Characterization and classification of soils from Bankanahalli micro-watershed in the semi-arid tropics of Karnataka

Introduction

To enhance or at least maintain the present level of agricultural productivity, the management of land resources on scientific principles is paramount. Soil resource inventory provides the indispensable information required to understand the potential and limitations of land relative to optimum utilization. This requires a profound understanding of the properties and pedogenic processes of different soils in a given area, allowing for accurate mapping, characterization, and the implementation of sustainable management practices. The study of soils within a micro-watershed framework has been increasingly adopted, as it encapsulates the diverse soil formations driven by local topographic and hydrological variations (Bhattacharyya et al., 2024) [1] . In semi-arid tropical regions, the interplay of erratic rainfall, frequent droughts, and varying degrees of land degradation poses severe challenges to agricultural sustainability. The Bankanahalli micro-watershed, located in the Mandya district of Karnataka, presents a quintessential semi-arid landscape characterized by undulating topography and a mix of rainfed and protective irrigation agricultural systems. Topography significantly influences soil depth, drainage, and texture. Shallower, eroded soils typically dominate uplands, while deeper, fine-textured soils accumulate in valleys due to prolonged deposition and pedogenesis. Previous studies in similar landscapes across Karnataka have demonstrated the profound impact of agro-climatic zones and relief on soil morphology, fertility gradients, and taxonomic classification ( [9,10,7] .

Therefore, this study was undertaken to systematically characterize and classify the soils of the Bankanahalli microwatershed across its toposequence. The resulting physical and chemical data are critical for devising effective, climate-resilient soil and water conservation strategies and ensuring sustainable agricultural intensification.

Materials and Methods

Site Description

The Bankanahalli micro-watershed is situated between 12° 35' 6.86" to 12° 36' 56.68" N latitude and 76° 45' 13.14" to 76° 46' 23.72" E longitude, covering a geographic area of 489 hectares. It spans multiple villages including Gerahalli, Bevakallu, Billenahalli, Bankanahalli, Kulikoppalu, Nallahalli, and Hulikere within the Mandya district of Karnataka, India. The area falls under the Agro Ecological Sub Region (AESR) 8.2, characterized as a hot, moist, semi-arid eco-sub region within the southern plateau and hill region. It is categorized under Agro-climatic Zone 6 (Southern Dry Zone of Karnataka). The underlying geological formations range from Archaean granitic gneiss to Dharwar schists, providing the primary parent material for local pedogenesis. Climatically, the region experiences three distinct seasons: Kharif, Rabi, and Summer. The average annual precipitation is 565.7 mm, predominantly concentrated during the south-west monsoon months (June to September). The daily mean maximum temperature reaches approximately 28.50°C, while the daily mean minimum temperature drops to 20.17°C. The natural vegetation is sparse, comprising dry deciduous species and grasses, which are heavily influenced by grazing and human interference.

Soil Profile Study and Field Sampling

A detailed traverse of the micro-watershed was conducted, and nine representative soil profiles were excavated across distinct physiographic positions-encompassing uplands, mid-lands, and lowlands. The horizons in each profile were delineated and described in situ for their morphological features, including depth, boundary distinctness, colour, texture, structure, consistency, root distribution, and effervescence, following standard USDA and ICAR-NBSS&LUP Soil Survey Manual protocols. Horizon-wise soil samples were collected, air-dried in the shade, gently crushed using a wooden mortar and pestle, and passed through a 2 mm sieve to prepare for comprehensive laboratory analysis.

Laboratory Analytical Procedures

The processed soil samples were analyzed for a suite of physical and chemical properties using standard analytical protocols. Particle size distribution (sand, silt, and clay fractions) was estimated using the international pipette method following dispersion with sodium hexametaphosphate (Piper, 2002) [8] . Bulk density and maximum water holding capacity (MWHC) were determined utilizing the Keen's cup method. Soil reaction (pH) and electrical conductivity (EC) were measured in a 1:2.5 soil-to-water suspension. Organic carbon (OC) content was determined using the Walkley and Black wet oxidation method (Walkley and Black, 1934) [14] . Free calcium carbonate equivalent was estimated via the rapid titration method (Black, 1965) [2] . Cation exchange capacity (CEC) was determined by saturating the soil with 1 N sodium acetate (pH 8.2) and displacing it with neutral normal ammonium acetate (Thomas, 1982) [13] . Exchangeable bases (Ca, Mg, Na, K) were extracted using 1 N neutral ammonium acetate; sodium and potassium were quantified via flame photometry, while calcium and magnesium were determined via complexometric Versenate titration (Jackson, 1973) [3] .

Results and Discussion

Mechanical Composition and Physical Pedogenesis

The mechanical composition of the soils across the Bankanahalli micro-watershed revealed significant spatial and vertical differentiation, heavily influenced by landscape position and active pedogenic processes. The particle-size distribution and coarse fragment data indicate a definitive textural progression from the surface to the subsurface horizons across all depth categories, as detailed in Table 1. [4] . Although these dense clay accumulations provide superior nutrient and water-holding capacities-making them ideal for intensive cropping-they simultaneously present risks of restricted internal drainage and potential waterlogging during intense monsoonal rains.

Structural development aligned closely with this textural gradient. The surface horizons (Ap) generally exhibited very friable to friable granular or sub-angular blocky structures, facilitated by regular cultivation and lower clay contents. However, the subsurface argillic horizons displayed moderate to strong sub-angular blocky structures. In the deepest, clay-dense profiles (e.g., SMH, BDK), firm, angular blocky structures with pressure faces were observed, indicating vertic shrink-swell tendencies. Coarse fragments played a crucial mitigating role in these deep pedons. The subsurface coarse fragment content reached up to 60% in the SMH and SNH series, and 40% in the BDK series. This high proportion of coarse fragments within the dense clay matrices improves subsoil aeration, assists in root penetration, and reduces the overall compaction risks that typically plague heavy clay soils.

Chemical Characterization and Fertility Dynamics

The chemical properties across the identified soil series highlight diverse fertility potentials and localized constraints directly linked to profile depth, leaching regimes, and clay mineralogy. Detailed chemical attributes of the soil horizons are presented in Table 2. Soil reaction (pH) across the micro-watershed spanned a remarkably broad spectrum, reflecting differential weathering, leaching, and depositional environments (Patil et al., 2017a) [6] . The surface horizons of the Kumchahalli (KMH) series are strongly acidic (pH 5.41), favoring the solubility and availability of most micronutrients but potentially limiting basic cations. Conversely, the deeper soils, particularly in the lowland Santemarahalli (SMH) and Shyanadrahalli (SNH) series, exhibit progressive alkalization. The pH peaks at a strongly alkaline 10.43 in the SMH Bt5 horizon (122-145 cm). This rising alkalinity is a direct consequence of the accumulation of calcium carbonates and basic cations in the subsoil, a phenomenon driven by the limited leaching capacity of the semi-arid climate. Despite the high pH in specific subsoils, the Electrical Conductivity (EC) remains universally low throughout the watershed, ranging from 0.05 dS/m to a maximum of 1.14 dS/m (SMH Bt5). This confirms that the entire micro-watershed is non-saline, thus posing no osmotic stress limitations for standard crop cultivation. The distribution of organic carbon (OC) follows a strict and expected depth-dependent decline. Surface horizons (Ap) demonstrated moderate biological activity and organic input, with OC peaking at 0.84% in the SMH series, 0.66% in the CSR series, and 0.60% in the MKH series. However, OC rapidly depletes in the subsurface layers, plummeting to as low as 0.15% in the deeper Bt horizons of the SMH and BDK series. This sharp vertical decline underscores the rapid oxidation and mineralization of organic matter in hot, semi-arid environments. The intrinsically low subsoil organic carbon highlights the necessity for agricultural management practices that incorporate organic amendments, such as mulching and green manuring, to build sustainable carbon pools. Cation Exchange Capacity (CEC) varies significantly across the pedons, closely mirroring the clay distribution pattern. It ranges from a low of 6.02 cmol(p+)kg⁻¹ in coarse, sandy surface soils to a robust 23.47 cmol(p+)kg⁻¹ in the SNH Ap horizon, extending deeply into the argillic profiles. Exchangeable calcium is the overwhelmingly dominant basic cation across all series, contributing to a strong Base Saturation (BS). BS remains consistently high, ranging from a minimum of 55.15% to an optimal 97.25% in the SNH Bw3 horizon. This indicates that the exchange complex is predominantly occupied by nutrient bases, affording the soils a high capacity for nutrient retention and supply. While the soils are generally non-saline and exhibit high base saturation, the Exchangeable Sodium Percentage (ESP) data reveals critical localized sodicity risks that require immediate attention. In the shallow to moderately deep series (CSR, HTI, MKH, BSR, GHT), ESP is well-controlled, generally remaining below 4.15%. However, in specific horizons of the deep and very deep series, sodium accumulation is pronounced. The ESP reaches 15.57% in the SNH Bw4 horizon and spikes to a concerning 18.18% in the SMH Ap horizon. These elevated ESP values (>15%) cross the threshold for sodicity, signifying a distinct risk of soil dispersion. Sodic dispersion compromises aggregate stability, severely restricts subsoil permeability, induces surface crusting, and negatively impacts root proliferation. Ameliorative measures, such as the application of gypsum combined with deep-rooted cover cropping, are essential to displace exchangeable sodium and restore structural integrity in these affected zones.

Taxonomic Classification

Based on the integration of pedogenic properties, horizonation, base saturation, and moisture regimes, the soils of the Bankanahalli micro-watershed were classified into two dominant taxonomic orders under the USDA Soil Taxonomy framework (Soil Survey Staff, 2022; Nayak et al., 2002) [12,5]: Alfisols and Inceptisols. Alfisols constitute the predominant soil order, characterized by the illuviation of silicate clays leading to distinct argillic (Bt) horizons. The Mukhadalli and Bisarahalli series exhibit extensive clay accumulation and reddish hues driven by iron oxide weathering; they are classified at the subgroup level as Typic Rhodustalfs. The deeply weathered profiles of the Kumchahalli series, featuring prolonged pedogenesis and intense red colors, are classified as Rhodic Paleustalfs. The Bidarakatte series, similarly deep but lacking the distinct rhodic color requirements, is grouped under Typic Paleustalfs. The relatively younger, fine-loamy, moderately weathered upland profiles of the Chikkasavanur and Hatti series are classified as Typic Haplustalfs. Inceptisols were identified in specific midland and lowland positions where soil development is progressive but lacks complete argillic expression (Sireesha and Naidu, 2013) [11] . The Shyanadrahalli (SNH) series is characterized by a developing cambic (Bw) subsurface horizon. This cambic horizon displays moderate structural development (blocky structure) and slight clay accumulation relative to the overlying ochric epipedon, indicating active but incomplete pedogenesis. Consequently, the SNH series is classified as fine, isohyperthermic Typic Haplustepts. The transition from highly leached, permeable upland Typic Haplustalfs to dense, clay-rich lowland Paleustalfs and Haplustepts perfectly encapsulates the profound influence of topography and localized hydrology on semi-arid soil formation.

Conclusion

The comprehensive pedological characterization of the Bankanahalli micro-watershed reveals a highly diverse landscape where soil formation is intrinsically linked to topography, drainage, and semi-arid climatic forcing. The soils, primarily derived from granitic gneiss, range from shallow, highly permeable upland soils to very deep, clay-rich lowland profiles. Upland soils suffer from severe physical constraints, notably restricted moisture retention and shallow rooting depths, which heighten vulnerability to drought. Conversely, the lowland Alfisols and Inceptisols offer substantial fertility potentials driven by high cation exchange capacity, active clay illuviation, and high base saturation. However, these deep soils face distinct chemical and physical constraints, notably localized strong alkalinity and marginal sodicity (ESP up to 18.18%) in subsurface horizons. These conditions can impede permeability, induce soil dispersion, and restrict micronutrient availability. The classification of these soils into Typic Rhodustalfs, Paleustalfs, Haplustalfs, and Haplustepts provides a precise taxonomic framework reflecting their weathering intensity. The findings underscore the necessity of abandoning generalized agricultural practices in favor of site-specific, climate-resilient agronomic interventions. Practices such as organic mulching, the incorporation of green manure, and contour farming are highly recommended. These interventions will mitigate the specific constraints of moisture loss and erosion in the uplands, and structural degradation in the lowlands, ultimately fostering enhanced soil organic carbon sequestration and ensuring longterm agricultural sustainability in the semi-arid tropics.

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