Elsevier

Geoderma

Volume 166, Issue 1, 30 October 2011, Pages 92-101
Geoderma

Modeling and correction of soil penetration resistance for varying soil water content

https://doi.org/10.1016/j.geoderma.2011.07.016Get rights and content

Abstract

For this study penetration resistance (PR) was measured within the profiles of four Oxisols for a wide range of water contents (θ) and bulk densities. Obtained data were utilized to parameterize 23 previously applied regression models. The most promising models were selected to illustrate effects of soil texture on PR. Finally, a new correction method based on normalization of PR with θ corresponding to a matric potential of − 10 kPa was introduced. Evaluation of texture effects revealed that for very wet soils PR was lowest, but increased with clay content. PR at − 1500 kPa exhibited a maximum at clay content of 35% and at − 10 kPa PR was least affected by texture. From all regression models three- and two-parametric exponential and power functions yielded closest matches to measured data. The proposed correction significantly dampened the influence of θ on PR, which allows better comparison for a specific soil or among different soils.

Highlights

►Soil PR was directly correlated to bulk density-BD and inversely to water content–WC. ►Modeling PR as function of BD and WC allowed correcting PR to a common WC value. ►The proposed correction method dampened the influence of water content on PR. ►At matric potentials of − 10 kPa PR was least affected by texture.

Introduction

Manual cone penetrometers are simple, user-friendly and relatively inexpensive devices for assessing the mechanical impedance or penetration resistance (PR) of soils. They have been extensively used to study tillage systems and best management practices (Cassel et al., 1978, Unger and Jones, 1998, Vazquez et al., 1991), soil compaction (Jung et al., 2010, Masaddeghi et al., 2000, Smith et al., 1997), seed emergence (Chi and Tessier, 1995), and soil crust formation (Baumhardt et al., 2004). Several studies revealed close correlations between PR and root elongation (Laboski et al., 1998, Lampurlanés and Cantero-Martinez, 2003, Pabin et al., 1998) and PR and crop yield (Sojka et al., 1997, Whalley et al., 2008).

In principle, PR can be estimated from basic bulk mechanical properties of soils (Bengough et al., 2001) considering the exerted pressure and the frictional resistance to the probe including resistance due to soil-metal adhesion (Farrell and Greacen, 1966). Though such approach is helpful for describing penetration resistance it is not useful for rapid PR assessment under field conditions because it requires knowledge of the soils compression characteristics (Whalley et al., 2007).

Alternatively, a number of laboratory and field studies correlated PR with soil parameters such as bulk density (ρb), porosity (ϕ), water content (θ), matric potential (ψ), soil texture, or plasticity via regression equations (Ayers and Bowen, 1987, Ayers and Perumpral, 1982, Busscher, 1990, Canarache, 1990, Dexter et al., 2007, Hernanz et al., 2000, Ohu et al., 1988, Whalley et al., 2007).

One the one hand, the PR changes with soil parameters that are relatively constant over time but can show significant spatial heterogeneity (e.g. particle size distribution, particle shape and density, soil mineralogy and organic matter). On the other hand, the PR varies with temporally and spatially highly dynamic soil properties such as water content, matric potential, bulk density or total porosity. Experiments conducted with various soils clearly reveal that PR is directly correlated to bulk density and it exhibits an inverse relationship to soil water content. These relationships are not linear over a wide range of water contents and bulk densities. At high water contents the PR is practically insensitive to changes in bulk density, and the water content has little effect in cohesionless soils (Bengough et al., 2001).

Because of the strong negative correlation between PR and water content or matric potential, several studies suggested to operate cone penetrometers at water contents close to a standardized matric potential to obtain comparable results (Busscher, 1990, Busscher et al., 1997, Smith et al., 1997).

According to Whalley et al. (2007) it is generally accepted that at PR values larger than 2.5 MPa root elongation is significantly restricted. Nevertheless, a number of field studies with visually healthy plants showed significantly higher PR values due to the influence of the soil water content at the time of the penetrometer readings (Busscher et al., 1997, Smith et al., 1997, Sojka et al., 2001, To and Kay, 2005, Whalley et al., 2007). Smith et al. (1997) conducted a comprehensive laboratory study measuring the PR for 29 South African soils with contrasting textures and large variations in both mass-based water content and bulk density. For a wide range of bulk densities they experienced only small changes in PR when water contents approached field capacity and saturation. The largest differences in PR were obtained for lower water contents. Texture was also found to significantly impact PR readings.

To allow comparison of measurements taken at different water contents, Busscher (1990) introduced the idea of normalizing PR readings to a common water content by fitting regression equations to PR, ρb and θm (mass-based water content) data. Busscher et al. (1997) presented a procedure to correct PR for water content differences based on the first term of a Taylor series expansion, assuming that a relationship between PR and gravimetric water content can be developed independent of other variables. They showed that corrected PR values were much better suited for illustrating statistically significant differences between treatments of a field trial than uncorrected values.

For this study, PR was measured in the field within the profiles of four Oxisols for a wide range of water content and bulk density values. Obtained data were correlated and fitted with common regression equations provided in literature. A new approach for correcting PR readings obtained at different water contents is proposed for better comparison and evaluation of field data.

Section snippets

Materials and methods

Four Oxisols (i.e. a Rhodic Eutrodox —NVef, a Rhodic Hapludox —LVdf, a Typic Hapludox —LVAd, and a Quartzipsamment —RQo) with textures ranging from loamy sand to clay were investigated for this study. The field experiments were conducted at the Embrapa Southeast Cattle Experimental Farm located in São Carlos, Brazil. Cone penetrometer resistances, water contents and bulk densities were measured on 10 × 10 m plots from January to August to cover a wide range of conditions during the wet and dry

Results and discussion

The PR measured with a cone penetrometer provides information regarding penetrability of the soil at a single point in space and time. It is common that obtained results show high spatial and temporal variability due to the presence of cavities or stones, density differences of aggregates and changes in water content and bulk density with time. To capture spatial and temporal variability, measurements were taken at different times of the year and four measurements taken at the corners of a

Summary and conclusions

For this study penetration resistance (PR) was measured with a dynamic hammer penetrometer within the profiles of four Oxisols for a wide range of water content and bulk density values during a comprehensive field campaign. Obtained data were utilized to parameterize and evaluate 23 previously applied regression models. A selection of the most promising models (based on RMSD and r2) was used to illustrate effects of soil textural contrasts on PR. Finally, a new correction method based on

Acknowledgements

The authors gratefully acknowledge support from the Brazilian Agricultural Research Corporation (EMBRAPA), the Brazilian National Council for Scientific and Technological Development (CNPq) under grant no. 301057/2009-5, and from the Arizona Agricultural Experiment Station (AAES).

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