ABSTRACT

The semiarid region of northeastern Brazil has a large area occupied by Planosols, where in the State of Pernambuco these soils are mainly used for livestock farming and subsistence crops. The knowledge on these soils is limited, which compromises the understanding on their behavior, potentialities and limitations.This study aimed to analyze morphological, chemical, physical and mineralogical attributes of Planosols developed under different geoenvironmental conditions. Morphological descriptions and chemical, physical and mineralogical analyses were performed in four profiles of Planosols along a rainfall gradient. An increase in rainfall allowed for an increase in the clay content in the Bt horizon and a reduction in ESP, EC, Na⁺, CEC, S, pH (water and KCl) and soil density. Horizons A and E were thicker in Planosols in more humid environments. The increase in ESP associated with the presence of expansive minerals (smectite and vermiculite) allowed the development of a prismatic structure in Haplic Planosols and a columnar structure in Natric Planosols. The mineralogical assembly is indicative of poorly weathered soils. The mineralogical assemblies of the silt and clay fractions were similar in the different geoenvironments, while higher contents of easily alterable minerals were observed in the composition of the sand fraction in environments with a drier climate.

soil mineralogy; weathering; climate; semiarid

INTRODUCTION

Planosols occupy large areas in subtropical and temperate regions with alternating dry and humid seasons, for example in Latin America (Brazil, Paraguay and Argentina), Africa, Eastern United States, Southeast Asia (Bangladesh and Thailand) and Australia. Its total extent is estimated at about 130 million hectares, 40 % of which are found in Latin America (FAO, 2006FAO. IUSS Working Group WRB. World reference base for soil resources. Roma: FAO; 2006. (World Soil Resources Reports, 103).). In Brazil, Planosols mainly occur in the state of Rio Grande do Sul, in approximately 27,763 km² (about 10 % of the State’s surface) (Lemos et al., 1973Lemos RC, Azolin MAD, Abrão PVR. Levantamento e reconhecimento dos solos do Rio Grande do Sul. Recife: Ministério da Agricultura – Divisão de Pesquisa Pedológica; 1973. (Boletim técnico, 30).) and in the northeast, where they are predominantly Natric and Haplic solodic (Northern Bahia to Ceará) (IBGE, 2007Instituto Brasileiro de Geografia e Estatística – IBGE. Manual técnico de pedologia. 2ª. ed. Brasília, DF: 2007.). In Pernambuco, Planosols encompass an area of 15,830 km² (16 % of the State’s surface) (Araújo Filho et al., 2000Araújo Filho JC, Burgos N, Lopes OFL, Silva FHBB, Medeiros LAR, Melo Filho HFR, Parayba RBV, Cavalcanti AC, Oliveira Neto MB, Silva FB, Leite AP, Santos JCP, Sousa Neto NC, Silva AB, Luz LRQP, Lima PC, Reis RMG. Levantamento de reconhecimento dos solos do Estado de Pernambuco. Rio de Janeiro: Embrapa Solos; 2000. (Boletim de Pesquisa, 11).).

These soils have been widely used in Pernambuco for extensive livestock farming, cotton, corn and bean in the regions of Agreste and Sertão, especially when they occur with thicker A and E horizons (Araújo Filho et al., 2000). The main limitations to the agricultural use of these soils are the lack of water in the dry period and a normally extremely hard and dense B horizon, which significantly limits crop development and water movement. The densification of the Bt horizon causes deficient drainage, higher mechanical resistance to root penetration and lower availability of water and nutrients to plants (Silva et al., 2002Silva MSLD, Klamt E, Cavalcanti AC, Kroth PL. Adensamento subsuperficial em solos do semiárido: processos geológicos e/ou pedogenéticos. Rev Bras Eng Agric Amb. 2002;6:314-20. doi:10.1590/S1415-43662002000200021; Cunha et al., 2010Cunha TJF, Petrere VG, Silva DJ, Mendes AMS, Melo RF, Oliveira Neto MB, Silva MSL, Alvarez IA. Principais solos do semiárido tropical brasileiro: caracterização, potencialidades, limitações, fertilidade e manejo. In: Sá IB, Silva PCG, editores. Semiárido brasileiro: pesquisa, desenvolvimento e inovação. Petrolina: Embrapa Semiárido; 2010.). These aspects have direct implications on the agricultural management of these soils, because they increase the erosive potential with the possibility of surface runoff of the non-infiltrated water. Some studies have shown a reduction in hydraulic conductivity, macro and microporosity in Planosols with an increase in depth (Lima et al., 2006Lima CLR, Pauletto EA, Gomes AS, Hartwig MP, Passianoto CC. Compactação de um Planossolo em função de sistemas de manejo. Rev Bras Agroci. 2006;12:179-82., 2008Lima CLR, Pillon CN, Suzuki LEAS, Cruz LEC. Atributos físicos de um Planossolo háplico sob sistemas de manejo comparados aos do campo nativo. Rev Bras Cienc Solo. 2008;32:1849-55. doi:10.1590/S0100-06832008000500006).

The large amount of 2:1 minerals in Planosols of semiarid regions, such as smectite and vermiculite, is associated with a dry climate, water deficiency and restricted drainage, which favor the process of bisialitization (Corrêa et al., 2003Corrêa MM, Ker JC, Mendonça ES, Ruiz HA, Bastos RS. Atributos físicos, químicos e mineralógicos de solos da região das várzeas de Souza (PB). Rev Bras Cienc Solo. 2003;27:311-24. doi:10.1590/S0100-06832003000200011). Smectites are generally neoformed from solutions rich in Al, Si and bases, derived from the weathering of primary and secondary soil minerals, subjected to low to moderate leaching of silica (Borchardt, 1989Borchardt G. Esmectites. In: Dixon JB, Weed SB, editors. Minerals in soil environments. Madison: Soil Science Society of America; 1989.).

The formation of the textural gradient in Planosols is still an issue to be elucidated. In general, it is attributed to the influence of transport and deposition of coarser material in surface, for having fragments of rocks and edgeless materials in the upper horizons. However, Parahyba et al. (2009Parahyba RDBV, Santos MCD, Rolim Neto FC. Evolução quantitativa de Planossolos do agreste do Estado de Pernambuco. Rev Bras Cienc Solo. 2009;33:991-9. doi:10.1590/S0100-06832009000400023, 2010Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000.), studying Planosols in the state of Pernambuco, concluded that these soils have an autochthonous origin, with little indication of reworking of materials at the surface, and that the remarkable textural contrast is formed by a combination of pedogenetic processes, such as clay eluviation-illuviation, in situ clay formation and selective clay loss in the surface horizon due to lateral movements through mechanical drag or dissolutions. Another widely cited hypothesis is the destruction of clay by ferrolysis in more superficial horizons (Brinkman, 1970Brinkman R. Ferrolysis, a hydromorphic soil forming process. Geoderma. 1970;3:199-206. doi:10.1016/0016-7061(70)90019-4), although van Ranst et al. (2011)van Ranst E, Dumon M, Tolossa AR, Cornelis JT, Stoops G, Vandenberghe RE, Deckers J. Revisiting ferrolysis processes in the formation of Planosols for rationalizing the soils with stagnic properties in WRB. Geoderma. 2011;163:265-74. doi:10.1016/j.geoderma.2011.05.002 question ferrolysis as the main process in the formation of Planosols.

Better management of soils, as well as the transfer of technology to different areas, is ensured when one has information on their genesis and classification, which require more detailed analyses of complete profiles. Since Planosols from semiarid regions have been little studied, it is necessary to search for information that provides a better understanding of their characteristics, differences and relationships, which can contribute to their sustainable use. Thus, this study aimed to analyze the morphological, chemical, physical and mineralogical attributes of Planosols developed under different geoenvironmental conditions in the state of Pernambuco, Brazil.

MATERIALS AND METHODS

Selection of soils and characterization of the studied areas

The study was conducted in the state of Pernambuco, Brazil, where four Planosol profiles were selected along a climatic gradient, in the municipalities of Timbaúba (Profile 1) (1,097.1 mm, 24.6 ^oC), Altinho (Profile 2) (633.2 mm, 23.1 °C), Arcoverde (Profile 3) (574.4 mm, 22.4 °C) and Jataúba (Profile 4) (510 mm, 22.7 °C) (Costa et al., 2013; ITEP, 2015). The studied profiles have the following geographical coordinates: profile 1 – UTM 25M 0256193 mE and 9172982 mN; profile 2 – UTM 24L 0827309 mE and 9062144 mN; profile 3 – UTM 24L 0710092 mE and 9069986 mN and profile 4 – UTM 24M 0773102 mE and 9118436 mN. The studied locations were selected using soil maps of the state of Pernambuco (Jacomine et al., 1973Jacomine PKT, Cavalcanti AC, Burgos N, Pessoa SCP, Silveira CO. Levantamento exploratório – reconhecimento de solos do Estado de Pernambuco. Recife: Ministério da Agricultura/Sudene; 1973. v.1. (Boletim técnico, 26; série pedologia, 14).; Araújo Filho et al., 2000; Embrapa, 2001Empresa Brasileira de Pesquisa Agropecuária – Embrapa. Zoneamento Agroecológico do Estado de Pernambuco [CD-ROM] Recife: Embrapa Solos/Governo do Estado de Pernambuco; 2001. (Documentos, 35).) and soil maps of the Usina Central Olho d’Água, in the municipality of Timbaúba, by choosing various profiles in similar topographic conditions. At each site, a soil pit was dug for the description and collection of disturbed and undisturbed samples, following the method proposed by Santos et al. (2005)Santos RD, Lemos RC, Santos HG, Ker JC, Anjos LHC. Manual de descrição e coleta de solo no campo. 5ª. ed. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2005.. The soils were classified according to the Brazilian Soil Classification System – SiBCS (Santos et al., 2013Santos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF. Sistema brasileiro de classificação de solos. 3ª. ed. Rio de Janeiro: Embrapa Solos; 2013.).

Chemical and physical analyses

The chemical analyses of soil and saturation paste extracts were performed according to the methods of Claessen (1997)Claessen MEC, organizador. Manual de métodos de análise de solo. 2ª ed. Rio de Janeiro: Centro Nacional de Pesquisa de Solos; 1997. and consisted of: pH in water and in 1 mol L^-1 KCl, exchangeable Ca²⁺, Mg²⁺, K⁺, Na⁺, Al³⁺, potential acidity (H+Al), organic carbon (OC) and extractable P. Electrical conductivity (EC), pH, Na⁺ and K⁺ were determined in the saturation paste extracts. From these data, the following parameters were determined: cation exchange capacity (CEC), sum of bases (SB), base saturation (V), aluminum saturation (m) and exchangeable sodium percentage (ESP).

The physical analyses were performed according to the recommendations of Claessen (1997)Claessen MEC, organizador. Manual de métodos de análise de solo. 2ª ed. Rio de Janeiro: Centro Nacional de Pesquisa de Solos; 1997.: granulometry of the fine earth fraction (coarse sand, fine sand, silt, total clay and clay dispersed in water), soil density, particle density and fractions larger than 2 mm, gravel (2-20 mm) and cobbles (20-200 mm). With the results, the degree of flocculation, total porosity and the silt/clay ratio were calculated. The textural differentiation index of the horizons (TDIh) was obtained according to Baize (1988)Baize D. Guide des analyses courantes en pédologie. Paris: INRA; 1988.. In order to evaluate the occurrence of lithological discontinuity in the profile, the uniformity value (UV) was calculated according to Bortoluzzi et al. (2008)Bortoluzzi EC, Pernes M, Tessier D. Mineralogia de partículas envolvidas na formação de gradiente textural em um Argissolo subtropical. Rev Bras Cienc Solo. 2008;32:997-1007. doi:10.1590/S0100-06832008000300009, adapted from Schaetzl (1998)Schaetzl R. Lithologic discontinuities in some soils on drumlins theory, detection, and application. Soil Sci. 1998;163:570-90. doi:10.1097/00010694-199807000-00006.

Cluster analysis was performed using the program Statistica 7 (Statsoft, 2007) for the chemical attributes of the Bt horizon (pH in water, pH in KCl, Ca²⁺, Mg²⁺, Na⁺, K⁺, SB, Al³⁺, H+Al, CEC, OC and P) and of the saturation paste (pH, EC, Na⁺ and K⁺), and for physical attributes (total sand, coarse sand, fine sand, silt, clay, clay dispersed in water, degree of flocculation, silt/clay, fine sand/total sand, soil density, particle density, total porosity and gravel). Dendrograms of similarity were constructed by measuring the Euclidean distance and using the complete-linkage mode with standardized data.

Mineralogical analyses

Coarse and fine sand fractions were separated from silt and clay fractions through wet sieving. The silt and clay fractions were separated through sedimentation according to Stokes’ Law.

The mineralogical composition of the clay fraction was determined through X ray diffractometry, after pre-treatment to remove carbonates, organic matter and Fe oxides using 1 mol L^-1 sodium acetate at pH 5.0, 30 % hydrogen peroxide and citrate-bicarbonate-dithionite, respectively (Jackson, 1975Jackson ML. Soil chemical analysis: advanced course. 29th. ed. Madison: 1975.). Then, the clay powder was mounted on a metal support after light pressure was applied on the sample using rough paper, in such a way as to minimize the preferential orientation of the particles. The silt powder did not receive any treatment and followed the same mounting procedure.

Treatments with K, Mg and Mg-glycerol saturation and heating of K treatments at 350 and 550 ^oC were performed in order to identify and characterize minerals in the clay fraction, which were then analyzed through X ray diffractometry, in the form of oriented microaggregates (Jackson, 1975Jackson ML. Soil chemical analysis: advanced course. 29th. ed. Madison: 1975.).

The diffractograms were obtained using a Shimadzu XRD-6000 diffractometer, operating at a tension of 40 kV and a current of 20 mA, with CuKα radiation and a graphite monochromator. A scanning amplitude of 5-70° (2θ) and a recording speed of 2^o 2θ min^-1 were used for clay and silt. For clay samples saturated with K at room temperature and heated at 350 and at 550 °C, the scanning amplitude was 3-35° (2θ) and the recording speed was 1.5^o 2θ min^-1. In samples saturated with Mg and Mg-glycerol, the scanning amplitudes were 2-35^o (2θ) and 2-15^o (2θ), respectively, with a recording speed of 1^o 2θ min^-1.

The criteria for the interpretation of diffractograms and the identification of minerals constituting silt and clay fractions were employed according to Grim (1968)Grim RE. Clay mineralogy. 2nd. ed. New York: McGraw-Hill; 1968., Jackson (1975)Jackson ML. Soil chemical analysis: advanced course. 29th. ed. Madison: 1975., Dixon and Weed (1977)Dixon JB, Weed SB. Minerals in soil enviroments. Madison: Soil Science of America; 1977., Brown and Brindley (1980)Brown G, Brindley GW. X-ray diffraction procedures for clay mineral identification. In: Brown G, Brindley GW, editors. Crystal structures of clay minerals and X-ray identification. London: Mineralogical Society; 1980. p.305-60., Whitting and Allardice (1986)Whitting LD, Allardice WR. X-ray diffraction techniques. In: Klute A, editor. Methods of soil analysis; physical and mineralogical methods. Madison: Soil Science Society of America; 1986. Pt 1. p.331-59. and Moore and Reynolds (1989)Moore DM, Reynolds RC. X-ray diffraction and the identification and analysis of clay minerals. New York: Oxford University Press; 1989..

The characterization of coarse and fine sand fractions was based on the usual methods described by Klein and Hurlbut Jr. (1999) and Leinz and Campos (1979)Leinz V, Campos JES. Guia para determinação de minerais. 8ª. ed. São Paulo: Nacional; 1979., which involve homogenization and quartering of samples; utilization of physical (magnetism) and chemical (addition of 10 % HCl for carbonates determination and 10 % H₂O₂ for Mn oxide determination) microtests; and description and characterization of physical properties of minerals, such as luster, color, cleavage, habit, fracture etc., performed using a binocular loupe.

The semi-quantitative determination of the percentages of minerals constituting coarse and fine sand fractions was based on the method of visual estimation proposed by Terry and Chilingar (1955)Terry RD, Chilingar GV. Comparison charts for visual estimation of percentage composition. J Sedim Petrol. 1955;25:229-34..

RESULTS AND DISCUSSION

Morphological attributes

The A horizon had a massive, moderately cohesive structure in all the studied Planosols, with sandy loam texture and color from brown to yellowish brown (Table 1). The E horizon had a texture ranging from loamy sand to sandy clay loam and lighter color, between brown and grayish brown.

In the A horizon, there was a reduction in the color value and chroma with an increase in rainfall rate; therefore, the soils become darker: profile 1 (10YR 3/3) > profile 2 (10YR 4/2) > profile 3 (10YR 4/3) > profile 4 (10YR 5/4) (Table 1). Barbosa et al. (2015)Barbosa WR, Romero RE, Souza Júnior VS, Cooper M, Sartor LR, Partiti CSM, Jorge FO, Cohen R, Jesus SL, Ferreira TO. Effects of slope orientation on pedogenesis of altimontane soils from the Brazilian semi-arid region (Baturité massif, Ceará). Environ Earth Sci. 2015;73:3731-43. doi:10.1007/s12665-014-3660-4, studying the effect of slope orientation on soil pedogenesis, observed that soils under drier conditions have higher value and chroma, and that the lower value and chroma in soils under humid conditions are related to the greater supply of organic matter to the soil and the more intense melanization process.

Profile 1, located in the Zona da Mata, in Northern Pernambuco state, a region with higher rainfall (1,097.1 mm), showed thicker A and E horizons, respectively 0.27 and 0.12 m (Figure 1). Profiles 2 and 3 were located in areas with higher water deficit, respectively, showing A horizons with similar thickness, around 0.20 m, and E horizons with only 0.06 m in P2 and 0.02 m in P3. Profile 4, in Jataúba, located in the most semiarid area compared with the others, had a poorly developed A horizon, only 0.15 m in thickness and an absent E horizon. These observations confirm the influence of rainfall on the development of soil profiles; as rainfall increases, there is better development of the A and E horizons in the studied soils (Figure 1).

Reduced development of the A and E horizons in areas with lower rainfall is explained by the lower rate of chemical weathering in the semiarid region, which is characterized by mean annual temperatures from 27 to 29 °C, rainfall between 800 and 400 mm yr^-1 and mean potential evapotranspiration of 2,000 mm yr^-1. In addition, rainfalls are irregular in time and space, concentrated in three to four months, with well-defined alternation of rainy periods with long, very dry periods (Ab’Saber, 1996Ab’Saber A. Domínios morfo-climáticos e solos do Brasil. In: Alvarez V VH, Fontes LEF, Fontes MPF, editores. O solo nos grandes domínios morfoclimáticos do Brasil e o desenvolvimento sustentável. Viçosa, MG: SBCS/UFV/DCS; 1996.; Silva et al., 2010Silva PCG, Moura MSB, Kiill LHP, Brito LTL, Pereira LA, Sá BI, Correia RC, Teixeira AHC, Cunha TJF, Guimarães Filho C. Caracterização do semiárido brasileiro: fatores naturais e humanos. In: Sá IB, Silva PCG, editores. Semiárido brasileiro: pesquisa, desenvolvimento e inovação. Petrolina: Embrapa Semiárido; 2010.). Additionally, there is lower supply of plant biomass to the soil in drier areas, which directly influences the melanization process, as observed by Barbosa et al. (2015)Barbosa WR, Romero RE, Souza Júnior VS, Cooper M, Sartor LR, Partiti CSM, Jorge FO, Cohen R, Jesus SL, Ferreira TO. Effects of slope orientation on pedogenesis of altimontane soils from the Brazilian semi-arid region (Baturité massif, Ceará). Environ Earth Sci. 2015;73:3731-43. doi:10.1007/s12665-014-3660-4 when studying the effect of slope orientation on the pedogenesis of soils in Ceará.

The absence of the E horizon in profile 4 may also be related to the presence of a sodic character and higher salinity compared with the other studied soils, which negatively influence the dispersion of clay particles (Tables 2 e 3), causing them to remain flocculated, which interferes with the process of lessivage.

The E horizon of profile 3 penetrated between the structural units of the Btn horizon and reached a depth of 0.26 m (Figure 2). Soil swelling and shrinking allows the structural units (prismatic and columnar) to move away from each other, creating fissures or channels between the structures, where material from the E horizon and roots can penetrate, forming the “tongues” observed in profile 3. The brown to grayish light brown color of the E horizon seems to result from a process of ferrolysis; however, according to van den Berg et al. (1987)Berg M van den, Lepsch IF, Sakai E. Solos de planícies aluviais do vale do rio ribeira de Iguape, SP – II Relações entre características físicas e químicas. Rev Bras Cienc Solo. 1987;11:315-21., for the confirmation of this process, complementary analyses are necessary, which would also be able to assess whether clay is currently being destroyed on the top of the B horizon.

These soils showed an abrupt transition and a clear interface between the A or E horizon and the Btn horizon, with a prismatic structure in Haplic Planosols (Profiles 1 and 2) and a very large columnar structure in Natric Planosols (Profiles 3 and 4) (Figure 2). The prismatic structure is usually related to the presence of high activity clays, which have more pronounced expansion and contraction due to soil wetting and drying cycles (Capeche, 2008Capeche CL. Noções sobre tipos de estruturas do solo e sua importância para o manejo conservacionista. Rio de Janeiro: Embrapa; 2008. p.1-6. (Comunicado técnico, 51).), while the columnar structure reflects high values of Na saturation.

The influence of the variation in climatic conditions and the exchangeable Na content on the formation of the structure in the Btn horizon was clearly observed when all the profiles were compared with profile 4, with a sodic character and salinity (Tables 1, 2 and 3).

All the Planosols clearly had expansive minerals (smectite and vermiculite) in the clay fraction. However, at higher ESP values, a columnar structure developed in Natric Planosols (Profiles 3 and 4) and, with a reduction in ESP, a prismatic structure developed in Haplic Planosols (Profiles 1 and 2) (Figure 2) (Tables 1 and 3).

The Btn horizons of all the studied Planosols showed a substantial increase in clay contents, with an abrupt transition and textural change in relation to the overlying A or E horizon (Tables 1 and 2), responsible for the formation of a suspended water table during the rainy period, which can lead to the appearance of mottles, as observed in profiles 1 and 2 (Table 1). This drainage deficiency may also have favored the action of ferrolysis in the contact area between A or E horizons and the Btn, and can lead to the destruction of clay minerals, thus contributing to the formation of textural gradient in Planosols (van den Berg et al., 1987Berg M van den, Lepsch IF, Sakai E. Solos de planícies aluviais do vale do rio ribeira de Iguape, SP – II Relações entre características físicas e químicas. Rev Bras Cienc Solo. 1987;11:315-21.; Fanning and Fanning, 1989Fanning DS, Fanning MCB. Soil, morphology, genesis and classification. New York: John Wiley & Sons; 1989.).

The structure of the Btn horizon in the studied profiles underwent clear changes with the alteration of climatic conditions and ESP. Profile 1, under a more humid climate condition, had a lower content of exchangeable Na (Table 3) and a Btn horizon with a moderate, large, prismatic structure (Table 1). In profile 2, the Btn horizon had a moderate, medium to very large, prismatic structure, with compression surfaces, i.e., a larger size and higher clay activity compared with profile 1, possibly due to the reduction in moisture and increase in exchangeable Na content. Profiles 3 and 4, in the Btn horizon, had a moderate to strong, large and very large, columnar structure, reflecting the increase of sodicity with a reduction in rainfall (Tables 1 and 3) (Figure 2).

The studied profiles have morphological features that indicate the influence of transported material in surface horizons, although from a short distance, because fragments of rocks, some times without edges (Profile 3), were observed in the transition between the horizons E and Btn in all the profiles. Some of these fragments penetrated cracks between the columnar structures of the Btn horizon, in profiles 3 and 4 (Table 1). Parahyba et al. (2009)Parahyba RDBV, Santos MCD, Rolim Neto FC. Evolução quantitativa de Planossolos do agreste do Estado de Pernambuco. Rev Bras Cienc Solo. 2009;33:991-9. doi:10.1590/S0100-06832009000400023, studying Planosols, also in the state of Pernambuco, observed some indication of reworking of more superficial materials and concluded that the studied soils had an autochthonous origin. However, as pointed out by Michelon et al. (2010)Michelon CR, Azevedo AC, Pedron FA, Dalmolin RSD, Stürmmer SK, Gonçalves J, Jesus SL. Causes of morphological discontinuities in soils of Depressão Central, Rio Grande do Sul State, Brazil. Sci Agric. 2010;67:319-26. doi:10.1590/S0103-90162010000300010, it is difficult to confirm the transport of material, because the processes of weathering and pedogenesis tend to level the differences that allow for the recognition of different materials.

Physical attributes

The dendrogram of similarity (Figure 3a) initially allows for observing similarities between the physical attributes of the Bt horizons in the profiles. Two different groups were formed; one comprising profiles 1 (Timbaúba) and 2 (Altinho), both with the highest rainfalls (1,097.1 and 633.2 mm). However, despite having similar attributes, which placed them in the same group (Euclidean distance of 5.0), this similarity was even less than that of the second group. The second group comprises profiles 2 (Arcoverde) and 3 (Jataúba), which are very similar with respect to their physical attributes (Euclidean distance of 3.8).

The textural contrast observed in the field during soil morphological descriptions (Table 1) was confirmed by the granulometric analysis and the textural differentiation index (TDIh) (Table 2), evidencing the abrupt textural change between A or E horizons and the Btn.

Profiles 1 and 2 had A and E horizons with medium texture and a Btn horizon with a clayey texture, with a lower amount of gravel in the Btn of profile 2, compared with the other Btn horizons (Table 2). The A horizon of profiles 3 and 4 and the E horizon of profile 3 had a sandy (light) texture, with less than 150 g kg^-1 of clay, and a Btn horizon with medium texture and 300 g kg^-1 of clay or more. Profiles 3 and 4 also showed higher amounts of gravel in the surface horizons, reinforcing the idea of lithological discontinuity between the A and E horizons, and the Btn horizon.

Water erosion is a concern in semiarid regions, because rainfalls are concentrated in a short period and due to the predominance of bushy, arboreal and herbaceous vegetation with a few or no leaves, which could reduce the impact of raindrops at the beginning of the rainy season (Sampaio et al., 2005Sampaio EVSB, Araújo MSB, Sampaio YSB. Propensão à desertificação no semi-árido brasileiro. Rev Geogr. 2005;22:67-76.; González-Botello and Bullock, 2012González-Botello MA, Bullock SH. Erosion-reducing cover in semi-arid shrubland. J Arid Environ. 2012;84:19-25. doi:10.1016/j.jaridenv.2012.04.002). These characteristics explain the increase in the gravel percentage in the A horizon of the Planosols with an increase in semiarid conditions (Table 2).

The high clay content in the Btn horizon of profile 1 may have resulted from rainfall in the region, which allowed for increased action of pedogenetic processes, such as lessivage, ferrolysis and leucinization and/or a higher clay content in the parent material. In general, profile 1 differs from the others by having the highest contents of clay in the Btn horizon and silt in the A and E horizons (Table 2).

The silt/clay ratio, in some situations, can indicate the weathering stage of soils in tropical regions (Anjos et al., 1998Anjos LH, Fernandes MR, Pereira MG, Franzmeier DP. Landscape and pedogenesis of an Oxisol-Inceptisol-Ultisol sequence in Southeastern Brazil. Soil Sci Soc Am J. 1998;62:1651-8. doi:10.2136/sssaj1998.03615995006200060024x). In all the profiles, the silt/clay ratio values were higher in the surface horizons, probably due to the loss of clay from the surface through eluviation, according to Silva et al. (2002)Silva MSLD, Klamt E, Cavalcanti AC, Kroth PL. Adensamento subsuperficial em solos do semiárido: processos geológicos e/ou pedogenéticos. Rev Bras Eng Agric Amb. 2002;6:314-20. doi:10.1590/S1415-43662002000200021, or even through the selective removal of fine material by water from the surface runoff of rainfalls (Chaves et al., 1985Chaves IB, Freire O, Amorim Neto MS. Características da precipitação e risco de erosão na região tropical semi árida brasileira. Pesq Agropec Bras. 1985;20:991-8.; Oliveira et al., 2009Oliveira LB, Fontes MPF, Ribeiro MR, Ker JC. Morfologia e classificação de Luvissolos e Planossolos desenvolvidos de rochas metamórficas no semiárido do nordeste brasileiro. Rev Bras Cienc Solo. 2009;33:1333-45. doi:10.1590/S0100-06832009000500026). However, the percentage of the silt fraction can increase as a function of the deposition of silica, which is present in plants in the form of phytoliths. Costa et al. (2010)Costa LM, Santos RF, Schaefer CEGR, Moreau AMSS, Moreau MS. Ocorrência de corpos silicosos em horizontes superficiais de solos de diferentes ecossistemas. Rev Bras Cienc Solo. 2010;34:871-9. doi:10.1590/S0100-06832010000300028, who studied phytoliths in many Brazilian ecosystems, pointed out that they mainly occur in the silt fraction of soils. Although it was not the objective of this study, the process of ferrolysis, which may occur in this environment, can corrode minerals from the fine sand fraction, transforming them into the silt fraction.

The degree of flocculation in the Btn horizon of profile 1 was higher compared with the Btn horizons of profiles 2, 3 and 4, with lower rainfalls (Table 2). Semiarid conditions can lead to higher Na contents in the exchange complex of the soils (Table 3), causing the dispersion of soil particles, similar to that observed by Galindo et al. (2008)Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036.

High values of soil density were observed, from 1.53 to 2.02 Mg m^-3, especially in the Btn horizons, confirming the higher densification of the planic B horizon compared with the others (Table 2). The Ap horizon of profile 3 had an extremely high density, possibly due to compaction caused by anthropic action.

The high soil density in the Bt horizon and lower total porosity (Table 2), associated with morphological characteristics (Table 1) (columnar and/or prismatic structures, ranging from medium to very large and with extremely hard consistency), promote an environment with deficient drainage, difficult aeration for plant roots and, consequently, poor root development. As a result, better root development is observed in the field between structures, possibly due to the densification of the Bt horizon of the soils (Reichardt and Timm, 2004Reichardt K, Timm LC. Solo, planta e atmosfera: conceitos, processos e aplicações. Barueri: Manole; 2004.; Galindo et al., 2008Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036). Some studies have shown a reduction in electrical conductivity, macro and microporosity in Planosols with an increase in depth (Lima et al., 2006Lima CLR, Pauletto EA, Gomes AS, Hartwig MP, Passianoto CC. Compactação de um Planossolo em função de sistemas de manejo. Rev Bras Agroci. 2006;12:179-82., 2008Lima CLR, Pillon CN, Suzuki LEAS, Cruz LEC. Atributos físicos de um Planossolo háplico sob sistemas de manejo comparados aos do campo nativo. Rev Bras Cienc Solo. 2008;32:1849-55. doi:10.1590/S0100-06832008000500006).

According to the uniformity values (UV) (Table 2), the possibility of lithological discontinuity is discarded for profiles 1 and 2, which showed UV below 0.30. Combined with that, the data on the fine sand/total sand (FS/TS) ratio between the horizons, with variation of only 0.14 and 0.12, respectively, confirmed the continuity of the same lithology, as also observed by Almeida et al. (1997)Almeida JA, Klamt E, Kämpf N. Gênese do contraste textural e da degradação do horizonte B de um Podzólico Vermelho-Amarelo da planície costeira do Rio Grande do Sul. Rev Bras Cienc Solo. 1997;21:221-33., Mafra et al. (2001)Mafra AL, Silva EF, Cooper M, Demattê JLI. Pedogênese de uma sequência de solos desenvolvidos de arenito na região de Piracicaba (SP). Rev Bras Cienc Solo. 2001;25:355-69. doi:10.1590/S0100-06832001000200012 and Bortoluzzi et al. (2008)Bortoluzzi EC, Pernes M, Tessier D. Mineralogia de partículas envolvidas na formação de gradiente textural em um Argissolo subtropical. Rev Bras Cienc Solo. 2008;32:997-1007. doi:10.1590/S0100-06832008000300009. Conversely, profiles 3 and 4, which had UV of 0.62 and 2.26, and FS/TS ratios with amplitudes of 0.04 and 0.32, respectively, show the possibility of having lithological discontinuity, especially in profile 4. However, more studies are necessary to confirm the presence of materials of allochthonous origin, because, according to Michelon et al. (2010)Michelon CR, Azevedo AC, Pedron FA, Dalmolin RSD, Stürmmer SK, Gonçalves J, Jesus SL. Causes of morphological discontinuities in soils of Depressão Central, Rio Grande do Sul State, Brazil. Sci Agric. 2010;67:319-26. doi:10.1590/S0103-90162010000300010, the transport of material similar to that from the basement rock can occur and the processes of weathering and pedogenesis can level the differences that allow for the recognition of different materials.

Sstudying Planosols in the state of Pernambuco, Parahyba et al. (2009Parahyba RDBV, Santos MCD, Rolim Neto FC. Evolução quantitativa de Planossolos do agreste do Estado de Pernambuco. Rev Bras Cienc Solo. 2009;33:991-9. doi:10.1590/S0100-06832009000400023, 2010Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000.) concluded that the studied soils had an allochthonous origin, with an expressive textural contrast formed by a combination of pedogenetic processes, such as clay eluviation-illuviation, in situ clay formation and selective clay loss in the horizon due to lateral movements through mechanical drag.

It is assumed that the studied soils have high erosive potential, due to the accumulation of clay in the Btn horizon and the presence of exchangeable Na dispersing clays, which makes water percolation difficult, as well as a gently undulating landscape and a high content of silt, coarse sand and fine sand in the surface horizons. The situation is more alarming in profiles 1 and 4, because they had much higher fine sand values than coarse sand values. Therefore, these are soils susceptible to erosion, which need conservational practices to avoid degradation. However, the erosive potential of profile 4 deserves more attention than the others, because its A horizon had the least depth, only 0.15 m, and can be easily eroded in case of inadequate soil use through the processes of laminar, rill and/or gully erosion.

Chemical attributes

For a better visualization of the behavior of soil chemical attributes, a dendrogram of similarity was constructed based on the chemical attributes of the Bt horizon of the profiles (pH(H₂O), pH(KCl), Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺, H+Al, OC, P, CEC and SB) (Figure 3b). Profiles 1 (Timbaúba) and 2 (Altinho) showed great similarity in their chemical attributes, with a Euclidean distance of only 4.8. Profiles 3 (Arcoverde) and 4 (Jataúba) had very different chemical characteristics; profile 4 was more similar to profiles 1 and 2, but with a Euclidean distance of only 5.6. This behavior, verified in the dendrogram of similarity, is similar to that observed in the relationship between rainfall and thickness of A and E horizons, which were thicker in profiles 1 and 2 (Figure 1).

The studied profiles are under different climate conditions; with an increase in rainfall, there was greater leaching of basic cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺), thus concentrating Al³⁺ and H+Al (Table 3). Profile 1, with a more humid climate, had lower SB values and higher contents of acidic cations (Table 3). In addition, it had the highest percentages of quartz in the sand fraction, with only a small reserve of nutrients in the structure of minerals from the silt (feldspar and amphibole) and clay (feldspar, vermiculite and smectite) fractions, which justifies the low contents of exchangeable cations and the more obvious presence of Al³⁺ and H+Al.

All the studied soils were eutrophic, with a clear contribution of Ca²⁺ and Mg²⁺ in all the profiles, corroborating the results of Corrêa et al. (2003)Corrêa MM, Ker JC, Mendonça ES, Ruiz HA, Bastos RS. Atributos físicos, químicos e mineralógicos de solos da região das várzeas de Souza (PB). Rev Bras Cienc Solo. 2003;27:311-24. doi:10.1590/S0100-06832003000200011, Alves et al. (2005Alves AGC, Marques JGW, Queiroz SB, Silva IF, Ribeiro MR. Caracterização etnopedológica de Planossolos utilizados em cerâmica artesanal no agreste paraibano. Rev Bras Cienc Solo. 2005;29:379-88. doi:10.1590/S0100-06832005000300008, 2007Alves AGC, Silva IF, Queiroz SB, Ribeiro MR. Sodium-affected Alfisols of the Agreste region, state of Paraíba, Brazil, as known by potter-farmers and agronomists. Sci Agric. 2007;64:495-505. doi:10.1590/S0103-90162007000500007), Galindo et al. (2008)Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036 and Parahyba et al. (2010)Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000.. Profile 1 had the lowest base saturation (V) among the studied profiles, possibly due to the high rainfall in this region.

The contents of Mg²⁺ in the deeper horizons in most of the profiles were higher than the Ca²⁺ contents. This can be explained by the alteration of minerals containing Mg²⁺ in their crystalline structure, such as smectite, vermiculite, mica and amphibole, which were observed in all of the studied Planosols (Table 4). Planosols with high Mg²⁺ contents are commonly found, as observed by Jacomine et al. (1973)Jacomine PKT, Cavalcanti AC, Burgos N, Pessoa SCP, Silveira CO. Levantamento exploratório – reconhecimento de solos do Estado de Pernambuco. Recife: Ministério da Agricultura/Sudene; 1973. v.1. (Boletim técnico, 26; série pedologia, 14)., Galindo et al. (2008)Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036, Oliveira et al. (2009)Oliveira LB, Fontes MPF, Ribeiro MR, Ker JC. Morfologia e classificação de Luvissolos e Planossolos desenvolvidos de rochas metamórficas no semiárido do nordeste brasileiro. Rev Bras Cienc Solo. 2009;33:1333-45. doi:10.1590/S0100-06832009000500026 and Parahyba et al. (2010)Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000..

The horizons of profile 1 were acidic, with pH values below 6.0, due to the higher rainfall in this region (1,194 mm). Profile 2 was moderately acidic in the A horizon, with a pH of 5.9, tending to neutrality in deeper horizons. Profile 3 was alkaline. Profile 4 had a moderately acidic pH, which tended to increase (7.0) in the deepest horizon (Btnz2) (Table 3). These pH values are consistent with the values of other Planosols in semiarid regions (Oliveira et al., 2009Oliveira LB, Fontes MPF, Ribeiro MR, Ker JC. Morfologia e classificação de Luvissolos e Planossolos desenvolvidos de rochas metamórficas no semiárido do nordeste brasileiro. Rev Bras Cienc Solo. 2009;33:1333-45. doi:10.1590/S0100-06832009000500026; Parahyba et al., 2010Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000.).

The contents of organic C were low and were similar in the A horizon of profiles 1, 2 and 3, and slightly lower in profile 4. These results are similar to those observed by Galindo et al. (2008)Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036 and Oliveira et al. (2009)Oliveira LB, Fontes MPF, Ribeiro MR, Ker JC. Morfologia e classificação de Luvissolos e Planossolos desenvolvidos de rochas metamórficas no semiárido do nordeste brasileiro. Rev Bras Cienc Solo. 2009;33:1333-45. doi:10.1590/S0100-06832009000500026, in Planosols under similar climatic conditions.

High Na⁺ values were observed in the subsurface horizons, in relation to the surface horizons, in all the profiles. This is due to the higher leaching of the element in surface horizons with a more sandy texture and its accumulation in the more clayey Bt horizons, with higher CEC. The high Na⁺ content in the profiles is possibly caused by the weathering of feldspars (Table 4), especially those of the series of calco-sodic plagioclases, which are frequently found in Planosols, according to the studies of Mota and Oliveira (1999)Mota FOB, Oliveira JB. Mineralogia de solos com excesso de sódio no Estado do Ceará. Rev Bras Cienc Solo. 1999; 23:799-806., Galindo et al. (2008)Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036, Oliveira et al. (2008)Oliveira LB, Fontes MPF, Ribeiro MR, Ker JC. Micromorfologia e gênese de Luvissolos e Planossolos desenvolvidos de rochas metamórficas no semiárido brasileiro. Rev Bras Cienc Solo. 2008;32:2407-23. doi:10.1590/S0100-06832008000600019 and Parahyba et al. (2010)Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000.. These contents, associated with semiarid conditions and poor drainage, favor the maintenance of high Na⁺ contents, according to Mota and Oliveira (1999)Mota FOB, Oliveira JB. Mineralogia de solos com excesso de sódio no Estado do Ceará. Rev Bras Cienc Solo. 1999; 23:799-806.. The effect of Na⁺ was more pronounced in profiles from more semiarid regions (profiles 3 and 4), which showed a sodic character (ESP equal to or higher than 15 %).

The electrical conductivity (EC) of the saturation paste extract and the content of soluble Na⁺ in the studied profiles were low in the surface horizons (A and/or E), but always tended to increase in the subsurface, except in profile 1. The highest values were observed in profile 4, with EC of 9.3 and 10.1 dS m^-1 in the horizons Btnz1 and Btnz2, respectively, which adds a salic character to the third categorical level of the Brazilian Soil Classification System (Santos et al., 2013Santos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF. Sistema brasileiro de classificação de solos. 3ª. ed. Rio de Janeiro: Embrapa Solos; 2013.).

Mineralogy of the silt and clay fractions

Most of the studied soils had a mineralogical assembly typical of poorly weathered soils, with the presence of feldspars, smectites, vermiculites, micas and amphiboles (Table 4). These minerals are easily altered in soils with good drainage, in a humid tropical climate (Kämpf et al., 2009Kämpf N, Curi N, Marques JJ. Intemperismo e ocorrências de minerais no ambiente do solo. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo: conceitos básicos. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. Pt.1. p.333-79.). However, profiles 2, 3 and 4 were found in a currently semiarid environment, with low rainfalls that are irregular in time and space, and high temperatures (Ab’Saber, 1996Ab’Saber A. Domínios morfo-climáticos e solos do Brasil. In: Alvarez V VH, Fontes LEF, Fontes MPF, editores. O solo nos grandes domínios morfoclimáticos do Brasil e o desenvolvimento sustentável. Viçosa, MG: SBCS/UFV/DCS; 1996.). Combined with the physical characteristics (imperfect drainage) of Planosols, these factors favor the process of bisialitization and neoformation of 2:1 clay minerals from solutions rich in Al, Si and bases, originating from the weathering of primary minerals in the soil under low to moderate leaching of silica (Borchardt, 1989Borchardt G. Esmectites. In: Dixon JB, Weed SB, editors. Minerals in soil environments. Madison: Soil Science Society of America; 1989.; Corrêa et al., 2003Corrêa MM, Ker JC, Mendonça ES, Ruiz HA, Bastos RS. Atributos físicos, químicos e mineralógicos de solos da região das várzeas de Souza (PB). Rev Bras Cienc Solo. 2003;27:311-24. doi:10.1590/S0100-06832003000200011; Melo and Wypych, 2009Melo VF, Wypych F. Caulinita e Haloisita. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo: conceitos básicos. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. p.427-504.).

The mineralogy of the studied Planosols is consistent with the concepts of Oliveira (2007), who claims that the Order of Planosols includes dystrophic and eutrophic soils, formed from materials with various origins, and can have a mineralogy from essentially kaolinitic to predominantly smectitic, but always with low contents of free Fe oxides, due to the formation conditions and the more or less pronounced hydromorphism to which they are susceptible during some parts of the year.

The silt fraction of the studied profiles was basically formed by quartz, feldspars and amphiboles in all the horizons (Table 4). These minerals were predominant in the A and Bt horizons of all the profiles, and are more evident in the profiles from Timbaúba (P1) and Jataúba (P4). Many Planosols in the Mesoregion of the Agreste of Pernambuco have a mineralogical assembly in the silt fraction composed of quartz, feldspars and micas (Galindo et al., 2008Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036).

Particles of feldspar, when found in the silt and clay fractions, are very susceptible to hydrolysis reactions and can undergo dissolution, with the release of silica, Al and K to the soil solution, and later recrystallization of 1:1 (kaolinite) and 2:1 (smectite and vermiculite) minerals, as observed in the studied Planosols (Melo and Wypych, 2009Melo VF, Wypych F. Caulinita e Haloisita. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo: conceitos básicos. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. p.427-504.) (Table 4). However, the neoformation of kaolinite in this environment is not facilitated, because the virtual absence of Na, Ca, Mg, Fe and K would be necessary, which does not happen in these Planosols. These soils have minerals that are the source of these cations and imperfect drainage, which hampers the leaching process, attenuating the weathering in this environment.

Despite being from different geoenvironmental conditions, the studied Planosols did not show considerable modification in the mineralogical assembly in the silt and clay fractions. Possibly, the condition of imperfect drainage, typical of Planosols, disfavors the more efficient action of weathering, allowing an environment with solutions rich in basic cations, Al and silica, which can neoform clay minerals such as smectite, vermiculite and kaolinite, observed in the clay fraction of these soils (Table 4).

In the profiles from Altinho (P2), Arcoverde (P3) and Jataúba (P4), mica was found in the clay fraction (Table 4), which may undergo weathering and the formation vermiculite and smectite, resulting from the loss of K from the interlayers, Fe²⁺ oxidation and reorientation of hydroxyls, as pointed out by Douglas (1989)Douglas LA. Vermiculites. In: Dixon JB, Weed SB. Minerals in soil environments. 2ª. ed. Madison: Soil Science Society of America; 1989. p.635-74. and Azevedo and Vidal-Torrado (2009)Azevedo AC, Vidal-Torrado P. Esmectita, vermiculita, minerais com hidróxientrecamadas e clorita. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo, conceitos básicos e aplicações. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. p. 381-426..

The characteristics of deficient drainage and alternating cycles of oxidation and reduction, typical of Planosols, promote an environment favorable to lower solubilization and loss of silica from the system, allowing the genesis of 2:1 layer minerals. This allows not only the neoformation of 2:1 minerals in the studied sites, but also their stability (Borchardt, 1989Borchardt G. Esmectites. In: Dixon JB, Weed SB, editors. Minerals in soil environments. Madison: Soil Science Society of America; 1989.; Corrêa et al., 2003Corrêa MM, Ker JC, Mendonça ES, Ruiz HA, Bastos RS. Atributos físicos, químicos e mineralógicos de solos da região das várzeas de Souza (PB). Rev Bras Cienc Solo. 2003;27:311-24. doi:10.1590/S0100-06832003000200011; Melo and Wypych, 2009Melo VF, Wypych F. Caulinita e Haloisita. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo: conceitos básicos. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. p.427-504.).

In the Cr horizon of profiles 2, 3 and 4 (Table 4), the presence of secondary minerals such as kaolinite and smectite in the clay fraction was common to all. This is due to the high contents of easily alterable primary minerals in the parent material of these soils, which can be altered to form the secondary minerals found in the clay fraction.

The kaolinite of the studied profiles can be being formed from the alteration of 2:1 clay minerals and directly from the alteration of micas and feldspars in some sites with better soil drainage.

Mineralogy of the coarse and fine sand fractions

The mineralogy of coarse and fine sand fractions was mainly formed by quartz (99 to 92 %) in A and Btn horizons (Table 4). Feldspar was the second most predominant mineral (1 to 2 %) in the coarse sand fraction of the horizons A and Btn of the profiles 2 (A – 1 %; Btn – 2 %) and 3 (A – 1 %; Btn – 1 %), which were the only ones with this mineral. In the Cr horizon, feldspar was found in all the profiles (P2 – 5 %; P3 – 8 % and P4 – 3 %), except in profile 1, in which the Cr horizon was not analyzed. In this horizon, feldspar dropped to the third position in terms of abundance and was surpassed by quartz (24 to 20 %) and biotite (5 to 19 %) (Table 4).

Unlike the coarse sand fraction, in the fine sand fraction feldspar was not present in the horizons A and Btn of the profiles, and was found only in the Cr horizon (6 to 2 %), where only quartz appeared as the most abundant mineral (Table 4). The absence of feldspar in the A and Btn horizons in the fine sand fraction is due to its lower stability with the reduction in particle size, which becomes more vulnerable to hydrolysis reactions and can suffer dissolution with release of silica, aluminum and potassium to the soil solution and later recrystallization of 1:1 (kaolinite) and 2:1 (smectite and vermiculite) minerals (Melo and Wypych, 2009Melo VF, Wypych F. Caulinita e Haloisita. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo: conceitos básicos. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. p.427-504.).

Evaluating the reserve and availability of nutrients in some soils cultivated with eucalyptus, Castro et al. (2010)Castro PP, Curi N, Furtini Neto AE, Resende AV, Guilherme LRG, Menezes MD, Araújo EF, Freitas DAF, Mello CR, Silva SRG. Química e mineralogia de solos cultivados com eucalipto (Eucalyptus sp.). Sci For. 2010;88:645-57. observed the presence of feldspar in the sand fraction of two Haplic Planosols and attributed to this element the high contents of total and exchangeable Ca and Mg in the subsurface horizons.

In the coarse sand fraction, biotite was observed in the Cr horizon of the profiles (P2 – 18 %; P3 – 19 % and P4 – 5 %). Possibly, this mineral underwent the action of physical weathering and was fragmented into smaller particles, which were observed in greater amount in the fine sand fraction (Table 4).

Biotite was found in the fine sand fraction of the A horizon of profiles 2 (1 %) and 3 (2 %) and only in the Btn horizon of profile 2 (2 %). However, in the Cr horizon, there was a large amount of biotite (P2 – 28 %; P3 – 48 % and P4 – 30 %) (Table 4). A reduction in the size of biotite particles favors the action of weathering and can lead to the formation of vermiculite in silt and clay fractions, resulting from the loss of K from the interlayers, Fe²⁺ oxidation and reorientation of hydroxyls, as pointed out by Douglas (1989)Douglas LA. Vermiculites. In: Dixon JB, Weed SB. Minerals in soil environments. 2ª. ed. Madison: Soil Science Society of America; 1989. p.635-74. and Azevedo and Vidal-Torrado (2009)Azevedo AC, Vidal-Torrado P. Esmectita, vermiculita, minerais com hidróxientrecamadas e clorita. In: Melo VF, Alleoni LRF, editores. Química e mineralogia do solo, conceitos básicos e aplicações. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2009. p. 381-426.. A large amount of biotite (52 %) in a Planosol from a semiarid region was also reported by Oliveira et al. (2008)Oliveira LB, Fontes MPF, Ribeiro MR, Ker JC. Micromorfologia e gênese de Luvissolos e Planossolos desenvolvidos de rochas metamórficas no semiárido brasileiro. Rev Bras Cienc Solo. 2008;32:2407-23. doi:10.1590/S0100-06832008000600019 and Parahyba et al. (2010)Parahyba RDBV, Santos MCD, Rolim Neto FC, Jacomine PKT. Pedogênese de Planossolos em topossequência do agreste pernambucano. Rev Bras Cienc Solo. 2010;34:1991-2000., the latter of whom associated the in situ weathering of this element with the remarkable textural contrast frequently observed in Planosols.

The presence of partially altered mica in the coarse and fine sand fractions of surface horizons of various Planosols in the state of Pernambuco was pointed by Galindo et al. (2008)Galindo ICDL, Ribeiro MR, Santos MFAV, Lima JFWF, Ferreira RFAL. Relações solo-vegetação em áreas sob processo de desertificação no município de Jataúba, PE. Rev Bras Cienc Solo. 2008;32:1283-96. doi:10.1590/S0100-06832008000300036.

Amphiboles were present in the coarse sand fraction, especially in the Cr horizon of the profiles 2 (12 %) and 4 (7 %). In the A and Btn horizons, only profile 2 had amphibole in the coarse sand fraction. The other profiles (1 and 3) did not have amphibole in the mineralogical assembly of the coarse sand fraction.

In the fine sand fraction, the presence of amphibole followed the distribution observed in the coarse sand fraction, where only profile 2 [A horizon (3 %) and Btn horizon (2 %)] had amphibole. In the other profiles, amphibole was only observed in the Cr horizon (except profile 1, where the Cr horizon was not analyzed), and was the third most abundant mineral in the mineralogical assembly (P2 – 16 %; P3 – 8 % and P4 – 12 %).

The lower percentage of amphiboles in the A and Btn horizons in the fine sand fraction, compared with coarse sand fraction, is due to the lower stability of this mineral when its size is reduced, because the specific area available to chemical weathering increases.

Amphiboles are double-chain inosilicates and can be a source of mainly Ca, Mg, Na, Fe, Si and Al, which can contribute to the neoformation of clay minerals in the soil. They are minerals with little mobility in the soil and are below the stability of albite in the Goldich series (Toledo et al., 2009Toledo MCM, Oliveira SMB, Melfi AJ. Da rocha ao solo: intemperismo e pedogênese. In: Teixeira W, Toledo MCM, Fairchild TR, Taioli F, editores. Decifrando a terra. 2ª. ed. São Paulo: Companhia Editora Nacional; 2009.). Nevertheless, amphiboles were observed in all the studied Planosols (Table 4), under the most varied climate conditions. This shows that the geochemical environment in these soils has favorable conditions for stabilization, since these minerals are easily alterable under the conditions of a humid tropical climate.

The mineralogical assembly of coarse and fine sand fractions was simplified with an increase in rainfall. However, possibly due to differences in the parent material, profile 4, located in the semiarid region of Pernambuco, with rainfall of only 510 mm, had a mineralogical assembly in the A and Btn horizons similar to that of profile 1, from a much more humid climate (1,097.1 mm).

Similar results were obtained by Parahyba et al. (2009)Parahyba RDBV, Santos MCD, Rolim Neto FC. Evolução quantitativa de Planossolos do agreste do Estado de Pernambuco. Rev Bras Cienc Solo. 2009;33:991-9. doi:10.1590/S0100-06832009000400023, studying the quantitative evolution of Planosols. These authors observed the presence of quartz, micas and feldspars as the main minerals constituting the coarse fraction of the soil, which also showed lower amounts of amphiboles and traces of zircon, tourmaline, rutile, epidotes and pyroxenes.

CONCLUSIONS

With an increase in rainfall, there was an increase in the clay contents in the Bt horizon, a reduction in ESP, EC, Na⁺, CEC, SB and soil density, and thicker A and E horizons.

The increase in ESP, associated with the presence of expansive minerals (smectite and vermiculite), allowed for the development of a prismatic structure in Haplic Planosols and a columnar structure in Natric Planosols with higher ESP.

The mineralogical assemblies of the silt and clay fractions were similar in the different geoenvironments studied, while higher contents of easily alterable minerals were observed in the composition of the sand fraction in environments with a drier climate.

ACKNOWLEDGMENTS

Thanks to the National Council for Scientific and Technological Development (CNPq) for granting the Master’s scholarship. In memoriam, to Prof. Mateus Rosas Ribeiro, for all the teaching. To the Agricultural Engineer José Fernando Wanderley Fernandes de Lima (Zeca) and the staff of the Usina Central Olho D’Água, for the logistic support during the study.

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