Petrocalcic Horizons Research Articles

Delineating the significance of sharp boundaries between horizons and their synergetic linkage in calcareous vineyard soils

The present work was undertaken to appraise and delineate the significance of sharp boundaries between horizons in calcareous vineyard soils by evaluating parent material uniformity and pedogenesis. To do so, 10 soil profiles (selected from 275) were investigated. The profiles up to a depth of approximately 150 cm were opened. Macromorphological features were described following FAO guidelines. Physico-chemical properties were analysed by the usual methods. X-ray fluorescence (XRF) was used to determine elemental Ti and Zn concentrations. In terms of the main morphological characteristics, soils are reddish, and have a well-developed solum, a subangular or prismatic structure and friable moist consistency (B horizon) unlike the C horizon, which generally varies from friable to hard or very hard in a massive structure. These latter are usually related to the pedogenesis of evolved soils containing accumulations of calcium carbonate to form calcic or petrocalcic horizons. Other genetic horizons of the studied soils are argillic or cambic, which all lie directly on parent materials, in addition to a surface Ap anthropic horizon. Although the considered soils displayed a significant sharp boundary between some horizons, and even though some soil parent materials were stratified in nature and apparently formed of multidepositional regimes, certainly the vertical distribution of Ti/Zr displayed minor variations with no clear inflection with depth. Therefore, the parent materials of the investigated soils are apparently formed of homogeneous depositional materials. The morphological contrast within a profile is due mainly to pedological processes (they are very mature) rather than to geogenic ones. Nevertheless, the marked contrasts between horizons cannot be explained only by pedogenetic processes, but by other secondary processes related to agricultural use (mechanical impact of tillage). This means a partly artificial distinction that entails surface erosion and, consequently, a manifest redistribution fundamentally on surface horizons. Based on all this, it can be deduced that these soils are formed from local and not from extraregional materials.

Calcareous termite mounds in South Africa are ancient carbon reservoirs

Ecosystems that offer carbon sequestration by leaching bicarbonate to groundwater are valuable natural capital. One region that may offer this service is the west coast of South Africa. Over 20 % is covered by soil mounds (“heuweltjies”) up to 40 m diameter, 2 m high, inhabited by the southern harvester termite Microhodotermes viator and enriched in soil organic and inorganic carbon and soluble minerals. We aimed to generate radiogenic and stable isotope data for soils and groundwater in a region where these data are absent, to 1) verify the atmosphere-soil-groundwater link, and 2) resolve the timing and pattern of calcite dissolution and water infiltration in the landscape. Results show that soil and groundwater sulfate have the same marine aerosol source. Episodic calcite dissolution in mound centers, which increased during periods of global cooling, has been set against background input of marine aerosols since before the Last Glacial according to radiocarbon (14C) ages. Our data push back soil organic carbon 14C ages of inhabited termite mounds to 13-19 ka (kiloannum, thousand years before present), nest carbonate 14C ages to 33 ka, and mound soil carbonate 14C ages to 34 ka, making these the oldest active termite features ever dated. These ages are consistent with soil organic carbon and carbonate 14C ages of regional, non-mound, coastal petrocalcic horizons formed by accumulation of carbonate leached from their overlying aeolian dune fields. Harvesting activities of termites inject younger organic material around nests >1 m deep, leading to continuous renewal of important soil carbon reservoirs at depth. Termite bioturbation increases the system's ability to dissolve carbonate. The central, bioturbated part of the mounds have greater infiltration depths and greater calcite dissolution, whereas surrounding soils experienced more surface runoff. Calcareous termite mounds offer a mechanism to sequester CO2 through dissolution and leaching of soil carbonate-bicarbonate to groundwater.

Carbonate morphological features of vineyard soils in a semiarid Mediterranean environment

AbstractMany vineyard soils in semiarid Mediterranean environments develop from carbonated materials and often show very diverse forms of pedogenic calcium carbonate accumulations. Based on 275 vineyard soil profiles in the Castilla La Mancha (CLM) territory (Spain), this paper presents an in‐depth comprehensive study of a large number of morphological expressions of these carbonate distributions, while exploring various pedogenesis formation hypotheses. The interactions among the calcareous nature of parent material, surface topography and climate are the factors that most affected their morphology and pedogenesis. The occurrence and distribution of carbonate features in CLM vineyard soils can be more complex than in other areas. Many accumulations are relatively ancient, which is the case of petrocalcic horizons, and in‐keeping with conservative means, which reveals the occurrence of successive carbonate accumulation cycles. More recently dissolved and precipitated seasonally pedogenic carbonates (sometimes as coatings) have appeared, which are nothing more than a simple recent redistribution of carbonates. A structural imprint is observed: pedogenic carbonates appeared much more abundantly on the lowermost horizons, which can be related to the repeated redistribution phases that occurred in different pedogenesis stages. The data provided herein help to improve the understanding of the nature of viticultural calcareous soils in semiarid Mediterranean regions.

Geochemical modeling of systems affected by irrigation: The case of Lerma basin (Spain, 2004–2020)

Geologically “saline” zones with scarce pluviometry, which are already susceptible to the salinization of natural drainage, can experience the acceleration of the salinization of the receiving water systems with the implementation of irrigation. The main objective of this paper is to analyze the geochemical processes that control the variations of the hydrosaline balance due to the implementation of irrigation of the Lerma basin (Spain) from the beginning of its transformation into irrigation land (2004) until the consolidation of irrigation (2020). The results of this study evidence the dissolution of some mineral phases, such as halite, gypsum, and dolomite, and the precipitation of others, such as calcite. Additionally, the final composition of the irrigation return flows cannot be explained without consideration of the NaCa exchange. Part of the dissolved Ca2+ is deposited in the soil, which, in turn, contributes with Na+ to the solution. These natural processes are accelerated with irrigation but progressively slow down as the soil salts are washed with time. Although less evident, there is an additional negative agroenvironmental effect associated with the precipitation of calcite and the possible formation of petrocalcic horizons in the soil. The results obtained herein indicate that studies focusing on the salinity of irrigated zones should go a step further and include the geochemical processes in quantifying the global mass of exported salts.

A morphological approach to evaluating the nature of vineyard soils in semiarid Mediterranean environment

AbstractLa Mancha (Central Spain) is one of the most extensive vineyard regions in the world, and ‘Valdepeñas’ is a representative Protected Denomination of Origin (PDO) in this region. However, what are their main soil types? what kind of horizons are the most common? and what is the role of the geomorphological positions in their pedodiversity? After describing and sampling 90 soil profiles in this area, Alfisols, Inceptisols and Entisols were mainly identified in Soil Taxonomy terms; in other words, Luvisols, Cambisols, Regosols, Leptosols and a highly significant proportion of Calcisols according to FAO‐UNESCO‐ISSS. The accumulation of carbonate, the thickness of which varies from a diffuse or powdery form to crusted forms, appear sometimes like polycyclic. The presence of red soils, with or without a calcic or petrocalcic horizon, indicates the most representative edaphic stages in this region. Consequently, the morphological signature is calcic or petrocalcic, followed by argillic and/or cambic horizons, under ochric horizons. It can be concluded that the nature of soils in Valdepeñas can be considered a differential factor to bear in mind for quality viticultural production.Highlights There are unknown conceptual zones in support production of wine. A comprehensive study in a local case was performed due to its traditional production of vineyards. This study highlights the importance and uniqueness of the calcic and petrocalcic horizons. A disconnect exists between some tradicional viticultural zones with low reputation and the real value of their soils.

Provenance, and environment context of pedogenic carbonates formation from MIS 3 to MIS 1 in the Teotihuacan Valley, Mexico

Pedogenic carbonates are considered important indicators of past climatic changes. Their formation is due to calcite accumulation through evaporation processes in arid or semi-arid regions. Over time, secondary carbonate accumulation results in the development of calcic and petrocalcic horizons in soil profiles. In this study, we conducted an analysis of pedogenic carbonates as records of paleoenvironmental characteristics in the Teotihuacan Valley, Basin of Mexico. Micromorphological, mineralogical, and geochemical characteristics of secondary carbonates from three paleosol profiles (Cerro Gordo 1, Cerro Gordo 2, and Maseca 1) were analyzed. Pedogenic carbonates were characterized by irregular and variable carbonate crystal sizes, inclusions of soil particles, and dark or light micrite and microsparite laminations. Micromorphology and scanning electron microscopy revealed weathered plagioclase minerals as a potential source of calcium for carbonate precipitation. The 87Sr/86Sr ratio of carbonate laminar crusts and nodules from the analyses pedons in the study area also indicated that rocks of the Transmexican Volcanic Belt were the principal source of Ca and Sr for carbonates, while contribution of a far-distance eolian input was minimal. The δ13C isotopic composition evidenced a mixture of C3 and C4 plants during the Pleistocene and a transition to a higher proportion of C4 vegetal cover during the Holocene. This agreed with an increase in modeled paleo-temperature from 10.9 °C to 13.7 °C based on 18O-temperature equations. Our results support those determined for the regional paleoenvironment during the Pleistocene, the Pleistocene-Holocene transition and Mid-Late Holocene.

INFLUENCE OF SOIL TEXTURE, CLIMATE AND VEGETATION COVER ON SECONDARY SOIL SALINIZATION IN PAMPAS PLAINS, SOUTH AMERICA

Land-use change from herbaceous to woody cover -or vice versa- largely affects water fluxes, which in turn can mobilize existing salts dissolved in the soil, and thereby negatively affect soil and/or water quality and site productivity. Secondary salinization was observed after afforestation at the Northern and Eastern subregions of the Río de la Plata Grassland. Based on this, our objective was to determine the degree of association between the climatic (e.g. water balance), edaphic (e.g. texture, presence of petrocalcic horizons) and biological factors (age of plantation, stand density and stand basal area) with the secondary salinization processes under the particular environmental conditions of the Austral Pampas. The lowest electrical conductivity (EC, µS.cm-1) values were found in sites with sandy-textured soils and negative water balance, regardless of tree canopy cover. When EC differences did occur between land uses of the same site (50% of the sites), in most cases - but not in all of them - the highest EC was measured under tree cover. When salinization was significant, no pattern was observed in the salt distribution along the soil profile. An increase of EC was associated with the oldest-unmanaged plantations, with mean age of 40 years, a period much longer than the productive rotation recommended. Under similar edaphic conditions where salinization was observed (i.e. clayed soils, with naturally high salt level), salinization was not significant in relatively young plantations (<rotation period), demonstrating the key role of forest management to decrease its negative impacts on soils.

Morphological, physical, and clay mineralogy of calcareous and gypsiferous soils in North of Lorestan, Iran

There is limited information about the genesis, classification, and properties of calcareous and gypsiferous soils of western Iran. This study investigated the morphological, physical, and mineralogical characteristics of soils on different physiographic units, including plateau, colluvial fans, and piedmont plain in the Aleshtar region. The results indicated that the parent materials (calcareous and gypsiferous) as well as topographic conditions had the most influence on the soil profile development, pedogenic processes, and clay mineralogy. Illite, chlorite, smectite, palygorskite, and kaolinite clay minerals were identified using X-ray powder diffraction, transmission electron microscopy, and scanning electron microscopy. Illite, chlorite, and kaolinite have genetically been inherited from parent rocks. Neoformation of smectite and palygorskite other than genetic inheritance was formed as a result of calcite and gypsum precipitation and poor drainage. Calcareous soils with the petrocalcic horizon and gypsiferous soils contained more pedogenic palygorskite. In conclusion, we suggest adding a new great group of Gypsixerepts to the soil taxonomy to reflect the presence of pedogenic gypsum in Inceptisols.

Estimations of rooting depths and sources of plant-available water (PAW) in flatland petrocalcic soils under different land uses

Plant-available water (PAW) is one of the most important issues in productive landscapes around the world. Besides climatic conditions, it depends on several factors, among them, land uses, the rooting and water table depths, and the presence of soil heterogeneities (e.g., petrocalcic horizons). In the present work, PAW evolution was assessed over 441 days in a vertical 1D domain of a monitored Pampean petrocalcic soil under two different land uses and contrasted water demand (i.e., natural and afforested grasslands). For this purpose, rooting depths were estimated using a process-based numerical model of the aquifer-soil-plant-atmosphere continuum constrained by both in situ measured atmospheric demand and water table fluctuations. This model was calibrated against soil water content measurements at different depths and finally, as validation, computed transpiration rates were compared with the sap flow measurements. Modeling results, and calibrated rooting depths, suggest that petrocalcic horizons induce a limited hydraulic connection between the shallow soil portions and the phreatic aquifer. Thus, water storage is enhanced above the petrocalcic horizon in the natural grassland plot particularly during the periods of low atmospheric demand, whereas it is immediately consumed by forest to soil wilting point below the afforested grassland (hydric stress). The numerical methodology followed here appears as an alternative to estimate the forest groundwater usage in afforested grasslands, a key issue in soil salinization research in great plain areas.

A process-based numerical approach to estimate forest groundwater consumption in flatland petrocalcic soils

Abstract Grasslands are extensively distributed in flatland areas around the world, such as the Pampas in South America. It is one of the most economically productive landscapes and, as in other regions, they are being replaced by forests at increasing rates. Soil salinization emerges as a negative consequence associated with water deficit and forest groundwater consumption (∼250–500 mm/yr, in this region). An assessment of forest groundwater consumption is crucial for risk evaluation of soil salinization on flatland environments. For this aim, numerical modeling based on physical/biological processes and atmospheric boundary conditions was successfully applied in monitored grassland and afforested plots. Modeling results suggested a partial hydraulic disconnection between forest and phreatic aquifer due to the presence of petrocalcic horizons. Forest transpiration estimates were approximately 13% of total groundwater usage. Forest water consumption was then restricted to that soil portions above the petrocalcic horizons. Estimated forest transpiration rates (∼723 mm/yr) were similar to and even exceeded those reported in salinized sites with similar features. However, the risk of salinization of these soils was unlikely, because forest transpiration was restricted to the upper soil portions filled with fresh rainwater. The petrocalcic horizon retained water and prevented both deep drainage and, indirectly, soil salinization.

Comparison of methods for evaluating soil quality of semiarid ecosystem and evaluation of the effects of physico-chemical properties and factor soil erodibility (Northern Plateau, Spain)

The objective of this study was to compare two different methods for the calculation of the soil quality index for agricultural land in the province of Salamanca in the semiarid ecosystem of the Northern Plateau, Spain. The integrated quality index (IQI) and Nemoro quality index (NQI) were applied using the following indicator selection methods: total data set (TDS) and minimum data set (MDS). A total of 16 soil parameters were used for the TDS method. The evaluation of the soil quality index using only the properties of surface soil provides incomplete information because crop productivity is influenced by both the surface and subsoil properties [e.g. lithic contact, petrocalcic horizon, clayey horizon (Bt)]. The quality indexes were calculated using data from 75 soil profiles considering the soil surface properties (between 0 and 25 cm depth) and control section properties (between 0 and 100 cm).The results show that the consideration of soil properties of both the surface and subsurface horizons is very important to establish a good relationship between soil quality, soil functions, and agricultural management.The results based on the IQI index provide a better estimate of the soil quality compared with the NQI index; higher values were obtained with the TSD than with the MSD. However, the results obtained from the IQIMSD method provide an adequate evaluation of the soil quality. This is relevant because the use of a limited number of indicators reduces the analysis cost and increases the sampling density.

Calcite accumulation in a South African heuweltjie: Role of the termite Microhodotermes viator and oribatid mites

The aim of the study was to understand calcium and organic matter cycling in a calcareous heuweltjie mound in Papendorp, inhabited by the termite Microhodotermes viator. We used micro-FTIR and Alizarin red staining techniques on soil micromorphology thin sections and compared these to non-calcareous M. viator excrement from a non-calcareous mound in Stellenbosch. The non-calcareous M. viator excrement was a source of lignin, cellulose and localised calcium oxalate. This suggests that the oxalate-carbonate pathway is likely to operate or have operated in the past in soils with an accumulation of termite excrement. In the calcareous Papendorp heuweltjie, phytoliths, amorphous organic material lining the termite tunnels and excrement from secondary decomposers (oribatids) were associated with elevated calcite concentrations and showed few lignin and cellulose micro-FTIR peaks. Our study suggests the primary role of M. viator is accumulating calcium- and/or oxalate-rich plant materials and introduces the importance of mites in further processing this material. The termites do not appear to biomineralize the calcite directly. Abiotic dissolution-reprecipitation appears to have been a volumetrically important process in the closing of the pores and development of the petrocalcic horizon in the Papendorp heuweltjie into its present form.

Investigation of limestone and dolomite clast alteration in petrocalcic horizons, southern Nevada, USA

Petrocalcic horizons formed on Quaternary surfaces in southern Nevada contain altered limestone and dolomite clasts in the enveloping carbonate matrix. Three profiles from two geomorphic surfaces contain pebble to cobble clasts with distinct voids and alteration textures at their tops. Clast and alteration zone characteristics were analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, electron microprobe analysis, and with an inductively-coupled plasma spectrophotometer. Limestone and dolomite clasts display considerable top-down alteration marked by vertical tubular voids and an increase in neoformed fibrous clay in the altered regions. Downward percolating soil waters moved through the cemented horizon and contacted the upper clast boundaries as evidenced by selective alteration and void development at the tops of clasts. The preservation of voids indicates that alteration and void development began no earlier than initial petrocalcic formation when soil matrix pore spaces were largely filled with pedogenic carbonate. Undersaturation of soil fluids with respect to primary clast components (Ca and/or Mg) and/or increased pCO2 under conditions enhanced during wetter (pluvial) climates is interpreted to drive dissolution and alteration of clasts in these horizons. Resulting morphological and mineralogical changes in these petrocalcic horizons may mark past climate and geomorphic influence on their development over time. Misinterpretation of parent material provenance as a result of selected removal of individual clast lithologies may occur.

Land subsidence and soil cracks monitoring by surveying on the clayey plain soils in Central Iran (case study: Yazd City)

Land subsidence and soil crack has become a global problem and cases of subsidence have been identified in many places around the world. The studied area is clayey plain lands, located in the east of the city of Yazd. In this area, two types of soil crack are created, such as longitudinal and polygonal. To show the land micro movements, as a result of soil diversity after surface wetting and drying intervals, this experiment was done. Direct surveying, soil mechanical and crystal-chemical measurements were essential method of the research. The results showed that all index points were moved in different directions to about 2.3 cm during 11 months. The geochemical and crystal-chemical processes as neoformation of duripan features from amorphous silica in subsurface, where is petrocalsic horizon, have created permanent cracks. The soils according to Soil Taxonomy are Vertisols, and because of aridic soil moisture regime, petrocalcic horizon and duripan have been classified in Leptic Calcitorrerts subgroups. The gilgai formation is the result of soil moisturing and drying alternation in wet and dry months. Because of unstable land in this area, avoiding urban and rural developments is recommended.

Roots point to water sources of Welwitschia mirabilis in a hyperarid desert

AbstractWelwitschia mirabilis is a long‐lived evergreen in the hyperarid Namib Desert; at our study site, rainfall is rare (mean annual precipitation = 31 mm), groundwater deep (57–75 m), and fog frequent (50–90 events per year). By examining root architecture in relation to soil moisture and analysing the isotopic composition of hydrogen and oxygen of plant and soil water, we established whether welwitschia sources water from a stable supply of deep groundwater, or from shallow moisture originating from fog and dew, or from rainwater at infiltration depth. Isotopes suggested rainwater as principal water source. Most (55%) major roots and fine roots occurred in 10‐ to 66‐cm‐deep layers of gypsum containing 10% moisture. A further 25% of both root types grew in moist sand in petrocalcic horizons at 93‐ to 125‐cm depths. A high density of fine roots (14% of total) grew upwards towards the ground surface in a 1.5‐m radius around plants, an area occasionally wetted by run‐off of fog and dew. We conclude that welwitschia mainly relies on rainwater obtained in perched horizons. Supplemental water is obtained from fog and dew from the surface and potentially from gypsum blocks. Multiple strategies enable this extremely long‐lived evergreen to be resilient against dehydration in hyperarid conditions.

Palygorskite occurrences and genesis in calcisol and groundwater carbonates of the Tensift Al Haouz area, Central Morocco

Palygorskite has been identified as a major constituent of the clay fraction in carbonate soils of arid and semi-arid regions. This fibrous clay has a great significance as a paleoclimatic indicator and is of fundamental importance for a more complete understanding of conditions under which petrocalcic horizons and groundwater carbonates precipitate. The present study was undertaken in order to evaluate the occurrence, the genesis as well as the micromorphological aspects of palygorskite in calcisol and groundwater carbonates. The samples were analyzed by X-ray Diffraction (XRD), scanning electron microscopy (SEM), and chemical analysis (XRF). Two modes of genesis for palygorskite were identified: (1) In the petrocalcic horizons, the palygorskite is the dominant clay mineral; palygorskite fibers and fiber bundles extend from euhedral or subhedral calcite crystals showing an authigenic origin of palygorskite. The palygorskite is formed in the vadose environment from infiltrating soil water through the intense evaporation of subsurface water after calcite precipitation. (2) In the groundwater carbonates, smectite flakes appear to terminate palygorskite fibers suggesting formation of palygorskite through the alteration of precursor smectite. The increasing aridity would have caused the intense evaporation conducive to the precipitation of calcite and an increase in pH, which lead to dissolution of smectite and formation of palygorskite.

Simple estimation of available water capacity in soils of semiarid and subhumid environments

ABSTRACTSoil available water capacity (AWC) is an important factor affecting soil productivity in semiarid and subhumid environments and is mainly determined by the soil textural composition. As the soils of these environments usually present fairly uniform textures across depth, we hypothesized that it would be possible to accurately estimate the whole-profile AWC using surface information. Our objective was to test this hypothesis in the Argentine Semiarid Pampas. Information was collected from 152 sites where AWC was measured in 20 cm layers up to a depth of 140 cm or up to the upper limit of the petrocalcic horizon, when present. In each case, whole profile AWC was estimated using a one-step and a two-step approach, comparing multiple regression and artificial neural networks as modeling techniques. Both modeling methods were effective (R2 > 0.76, P < 0.05), however the former was chosen as no special software is required to run it, thus favoring simplicity. Models showed a strong interaction between surface AWC and soil depth and a simple nomogram was developed to estimate whole-profile AWC. Sampling and laboratory efforts should be significantly reduced using the model proposed in this paper.

An early carboniferous humus from South Wales preserved by marine hydromorphic entombment

Irregular, sub-metre scale occurrences of a highly unusual calcified peloidal (pelletal) limestone are described from a polygenetic paleosol from the sub-Arundian landsurface (Carboniferous, 343Ma) of South Wales. The pedogenic origin is supported by a suite of pedogenic features, and the textures in the peloidal carbonate mimic those of modern mull-like humus indicating the likelihood that the original soil biota was diverse. The occurrence of such material, normally with such a poor preservation potential, is explained by it being partially cemented and entombed by a brackish dolomite petrocalcic horizon, both formed as the landsurface was flooded by marine waters.

Modelling effective soil depth at field scale from soil sensors and geomorphometric indices

The effective soil depth (ESD) affects both dynamic of hydrology and plant growth. In the southeast of Buenos Aires province, the presence of petrocalcic horizon constitutes a limitation to ESD. The aim of this study was to develop a statistic model to predict spatial patterns of ESD using apparent electrical conductivity at two depths: 0-30 (ECa_30) and 0-90 (ECa_90) and geomorphometric indices. To do this, a Random Forest (RF) analysis was applied. RF was able to select those variables according to their predictive potential for ESD. In that order, ECa_90, catchment slope, elevation and ECa_30 had main prediction importance. For validating purposes, 3035 ESD measurements were carried out, in five fields. ECa and ESD values showed complex spatial pattern at short distances. RF parameters with lowest error (OOBerror) were calibrated. RF model simplified which uses main predictors had a similar predictive development to it uses all predictors. Furthermore, RF model simplified had the ability to delineate similar pattern to those obtained from in situ measure of ESD in all fields. In general, RF was an effective method and easy to work. However, further studies are needed which add other types of variables importance calculation, greater number of fields and test other predictors in order to improve these results.

Sampling scheme optimization to map soil depth to petrocalcic horizon at field scale

Soil depth has played a key role in the development of soil survey, implementation of soil-specific management and validation of hydrological models. Generally, soil depth at field scale is difficult to map due to complex interactions of factors of soil formation at field scale. As a result, the conventional sampling schemes to map soil depth are generally laborious, time consuming and expensive. In this study, we presented, tested and evaluated a method to optimize the sampling scheme to map soil depth to petrocalcic horizon at field scale. The method was tested with real data at four agricultural fields localized in the southeast Pampas plain of Argentina. The purpose of the method was to minimize the sample dataset size to map soil depth to petrocalcic horizon based on ordinary cokriging, five calibration sample sizes (returned by Conditioned Latin hypercube –cLHS-), and apparent electrical conductivity (ECa) or elevation as variables of auxiliary information.The results suggest that (i) only 30% of samples collected on a 30-m grid are required to provide high prediction accuracy (R2>0.95) to map soil depth to petrocalcic horizon; (ii) an independent validation dataset based on 50% of the samples on a 30-m grid is adequate to validate the most realistic accuracy estimate; and (iii) ECa and elevation, as variables of auxiliary information, are sufficient to map soil depth to petrocalcic horizon. The method proposed provides a significant improvement over conventional to map soil depth and allows reducing cost, time and field labour. Extrapolation of the results to other areas needs to be tested.