of Fe and Mn under acidic conditions may induce micronutrient toxicity and antagonistic interactions with phosphorus and zinc (Lindsay and Norvell, 1978; Prasad et al., 2019) [16,24] . In contrast, Eastern Dry Zone soils exhibited marginal availability of zinc, copper, and boron. Hot water soluble boron content was notably lower in Eastern Dry Zone soils (mean 0.24 mg kg⁻¹) compared to Coastal Zone soils (mean 0.41 mg kg⁻¹) (Table 3), indicating potential boron deficiency under intensive cropping systems.

Integrated Assessment of Soil Fertility Constraints

A combined evaluation of Tables 2 and3 and Figures 1 and2 indicates that Coastal Zone soils are constrained primarily by strong acidity, high exchangeable aluminium, low base cation status, phosphorus fixation, and excess iron and manganese. In contrast, soils of the Eastern Dry Zone exhibit moderate acidity with improved base status and phosphorus availability but are limited by low organic carbon and marginal micronutrient availability. The results clearly demonstrate that agro-climatic conditions exert a dominant control on soil acidity and nutrient dynamics in Karnataka. Coastal Zone soils are affected mainly by acidityinduced toxicity and nutrient fixation, whereas Eastern Dry Zone soils face nutrient depletion and imbalance. These contrasting constraints underscore the necessity for zone-specific soil management strategies rather than uniform recommendations.

Conclusion

The study characterized low pH soils of the Coastal and Eastern Dry Zones of Karnataka and revealed distinct zone-specific soil acidity and fertility constraints. Coastal Zone soils were strongly acidic with high exchangeable aluminium, low calcium and magnesium, low phosphorus availability, and excessive iron and manganese, reflecting intense leaching and advanced weathering under humid conditions. Despite higher organic carbon content, nutrient availability in these soils remained restricted due to acidity-induced fixation and metal toxicity.

In contrast, soils of the Eastern Dry Zone were slightly acidic with comparatively higher base cation status and phosphorus availability but were constrained by low organic carbon and marginal micronutrient availability, particularly zinc, copper, and boron. The inverse relationship between soil pH and exchangeable aluminium and the positive influence of pH on phosphorus availability highlight the dominant role of soil reaction in regulating nutrient dynamics across both zones. Overall, the findings emphasize the need for zone-specific soil management strategies, with liming and phosphorus management being critical for Coastal Zone soils, and integrated nutrient management focusing on organic carbon enhancement and micronutrient supplementation for Eastern Dry Zone soils.

This study provides a scientific basis for developing locationspecific soil fertility management practices to improve soil health and sustain crop productivity in Karnataka.

References

1. Anil Kumar KS, Basavaraj B, Patil PL, Shankar MA. Soil fertility status of major cropping systems in Eastern Dry Zone of Karnataka. J Indian Soc Soil Sci. 2020;68(2):155162.
2. Bhat R, Hegde R, Prakash NB. Characterization and management of acidic soils of coastal Karnataka. Agropedology. 2015;25(1):45-54.
3. Berger KC, Truog E. Boron determination in soils and plants. Ind Eng Chem Anal Ed. 1939;11:540-545.
4. Blake GR, Hartge KH. Bulk density. In: Klute A, editor. Methods of Soil Analysis. Part 1: Physical and Mineralogical Methods. 2nd ed. Madison (WI): ASA and SSSA; 1986. p. 363-375.
5. Brady NC, Weil RR. The Nature and Properties of Soils. 15th ed. New York: Pearson Education; 2017.
6. Bray RH, Kurtz LT. Determination of total, organic, and available forms of phosphorus in soils. Soil Sci. 1945;59:39-45. doi:10.1097/00010694-194501000-00006.
7. Chapman HD. Cation-exchange capacity. In: Black CA, editor. Methods of Soil Analysis. Part 2: Chemical and Microbiological Properties. Madison (WI): ASA; 1965. p. 891-901.
8. Fageria NK, Baligar VC. Ameliorating soil acidity of tropical oxisols by liming for sustainable crop production. Adv Agron. 2008;99:345-399. doi:10.1016/S00652113(08)00407-0.
9. Gomez KA, Gomez AA. Statistical Procedures for Agricultural Research. 2nd ed. New York: John Wiley & Sons; 1984.
10. Guo JH, Liu XJ, Zhang Y, et al. Significant acidification in major Chinese croplands. Science. 2010;327:1008-1010. doi:10.1126/science.1182570.
11. Gupta RK, Sharma SK, Singh B. Soil acidity and its management in Indian agriculture. Indian J Fertil. 2017;13(5):16-27.
12. Hegde R, Bhat R. Acid soils of coastal Karnataka: distribution, properties and management. J Indian Soc Soil Sci. 2014;62(3):259-266.
13. ICAR-NBSS&LUP. Soil Resource Mapping of Karnataka. Nagpur: National Bureau of Soil Survey and Land Use Planning; 2020.
14. Jackson ML. Soil Chemical Analysis. New Delhi: Prentice Hall of India Pvt. Ltd.; 1973.
15. Kochian LV, Piñeros MA, Liu J, Magalhaes JV. Plant adaptation to acid soils: the molecular basis for crop aluminum resistance. Annu Rev Plant Biol. 2015;66:571598. doi:10.1146/annurev-arplant-043014-114822.
16. Lindsay WL, Norvell WA. Development of a DTPA soil test for zinc, iron, manganese, and copper. Soil Sci Soc Am J. 1978;42:421-428. doi:10.2136/sssaj1978.03615995004200030009x.
17. Mandal AK, Sharma RC, Singh G. Acid soils of India: distribution, extent and management. Indian J Fertil. 2019;15(4):16-27.
18. McLean EO. Soil pH and lime requirement. In: Page AL, Miller RH, Keeney DR, editors. Methods of Soil Analysis. Part 2. 2nd ed. Madison (WI): ASA and SSSA; 1982. p. 199-224.
19. Nayak AK, Prakash NB, Shivakumar BG. Phosphorus fixation characteristics of acidic soils of coastal Karnataka. J Soil Sci Plant Nutr. 2017;17(2):467-480.
20. Panda BB, Mishra S, Nanda SS. Soil acidity management for sustainable agriculture in India. J Soil Water Conserv. 2020;19(2):99-110.
21. Patil PL, Shankar MA, Basavaraj B. Soil buffering capacity and acidity status in granitic soils of Eastern Dry Zone of Karnataka. Agropedology. 2016;26(1):61-69.
22. Piper CS. Soil and Plant Analysis. Bombay: Hans Publishers; 1966.
23. Prakash NB, Hegde R, Bhat R. Properties and management of lateritic soils of coastal Karnataka. Agropedology. 2016;26(2):95-104.
24. Prasad J, Rao S, Ramesh T. Micronutrient availability in acidic soils of humid regions of Karnataka. J Indian Soc Soil Sci. 2019;67(4):411-420.
25. Rao KV, Hegde R, Shivakumar BG. Organic carbon dynamics in acid soils of coastal Karnataka. J Soil Sci Plant Nutr. 2016;16(3):701-712.
26. Ramesh T, Anil Kumar KS, Patil PL. Soil fertility constraints in Eastern Dry Zone of Karnataka. J Indian Soc Soil Sci. 2018;66(3):257-266.
27. Rengel Z. Soil pH, soil health and climate change. In: Soil Health and Climate Change. Berlin: Springer; 2011. p. 6985.
28. Sarkar D. Acid Soils of India: Their Distribution, Management and Future Strategies. New Delhi: ICAR; 2002.
29. Sehgal JL, Mandal DK, Mandal C, Vadivelu S. AgroEcological Regions of India. Nagpur: NBSS&LUP; 1995.
30. Shankar MA, Patil PL, Basavaraj B. Soil acidity and crop response in different agro-climatic zones of Karnataka. Indian J Agric Res. 2017;51(6):529-535.
31. Shivakumar BG, Prakash NB, Nayak AK. Organic carbon and nutrient dynamics in acidic soils of coastal Karnataka. J Indian Soc Soil Sci. 2018;66(2):183-191.
32. Srinivas S, Anil Kumar KS, Ramesh T. Influence of parent material on soil acidity and buffering capacity in semi-arid regions of Karnataka. Geoderma Reg. 2021;24:e00364. doi:10.1016/j.geodrs.2021.e00364.
33. Subbiah BV, Asija GL. A rapid procedure for the estimation of available nitrogen in soils. Curr Sci. 1956;25:259-260.
34. Von Uexküll HR, Mutert E. Global extent, development and economic impact of acid soils. Plant Soil. 1995;171:1-15. doi:10.1007/BF00009558.
35. Walkley A, Black IA. An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci. 1934;37:29-38. doi:10.1097/00010694-193401000-00003.