Computed tomographic measurement of macroporosity in chisel-disk and no-tillage seedbeds

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Abstract

Methods for evaluating soil conditions as influenced by tillage are often limited to analysis of bulk samples. The application of medical computed tomography (CT) to the characterization of tillage effects on soil provides an alternative tool for measurement since it is at a more detailed scale. The objective of this study was to compare soils under conventional and no-tillage (NT) systems using X-ray CT. Chisel-disk-disk (CDD) conventional tillage and NT systems were compared for a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs) soil. Five replicate soil cores (75 mm long by 75 mm diameter) were collected from each treatment. Two CT systems were used in this study: a medical CT scanner (MCT, 1 mm thick scans) and an ultra-high resolution CT scanner (UHCT, 0.1 mm thick scans). Significantly higher soil density was found for the NT treatment using the MCT scanner (P=0.05). Data from the UHCT scanner were used to compare the effects of scan thickness and to evaluate macropore characteristics. Macropore area was significantly higher (P<0.001) for CDD as compared to the NT treatment: 11 versus 5%. The number of macropores in the CDD treatment were twice those in NT; their perimeter was 62% longer; and their circularity was 94% of that for pores from the NT tillage treatment. The macropore box-counting fractal dimension (D) was significantly greater (P<0.001) for CDD (D=1.44) as compared to the NT treatment (D=1.26), reflecting the greater space-filling behavior of the CDD treatment. This study shows that the UHCT scanner can characterize differences in soil macroporosity more precisely than standard MCT scanners. The use of ultra-high resolution tomography can aid in the discrimination of differences between seedbeds created by different tillage systems.

Introduction

The rationale for tillage of soils is to improve their physical condition, to produce a more favorable environment for seed germination and plant growth. To improve means to do it in a better way, implying the difference between good and poor conditions is known (Kuipers, 1963). A major effect of tillage on the soil physical condition is to alter the soil structure. Soil structure has been defined by Dexter (1988) as “the spatial heterogeneity of the different components or properties of soil …”. Soil structure defines the framework of soil in which chemical, physical and biological reactions take place. Traditional methods for determining soil structure include measurements of soil bulk density, pore-size distribution, aggregate size distribution and the stability of aggregates and pores. These measurements are usually destructive and do not provide information on the spatial heterogeneity of the different components which constitute soil structure. Questions such as how to till soil with the least damage to soil structure, and with the least cost are difficult to answer precisely with these traditional methods because of the complexity of soil structure.

One important feature of soil structure is the number and size of macropores (tillage-induced structure, wormholes, root channels, etc.). Macropores have a great influence on many soil properties, as shown by numerous investigations. Pore shape, size, orientation, and size distribution affect the rate of water flow and retention (Rasiah and Aylmore, 1998). However, differences in hydraulic conductivity under different tillage treatments tend to be inconsistent. It is sometimes thought that no-tillage (NT) promotes infiltration; however, some workers have found that lower conductivity values occur with NT versus conventional tillage (Gantzer and Blake, 1978, Lindstrom and Onstad, 1984). This is attributed to decreased total porosity, and specifically macroporosity. Other workers have found that NT increases macroporosity compared to conventional tillage (Wu et al., 1992, Benjamin, 1993). Tollner et al. (1984) and Culley et al. (1987) found that differences between NT and conventional tillage treatments were not different. A tool to assist in better understanding these differing results is needed.

One method for direct measurement of intact soil structure is the use of X-ray computed tomography (CT) scanners for determining the relative density of soil and its spatial arrangement in intact cores (Anderson et al., 1988). CT measures soil structure non-intrusively and non-destructively and provides spatial information on pores and soil solids at less than millimeter scales. Soil macropores can be identified from the soil matrix with CT (Anderson et al., 1990, Peyton et al., 1992, Zeng et al., 1996, Olsen and Børresen, 1997, Asare et al., 1999). However, in nearly all studies, the volume element (voxel) resolution of medical CT scanners (hereafter referred to as MCT) is not uniformly scaled. Scans are usually thicker (1–4 mm) than the cross-sectional picture element (or pixel) dimensions (0.1mm×0.1mm). This limits medical CT imaging resolution to detecting only “large” macropores (>1–2 mm). While high-energy synchrotron scanning is capable of spatial resolution on the order of a few micrometers, the small 1–2 mm sample size requirements limits its use for intact soil cores containing large aggregates and voids produced by tillage (Spanne et al., 1994).

Because voxel thickness of MCT scanners is relatively large, macropore detail is not well characterized with these systems. The goal of this research is to evaluate seedbed macroporosity using a high-resolution CT scanner to measure relative attenuation values (RAV) of intact soil samples from conventional tillage chisel-disk-disk (CDD) and NT seedbeds. The objectives are to estimate and compare CT-measured relative density and to estimate and compare CT-measured macropore characteristics from the CDD and NT treatments.

Section snippets

Field experiment

The experimental site was located at the Bradford Research Center, 8 km east of Columbia, MO. The soil is a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs). The Ap (0–178 mm) is very dark grayish brown (10YR 3/2, wet) or grayish brown color (10YR 5/2, dry) having an organic matter content of approximately 1.9%. It has a silt loam texture (clay ∼20%, silt ∼68%), with weak fine granular structure, and is very friable. The site had been in continuous NT corn (Zea mays), soybeans (

Bulk sample properties

Fig. 1 presents average RAV of the MCT scans for the replicate samples from the CDD and NT treatments. Mean RAV for the CDD samples was 99±1.6 (mean±standarderror) and that for the NT was 134±0.9. A two-sample t-test indicated these means were significantly different at the <0.001 probability level. These values correspond to wet bulk densities of 1.35Mg/m3±0.05 for the CDD and 1.80Mg/m3±0.03 for the NT samples (dry bulk densities were 1.1Mg/m3±0.04 and 1.44Mg/m3±0.04). Fig. 2a and Table 2

Discussion and conclusions

This study shows that use of a UHCT scanner improves the resolution for measurement of macropore characteristics compared to an MCT scanner because of reduced voxel size. Continued work is necessary to improve UHCT scanner techniques. First is the development of improved methods to calibrate the instrument for specific soil samples and to better adjust scanner results for beam hardening and partial volume artifacts. Work is also necessary to more accurately measure the spatial heterogeneity of

References (23)

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    Contribution from the Department of Soil and Atmospheric Sciences, Missouri Agricultural Experimental Station.

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