Elsevier

CATENA

Volume 190, July 2020, 104541
CATENA

Tensile strength values for the degrees of soil consistency using human perception and TS-Soil device

https://doi.org/10.1016/j.catena.2020.104541Get rights and content

Highlights

  • Tensile strength is useful in the evaluation of cohesion between soil particles.

  • TS-Soil can be combined with human perception in the evaluation of soil consistency.

  • Human-machine interaction gives quality to the evaluation of dry soil consistency.

Abstract

Tensile strength of aggregates, defined as the force per unit of area required to cause rupture of aggregates, is useful to quantify the effects of both use and management on soil quality. The present study considered the hypothesis that the evaluator’s perception, combined with the force applied to break the aggregate, measured by the TS-Soil device, can be used to define ranges of tensile strength for the degrees of dry soil consistency. The objective in this study was to define ranges of tensile strength for the degrees of dry soil consistency. For this purpose, aggregates (mean diameter of 22 mm after preparation) were collected in five horizons: A and Bi of a Cambissolo Háplico (Cambisol), Bt with a cohesive character of an Argissolo Vermelho Amarelo (Lixisol), Bt of a Luvissolo Crômico (Luvisol) and Bw of a Latossolo Vermelho-Amarelo (Ferralsol). One hundred aggregates of each horizon were evaluated for tensile strength, measured in the TS-Soil device, and, concomitantly, when pressed, were analyzed by an evaluator to assign the following scores to their consistency: 1- loose, 2- soft, 3- slightly hard, 4- hard, 5- very hard, and 6- extremely hard. Data were initially analyzed by descriptive statistics. Then, evaluator’s scores and tensile strength measured by the TS-Soil device were considered to define the ranges of values for the degrees of dry soil consistency. The following tensile strength (kPa) ranges were obtained: >0 to <21 for soft consistency, ≥21 to <38 for slightly hard, ≥38 to <65 for hard, ≥65 to <110 for very hard and ≥110 for extremely hard. It was concluded that the TS-Soil device makes it possible to combine the evaluator’s perception with the force applied to break the aggregate, enabling the definition of ranges of values for the degrees of dry soil consistency, and that the qualitative perception (evaluator) associated with tensile strength values (TS-soil device) gives better quality to the process of evaluating dry soil consistency.

Introduction

Tensile strength of aggregates, defined as the force per unit of area required to cause rupture of aggregates (Dexter and Krosbergen, 1985), has been used to quantify the effects of the various uses of soil on its quality. It is a dynamic property under field conditions (Kay and Dexter, 1992) and reflects the effects of natural factors and of use and management on the hierarchical organization of aggregates in soil structure.

Direct and indirect methods can be applied to determine tensile strength. Direct methods are less used in most studies with soils, especially because of the difficulty in obtaining the tensile strength measurement. In order to make it easier, in the indirect determination the stress is not directly applied to the aggregate; a compressive force is applied to a plate, which transmits it to the aggregate, producing increased stress in its interior (Dexter and Kroesbergen, 1985, Dexter and Watts, 2000).

In the pedological perception, dry soil consistency can be confused with tensile strength, as it is determined according to the evaluator’s perception of the force applied to break the aggregate either between the fingers or between the hands. Santos et al. (2013) consider the terms loose (when there is no cohesion), soft (weak cohesion, aggregate is fragmented under slight pressure), slightly hard (aggregate breaks easily between the index finger and thumb), very hard (aggregate resistant to pressure; only with difficulty can be broken in the hands, but it is not breakable between the index finger and thumb), and extremely hard (aggregate does not break with pressure applied between the hands).

Given the possibility of improving the evaluation of soil consistency, with better accuracy in the approach on tensile strength of aggregates, it is important that a quantitative parameter be associated with the evaluation protocol (Lima et al., 2004).

In soil science, scales defined by ranges of values are widely used to group similar data and commonly employed as an effective tool to define ranking of soil attributes in evaluative procedures. It is important to point out that these scales should be easy to use, applicable in different soils and enable immediate evaluation of the attribute considered. In this context, there are already scales for inferences on resistance to penetration, density, hydraulic conductivity, losses by erosion, tensile strength, aggregate stability and friability (Soil Science Division Staff, 2017).

The Soil Science Division Staff (2017) proposes, for aggregates with diameter of 25–30 mm, ranges of values of resistance to rupture associated with consistency in moderately dry to very dry soil and slightly dry to moist soil. It is important to highlight the advancement brought by the tensile strength-consistency combination, but one can also criticize the subjectivity in defining the moisture states still associated with consistency, mainly because slight variations in moisture greatly influence changes in aggregate tensile strength. Thus, there is a gap that needs to be filled in order to provide standardization in terms of moisture for the experimental protocol and, particularly, in terms of instrumentation, which, coupled with human qualitative perception, needs to be sufficiently sensitive to quantify soil aggregate resistance to rupture. Recently, Queiroz et al. (2018) presented a proposal of tensile strength of aggregates considering the degrees of dry soil consistency.

Several devices and some adaptations over the years have been proposed to obtain the maximum rupture strength of soil aggregates, for instance, those described by Rogowski et al., 1968, Öztas et al., 1999. However, no portable device has been developed to either measure the evaluator’s perception when attributing quality to dry soil consistency or to be used in the evaluation of soil structural quality. Therefore, TS-Soil device becomes a useful tool in this context.

This study considered the hypothesis that, with the evaluator’s perception, combined with the force applied to break the aggregate, measured by TS-Soil device, it is possible to define ranges of tensile strength for the degrees of dry soil consistency. Thus, the objective was to define ranges of tensile strength for the degrees of dry soil consistency.

Section snippets

Soil collection and analyses

The soils used in this study were classified according to the Brazilian Soil Classification System (Santos et al., 2018) and World Reference Base for Soil Resources (FAO, 2015). Regarding the particle size of the soils, the contents of sand, silt and clay fractions with their respective textural classes are presented in Table 1. The morphological descriptions of the soils are shown in Table 2. Samples were collected in such a way to cover all degrees of dry soil consistency: loose, soft,

Results and discussion

In relation to the scores attributed by the evaluator to the A horizon, there was a predominance of scores corresponding to soft consistency (67%), 30% to slightly hard, and 3% to hard (Table 3) – partially differing from the morphological description (Table 2). For the Bi horizon, almost all scores (94%) corresponded to the soft consistency (Table 3), which was also different from the description presented in Table 2. For the Bt horizon with cohesive character, the attributed scores were

Conclusions

The TS-Soil device makes it possible to combine the evaluator’s perception with the force applied to break the aggregate, enabling the definition of ranges of values for the degrees of dry soil consistency.

The qualitative perception associated with values of tensile strength gives better quality to the process of evaluating dry soil consistency.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

To the Coordination for the Improvement of Higher Education Personnel (CAPES), for providing the research grant for the first author and partially funding this study through the Finance Code 001 and Pró-integração Project n° 55/2013. To the National Council for Scientific and Technological Development (CNPq), for providing the research grant for the fifth and last authors.

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    Thus, as the aggregates/clods were drying, pairs of values (numerical values and gravimetric water content) were obtained for the water content in the range between field capacity and slightly moist soil. We started from the slightly dry soil water content, while the aggregates/clods were drying in the air, the same evaluator have done the consistency classification of aggregates/clods, according to Santos et al. (2015) and Oliveira et al. (2020), in which numerical values were assigned as follows: 1 - loose (no cohesion), 2 - soft (weak cohesion, aggregate breaks under light pressure), 3 - slightly hard (aggregate breaks easily between thumb and forefinger), 4 - hard (aggregate moderately resistant to pressure, can be easily broken between hands, but breakable between the index finger and thumb), 5 - very hard (aggregate highly resistant to pressure; can only be broken by hand with great difficulty and cannot be broken between thumb and forefinger), and 6 - extremely hard (does not break between hands). In this case, the consistency description was done considering others ten /aggregates/clods.

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