Characterization and Classification of Soils of the Terra da Esperança Settlement Project in Chapada do Apodi , Brazil

Soil characterization and pedological classification are essential to define its main potentials and restrictions. The objective of this work was to classify the morphological, physical, chemical, and pedological attributes of soils of the Terra da Esperança Settlement Project (TESP) in Chapada do Apodi, Brazil, and find the most sensitive attributes for distinguishing them using multivariate analysis. The research was carried out in the TESP, in the municipality of Governor Dix-Sept Rosado, state of Rio Grande do Norte, Brazil. Ten sites were chosen to open representative soil profiles: Native Forest Area 1 (NFA1), 2 (NFA2), and 3 (NFA3), Collective Area with Native Forest (CNF), Agroecological Area (AEA), Cashew crop Area (CCA) Collective Area with Pasture 1 (CAP1), and 2 (CAP2), Permanent Preservation Area (PPA), and Cajaraneira (Spondia sp.) Orchard Area (COA). Disturbed and undisturbed soil samples were collected and subjected to physical and chemical analysis for soil classification. The soils classes found were: Cambissolo Haplico Carbonatico vertissolico (NFA1), Cambissolo Haplico Carbonatico tipico (CNF, and AEA), Cambissolo Haplico Ta Eutrofico tipico (CAP2, NFA2, and COA), Cambissolo Haplico Ta Eutrofico vertissolico (NFA3), Argissolo Vermelho Distrofico latossolico (CCA), Chernossolo Rendzico Ortico saprolitico (CAP1), and Neossolo Fluvico Ta Eutrofico tipico (PPA). The material of origin of the soils contributed to the presence of a calcic horizon in the profiles NFA1, CNF, AEA, CCA (Cambissolos), and CAP1 (Chernossolos). The textural class of the soils varied from sand to clay. The Argissolo (CCA) presented acid character, high aluminum saturation, low base saturation, dystrophic character, and low cation exchange capacity, forming horizons with chemical limitations, due to its latossolico character. The most sensitive attributes for distinguishing the soil classes were related to the source material, which directly influenced the soil physical (silt and clay) and chemical (acidity, salinity, nutrient availability, and clay activity) attributes.


Introduction
The Chapada do Apodi geological formation encompasses twelve municipalities-Apodi, Baraúna, Felipe Guerra, Governador Dix-Sept Rosado, Mossoro, Upanema, and Assu in the state of Rio Grande do Norte, and Alto Santo, Jaguaruana, Limoeiro do Norte, Quixeré, and Tabuleiro do Norte in the state of Ceara, Brazil.It is part of the Potiguar basin, whose stratigraphic units are predominantly represented by limestones of the Jandaira Formation at the top, and sandstones of the Açu Formation at the base (Pessoa Neto et al., 2007).
The spatial variability of the soil attributes of the Chapada do Apodi is probably due to its different micro-relieves and water flows (Costa et al., 2011;Oliveira et al., 2009).This explains its soil diversity, which includes Cambissolos, Chernossolos, and Neossolos Fluvicos in the fluvial plain formed by sediments from Cretaceous rocks of the Apodi group (Maia, 2005;Melo et al., 2017), and Argissolos, Latossolos, and Neossolos Quartzarenicos in the flat landscape formed from sandstones of the Açu Formation (Ernesto Sobrinho, 1980).
Since the occupation of the land by the settlers, the disorderly agricultural exploitation has changed significantly the organization and promoted degradation of this rural area in the Chapada do Apodi microregion, which is an important agricultural region in the Northeast of Brazil (Mota et al., 2007).This was the result of erosion due to the non-observance of agricultural suitability or land use capacity, intensive land use without maintenance of soil The NFA1, NFA2, and NFA3 were in an environmental reserve with preserved hyper-xerophilous Caatinga vegetation, and vegetation with deciduous species, however, some wood had been removed for fencing of other areas.These areas were reference areas because of the low anthropogenic influence on them.
The CNF had a preserved hyper-xerophilous Caatinga vegetation next to a crop area with soil conventional management (CCA).
The AEA was delineated and implemented in 2005.The use of this area was defined by the settlers, who intended to produce fruits and forage on it to meet the needs of local families and animals.This area presented some agro-ecological practices of soil conservation, and honey production by Africanized bees.
The CCA had cashew crops intercropped with the hyper-xerophilous Caatinga native vegetation, using traditional managements.
The CAP1, and CAP2 were natural pasture areas for goats reared by the settlers.CAP2 naturally presented accumulation of water in rainy periods due to its low altitude, and it was mechanically excavated by the settlers to provide water for the animals.
The PPA had outcropping and accumulation of water in rainy periods.It was in the lowest point of the landscape and, therefore, an area of deposition of sediments.
The COA had cajaneiras, species of the genus Spondia, which were planted by the former owner in the 1970s.This species had great economic importance for the settlers due to its great production of fruits.In the dry season, this area presents a great input of organic matter to the soil due to the leaf loss of this species.

Pedological Classification
Ten representative soil profiles were opened, one in each study site.The profile opening locations were chosen based on photointerpretation of satellite images (Google Earth Pro) from 2002 to 2016, considering the different soil shades, which were confirmed in field.The morphological analysis consisted of descriptions of soil order, depth, transition between horizons, color (Munsell chart), structure, and consistency (Santos et al., 2006).Physical and chemical analyzes were performed to obtain the soil taxonomic classification up to the fourth categorical level, according to the Brazilian Soil Classification System (SiBCS) (Santos et al., 2013, Teixeira et al., 2017).The soil profiles were georeferenced, including altitude, using a GPS device (GPSMAP GARMIN 64s).

Collection and Preparation of Samples
Disturbed and undisturbed soil samples were collected from all horizons of the representative profiles.The samples were placed in identified plastic bags and taken to the Laboratory of Soil, Water and Plant Analysis of the Agricultural Sciences Center of the Federal Rural University of the Semiarid Region (LASAP/CCA/UFERSA).Undisturbed samples were collected using four volumetric rings with height of 5.0 cm and diameter of 5.0 cm.Disturbed samples were air-dried, disaggregated and passed through 20 mm, and 2 mm mesh sieves to obtain the gravel fraction (> 2.00 mm to < 20.00 mm) by washing them with a NaOH 1 mol L -1 and distilled water solution, and the air-dried fine earth (ADFE) (< 2.00 mm), according to Teixeira et al. (2017).

Physical and Chemical Analyzes
The results of the physical and chemical analyzes were expressed as means of three replications.The soil granulometry and particle density (ADFE) was evaluated using disturbed samples, and soil density was evaluated using undisturbed samples, according to Teixeira et al. (2017).
The soil chemical analysis was performed according to Teixeira et al. (2017) using the ADFE.The soil attributes evaluated were pH in water at ratio of 1:2.5 (soil:water), and in KCl; Ca 2+ , Mg 2+ , and exchangeable Al 3+ extracted with KCl 1 mol L -1 and analyzed by titration; potential acidity (H+Al), using calcium acetate; available P, Na + , and K + , using the Mehlich 1 extractor, analyzes by colorimetry, and flame photometry, respectively; electrical conductivity of the soil saturation extract in water (EC), using a conductivity meter, and calcium carbonate equivalent.Subsequently, the delta pH, sum of bases (SB), effective cation exchange capacity (t); cation exchange capacity at pH 7.0 (T), base saturation (V %), exchangeable aluminum saturation (m %), exchangeable sodium percentage (ES %), and clay activity were calculated.Total organic carbon (TOC) was determined by the titration of the remaining 0.167 mol L -1 potassium dichromate with ammoniacal ferrous sulfate after the wet oxidation process (Yeomans & Bremner, 1988).

Soil Class Mapping of the Terra da Esperança Settlement Project
The photointerpretation of the soil shades from the satellite images of the TESP perimeter, which was defined by the shapefile of the INCRA, was carried out using vector files in KML format manually vectorized and later converted into shapefile in the Geographic Information System (GIS) Qgis 2.8.9, in WGS 84 Datum.Field surveys of the areas with different soil shades were performed to identify their boundaries and georeferencing them for a better precision, and the opening of the profiles for pedological classification.After defining the shapes of the different soils, their predominant soil classes were determined.These shapes were converted to the cartographic projection UTM 24S, generating a map of their respective areas in hectare.

Pedagogical Classification
The classification of the soils of the profiles, evaluated up to the fourth categorical level according to the Brazilian Soil Classification System (SiBCS) (Santos et al., 2013;Teixeira et al., 2017) According to the soil classes found in this geological formation (Chapada do Apodi), the predominant class in this region is Cambissolo.Jarbas et al. (2018) also found a great extension of this class in parts of the states of Rio Grande do Norte and Ceara, in Brazil.Soils of this class originated from the Jandaira Formation, and consist of layers of light gray to white or yellow calcite limestones of fine to medium granulation, and gray or yellow dolomitic limestones of coarser granulation (Mota et al., 2007), and other homogeneous formation factors.However, the area also presented Argissolos, Chernossolos, and Neossolos, complementing the Soil Exploration-Reconnaissance Survey of the state of Rio Grande do Norte (Jacomine et al., 1971), which found only Cambissolos in the study area due to its work scale.
The taxonomic classification showed all Cambissolos classified as Haplicos at the second categorical level because they did not fit to the other classes in this level, i.e., due to absence of other differential properties.The third categorical level of the soil profiles NFA1, CNF, and AEA were Carbonatico, because they presented a calcium horizon within the 120 cm soil layer, characterized by the accumulation of calcium carbonate in the C horizon, containing at least 150 g kg -1 of calcium, and with width of at least 15 cm, due to the pedogenetic process of calcification (Table 3) (Santos et al., 2013).The other profiles (CAP2, NFA2, NFA3, and COA) were classified as Ta Eutrofico because they presented high clay activity, i.e., equal to or greater than 27 cmol kg -1 of clay, and saturation by bases greater than 50 %, at least on the B horizon.The fourth classification level showed the profiles CNF, AEA, CAP2, NFA2, and COA classified as tipico because they did not present any restrictive characteristic for this level.The profiles NFA1 and NFA3 were classified as vertissolico, since a vertic horizon was noticeable, consisting of a subsurface mineral horizon that presents cracks in dry periods of the year due to the expansion and contraction of clays, with at least 1 cm of width, and it presented slinckensides in the profile NFA3, which are smooth surfaces of friction, also caused by the expansion and contraction of clays (Tabela 2) (Santos et al., 2013).
The Argissolo in the CCA profile was classified up to the fourth categorical level as Argissolo Vermelho Distrofico latossolico (PVd) because it presented a 2.5YR soil shade and basal saturation of less than 50% in most of the 100 cm of the B horizon (including the BA horizon), and presence of a latossolico B horizon (formed by the latolization process) below the textural diagnostic B horizon within the 150 cm soil layer, and textural B to A ratio greater than 1.8, which differentiated this soil class from the Latossolo class.Moreover, it was the profile with the highest altitude (97 m) (Tables 1 and 3).
The Chernossolo in the CAP1 profile was classified as Chernossolo Rendzico Ortico saprolitico (MDo), due to its chernozemic A horizon (Ak), calcic horizon coinciding with the Ak horizon and C horizon, and no restrictive characteristic for the third categorical level, presenting a mild Cr horizon, and absence of lytic contact within the 100 cm soil layer.
The Neossolo of the PPA profile was classified as Neossolo Fluvico Ta Eutrofico tipico (RYve) due to the presence of alluvial sediments, and A horizon over the C horizon, and its fluvic character within the 150 cm soil layer, since the area is characterized by outcrop of water in rainy periods and a lower altitude (74 m) (Table 1); this is a permanent preservation area because of this characteristic.Moreover, it presented high clay activity, and saturation by bases greater than 50% in most of the C horizon within the 120 cm soil layer, and it was classified as tipico because it did not fit to other classes of the fourth categorical level.

Morphological Attributes
The Cambissolos found were mineral soils derived from Jandaira limestones from the Cretaceous Period, presenting an incipient diagnostic B horizon (Bi).In general, they were shallow soils, especially CAP2, which had depth of 27 cm (Table 1), and present clear and flat transitions between surface horizons.
The profiles NFA1, CNF, AEA, CAP2, NFA2, NFA3, and COA did not present significant differences in color, with hue angles ranging from 7.5YR to 10YR, except NFA1 and NFA3, which presented differences in deeper horizons, with 2.5Y, and yellowish shades in all horizons.
The structure of almost all profiles showed subangular blocks; the soil of the profiles PPA (Neossolo), NFA3, and COA (Cambissolos) disintegrate in granules.
The wet and dry consistencies of all profiles were similar, varying from friable to very friable, and from soft to slightly hard, respectively, with variation in consistency in subsurface (friable to firm, slightly hard to hard, hard to very hard), except the profiles CNF and CAP2 (Cambissolos).The wet consistency was slightly plastic to plastic, and slightly sticky to sticky.However, the Argissolo was the only soil that did not present plasticity and stickiness in the surface layer due to its sandy texture (Table 3).Large size particles of coarse sands have low specific surface area, little capacity to adsorb water and nutrients, and do not adhere to each other forming a coherent mass (Brady & Weil, 2013).
The Argissolo Vermelho Distrofico latossolico (CCA) is a mineral soil derived from sandstones of the Açu Formation, with a textural diagnostic B horizon (Bt).It is a well-developed and deep soil, which contributes to a diffuse flat transition between its horizons, and has reddish color with shades ranging from 5YR (surface layer) to 2.5YR (deeper layers).The other soils presented no clear chroma as the Chernossolo Rendzico (CAP1), with lighter greyish colors, denoting accumulation of CaCO 3 and its effect from surface to deep layers, and lighter and purer color in the Neossolo Fluvico (PPA).This soil is characterized by the influence of alluvial sediments from Cretaceous rocks of the Apodi group (Maia, 2005) and sediments from erosion of surrounding soils in higher altitudes that are deposited in the Barreiro watershed.

Physical Attributes
According to the distribution of the particle size that shows the physical attributes of the profiles (Table 2), the highest clay contents in the surface layer were found in the profiles NFA1, NFA2, NFA3, and COA (Cambissolos Haplicos), ranging from 360 g kg -1 to 408 g kg -1 , and in the PPA (Neossolo Fluvico) (550 g kg -1 ), which is in an area of sediment deposition and groundwater oscillations and present sandy clay, and clay loam to clay textures.Souza et al. (2015) evaluate physical and chemical properties of a Cambissolo Eutrofico under different agricultural uses in the Chapada do Apodi and found predominance of the clay fraction in areas with native forests, and Spondia sp., with similar values ranging from 373 to 478 g kg -1 .However, the clay fraction increased in depth, mainly in the Argissolo (CCA), presenting accumulation of clay of 422 g kg -1 in the 25-50 cm layer, denoting the evolution to a textural B horizon (Bt) formed by the pedogenic process of argiluviation (Table 2).
The Argissolo Vermelho (CCA) had the highest sand content in the A horizon (894 g kg -1 ), showing sandy, and sandy clay loam to sandy clay textures along the profile (Table 2).Its higher soil and particle densities are explained by the sand fraction, due to the dominance of quartz; solid particles in coarser soils are less susceptible to aggregate formation, and these soils have normally higher density than fine-texture soils (Brady & Weil, 2013).
The Bt horizon of the Argissolo (CCA) presented higher clay content than the A horizon (Table 2).This was expected due to the soil class, and its sandy clay texture.The lowest silt to clay ratio found in this soil denoted its more weathered condition in depth.
The Chernossolo Rendzico had the highest silt contents, ranging from 593 g kg -1 (chernozemic A horizon) to 642 g kg -1 (C horizon), presenting silt loam texture (Table 2).This was due to the strong influence of the source material (Jandaira limestone), which contributed to its higher silt to clay ratio (2.52 to 3.42) compared to the other soil classes, and indicates a low weathering degree.Similar results were found by Melo et al. (2017), who attributed these results to the fossiliferous limestone (low-weathered source material that can originate different soils), and its fine granulation; they found silt to clay ratios ranging from 3.00 to 6.00.
The highest degrees of flocculation were found for the Neossolo Fluvico (PPA) (Table 2), in the A horizon (24.76%), and for the Chernossolo Rendzico (Ak = 34.65 %, and Ck1 = 36.49%); these results were probably influenced by the silt and clay fractions, and the exchangeable bases Ca 2+ and Mg 2+ (Table 3).Note.CDW = clay dispersed in water; DF = degree of flocculation; SiBCS-Brazilian Soil Classification System; Dp = particle density; Ds = soil density.

Chemical Attributes
The total organic carbon (TOC) of all profiles decreased with depth (Table 3), however with high and good levels (Ribeiro et al., 1999), denoting their representativeness for semiarid environments (Marinho et al., 2016), especially in the surface of the profiles CNF, and CAP2, which presented 48.42, and 45.52 g kg-1, respectively, in the surface layer.
The pH in water of the Argissolo Vermelho Distrofico latossolico (CCA) was acidic (5.6 to 6.5), confirming its aluminum contents (0.48 to 0.58 cmolc kg -1 ), potential acidity (2.31 to 3.74 cmolc kg -1 ), and, consequently, aluminum saturation (15 % to 20%).However, it exhibited the lowest cation exchange capacity at pH 7.0 (from 5.17 (Bw Horizon) to 7.31 (A Horizon) cmolc kg -1 ), and base saturation (41% to 55%) and, thus, this soil was classified as dystrophic.This soil presented advanced actions of chemical weathering in the AB and diagnostic Bt horizons (V = 41%), which gave it the latossolico character at pedogenesis, despite the low precipitation in the semiarid region, which favors the leaching of basic cations, forming horizons with chemical limitations.However, the A and Bw horizons maintained enough Ca 2+ and Na + contents to classify them as eutrophic (V = 50%, and 55%, respectively).According to Kiehl (1979), pH lower than 5.0 cause deficiency in the Ca 2+ , Mg 2+ , and P divalent bases, and toxicity by Al 3+ because of its greater solubility in this pH range, whereas pH of 8.0 to 8.5 indicate the presence of calcium and magnesium carbonate.
The other soil classes had, in general, alkaline pH in water, especially the Chernossolo Rendzico (CAP1) (8.0 to 8.3) and Neossolo Fluvico (PPA) (8.0 to 8.1) (Table 3).This high pH values is due to the influence of the sandstones of the Jandaira geological formation, which according to Melo et al. (2017), presents strata with basic rocks rich in carbonates that provide significant Ca and Mg contents, increasing base saturation, and classify them as eutrophic, especially the Neossolo, which is in a sediment deposition area.Moreira et al. (2007) evaluated soil chemical and physical attributes in Chapada do Apodi and found effects of the lithology of the region on pH, showing that soils derived from limestones present a neutral to alkaline pH due to their high carbonate and exchangeable base contents.Similar results were found in the study profiles, denoted by the high Ca and Mg contents, with variations in Ca from 6.63 (Chernossolo; CAP1; Ck3) to 24.07 cmolc kg -1 (Cambissolo; NFA1; C), and Mg from 1.35 (Chernossolo; AEA; C) to 13.84 cmolc kg -1 (Cambissolo; NFA1; Bi).Alkalinity is common in semiarid regions due to their low precipitation rates (Souza et al., 2007).
The pH in KCl was lower than in water in all profiles, indicating the predominance of negative net charges on the particle surfaces, which is an indicator of the soil weathering stage.The lower the ΔpH (more negative), the lower the soil weathering degree; however, negative net charges predominate even in the more weathered soil (Argissolo Vermelho Distrofico latossolico; CCA) (ΔpH of -1.5 to -1.8) (Table 3) due to the leaching of exchangeable bases, indicating an advanced weathering degree (Melo et al., 2006).
In general, phosphorus (P) were very low (Table 3), decreasing with depth, especially in the Argissolo Vermelho (CCA), ranging from 0.46 (Bw Horizon) to 7.01 mg kg -1 (A Horizon), and even lower in the more weathered horizons (AB, Bt, and Bw).Chaves et al. (2009) evaluated adsorption characteristics of P in different soil classes in the state of Paraiba, semiarid region of Brazil, and found similar results, with lower P contents in weathered soils due to the clay content and type, compared to soils that have a predominance of Fe and Al +3 oxides, which present higher adsorption force on P due to positive electric charges, strongly binding it to mineral colloids, and decreasing their availability in the soil solution (Machado & Souza, 2012).
The Cambissolos (NFA1, CNF, AEA, CAP2, NFA2, NFA3, and COA) also presented low P contents due to the low P content on the source material (limestone of the Jandaira formation) (Costa et al., 2016).However, the profile CAP2 (Cambissolo Haplico Ta Eutrofico tipico) presented P contents of 21.03 mg kg -1 in A horizon (0-5 cm) to 9.21 mg kg -1 in the Bi horizon (Ribeiro et al., 1999).This soil presented the second highest TOC content (45.52 g kg -1 ) on the surface layer due to presence of litterfall, and was shallower than the others.Soil P availability also depends on organic matter content, and its weathering degree.Less developed soils can be sources of P because it has natural reserves of primary phosphates from rocks that can be released to the soil (Pavinato & Rosolem, 2008).This confirms the positive correlation between TOC and P in the correlation matrix of physical and chemical attributes of the soil profiles (Table 5).
Table 3.Chemical attributes of representative soil profiles of the Terra da Esperança Settlement Project, in Governor Dix-Sept Rosado, RN, Brazil Horizon/Profile CaCO 3 TOC pH (1:2.5)

Soil Class Mapping of the Terra de Esperança Settlement Project (TESP)
The Soil Exploration-Reconnaissance Survey of the state of Rio Grande do Norte (Jacomine et al., 1971) reported that the region of the TESP in Governador Dix-Sept Rosado had only Cambissolos.However, three  Note.CaCO 3 = Calcium Carbonate Equivalent; TOC = Total Organic Carbon; EC = electrical conductivity; H+Al = potential acidity; T = cation exchange capacity at pH 7.0; V = base saturation; m = saturation by aluminum; ES % = exchangeable sodium percentage.
The principal component analysis (PCA) and factorial analysis were performed using a data matrix with 18 soil attributes.Delta pH (ΔpH), and aluminum saturation (m) were necessary due to their multicollinearity in the correlation matrix.
The principal components analysis generated vector projection diagrams for physical and chemical attributes of the soils for Factors 1 and 2 (Figure 3), and 3 and 4 (Figure 4), which had greater weights in the classification of soil attributes, and allowed confirming their influences for the distinction of the soil classes.
Figure 3a shows the predominance of physical attributes that discriminate the most environments, especially silt and clay, since they are not close in the correlation circle, since and the studied soils presented varied texture classes.
The distribution of the selected variables presented accumulated variance of 57.71% for the Factor 1 and Factor 2 axes, explaining 32.83 % and 24.88 % of the variance, respectively, forming three groups (Figure 3).The most sensitive attributes for the differentiation of the soil classes were related to the source material, which directly influenced the soil physical (silt and clay), and chemical (acidity, salinity, nutrient availability, and clay activity) attributes.

Table 2 .
Physical attributes of representative soil profiles of the Terra da Esperança Settlement Project, in Governor Dix-Sept Rosado, RN, Brazil

Table 5 .
Correlation matrix between the physical and chemical attributes of the soil profiles in the Terra de Esperança Settlement Project, Governor Dix-Sept Rosado, RN, Brazil Attributes Sand Silt Clay CaCO 3 TOC pH H2O pH KCl ∆pH CE P K + Na + Ca 2+ Mg 2+ Al 3+ (H+Al) T