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Genesis of cohesive soil horizons from north-east Brazil: role of argilluviation and sorting of sand.

Introduction

The Coastal Tablelands represent a geomorphological unit of great social and economic importance in Brazil, extending from Rio de Janeiro to the Amazon (Amapa state) of Brazil. These coastal table landforms, with altitudes ranging from 30 to 200 m above sea level, contain approximately 7% of the agricultural soils from Brazil and cover more than 10 million ha (Rezende 2000). Characteristics such as the presence of deep soils, a flat or gently wavy landscape, humid climate conditions and proximity to large consumer centres, make the Coastal Tablelands one of the most favourable areas for agriculture in Brazil (Jacomine 2001).

From the perspective of the Brazilian north-east region, the Coastal Tableland soils account for more than 65% of the area cultivated for most crops, demonstrating their potential for activities such as fruit plantations (i.e. cashew and coconut), sugar cane and eucalyptus cultivation (Pacheco and Cantalice 2011; Gomes etal. 2012), as well as pasture (Costa etal. 2009). Despite this intensive agricultural exploitation and importance for the national and regional agroeconomy, Coastal Tableland soils exhibit a natural, singular agricultural limitation, namely dense subsoil layers referred to locally as solos coesos (Araujo et al. 2005; Ajayi et al. 2009a, 2009b), or cohesive soils, similar to the hard-setting soils of Australia (Mullins et al. 1990; Harper and Gilkes 1994; Chan 1995; Greene et al. 2002; Abbott and Christoph 2012) and Africa (McKyes et al. 1994; Materechera 2009; Moroke et al. 2009), as well as other hard soil horizons (Lamotte et al. 1997a, 1997b). These soils are mostly classified as Latossolos and Argissolos (Oxisols and Ultisols), all formed from the Barrciras Group, a loose sedimentary deposit of Miocene to Lower Pleistocene age (Vilas Boas et al. 2001), which covers approximately 10 million hectares, only in northeast Brazil (Rezende 2000).

In the Brazilian system of soil classification (EMBRAPA 2013) the diagnostic soil characteristic called 'cohesive character' (in Portuguese, carater coeso) has been used to describe soils with pedogenetically formed, dense subsurface horizons that present hard to extremely hard dry consistencies and become friable or firm under moist condition (Jacomine 2001; Santos et al. 2005). These dense horizons present a slow deformation when subjected to compression under wet conditions, unlike fragipan, which shatters suddenly (EMBRAPA 2013). According to Giarola et al. (2001), with regard to morphological and physical properties, cohesive soil horizons and hard-setting horizons may be considered as analogous soil materials. However, in contrast with the solos coesos, the hard-setting behaviour was evident in surface layers and over a wider range of climatic regions (i.e. tropical arid, semi-arid and Mediterranean), occurring in approximately 20% of the Australian territory, mostly in Luvisols, Planosols and Solonetz (see Giarola and Silva 2002).

In Brazilian soils, these dense horizons have been observed in subsurface horizons occurring at a wide range of depths (from 0.2 up to 1.42 m; Lima et al. 2004, 2006; Correa et al. 2008; Lima Neto et al. 2009, 2010; Vieira et al. 2012; EMBRAPA 2013). However, with regard to agricultural limitations, especially in fruit growing, previous studies have shown that moisture stored below a depth of 1.0 m soil is readily extractable (i.e. by coconut roots) and that this extraction may be extended up to 2 m (Vidhana Arachchi 1998). Similarly, cashew plants were shown to extract water up to a depth of 1.2 m, indicating a highly deep and active root system (O'Farrell 1994), and to penetrate to 9.5 m depth in well-drained soils (Abdul Khader 1987). Thus, the presence of this diagnostic soil characteristic may reduce not only the effective soil depth, limiting root growth (Vieira et al. 2012), but also cause some constraint to water uptake.

With regard to the genesis of cohesive soil horizons, researchers still disagree as to the processes involved. The following mechanisms have been suggested to explain this phenomenon: deposition of dispersed clay into pores and natural compaction promoted by drying and desiccation of soil material (Ribeiro 1986, 2001; Moreau et al. 2006). Other studies also indicate that siliceous and silico-aluminous compounds may act as temporary cementing agents in both hard-setting and cohesive soil horizons (Chartres et al. 1990; Vieira et al. 2012). In this case, these compounds would undergo polymerisation and precipitation during the dry period (causing hard consistencies) and depolymerisation during wet seasons (causing friability). In addition, Ajayi et al. (2009a, 20096) have emphasised a mineralogical factor, where the absence of gibbsite and low amounts of iron oxides would favour the face-to-face arrangement of kaolinite and thus contribute to the genesis of these cohesive horizons. Greene et al. (2002) also suggested the concept of face-to-face packing of clay minerals to explain the occurrence of hard setting in Australian soils.

Recent studies in Brazilian soils (Correa et al. 2008; Lima Neto et al. 2009, 2010) have suggested that the formation of cohesive soil horizons (at depths of ~0.20 and 0.40 m) may be related to the translocation of fine clay (<0.2 (tm; very fine clays), which, after dispersion, would increase the contact between larger soil particles and thus increase soil strength.

Although different studies (Silva et al. 1998; Correa et al. 2008; Lima Neto et al. 2010) have reported illuviation features (argillans or illuviated clay cutans) in cohesive soils from different parts of Brazil (Pernambuco and Alagoas), their presence has either not been recognised in many soils or has been observed in small quantities (Melo and Santos 1996; Lima et al. 2006).

In contrast with these pedogenic theories, some authors have focused on the role of geogenic processes in cohesive soil formation. According to Abrahao et al. (1998), the enhanced strength of soils formed from the Barreiras Group could be related to the poorly sorted sand particles (inherited from the sedimentary environment; see Morais et al. 2006), which would increase their packing density and thus soil strength. In fact, sand parameters such as size distribution, roundness, sphericity and roughness have long been highlighted as influencing pore size distribution, void ratio, compressibility and cohesion of sandy soil material (Coulon and Bruand 1989; Panayiotopoulos 1989). However, it must be mentioned that the degree of sand sorting within a soil layer may also be related to bioturbation (see Cooper et al. 2005). In addition, according to Vilas Boas et al. (2001), the Barreiras Group can contain vertical accretion deposits of massive mud levels that are very seldom laminated, which could be transformed into cohesive soil horizons.

Other studies on cohesive soils from Brazil (Lima et al. 2004, 2005, 2006) have reported the occurrence of cohesive soil horizons diagnostic soil characteristic mostly in sandy clay loam soils and in sand-dominated horizons (70%-80%). Vieira et al. (2012), studying different cohesive Ultisols, observed the highest soil strength in horizons with the higher sand proportions (~80%). In addition, different studies of coarse textured soils from Asia have shown that packing of the sand grains may limit root development and extension (Bruand et al. 2004; Hartmann et al. 2008a, 20086).

Identification of factors involved in the genesis of cohesive soils is crucial for evaluating the effects of this phenomenon and to the establishment of adequate soil management techniques. The aim of the present study was to evaluate the contribution of both argilluviation and sorting of sand on the formation of cohesive Ultisols from north-east Brazil, with dense (cohesive) horizons occurring at greater depths (from 0.9 to >1.7m).

Methods

Study area

The study area is located in the city of Trairi, Ceara, Brazil (Fig. 1; 3[degrees]16'40"S and 39[degrees]16'08"W). According to Koppen's classification (Kottek et al. 2006), the area has a tropical warm semi-arid climate type (BSw'h'). The region has a mean annual temperature of 24[degrees]C and a mean annual rainfall of 1589 mm, which is mostly concentrated between January and April (IPECE 2011).

[FIGURE 1 OMITTED]

The soil survey map of north-east Brazil (Brasil 1973) indicates eight soil associations in the Trairi district, with the predominance of Ultisols, Entisols (Quartzipsamments and Psamments) and Oxisols (Soil Survey Staff 2010). Vieira et al. (2012) identified an Ultisol toposequence with four soil profiles, three of which exhibited cohesive horizons. In those cases, cohesive soil horizons were identified in the field by presenting hard to extremely hard dry consistencies, which contrasted with friable or firm consistencies under moist conditions (Jacomine 2001; Santos et al. 2005). The classification of the profiles studied, according to soil taxonomy (Soil Survey Staff 2010) and WRB (2006), their location, topographic position and soil use, is listed in Table 1. All four Ultisols arc highly weathered soils with low-activity clay and a low fertility status (see Vieira et al. 2012).

The geology of the studied site is characterised by the presence of Tertiary sediments from the Barreiras Group, which are mostly composed of claystones (or mudstones), siltstones and sandstones, usually with white to yellow tonalities (Behling and Costa 2004). The coarse fraction of the deposits consists predominantly of quartz, with ferruginous concretions, whereas kaolinite is the dominant mineral in the clay fraction (Duarte et al. 2000; dos Santos Moreau et al. 2006; Correa et al. 2008).

Soil sampling

For the present study, soil samples from all four soil profiles (P1, P2, P3 and P4; Fig. 1) were collected and analysed. For subsequent analyses, soil samples were collected in three different ways. Bulk samples were collected from each soil horizon (both cohesive and non-cohesive horizons, which were identified in the field during sampling and morphological description), air dried, ground and sieved to <2 mm for particle size distribution and sand fractionation. In addition, for the determination of soil bulk density (BD; Blake and Hartge 1986) and soil penetration resistance (PR; MA 933 Model Electronic Penetrometer), undisturbed soil cores (5 cm diameter, 5 cm length) of cohesive and non-cohesive soils were collected from the Bt (argic) horizons of the four profiles.

For micromorphological analysis, undisturbed soil samples of cohesive and non-cohesive soil horizons were collected in Kubiena boxes (10x6 x 5cm) as follows: non-cohesive horizons PIBtl (90-138cm) and PlBt2 (138-167+ cm); cohesive horizon P2Btl (92-125 cm); cohesive horizons P3Btl (91-142 cm) and P3Bt2 (140-170+ cm); and cohesive horizon P4Btl (142-170+ cm).

Physical analyses

Particle size distribution was determined by the pipette method (Gee and Bauder 1986), using chemical (sodium hexametaphosphate) and physical (fast agitation for 10 min) dispersion. The different sand fractions were determined by dry sieving according to Soil Survey Division Staff (1993): very coarse sand (2-1 mm), coarse sand (1-0.5 mm), medium sand (0.5-0.25 mm), fine sand (0.25-0.1 mm) and very fine sand (0.1-0.05 mm).

From the sand fraction results, different particle diameters, defined by Atterberg limits (mm), were transformed to the Krumbein phi scale ([phi]), the units of which are determined by the equation [phi] = -[log.sub.2]D (Krumbein 1934), where D is diameter of grains (in mm). The equivalence between these two units for particle size was established as follows: [phi] from -1 to 0=sand particles 2-1 mm; [phi] 0-1= sand particles 1-0.5 mm; [phi] values 1-2 = sand particles 0.5-0.25 mm; [phi] 2-3.32 = sand particles 0.25-0.1 mm; [phi] 3.32- 4.24-sand particles 0.1-0.05 mm.

The [phi] values were analysed statistically using the microcomputer program for sediment particle size distribution (see Jong van Lier and Vidal-Torrado 1992), which calculates different statistical parameters (relative and cumulative frequency; mean and standard deviation) according to Folk and Ward (1957). Histograms were built with the percentage fractions of each granulometric sand fraction to evaluate the particle size distribution of sands in both the cohesive and non-cohesive Bt horizons.

The degree of sorting of sand in each horizon was determined through the standard deviation ([sigma]) of [sigma] values based on the following intervals (from Folk and Ward 1957): very well sorted ([sigma] <0.35), well sorted (0.35 [less than or equal to] [sigma] <0.50), moderately sorted (0.50 [less than or equal to] [sigma] 1.00), poorly sorted (1.00 [less than or equal to] [sigma] 2.00), very poorly sorted (2.00 [less than or equal to] [sigma] <4.00) and extremely poorly sorted ([sigma] [greater than or equal to] 4.00).

Soil PR was determined for each undisturbed soil sample (1-4 cm depth in the cylinder) after equilibration to a matric potential of -10 kPa. An electronic penetrometer (Marconi, Piracicaba, Sao Paulo, Brazil) was used, with receptor and interface attached to a computer to record readings using its own software. The penetration speed was 1 cm [min.sup.-1] with one reading recorded per second; the cone angle was 60[degrees] and the area 12.6 [mm.sup.2]. The tests were performed in three replicates in the core section of each sample, with 180 readings per determination. The soil PR is given as the mean value of 540 readings (Tormena et al. 1998).

Soil BD was determined on undisturbed 105[degrees]C oven-dried cores by calculating mass per unit volume.

Micromorphological analysis

After soil collection, soil samples were air dried for 3 days. Before impregnation, all soil samples were further dried in a convection oven at 60[degrees]C, for 24 h. After drying, samples were impregnated by capillarity (Murphy 1986) with an unsaturated polyester resin (Arazyn1.0#00 LT; Redelease, CEP, Campinas, SP, Brazil) mixed with styrene monomer (to reduce resin viscosity).

Impregnated samples were cut into approximately 0.5-cm slabs using a saw and polished with abrasives up to 600 mesh. The polished blocks were mounted onto glass slides, followed by polishing, to obtain 30-p.m thin sections. The thin sections were analysed using a polarising microscope (optic transmission microscopy (OTM) level; Carl Zeiss, Sao Paulo, Brazil) with an attached digital camera. The micromorphological descriptions were made using the criteria proposed by Bullock et al. (1985) and Stoops et al. (2010).

Results and Discussion

Soil physical attributes

Soil BD and PR values (Table 2) varied depending on whether the samples came from cohesive or non-cohesive horizons, as identified in the field. The lowest BD was recorded in PIBtl (1.53 Mg [m.sup.-3]), a non-cohesive horizon. Conversely, higher BDs were registered in cohesive horizons (P2Btl = 1.66 Mg [m.sup.-3]; P3Btl = 1.67 Mg [m.sup.-3]; P3Bt2= 1.73 Mg [m.sup.-3]; Table 2), with the highest value found in P4Btl (1.80Mg [m.sup.-3]).

Similarly, the lowest value of soil PR was found in the non-cohesive soil horizon PIBtl (0.74 MPa), whereas significantly higher values were recorded in the cohesive horizons (P2Btl =2.73 MPa; P3Btl =2.61 MPa; P3Bt2 = 3.56 MPa; P4Btl =7.71 MPa; Table 2).

Soil BD and PR values were similar to those found in other studies on cohesive soils at depths of 0.74 and 0.97 m from north-east Brazil (Lima et al. 2004, 2005). In addition, the BD and PR data are consistent with the micromorphological results (see below), which evidenced both void area reduction and an increased compaction in cohesive horizons.

In fact, it should be highlighted that BD alone does not provide enough information on physical soil quality. Additional data, such as micromorphological analyses, providing measurements or estimates of soil porosity and pore connectivity should be provided in order to support interpretations regarding the dynamics of physical processes occurring in the soil. Thus, BD alone is not a safe parameter to identify cohesive soils because it may, for example, be altered in response to textural variations. Therefore, in studies of cohesive soils, additional data on pore network (such as those provided in micromorphological analyses) are essential for better interpretations of the pedogenesis of this soil attribute.

With regard to the cohesive and non-cohesive horizons studied herein, the soil texture ranged from loamy to sandy loam (Fig. 2). These results are commonly reported for Coastal Tableland soils, which usually have vertical textural contrasts from sandy surface horizons to clayey subsurface horizons. Particle size distribution data (Table 2) show a clear predominance of the sand fraction (60%-91%) in all horizons studied, which confirms the sandy nature of the Barrciras Group sediments (Behling and Costa 2004; Balsamo et al. 2010). In fact, Furrier et al. (2006) have described the deposits from the Brazilian Coastal Tablelands as poorly consolidated clayey-sandy sediments of predominantly continental origin (Andrade and Dominguez 2002). Sand fractionation revealed similar results to those reported by Lima et al. (2004), with a predominance of medium and fine sand fractions that, combined, represent 77% of the total sand in the horizons studied (Table 2).

Statistical analysis of sand particle distribution, based on the parameters of Folk and Ward (1957), as well as the percentage histograms of each particle size fraction are presented in Table 3 and Fig. 3, respectively. Both parameters indicate the predominance of poorly sorted sand in cohesive horizons. With the exception of P2Btl, the standard deviation for [phi] values ranged from 1.018 to 2.160 (Table 3) in horizons with cohesive character, evidencing the poor degree of sorting (1.00 < a < 2.00) of sand, according to the limits established by Folk and Ward (1957).

In addition, considering that the coefficient of kurtosis may indicate peaked (positive excess kurtosis) or flat (negative excess kurtosis) distribution of values, the histograms in Fig. 3 show evidence that all cohesive horizons (except P1 Bt 1) contain sand distributions that follows mesokutic to platykurtic distributions. In these horizons, the relatively homogeneous distribution of sands of different sizes confirms the poor degree of sorting. In contrast, both non-cohesive horizons presented better-sorted sands, as evidenced by the leptokurtic distributions (Fig. 3) and the lower standard deviation values (Table 3).

These results indicate a possible contribution of the degree of sand sorting in the formation of cohesive horizons. Nunes et al. (2011) considered the hypothesis that dense soil horizons (i.e. duripans, fragipans and cohesive layers) often recorded in soils formed from the Barriers Group may have, in fact, a sedimentary origin rather than a pedogenetic one. Others have suggested that the hardening of these subsurface horizons may result from the compression caused by the static weight of the upper sediment layers and, therefore, is an inherited characteristic from the sediment itself (see Correa 2005). In addition, because it is a sedimentary cover from the Tertiary period (Mioccnic to Pleistocenic age; Suguio and Nogueira 1999), the relative youthfulness of this material may have retarded soil ripening (physical, chemical and biological) and, therefore, the material may reflect the original sediment properties rather than properties resulting from the pedological processes.

[FIGURE 2 OMITTED]

In fact, Abrahao et al. (1998) observed that higher contents of fine sand and wide variations in sand sizes may actually favour a compact arrangement of particles, increasing compactness and the PR of soils originating from Barreiras Group sediments. This natural packing would be related to the sedimentation conditions and/or the sedimentation environment. In this case, small sand grains would have been accommodated within interspaces between larger grains, along with the fine material, decreasing the existing porosity and increasing the compacity of soil material. The positive significant correlations between the standard deviation of [phi] and soil BD (r=0.887; P<0.05; n = 5) and soil PR (1 = 0.933; P<0.05; n = 5) support this hypothesis (Fig. 4a, b).

Although different studies have pointed out the poor degree of sorting of Barreiras Group sediments (Abrahao et al. 1998; Behling and Costa 2004; Balsamo et al. 2010), very few have considered its possible contribution to the formation of cohesive horizons (Lima et al. 2004; cohesive horizon at 0.74 and 0.97 m). In addition, to our knowledge, no study has established a causal relationship between the degree of sand sorting and the genesis of cohesive horizons.

Micromorphological characterisation

Some important aspects of the genesis and structural evolution of cohesive and non-cohesive horizons could be observed upon the description of thin sections (summarised in Table 4). In cohesive horizons, the coarse/fine-related distribution was mostly porphyric, confirming the observations made by Ferreira et al. (1999) and Giarola et al. (2003). However, some cohesive horizons (P2Btl and P4Btl; Table 4, Fig. 5) showed less dense and more porous areas, confirming what was reported previously by Silva et al. (1998) and Lima et al. (2006). According to these authors, the matrix of a cohesive horizon is not completely dense and thus may contain less dense areas, with an enaulic related distribution (Table 4).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Similarly, dense areas with a porphyric c/f-related distribution were found within non-cohesive soil horizons (PIBtl; Table 4, Fig. 5). According to Lima et al. (2006), the presence of dense areas in the matrix of non-cohesive horizons may indicate an ongoing formation processes in these horizons, where less dense areas may be experiencing a consolidation process and therefore represent a developing stage of the cohesive horizons. Similarly, the presence of more porous areas in cohesive horizons may indicate an early stage of the consolidation process.

With regard to porosity, non-cohesive horizons showed a predominance of complex packing voids (~80%), followed by large vughs (15 to 40%), planar voids (20%) and channels (5%; Table 4). These horizons showed many interconnected pores and loose coarse and fine material without orientation patterns.

The cohesive soil horizons showed a significant reduction in void sizes, associated with a predominance of polyconcave vughs (35%-65%; Table 4). This void pattern commonly occurs in dense horizons (Cooper and Vidal-Torrado 2005; Cooper et al. 2010) and may have formed from the effects of natural consolidation processes on a pre-existing complex packing void pattern, in which kaolinitic microaggregates were welded by natural coalescence by setting, as reported previously in similar tableland soils (Schaefer 2001; Schaefer et al. 2004). The higher BD and PR values observed in the cohesive horizons (Table 2) are probably related to this poorly interconnected void network.

The coarse fraction present in thin sections from both cohesive and non-cohesive horizons showed some similarities with regard to shape, size and mineralogical composition. Most coarse fractions consisted of quartz grains, with and without edges, and thus with variable sphericity. With respect to the degree of sorting, all thin sections showed a coarse material consisting basically of poorly to moderately sorted quartz (Fig. 5; Table 4).

[FIGURE 6 OMITTED]

Although previous studies on cohesive Ultisols at depths of approximately 0.20 and 0.40 m have suggested the participation of argilluviation on the formation of cohesive horizons (Correa et al. 2008; Lima Neto et al. 2009, 2010), textural pedofeatures, indicative of clay illuviation, were found in one of the cohesive horizons (P4Btl; Fig. 5) and in small amounts (<5%; Table 4). This indicates that other pedogenetic processes are probably involved in the consolidation of cohesive soil horizons, as suggested by others (Lima et al. 2006; Vieira et al. 2012) who have studied cohesive horizons at depths of 0.91 and 1.42 m.

Despite the fact that most cohesive horizons studied showed an absence of textural pedofeatures, such as clay cutans (argillans), the contribution of argilluviation in the genesis of cohesive horizons cannot be ruled out. In fact, most probably the genesis of these soil horizons is the result of a combination of different processes and thus the mineralogical and chemical cementation pathways proposed by Ajayi et al. (2009a, 20096) and Chartres et al. (1990), respectively, may also contribute to the genesis of these cohesive horizons. In fact, Vieira et al. (2012) in an earlier study conducted in the same area as the present study concluded that the cohesive behaviour may be lost after the extraction of amorphous Si compounds from the soil.

However, based on the data presented herein, the occurrence of illuvial clay skins partially obstructing voids in P4Bt (Fig. 5; Table 4), along with the maximum values of soil BD (1.80 Mg itT3) and PR (7.71 MPa; Table 2), may indicate the participation of argilluviation in the genesis of cohesive horizons, as suggested recently by Lima Neto et al. (2010)

working on cohesive horizons at depths of approximately 0.20 and 0.40 m. In fact, clay illuviation may play a complementary role in the genesis of cohesive horizons from a pre-existing poorly sorted coarse material. In that case, argilluviation, especially of fine clay (Correa et al. 2008; Lima Neto et al. 2010), would obstruct void spaces between the poorly selected sand grains and act as bridges between these soil particles (Fig. 6).

Conclusions

1. The formation of cohesive horizons is a hard-setting phenomenon and may be partially inherited from the original sediment. The predominance of poorly sorted sand particles plays a key role in increasing BD, soil PR and cohesion of these soil horizons.

2. Statistical analysis of sand particle distribution presents itself as an important tool for the identification and characterisation of cohesive horizons.

3. The occasional presence of textural pedofeatures (illuvial clayskins) in cohesive horizons may indicate a combined action of poorly sorted sands and fine clay argilluviation in their genesis, corroborating previous studies. The role of amorphous silica cementation cannot be ruled out.

4. Overall, the results of the present study show that subtle variations in the distribution of basic soil components (i.e. particle size fractions) may determine the formation of cohesive horizons and thus must be taken into account in future studies aiming to predict their spatial distribution and in the definition of more efficient management strategies.

Acknowledgements

The first author was supported by a scholarship from Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (National Council for Scientific and Technological Development; CNPq). The authors thank Gabriel Nuto Nobrega for assistance with the preparation of the figures.

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http://dx.doi.org/10.1071/SR13188

C. E. E. Bezerra (A), T. O. Ferreira (B,C), R. E. Romero (A), J. C. A Mota (A), I. M. Vieira (A), L. R. S. Duarte (A), and M. Cooper (B)

(A) Departamento de Ciencias do Solo, Universidade Federal do Ceara, UFC, M.B. 12168, Av. Mister Hull, 2977--Campus do Pici--Bloco 807 60440-554--Fortaleza, Brazil.

(B) Departamento de Ciencia do Solo, Universidade de Sao Paulo, ESALQ/USP, Avenida Padua Dias, 11--CEP 13418-260 Piracicaba--Sao Paulo, Brazil.

Corresponding author. Email: toferreira@usp.br
Table 1. Location, classification, occurrence of cohesive character
and use of soils in the toposequence studied, Trairi, Ceara state,
Brazil

a.s.l., above sea level; WRB, World Reference Base for Soil Resources

Soil           Geographic        Elevation     Soil classification
profile       coordinates        (m a.s.l.)      Brazilian system
                                              of soil classification

P1        3[degrees]20'17.8"S,       48         Argissolo Amarelo
          39[degrees]19'36.5"W                  Eutrofico solodico
P2        3[degrees]19'53.8"S,       42         Argissolo Amarelo
          39[degrees]19'13.1"W                 Distrocoeso arenico
P3        3[degrees]19'51.8"S,       32         Argissolo Amarelo
          39[degrees]18'57.9"W                  Distrocoeso tipico
P4        3[degrees]19'50.6"S,       20       Argissolo Acinzentado
          39[degrees]18'48.7"W                 Eutrofico abniptico

Soil                                             Occurrence of
profile      Soil              WRB/FAO          cohesive horizon
           taxonomy

P1          Arenic      Haplic Lixisol          Absent
          Kandiustult     (Profondic, Arenic)
P2          Arenic      Haplic Acrisol          92-125 cm
          Kandiustult     (Profondic, Arenic)
P3          Arenic      Haplic Acrisol          91-142 and
          Kandiustult     (Profondic, Arenic)     140-170 cm
P4           Aquic      Stagnic Lixisol         142-170 cm
          Kandiustult     (Abmptic, Arenic)

Soil                 Soil use
profile

P1        Perennial dwarf cashew
            (Anaccirdium occidentale)
            cultivation
P2        Perennial great morinda
            (Morinda citrifolia)
            cultivation
P3        Perennial coconut
            (Cocus nucifera)
            cultivation
P4        Perennial dwarf cashew
            (A. occidentale) cultivation

Table 2. Particle size distribution, sand fractionation, bulk density
and penetration resistance of the cohesive and non-cohesive horizons
from the soil profiles studied

Unless indicated otherwise, data are the meanis.d. P1-P4, Profile
1-Profile 4; VCS, very coarse sand; CS, coarse sand; MS, median sand;
FS, fine sand; VFS, very fine sand; BD, bulk density; PR, penetration
resistance; nd, not determined

Horizon                 Depth     Clay   Silt   Sand
                         (cm)     (%)    (%)    (%)

P1 Uaplic Lixisol (Profondic, Arenic)

P1Bt1 (non-cohesive)    90-138    14.8   4.8    80.4
P1Bt2 (non-cohesive)   138-167+   18.0   3.7    78.3

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1 (cohesive)        92-125    14.6   6.5    78.9

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1 (cohesive)        91-142    25.6   13.0   61.4
P3Bt2 (cohesive)       140-170+   30.8   8.5    60.7

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1 (cohesive)       142 170+   13.0   7.6    79.4

Horizon                         Sand classes

                       VCS    CS     MS     FS    VFS
                       (%)   (%)    (%)    (%)    (%)

P1 Uaplic Lixisol (Profondic, Arenic)

P1Bt1 (non-cohesive)   2.9   12.8   40.6   21.5   2.6
P1Bt2 (non-cohesive)   3.6   9.0    36.3   26.5   2.9

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1 (cohesive)       2.1   6.9    31.3   33.6   5.0

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1 (cohesive)       4.1   9.4    22.4   23.2   2.3
P3Bt2 (cohesive)       3.8   10.1   22.4   22.6   1.8

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1 (cohesive)       5.8   14.4   28.8   27.6   2.8

Horizon                BD (mg [m.sup.-3];        PR (MPa;
                             n = 4)               n = 3)

P1 Uaplic Lixisol (Profondic, Arenic)

P1Bt1 (non-cohesive)   1.53 [+ or -] 0.06   0.74 [+ or -] 0.07
P1Bt2 (non-cohesive)           nd                   nd

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1 (cohesive)       1.66 [+ or -] 0.05   2.73 [+ or -] 0.08

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1 (cohesive)       1.67 [+ or -] 0.11   2.61 [+ or -] 0.38
P3Bt2 (cohesive)       1.73 [+ or -] 0.06   3.56 [+ or -] 0.30

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1 (cohesive)       1.80 [+ or -] 0.03   7.71 [+ or -] 0.28

Table 3. Statistical parameters (mcan [+ or -] s.d.) of sand
fractionation and sorting degree according to Folk and Ward (1957)
for horizons with and without cohesive character

P1-P4, Profile 1-Profile 4; ([phi]), particle diameter on the
Krumbein (1934) scale

Horizon                Depth (cm)          [phi]

P1 Haplic Lixisol (Profondic, Arenic)

P1Bt1 (non-cohesive)     90-138     1.635 [+ or -] 0.857
P1Bt2 (non-cohesive)    138-167+    1.798 [+ or -] 0.896

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1 (cohesive)         92-125     2.001 [+ or -] 0.878

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1 (cohesive)         91-142     1.717 [+ or -] 1.018
P3Bt2 (cohesive)        140-170+    1.227 [+ or -] 1.227

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1 (cohesive)        142-170+    1.639 [+ or -] 1.026

Horizon                Degree of sorting

P1 Haplic Lixisol (Profondic, Arenic)

P1Bt1 (non-cohesive)   Moderately sorted
P1Bt2 (non-cohesive)   Moderately sorted

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1 (cohesive)       Moderately sorted

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1 (cohesive)         Poorly sorted
P3Bt2 (cohesive)         Poorly sorted

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1 (cohesive)         Poorly sorted

Table 4. Micromorphological characteristics of cohesive and
non-cohcsive horizons from the soil profiles studied

P1-P4, Profile 1-Profile 4; Bt, argic horizon; c/f, coarse/fine ratio

Horizon             Depth     c/f ratio     c/f-related
                     (cm)                  distribution

Pt Haplic Lixisol (Profondic, Arenic)

P1Bt1               90-138       9/8         Porphyric
  (non-cohesive;
  Area 1)

P1Bt1               90-138      10/3          Enaulic
  (non-cohesive;
  Area 2)

P1Bt2              138-167+     10/3          Enaulic
  (non-cohesive)

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1               92-125       2/1       Porphyric and
  (cohesive)                              Enaulic in less
                                            dense areas
                                               (15%)

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1               91-142       7/9           Dense
  (cohesive)                                 porphyric

P3Bt2              140-170+      7/9           Dense
  (cohesive)                                 porphyric

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1              142-170+      9/5       Porphyric and
  (cohesive)                              enaulic in less
                                            dense areas
                                               (10%)

Horizon               Microstructure             Voids

Pt Haplic Lixisol (Profondic, Arenic)

P1Bt1               Massive (dominant)       Vughs (40%),
  (non-cohesive;   and weakly developed       polyconcave
  Area 1)           subangular blocky         vughs (40%)
                                           and planar (20%)

P1Bt1               Massive (dominant)      Complex packing
  (non-cohesive;   and weakly developed      voids (80%),
  Area 2)           subangular blocky         polyconcave
                                              vughs (15%)
                                           and channels (5%)

P1Bt2                   Intergrain          Complex packing
  (non-cohesive)     microaeareeates         voids (80%),
                                              polyconcave
                                              vughs (15%)
                                           and channels (5%)

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1               Massive (dominant)        Polyconcave
  (cohesive)           with weakly            vughs (60%)
                        developed         and complex packing
                       peds in open           voids (40%)
                          areas

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1                    Massive              Polyconcave
  (cohesive)                                  vughs (60%)
                                              and complex
                                          packing voids (40%)

P3Bt2                    Massive              Polyconcave
  (cohesive)                                  vughs (60%)
                                              and complex
                                          packing voids (40%)

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1               Massive (dominant)       Vughs (35%),
  (cohesive)           with weakly            polyconcave
                    developed peds in         vughs (45%)
                        open areas            and complex
                                          packing voids (20%)

Horizon                 Micromass             Pedofeatures

Pt Haplic Lixisol (Profondic, Arenic)

P1Bt1                    Yellow,                   --
  (non-cohesive;    undifferentiated
  Area 1)

P1Bt1                    Yellow,                   --
  (non-cohesive;    undifferentiated
  Area 2)

P1Bt2                    Yellow,                   --
  (non-cohesive)    undifferentiated

P2 Haplic Acrisol (Profondic, Arenic)

P2Bt1               Brownish yellow,               --
  (cohesive)        undifferentiated
                       (dominant)
                        with some
                    stipple-speckled

P3 Haplic Acrisol (Profondic, Arenic)

P3Bt1                Reddish-yellow,               --
  (cohesive)        undifferentiated
                       (dominant)
                   with some stipple-
                        speckled

P3Bt2                Reddish-yellow,               --
  (cohesive)        undifferentiated
                       (dominant)
                        with some
                    stipple-speckled

P4 Stagnic Lixisol (Abruptic, Arenic)

P4Bt1              Light brownish grey      Occasional (<5%)
  (cohesive)        undifferentiated     textural pedofeatures:
                       (dominant)             crescentic,
                        with some         dusty clay coatings
                    stipple-speckled
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Author:Bezerra, C.E.E.; Ferreira, T.O.; Romero, R.E.; Mota, J.C.A.; Vieira, I.M.; Duarte, L.R.S.; Cooper, M
Publication:Soil Research
Article Type:Report
Geographic Code:3BRAZ
Date:Feb 1, 2015
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