Tensile strength and friability of an Alfi sol under agricultural management systems

Management systems may infl uence the structural quality of soils. Tensile strength (TS) and friability (F) are indicators of soil structural quality. The aim of this study was to evaluate the TS and F of an Alfi sol under different management systems. The treatments were as follows: (i) soil under conventional system with growing maize after tobacco cultivation, (ii) soil under conventional system with growing maize after use as pasture, (iii) soil under natural pasture, and (iv) a natural area with predominance of spontaneous vegetation. TS and F were evaluated at depths of 0.00-0.05 and 0.05-0.10 m. The water content of soil aggregates, soil particle-size distribution, total organic carbon, carbon in the coarse fraction and carbon associated with minerals were also determined. The increase in clay content and soil organic carbon infl uences the values of TS. The lowest TS was for the soil under maize cultivation after tobacco in the conventional system. Soil under natural area in the 0.05-0.10 m layer was classifi ed as slightly friable, while other systems were classifi ed as friable. Evaluations of the structural quality of soils under management systems can be performed using TS. However, F was not effi cient in detecting changes between the different management systems.


Introduction
The tensile strength (TS) of aggregates is defi ned as the force per unit area required to cause the disruption of aggregates, being the most useful measure of individual resistance of soil aggregates consisting of an indicator sensitive to the structural soil condition, that refl ects the effects of natural factors, as well as land use and management (Dexter and Kroesbergen, 1985;Dexter and Watts, 2000).
The distribution of TS could be utilized as an index of soil friability (Utomo and Dexter, 1981).The friability (F) of the soil can be defi ned as the tendency of a soil mass to break into smaller sizes of aggregates under application of stress or load.As a result of its heterogeneous nature, TS responds to the weakness planes or fault zones between aggregates in this way and it can be estimated by the coeffi cient of variation of TS (Watts and Dexter, 1998).
F is synonymous with soil quality and is indicative of the soil structure being classifi ed by these authors from measurements obtained by the volume of aggregates method from sandy loam soils that produces F values smaller than soils with different textures (Utomo and Dexter, 1981).The volume of aggregates method estimates F values (F') as being the slope of the straight line that relates the logarithm of TS to the logarithm of the sample size (aggregate volume).This method produces F values smaller than those determined from the coeffi cient of variation method (F).Chan et al. (1999) found a mean ratio of F/F' of ~ 2 for clayey soils.Imhoff et al. (2002) used the classifi cation of F proposed by Utomo and Dexter (1981) multiplied by 2 to defi ne classes of F for an Oxisol.The F classifi cation proposed by Imhoff et al. (2002) was used for the F values in this work.
Textural class, clay mineralogy and dispersion of these, content of clay and silt, organic matter and their interactions are factors that affect the aggregation and stability of soil aggregates.Furthermore, the greater the concentration of poorly crystalline Fe oxides, conversely, the greater the impact of crystalline Fe forms on the TS of aggregates and soil F (Ley et al., 1993;Imhoff et al., 2002).Thus, the objective of this study was to evaluate the effect of agricultural management systems on the tensile strength and friability of an Alfi sol.

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The study was carried out on eroded granite and granitic sediments from the Rio Grande do Sul Shield.The relief varies from wavy to smooth wavy and all soils are shallow.The soil is a sandy loamy textured Hapludalf (USDA, 2012).The region where the city of Turuçu is located is composed of pioneer vegetation characterized by shrub-herbaceous species, typical of the lagoon, with forest formations and alluvial lowland, typical of the deciduous and semi-deciduous forests, with natural pastures as the primary land use (IBGE, 2004).
and loaded progressively, across a diameter, between a fi xed lower plate and an upper parallel mobile plate that was assembled to accept an electronic load cell of 20 kg capacity.The electrical output was recorded by a data acquisition system.After each test, the aggregates of each set were oven dried at 105 to calculate the soil water content The gravimetric water content and particle size fraction analysis (Embrapa, 1997) are presented in Table 1 and Table 2 respectively.The TS (kPa) was calculated in accordance with the expression formulated by Dexter and Kroesbergen (1985): ).The equation was elucidated from the theory of loading spherical samples of linearly elastic material.Because of its simplicity it has been used to estimate the TS of ag-gregates (Chan et al., 1999).On the assumption that aggregate density is constant, the e g), and . Disturbed samples were obtained after passing through a sieve (mesh of 2 mm) for the physical fractionation of soil organic matter according to Cambardella and Elliott (1992).The concentration of total organic carbon (TOC) and carbon of the coarse fraction (CCF) in the soil mass were quantifi ed by dry oxidation in an elemental analyzer.The carbon associated with minerals (CAM) was obtained by difference between TOC and CCF.
Data were tested for normality using Shapiro-Wilk statistics.Analysis of variance (Anova) and Duncan test (p ≤ 0.05) were performed per treatment and per layers.Regression analysis was carried out on the values of TS and TOC, CCF and CAM.These statistical tests were performed using the SAS software (Statistical Analyses System Institute, version 5, 1999).Sandy loam 1 MT: soil under maize cultivation after tobacco in conventional system; MNP: soil under maize cultivation after natural pasture in conventional system; NP: natural pasture; NAT: natural area.Czarnes et al. (2000) reported that tensile strength was affected by organic exudates, rooting and plant growth.Accordingly, reductions in organic carbon caused by cropping practices, led to changes in the values of tensile strength of aggregates, making them less resistant to management processes.
Agricultural exhibit little stability against applied tensile stresses.Nevertheless, tensile linkages in soils are the most effective factors in soil resistance against cultivation operations, execution of horizontal load (in tensile processes), and vertical load (in displacement processes) under fi eld conditions (Chancellor, 1994).Soils which possess aggregates with greater TS are more resistant to mechanical dispersion when subjected to soil preparation.
Table 3 -Statistic moments of total organic carbon (TOC), carbon of the coarse fraction (CCF), carbon associated with minerals (CAM) and tensile strength (TS) of an Hapludalf under several management systems.Correlations between total organic carbon carbon of the coarse fraction and carbon associated with minerals suggest increases in TS present a signifi cant linear relationship (p < 0.01) (Figure 1).Similar results were obtained by Blanco-Canqui et al. (2005).
¸The relationships of TS and other soil properties, such as organic carbon, have not been well resolved due to the many factors involved, e.g.soil texture, porosity, water content (Munkholm and Kay, 2002).TS is more strongly related to TOC and CAM than CCF which can be attributed to the formation of stable bonds between soil organic matter and the metal ions and minerals from the soil, that are signifi cantly affected by soil management.
There are two opposing effects of soil organic matter on TS: an increase in the number and strength of linkages between the particles and the effect of dilution that would reduce the bulk density or increase the porosity of aggregates (Zhang, 1994).The increase in soil organic matter results in higher porosity of the aggregate which reduces the number of bonds between particles, in which case the strength of these links is not increased, and there is a consequent reduction in TS.
The predominance of one or another mechanism will determine the direction of correlation between TS of aggregates and soil organic matter, as verifi ed in studies by Rahimi et al. (2000).Zhang (1994) found that there is reduction in TS with increasing organic matter due to the increase the porosity of aggregates.The degree of humidifi cation also infl uences TS, demonstrating that the more humidifi ed the soil organic matter becomes, the less its effect in reducing TS (Zhang, 1994).
The highest average values of TS were obtained in the 0.00 to 0.05 m layer.This fact can be attributed to the content of TOC, which in general was also high in the surface layer.The MT and MNP systems had the lowest TS in the two soil layers (Table 5), probably due to the land use history, in line with the common fact that management systems promote changes in soil   , 1994).
TOC, CCF and CAM were classifi ed ¾ Ì Ï Â Ë ¾ È Ç Ã , except for ß ç ¼ , in the 0.05 to 0.10 m layer, that was classifi ed Levels of friability that fall into the category "slightly friable" indicate that the aggregates are resistant to fracture when subjected to load or pressure.Macks et al. (1996) suggest friability as an indicator of the restriction of the structural condition of the soil for crop establishment in no-tillage.The condition of the soil in the "friable" category requires reduced cropping intensity for the production of small aggregates and, consequently, a planting bed suitable for germination and plant establishment.
wrapped in plastic fi lm immediately after removal to maintain the natural soil water content and the integrity of the samples until their arrival at the laboratory.They were carefully and manually fragmented along a b were selected per depth in the four areas studied, totaling 480 soil aggregates samples, speed of 4 mm s -1 until the aggregate failed i.e., until the formation of a continuous tensile crack which ran approximately between the polar diameters of measured values of TS, Y a b e

Figure 1 -
Figure 1 -Correlation between tensile strength (TS) and total organic carbon (TOC) (A), carbon associated with minerals (CAM) (B), carbon of the coarse fraction (CCF) (C) of an Hapludalf under several management systems.

Table 1 -
Gravimetric water content of analyzed aggregates of an Hapludalf under several management systems.
1MT: soil under maize cultivation after tobacco in conventional system; MNP: soil under maize cultivation after natural pasture in conventional system; NP: natural pasture; NAT: natural area.Numbers followed by the same letters in the column are not different by Duncan test (p ≤ 0.05).

Table 2 -
Particle size fraction of an Hapludalf under several management systems.

Table 5 -
Imhoff et al. (2002)ngth (TS) and friability (F) values of two layers of an Hapludalf under several management systems.MT: soil under maize cultivation after tobacco in conventional system; MNP: soil under maize cultivation after natural pasture in conventional system: NP: natural pasture; NAT: natural area; 2 Based onImhoff et al. (2002).Same letters in the same column means no difference between values (Duncan test p ≤ 0.05).