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ISSN 1064-2293, Eurasian Soil Science, 2019, Vol. 52, No. 8, pp. 954–962. © Pleiades Publishing, Ltd., 2019. Published in Russian in Pochvovedenie, 2019, No. 8, pp. 956–965.
SOIL PHYSICS
Quantifying Coarse Fragments in Soil Samples Using a Digital Camera Y. Zhanga, A. E. Harteminka, *, and J. Huanga aUniversity
of Wisconsin-Madison, Department of Soil Science, 1525 Observatory Drive, Madison, WI 53706 USA *e-mail: [email protected] Received December 5, 2018; revised December 13, 2018; accepted December 25, 2018
Abstract—We developed a novel method using image analysis to quantify the content and shape characteristics of coarse fragments in an Alfisol developed from loess over outwash. A total of 45 soil samples (about 600–1000 g each) were collected at 10-cm depth interval from 0 to 150-cm deep at three transects in the profile. The coarse fragments were separated from the soil fine-earth particles and photographed in the laboratory. The images were processed using ImageJ processing software that provides total count, total area, and mean circularity of each sample, as well as shape characteristics (circularity, aspect ratio, roundness, and solidity) of each coarse fragment. The amount of coarse fragments (coarse fragments, total count/mass, and total area/mass) varied with depth and was highest in the 2C1 and BC horizons where the soil was developed from coarse glacial outwash and a mixing of coarse outwash and loess. The coarse fragments content was lower in the loess and fine glacial outwash horizons. Approximately 9.6% of coarse fragments in weight or 14.5% by count moved from the glacial outwash layer into the loess layer (Ap and Bt horizons). The shape characteristics showed large variations at 50–110 cm depth where the soil was developed from coarse glacial outwash and a mixing of coarse outwash and loess. The mean circularity, roundness, and solidity of the coarse fragments at the topsoil were larger than those at the 50-110 cm, indicating that rounder stones tend to move faster than irregularly shaped stones during the frost heaving. This method provides a robust, fast and quantitative way to estimate the distribution and the shape characteristics of coarse fragments in a soil sample that can be applied to help better understand soil formation. Keywords: glacial outwash, loess, circularity, roundness, frost heaving, gravelization DOI: 10.1134/S1064229319080179
INTRODUCTION Coarse fragments in the soil refer to particles larger than 2 mm and smaller than 25 cm in diameter [27]. Coarse fragments are not the same as rock fragments which do not have an upper limit in diameter and include large-size stones (25–60 cm diameter) and boulders (>60 cm diameter) [24]. Coarse fragments can be expressed in three ways: percentage by weight (Rm), percentage by volume (Rv), and percent coverage of soil surface (Rc) [19]. It is described in size and shape (or roundness). The shape (or roundness) includes six classes: very angular, angular, subangular, subrounded, rounded, and well-rounded [19]. Coarse fragments may originate from any igneous, sedimentary, or metamorphic rock [15]. Soils rich in coarse fragments are widespread, usually found in erosional and depositional landforms or soils formed in glacial outwash deposits [15, 19]. Quantification of coarse fragments is important for soil and water management [12]. Soil physical properties are influenced by the content of coarse fragments, as well as the shape, position, distribution, and orientation [10]. The bulk density of fine-earth materials
decreases with increasing coarse fragments [29]. Soils with higher amounts of coarse fragments can also increase saturated hydraulic conductivity and macropores, and decrease total porosity, water holding capacity, volumetric water content, water infiltration, and water retention capacity [4, 12]. Coarse fragments on the soil surface and within the soil matrix affect rainfall interception, infiltration, surface runoff, evaporation, percolation, and capillary rise of soils in the hydrological processes [19]. However, it can reduce surface runoff and soil erosion, and increase soil carbon sequestration [32, 33]. The presence of coarse fragments in soils affects carbon and nitrogen stocks of soil [23]. In summary, coarse fragments reduce the rooting volume and amount of soil available for storing water and nutrients. The influence of coarse fragments on plant growth depends on soil properties, vegetation type, and climate [19]. It has been reported that soils with high coarse fragments produce lower crop yields [4, 8], and a low coarse fragments content provides a more favorable condition for plant growth especially in clay soils [14]. In the temperate regions, soils with coarse frag-
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ments may warm up more rapidly in spring which is favorable for seed germination [3]. Pedoturbation processes and human activities affect the vertical and lateral movement of coarse fragments [11]. Some studies have reported more coarse fragments at the soil surface than below the surface [25] due to the upward movement of rocks by the freezing and thawing cycles [5]. A semivariogram of coarse fragments has been used to quantify the lateral variation in the field [35]. The shape of the coarse fragments is usually described by roundness and sphericity [20, 24] which reflect sedimentation processes or past environmental conditions [13, 18]. Traditional methods of assessing soil coarse fragments take into account the coarse fragments by percent weight or volume. Describing the shapes of all the coarse fragments in a soil sample can be subjective and time-consuming. Advances in image analysis provide opportunities to study and quantify soil phenomena that were previously described semi-quantitatively [9]. For example, Krotovinas in a soil profile have been quantified using digital image analysis [31]. Redoximorphic features have been identified and quantified using image processing [16]. Little research has been conducted to quantify the soil coarse fragments content and shape characteristics. The objective of this paper is to develop a novel method to quantify the content and shape of coarse fragments in a soil profile using image analysis, and investigate the distribution of coarse fragments in a soil profile. MATERIALS AND METHODS The soil profile An Alfisol (Fine-loamy, mixed, superactive, mesic Typic Hapludalfs, Pecatonica series) was studied at the O’Brien farm, in southcentral Wisconsin (WGS84 42.88° N, 89.40° W) (Fig. 1). The annual average minimum temperature (1981−2010) was 2.7°C, and the annual average maximum temperature (1981−2010) was 13.3°C. The annual precipitation (1981−2010) was about 876 mm, and the annual evaporation (1971−2000) was about 902 mm. The soil was formed in loess over glacial outwash. The soil was described down to 150-cm depth at the left, middle, and right side of the profile over a width of 2 m. On the left side of the soil profile, eight horizons were identified in the field: Ap (0–10 cm), Bt1 (10–33 cm), Bt2 (33–49 cm), BC (49– 72 cm), 2C1 (72–85 cm), 2C2 (85–100 cm), 2C3 (100–120 cm), 2C4 (120–150 cm). On the middle of the soil profile, nine horizons were identified: Ap (0– 14 cm), Bt1 (14–30 cm), Bt2 (30–38 cm), BC (38– 59 cm), 2C1 (59–83 cm), 2C2 (83–97 cm), 2C3 (97– 102 cm), 2C4 (102–130 cm), 2C5 (130–150 cm). On the right of the soil profile, eight horizons were identified: Ap (0–12 cm), Bt1 (12–30 cm), Bt2 (30–47 cm), BC (47–68 cm), 2C1 (68–80 cm), 2C2 (80–95 cm), EURASIAN SOIL SCIENCE
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Ap Loess
Вt1 Вt2
Mixing
ВC 2C1 2C2
Coarse outwash 2C3 Fine outwash
2C4
Fig. 1. The Alfisol profile (Fine-loamy, mixed, superactive, mesic Typic Hapludalfs, Pecatonica series) at O’Brien Farm, Wisconsin, USA.
2C3 (95–128 cm), 2C4 (128–150 cm). The depth of the Ap horizon ranged from 10 cm to 20 cm. Soil sampling and analysis A total of 45 soil samples (about 600–1000 g each) were collected at 10-cm depth interval from 0 to 150-cm deep at the left, middle, and right of the profile. The soil samples were air-dried, ground, and sieved to a diameter smaller than 2 mm. The particle size distribution of the soil was determined by hydrometer method. The coarse fragments of each sample were bagged separately from the soil. The mass of the soil and the coarse fragments were weighed. The coarse fragments by weight was calculated (Equation 1):
Rm =
Weight of coarse fragments . Total weight
(1)
Image analysis All coarse fragments were photographed in the laboratory. The coarse fragments of each soil sample were evenly spread on a transparent paper on a light slide table (Fig. 2). Images (JPEG format) were taken by a Nikon v1 camera (Nikon Corporation, Tokyo, Japan), and an internal white balance test was carried out before taking the images. The images were processed in ImageJ image-processing program [22], with the following procedures: (1) Set a scale of the image using Set scale function. (2) Crop the image to include only the coarse fragments. (3) Convert the color image into a binary image by Make binary function. If the color of some coarse fragments is too light, convert the color image to 8-bit image and adjust the threshold to include all the gravels, and then use the Make binary function.
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Fig. 2. The setup of taking images of coarse fragments with a digital camera.
Fig. 3. Outlines of coarse fragments and the summary and individual results from image analysisusing the ImageJ software (Rasband, 1997).
(4) Use Analyze particles function to calculate the properties of the boundary of each coarse fragment and the summary of all the coarse fragments. The lower limit of size was set to 0.04 cm2 to exclude the smaller soil particles (<0.2 cm in diameter) from the coarse fragments. This returned two tables (Fig. 3): a summary table with total count (number of coarse fragments), total area, and mean circularity of each sample, and a result table with the shape characteristics (circularity, aspect ratio, roundness, and solidity, see below) of each coarse fragment in the sample. The total count and total area were divided by the total weight of the soil sample. Shape descriptors Each coarse fragment was described by its roundness and sphericity [20] using six roundness classes to
indicate the smoothness of the boundary, and five sphericity classes to indicate how the shape is similar to a perfect circle (Fig. 4) [24]. The circularity, aspect ratio, roundness, and solidity of each coarse fragment in each sample was calculated in ImageJ [37]. The circularity (Equation (2)) indicates the degree of similarity to a perfect circle which ranges from 0 (infinitely elongated polygon) to 1 (perfect circle). The aspect ratio (Equation (3)) is the ratio of the major axis and the minor axis of the best-fitted ellipse of the gravel. The roundness (Equation (4)) measures how closely the shape approaches a perfect circle which ranges from 0 to 1 (perfect circle), and it takes into consideration the major axis of the best-fitted ellipse. Roundness is similar to circularity but is insensitive to irregular borders along the perimeter of the gravel. Solidity (Equation (5)) describes the extent to which a shape is convex or concave. It is calculated by dividing the area EURASIAN SOIL SCIENCE
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Vary angular 0.5
Roundness SubSubangular rounded 2.5 3.5
Rounded 4.5
Well rounded 5.5
Spherical 2.5 Prismoidal 4.5
Subprismoidal 3.5
Sphericity
Subdiscoidal 1.5
Discoidal 0.5
Angular 1.5
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Fig. 4. The roundness and sphericity classes of coarse fragments in soils, adapted from Schoeneberger et al. (2012).
of the gravel by the area enclosed by a convex hull. The solidity of a completely convex shape is 1, the farther the solidity deviates from 1, the greater the extent of concavity in the structure.
Circularity = 4π
Area , (Perimeter )2
Major axis Aspect ratio = , Minor axis Area Roundness = 4 , 2 π × (Major axis ) Area Solidity = . Convex area
(2) (3) (4) (5)
Data analysis The mean and standard deviation of the coarse fragments of each depth were calculated. Scatterplots were used to investigate the relationships between shape characteristics and between the coarse fragments and particle size distribution. RESULTS Distribution of coarse fragments with depth The coarse fragments, total count/mass, and total area/mass showed similar trends with depth with two EURASIAN SOIL SCIENCE
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peaks at 80 and 110 cm (Fig. 5). Coarse fragments increased from 0 to 80 cm but decreased from 80 to 90 cm. It increased from 90 to 110 cm but decreased from 110 to 150 cm. There was large variation at around 80-cm depth at the boundary of the loess and the underlying outwash. The mean circularity had a large variation in the topsoil (0–30 cm), but little variation below 30 cm soil depth. Sand content increased from 30% to 90% with depth, while the silt content decreased from 50% in the loess layer and there was no silt with depth (Fig. 6). Clay content increased to 30% from 0 to 30 cm and decreased to less than 5% from 30 to 150 cm. Coarse fragments increased from 0 to 100 cm and showed large variations at around 50 to 100 cm. The coarse fragments did not have correlations with the sand, silt, or clay content, but high coarse fragments corresponded with high sand content. Shape characteristics of coarse fragments
A total of 14573 coarse fragments were separated from the 45 soil samples. A wide range in roundness and circularity was found. The correlations between the roundness and circularity of some coarse fragments are shown in Fig. 7 where the blank regions indicate that coarse fragments did not have such shape characteristic. When the roundness and circularity approach 1, the shape of the gravel is a perfect circle.
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Coarse fragments, % 10 20 30 40 50
0
0
Total count/mass, g–1 0 100 200 300
0
Total area/ mass, cm2/g 20 40 60
Mean circularity 0.8 0.9
0.7 0
Ap
Depth, cm
Вt1 30
30
30
30
60
60
60
60
ВC 2C1
90
90
90
90
2C2
Вt2
2C3 120
120
120
120 2C4
150
150
150
150
Depth, cm
Fig. 5. The distribution of coarse fragments and their properties to 150 cm depth in an Alfisol. Error bars are the standard deviations calculated from the samples of three transects. The lines were created by a spline function. Horizons at the right side of the profile.
150 100 50 0
Clay, %
CF, %
40 30 20 10 0 20 10
Silt, %
0 40 20 0 Sand, %
100 80 60 40 20
0
50 100 150 0 Depth, cm
20 CF, %
40
0
10 20 Clay, %
0
20 40 Silt, %
20 40 60 80 100 Sand, %
Fig. 6. Scatterplots of coarse fragments (CF)and particle size fractions of the 45 soil samples. EURASIAN SOIL SCIENCE
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0.4–0.5
Roundness 0.5–0.6 0.6–0.7 0.7–0.8
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0.9–1.0
0.3–0.4
0.4–0.5
0.6–0.7
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0.8–0.9
When the circularity increases, the boundary of the gravels becomes smoother. When the roundness decreases, the shape becomes more elongated. It should be noted that the roundness and the circularity are positively correlated. The scatterplot of the shape characteristics of all the 14573 coarse fragments is shown in Fig. 8. The shape characteristics of all the coarse fragments were normally distributed. The shape characteristics showed large range and variation at around 50- and 110-cm depths. The circularity was negatively correlated with the aspect ratio but positively correlated with the roundness and solidity. The aspect ratio showed a negatively curvilinear relationship with roundness.
0.9–1.0
DISCUSSION The method The method presented here of counting coarse fragments and calculating the shape characteristics is faster and more quantitative compared to the reference charts used to estimate coarse fragments counts and
Fig. 7. The relationship between the roundness and circularity of the coarse fragments. The binary examples of coarse fragments were selected from the 45 soil samples. Blank regions indicate that coarse fragments in these 45 soil samples lacked such shape.
Depth, cm
150 100 50 1.0 0.8 0.6 0.4
Roundness
Aspect ratio
Circularity
0
5 4 3 2 1
1.0 0.8 0.6 0.4 0.2 1.0
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50 100 150 0.4 0.6 0.8 1.0 Depth, cm Circularity
1
2 3 4 Aspect
5
0.2 0.4 0.6 0.8 1.0 0.7 0.8 0.9 1.0 Roundness Solidity
Fig. 8. Scatterplots of shape characteristics of 14573 coarse fragments in 45 soil samples. EURASIAN SOIL SCIENCE
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shapes. It provides a robust way to estimate the distribution of the shape characteristics of all coarse fragments in a soil sample. The method used in this study also has some restrictions. Firstly, preparation of the sample is timeconsuming, which requires grinding and laying out gravels on a transparency. Rock recognition and quantification on the natural images with a more complex background (e.g. profile images) could be explored in the future [6]. Secondly, most of the coarse fragments showed darker colors on the transparent paper due to the lighting effects. This masked the color and other features of the coarse fragments which may reflect their geologic origin. Thirdly, if a coarse fragment had a flat shape, the flat face tend to lay down on the transparency which may affect the estimated area of the coarse fragment and misrepresent the shape characteristics. Coarse fragments distribution with depth The Alfisol profile had the highest amount of coarse fragments in the 2C1 and BC horizons where the soil was developed from coarse glacial outwash and a mixing of coarse outwash and loess. The 2C2 and 2C3 horizons also had high amount of coarse fragments that were developed from coarse outwash. The coarse fragments content was lower in topsoils and deep soils which were developed from loess and fine glacial outwash. The glacial outwash was deposited by melting glaciers with substantial amounts of glacial sediments. Such sediments contain sands and gravels sorted by flowing water. Loess is windblown material composed primarily of silt with some fine sand and coarse clay [21]. The loess of the study area was mainly from the great barren expanses of till and outwash left in the Missouri and Mississippi River valleys by the retreating glaciers of the last Ice Age [36]. Only the fine materials were picked up by winter winds and thus did not contain gravels. The distribution of coarse fragments in the Alfisol was mainly caused by the parent materials and the physical soil processes (e.g. frost heaving and granular convection − see below). Gravelization The frost heaving is an upwards swelling of soils during a freezing period which affects the vertical distribution of coarse fragments [17]. Soil freezes from top to bottom. Coarse fragments are a good heat (or cold) conductor than the surrounding soils which will cause the soils freezing faster under the coarse fragment than the surrounding soils. The freezing process requires a water supply that promotes the formation of ice lens (freezing front). The water expands when it freezes and it will lift up the coarse fragment relative to the surrounding soils [30]. When the ice starts to melt in spring, the soil under the coarse fragment will melt
faster than surrounding soils. The smaller particles can fill the open space below, while the coarse fragment will remain raised. The periodic cycles of freezing and thawing will finally cause the coarse fragment moving towards the surface. Soil mineralogy, particle shape and size distributions can affect frost heaving behaviors. The frost heaving in soils developed from montmorillonite was less than that in soils developed from kaolinite [26]. A logistic regression was established between the frost heaving ratio and specific surface area of soils for different soil types [26]. Soil horizons, texture, and soil disturbance also affect the frost heaving. Disturbed B horizon with loamy sand and sandy loam textures had most frost heaving compared to undisturbed Ae horizon and B horizon with sand texture [2]. In the Alfisol profile, approximately 9.6% of coarse fragments in weight or 14.5% by count has moved from the glacial outwash layer to the loess layer (Ap and Bt horizons). Here we name it as a new soil process – gravelization. It refers to upward movements of gravels in the soils where periodic freezing and thawing occur. This process is distinguished with cryoturbation which is in permafrost-affected soils (Gelisols) and refers to the mixing of materials from various horizons [1]. This process is a physical process, involving energy and water exchange and fits in translocation category. Shape characteristics Some studies have investigated the relationship between the shapes of the rocks and environmental factors. It was found the mean size of coarse fragments can be used to differentiate depositional processes (deltas, eskers, ice-contact, and moraine deposits) in a formerly glaciated tundra area [13]. The standard deviation of the stones shows how far the stones have been transported – the farther the stones are transported, the smaller the standard deviation. The roundness is a diagnostic factor to distinguish depositional environments [13]. It has been found that the correlation of roundness and sphericity is the result of abrasion in earlier cycles of sedimentation [18] because water currents, sorting the sands of the beach in the down-current direction, carries the less spherical grains further than the more spherical. In our study, the shape characteristics showed higher variations around 50−110 cm depths indicating the soil was developed in coarse glacial outwash and a mixing of coarse outwash and loess. The variation was lower when the parent material was loess or fine glacial outwash. In terms of the shape characteristics, the mean values at the depth 0–50 cm were different than those at 50–110 cm. For example, the mean circularity, roundness, and solidity of the coarse fragments at the topsoil were larger than those at the 50–110 cm. The mean aspect ratio of the coarse fragments at the topsoil was smaller than that at the 50–110 cm. This EURASIAN SOIL SCIENCE
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suggested that rounder stones tend to move faster than irregular stones during the frost heaving. Pedological and management implications Improved assessment and quantification of coarse fragments may help better understand soil formation and process, and improve soil classification by including more diagnostic features (e.g. shape characteristics of the coarse fragments) [28]. The image analysis provides a method to map coarse fragments vertically and laterally in the soil that may help to quantify root restrictive layers [27]. It may offer potential guidance for understanding hydrological or physical processes in the soil, such as unstable water and nutrient flows [7, 34]. The variations of coarse fragments and their shape characteristics with depth have several management implications. Firstly, the amount of coarse fragments increased from soil surface to 80 cm and decreased slightly with depth. This suggested that this layer of coarse fragments will act as a root-restrictive layer to most crops (e.g. corn and soybean) that normally have a rooting depth of 60 cm. Secondly, quantification of coarse fragments is important for soil and water management in the field [12]. The layer of large amounts of coarse fragments can increase saturated hydraulic conductivity and macropores, and decrease total porosity, water holding capacity, and available water content [4, 12]. Besides, the presence of high amount of coarse fragments at subsurface may improve drainage and aeration and increase the biodiversity of the soil. Lastly, the presence of coarse fragments in soils would adversely affect carbon and nitrogen stocks of soil and needs to be considered when estimating soil carbon stocks. CONCLUSIONS In this study, we developed a novel method using image analysis to quantify the content and shape characteristics of coarse fragments of an Alfisol. We can conclude: • The amount of coarse fragments (coarse fragments, total count/mass, and total area/mass) varied with depth and showed peaks at 80 and 110 cm. The coarse glacial outwash layers and the mixing layer of coarse outwash and loess had a higher amount of coarse fragments, while the amount of coarse fragments content was lower in the loess and fine glacial outwash layers. • The variation of shape characteristics was lower when the parent material was loess or fine glacial outwash, but higher when the parent material was coarse outwash. The mean circularity, roundness, and solidity of the coarse fragments at the topsoil were larger than those at the 50–110 cm, indicating that rounder stones tend to move faster than irregular stones during the frost heaving. EURASIAN SOIL SCIENCE
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• The method provides a robust, fast and quantitative way to estimate the distribution of coarse fragments and the shape characteristics in a soil sample compared to the reference charts used to estimate coarse fragments counts and shapes. • The method can be applied elsewhere to help better understand soil formation and process, and improve soil classification by including more diagnostic features (e.g. shape characteristics of the coarse fragments). It can be also applied to map coarse fragments vertically and laterally in soil that may help to quantify root restrictive layers, manage drainage and aeration, and better estimate carbon and nitrogen stocks of soil. ACKNOWLEDGMENTS We would like to thank Dr. Yin Zhou for her assistance in the sample collection, and Richard Lee and Marina Steiner for their help in the sample preparation and image analysis.
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Quantifying Coarse Fragments in Soil Samples Using a Digital Camera Y. Zhanga, A. E. Harteminka, *, and J. Huanga aUniversity
of Wisconsin-Madison, Department of Soil Science, 1525 Observatory Drive, Madison, WI 53706 USA *e-mail: [email protected] Received December 5, 2018; revised December 13, 2018; accepted December 25, 2018
Abstract—We developed a novel method using image analysis to quantify the content and shape characteristics of coarse fragments in an Alfisol developed from loess over outwash. A total of 45 soil samples (about 600–1000 g each) were collected at 10-cm depth interval from 0 to 150-cm deep at three transects in the profile. The coarse fragments were separated from the soil fine-earth particles and photographed in the laboratory. The images were processed using ImageJ processing software that provides total count, total area, and mean circularity of each sample, as well as shape characteristics (circularity, aspect ratio, roundness, and solidity) of each coarse fragment. The amount of coarse fragments (coarse fragments, total count/mass, and total area/mass) varied with depth and was highest in the 2C1 and BC horizons where the soil was developed from coarse glacial outwash and a mixing of coarse outwash and loess. The coarse fragments content was lower in the loess and fine glacial outwash horizons. Approximately 9.6% of coarse fragments in weight or 14.5% by count moved from the glacial outwash layer into the loess layer (Ap and Bt horizons). The shape characteristics showed large variations at 50–110 cm depth where the soil was developed from coarse glacial outwash and a mixing of coarse outwash and loess. The mean circularity, roundness, and solidity of the coarse fragments at the topsoil were larger than those at the 50-110 cm, indicating that rounder stones tend to move faster than irregularly shaped stones during the frost heaving. This method provides a robust, fast and quantitative way to estimate the distribution and the shape characteristics of coarse fragments in a soil sample that can be applied to help better understand soil formation. Keywords: glacial outwash, loess, circularity, roundness, frost heaving, gravelization DOI: 10.1134/S1064229319080179
INTRODUCTION Coarse fragments in the soil refer to particles larger than 2 mm and smaller than 25 cm in diameter [27]. Coarse fragments are not the same as rock fragments which do not have an upper limit in diameter and include large-size stones (25–60 cm diameter) and boulders (>60 cm diameter) [24]. Coarse fragments can be expressed in three ways: percentage by weight (Rm), percentage by volume (Rv), and percent coverage of soil surface (Rc) [19]. It is described in size and shape (or roundness). The shape (or roundness) includes six classes: very angular, angular, subangular, subrounded, rounded, and well-rounded [19]. Coarse fragments may originate from any igneous, sedimentary, or metamorphic rock [15]. Soils rich in coarse fragments are widespread, usually found in erosional and depositional landforms or soils formed in glacial outwash deposits [15, 19]. Quantification of coarse fragments is important for soil and water management [12]. Soil physical properties are influenced by the content of coarse fragments, as well as the shape, position, distribution, and orientation [10]. The bulk density of fine-earth materials
decreases with increasing coarse fragments [29]. Soils with higher amounts of coarse fragments can also increase saturated hydraulic conductivity and macropores, and decrease total porosity, water holding capacity, volumetric water content, water infiltration, and water retention capacity [4, 12]. Coarse fragments on the soil surface and within the soil matrix affect rainfall interception, infiltration, surface runoff, evaporation, percolation, and capillary rise of soils in the hydrological processes [19]. However, it can reduce surface runoff and soil erosion, and increase soil carbon sequestration [32, 33]. The presence of coarse fragments in soils affects carbon and nitrogen stocks of soil [23]. In summary, coarse fragments reduce the rooting volume and amount of soil available for storing water and nutrients. The influence of coarse fragments on plant growth depends on soil properties, vegetation type, and climate [19]. It has been reported that soils with high coarse fragments produce lower crop yields [4, 8], and a low coarse fragments content provides a more favorable condition for plant growth especially in clay soils [14]. In the temperate regions, soils with coarse frag-
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ments may warm up more rapidly in spring which is favorable for seed germination [3]. Pedoturbation processes and human activities affect the vertical and lateral movement of coarse fragments [11]. Some studies have reported more coarse fragments at the soil surface than below the surface [25] due to the upward movement of rocks by the freezing and thawing cycles [5]. A semivariogram of coarse fragments has been used to quantify the lateral variation in the field [35]. The shape of the coarse fragments is usually described by roundness and sphericity [20, 24] which reflect sedimentation processes or past environmental conditions [13, 18]. Traditional methods of assessing soil coarse fragments take into account the coarse fragments by percent weight or volume. Describing the shapes of all the coarse fragments in a soil sample can be subjective and time-consuming. Advances in image analysis provide opportunities to study and quantify soil phenomena that were previously described semi-quantitatively [9]. For example, Krotovinas in a soil profile have been quantified using digital image analysis [31]. Redoximorphic features have been identified and quantified using image processing [16]. Little research has been conducted to quantify the soil coarse fragments content and shape characteristics. The objective of this paper is to develop a novel method to quantify the content and shape of coarse fragments in a soil profile using image analysis, and investigate the distribution of coarse fragments in a soil profile. MATERIALS AND METHODS The soil profile An Alfisol (Fine-loamy, mixed, superactive, mesic Typic Hapludalfs, Pecatonica series) was studied at the O’Brien farm, in southcentral Wisconsin (WGS84 42.88° N, 89.40° W) (Fig. 1). The annual average minimum temperature (1981−2010) was 2.7°C, and the annual average maximum temperature (1981−2010) was 13.3°C. The annual precipitation (1981−2010) was about 876 mm, and the annual evaporation (1971−2000) was about 902 mm. The soil was formed in loess over glacial outwash. The soil was described down to 150-cm depth at the left, middle, and right side of the profile over a width of 2 m. On the left side of the soil profile, eight horizons were identified in the field: Ap (0–10 cm), Bt1 (10–33 cm), Bt2 (33–49 cm), BC (49– 72 cm), 2C1 (72–85 cm), 2C2 (85–100 cm), 2C3 (100–120 cm), 2C4 (120–150 cm). On the middle of the soil profile, nine horizons were identified: Ap (0– 14 cm), Bt1 (14–30 cm), Bt2 (30–38 cm), BC (38– 59 cm), 2C1 (59–83 cm), 2C2 (83–97 cm), 2C3 (97– 102 cm), 2C4 (102–130 cm), 2C5 (130–150 cm). On the right of the soil profile, eight horizons were identified: Ap (0–12 cm), Bt1 (12–30 cm), Bt2 (30–47 cm), BC (47–68 cm), 2C1 (68–80 cm), 2C2 (80–95 cm), EURASIAN SOIL SCIENCE
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Ap Loess
Вt1 Вt2
Mixing
ВC 2C1 2C2
Coarse outwash 2C3 Fine outwash
2C4
Fig. 1. The Alfisol profile (Fine-loamy, mixed, superactive, mesic Typic Hapludalfs, Pecatonica series) at O’Brien Farm, Wisconsin, USA.
2C3 (95–128 cm), 2C4 (128–150 cm). The depth of the Ap horizon ranged from 10 cm to 20 cm. Soil sampling and analysis A total of 45 soil samples (about 600–1000 g each) were collected at 10-cm depth interval from 0 to 150-cm deep at the left, middle, and right of the profile. The soil samples were air-dried, ground, and sieved to a diameter smaller than 2 mm. The particle size distribution of the soil was determined by hydrometer method. The coarse fragments of each sample were bagged separately from the soil. The mass of the soil and the coarse fragments were weighed. The coarse fragments by weight was calculated (Equation 1):
Rm =
Weight of coarse fragments . Total weight
(1)
Image analysis All coarse fragments were photographed in the laboratory. The coarse fragments of each soil sample were evenly spread on a transparent paper on a light slide table (Fig. 2). Images (JPEG format) were taken by a Nikon v1 camera (Nikon Corporation, Tokyo, Japan), and an internal white balance test was carried out before taking the images. The images were processed in ImageJ image-processing program [22], with the following procedures: (1) Set a scale of the image using Set scale function. (2) Crop the image to include only the coarse fragments. (3) Convert the color image into a binary image by Make binary function. If the color of some coarse fragments is too light, convert the color image to 8-bit image and adjust the threshold to include all the gravels, and then use the Make binary function.
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Fig. 2. The setup of taking images of coarse fragments with a digital camera.
Fig. 3. Outlines of coarse fragments and the summary and individual results from image analysisusing the ImageJ software (Rasband, 1997).
(4) Use Analyze particles function to calculate the properties of the boundary of each coarse fragment and the summary of all the coarse fragments. The lower limit of size was set to 0.04 cm2 to exclude the smaller soil particles (<0.2 cm in diameter) from the coarse fragments. This returned two tables (Fig. 3): a summary table with total count (number of coarse fragments), total area, and mean circularity of each sample, and a result table with the shape characteristics (circularity, aspect ratio, roundness, and solidity, see below) of each coarse fragment in the sample. The total count and total area were divided by the total weight of the soil sample. Shape descriptors Each coarse fragment was described by its roundness and sphericity [20] using six roundness classes to
indicate the smoothness of the boundary, and five sphericity classes to indicate how the shape is similar to a perfect circle (Fig. 4) [24]. The circularity, aspect ratio, roundness, and solidity of each coarse fragment in each sample was calculated in ImageJ [37]. The circularity (Equation (2)) indicates the degree of similarity to a perfect circle which ranges from 0 (infinitely elongated polygon) to 1 (perfect circle). The aspect ratio (Equation (3)) is the ratio of the major axis and the minor axis of the best-fitted ellipse of the gravel. The roundness (Equation (4)) measures how closely the shape approaches a perfect circle which ranges from 0 to 1 (perfect circle), and it takes into consideration the major axis of the best-fitted ellipse. Roundness is similar to circularity but is insensitive to irregular borders along the perimeter of the gravel. Solidity (Equation (5)) describes the extent to which a shape is convex or concave. It is calculated by dividing the area EURASIAN SOIL SCIENCE
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Vary angular 0.5
Roundness SubSubangular rounded 2.5 3.5
Rounded 4.5
Well rounded 5.5
Spherical 2.5 Prismoidal 4.5
Subprismoidal 3.5
Sphericity
Subdiscoidal 1.5
Discoidal 0.5
Angular 1.5
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Fig. 4. The roundness and sphericity classes of coarse fragments in soils, adapted from Schoeneberger et al. (2012).
of the gravel by the area enclosed by a convex hull. The solidity of a completely convex shape is 1, the farther the solidity deviates from 1, the greater the extent of concavity in the structure.
Circularity = 4π
Area , (Perimeter )2
Major axis Aspect ratio = , Minor axis Area Roundness = 4 , 2 π × (Major axis ) Area Solidity = . Convex area
(2) (3) (4) (5)
Data analysis The mean and standard deviation of the coarse fragments of each depth were calculated. Scatterplots were used to investigate the relationships between shape characteristics and between the coarse fragments and particle size distribution. RESULTS Distribution of coarse fragments with depth The coarse fragments, total count/mass, and total area/mass showed similar trends with depth with two EURASIAN SOIL SCIENCE
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peaks at 80 and 110 cm (Fig. 5). Coarse fragments increased from 0 to 80 cm but decreased from 80 to 90 cm. It increased from 90 to 110 cm but decreased from 110 to 150 cm. There was large variation at around 80-cm depth at the boundary of the loess and the underlying outwash. The mean circularity had a large variation in the topsoil (0–30 cm), but little variation below 30 cm soil depth. Sand content increased from 30% to 90% with depth, while the silt content decreased from 50% in the loess layer and there was no silt with depth (Fig. 6). Clay content increased to 30% from 0 to 30 cm and decreased to less than 5% from 30 to 150 cm. Coarse fragments increased from 0 to 100 cm and showed large variations at around 50 to 100 cm. The coarse fragments did not have correlations with the sand, silt, or clay content, but high coarse fragments corresponded with high sand content. Shape characteristics of coarse fragments
A total of 14573 coarse fragments were separated from the 45 soil samples. A wide range in roundness and circularity was found. The correlations between the roundness and circularity of some coarse fragments are shown in Fig. 7 where the blank regions indicate that coarse fragments did not have such shape characteristic. When the roundness and circularity approach 1, the shape of the gravel is a perfect circle.
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Coarse fragments, % 10 20 30 40 50
0
0
Total count/mass, g–1 0 100 200 300
0
Total area/ mass, cm2/g 20 40 60
Mean circularity 0.8 0.9
0.7 0
Ap
Depth, cm
Вt1 30
30
30
30
60
60
60
60
ВC 2C1
90
90
90
90
2C2
Вt2
2C3 120
120
120
120 2C4
150
150
150
150
Depth, cm
Fig. 5. The distribution of coarse fragments and their properties to 150 cm depth in an Alfisol. Error bars are the standard deviations calculated from the samples of three transects. The lines were created by a spline function. Horizons at the right side of the profile.
150 100 50 0
Clay, %
CF, %
40 30 20 10 0 20 10
Silt, %
0 40 20 0 Sand, %
100 80 60 40 20
0
50 100 150 0 Depth, cm
20 CF, %
40
0
10 20 Clay, %
0
20 40 Silt, %
20 40 60 80 100 Sand, %
Fig. 6. Scatterplots of coarse fragments (CF)and particle size fractions of the 45 soil samples. EURASIAN SOIL SCIENCE
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0.4–0.5
Roundness 0.5–0.6 0.6–0.7 0.7–0.8
0.8–0.9
0.9–1.0
0.3–0.4
0.4–0.5
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0.8–0.9
When the circularity increases, the boundary of the gravels becomes smoother. When the roundness decreases, the shape becomes more elongated. It should be noted that the roundness and the circularity are positively correlated. The scatterplot of the shape characteristics of all the 14573 coarse fragments is shown in Fig. 8. The shape characteristics of all the coarse fragments were normally distributed. The shape characteristics showed large range and variation at around 50- and 110-cm depths. The circularity was negatively correlated with the aspect ratio but positively correlated with the roundness and solidity. The aspect ratio showed a negatively curvilinear relationship with roundness.
0.9–1.0
DISCUSSION The method The method presented here of counting coarse fragments and calculating the shape characteristics is faster and more quantitative compared to the reference charts used to estimate coarse fragments counts and
Fig. 7. The relationship between the roundness and circularity of the coarse fragments. The binary examples of coarse fragments were selected from the 45 soil samples. Blank regions indicate that coarse fragments in these 45 soil samples lacked such shape.
Depth, cm
150 100 50 1.0 0.8 0.6 0.4
Roundness
Aspect ratio
Circularity
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5 4 3 2 1
1.0 0.8 0.6 0.4 0.2 1.0
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50 100 150 0.4 0.6 0.8 1.0 Depth, cm Circularity
1
2 3 4 Aspect
5
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Fig. 8. Scatterplots of shape characteristics of 14573 coarse fragments in 45 soil samples. EURASIAN SOIL SCIENCE
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shapes. It provides a robust way to estimate the distribution of the shape characteristics of all coarse fragments in a soil sample. The method used in this study also has some restrictions. Firstly, preparation of the sample is timeconsuming, which requires grinding and laying out gravels on a transparency. Rock recognition and quantification on the natural images with a more complex background (e.g. profile images) could be explored in the future [6]. Secondly, most of the coarse fragments showed darker colors on the transparent paper due to the lighting effects. This masked the color and other features of the coarse fragments which may reflect their geologic origin. Thirdly, if a coarse fragment had a flat shape, the flat face tend to lay down on the transparency which may affect the estimated area of the coarse fragment and misrepresent the shape characteristics. Coarse fragments distribution with depth The Alfisol profile had the highest amount of coarse fragments in the 2C1 and BC horizons where the soil was developed from coarse glacial outwash and a mixing of coarse outwash and loess. The 2C2 and 2C3 horizons also had high amount of coarse fragments that were developed from coarse outwash. The coarse fragments content was lower in topsoils and deep soils which were developed from loess and fine glacial outwash. The glacial outwash was deposited by melting glaciers with substantial amounts of glacial sediments. Such sediments contain sands and gravels sorted by flowing water. Loess is windblown material composed primarily of silt with some fine sand and coarse clay [21]. The loess of the study area was mainly from the great barren expanses of till and outwash left in the Missouri and Mississippi River valleys by the retreating glaciers of the last Ice Age [36]. Only the fine materials were picked up by winter winds and thus did not contain gravels. The distribution of coarse fragments in the Alfisol was mainly caused by the parent materials and the physical soil processes (e.g. frost heaving and granular convection − see below). Gravelization The frost heaving is an upwards swelling of soils during a freezing period which affects the vertical distribution of coarse fragments [17]. Soil freezes from top to bottom. Coarse fragments are a good heat (or cold) conductor than the surrounding soils which will cause the soils freezing faster under the coarse fragment than the surrounding soils. The freezing process requires a water supply that promotes the formation of ice lens (freezing front). The water expands when it freezes and it will lift up the coarse fragment relative to the surrounding soils [30]. When the ice starts to melt in spring, the soil under the coarse fragment will melt
faster than surrounding soils. The smaller particles can fill the open space below, while the coarse fragment will remain raised. The periodic cycles of freezing and thawing will finally cause the coarse fragment moving towards the surface. Soil mineralogy, particle shape and size distributions can affect frost heaving behaviors. The frost heaving in soils developed from montmorillonite was less than that in soils developed from kaolinite [26]. A logistic regression was established between the frost heaving ratio and specific surface area of soils for different soil types [26]. Soil horizons, texture, and soil disturbance also affect the frost heaving. Disturbed B horizon with loamy sand and sandy loam textures had most frost heaving compared to undisturbed Ae horizon and B horizon with sand texture [2]. In the Alfisol profile, approximately 9.6% of coarse fragments in weight or 14.5% by count has moved from the glacial outwash layer to the loess layer (Ap and Bt horizons). Here we name it as a new soil process – gravelization. It refers to upward movements of gravels in the soils where periodic freezing and thawing occur. This process is distinguished with cryoturbation which is in permafrost-affected soils (Gelisols) and refers to the mixing of materials from various horizons [1]. This process is a physical process, involving energy and water exchange and fits in translocation category. Shape characteristics Some studies have investigated the relationship between the shapes of the rocks and environmental factors. It was found the mean size of coarse fragments can be used to differentiate depositional processes (deltas, eskers, ice-contact, and moraine deposits) in a formerly glaciated tundra area [13]. The standard deviation of the stones shows how far the stones have been transported – the farther the stones are transported, the smaller the standard deviation. The roundness is a diagnostic factor to distinguish depositional environments [13]. It has been found that the correlation of roundness and sphericity is the result of abrasion in earlier cycles of sedimentation [18] because water currents, sorting the sands of the beach in the down-current direction, carries the less spherical grains further than the more spherical. In our study, the shape characteristics showed higher variations around 50−110 cm depths indicating the soil was developed in coarse glacial outwash and a mixing of coarse outwash and loess. The variation was lower when the parent material was loess or fine glacial outwash. In terms of the shape characteristics, the mean values at the depth 0–50 cm were different than those at 50–110 cm. For example, the mean circularity, roundness, and solidity of the coarse fragments at the topsoil were larger than those at the 50–110 cm. The mean aspect ratio of the coarse fragments at the topsoil was smaller than that at the 50–110 cm. This EURASIAN SOIL SCIENCE
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suggested that rounder stones tend to move faster than irregular stones during the frost heaving. Pedological and management implications Improved assessment and quantification of coarse fragments may help better understand soil formation and process, and improve soil classification by including more diagnostic features (e.g. shape characteristics of the coarse fragments) [28]. The image analysis provides a method to map coarse fragments vertically and laterally in the soil that may help to quantify root restrictive layers [27]. It may offer potential guidance for understanding hydrological or physical processes in the soil, such as unstable water and nutrient flows [7, 34]. The variations of coarse fragments and their shape characteristics with depth have several management implications. Firstly, the amount of coarse fragments increased from soil surface to 80 cm and decreased slightly with depth. This suggested that this layer of coarse fragments will act as a root-restrictive layer to most crops (e.g. corn and soybean) that normally have a rooting depth of 60 cm. Secondly, quantification of coarse fragments is important for soil and water management in the field [12]. The layer of large amounts of coarse fragments can increase saturated hydraulic conductivity and macropores, and decrease total porosity, water holding capacity, and available water content [4, 12]. Besides, the presence of high amount of coarse fragments at subsurface may improve drainage and aeration and increase the biodiversity of the soil. Lastly, the presence of coarse fragments in soils would adversely affect carbon and nitrogen stocks of soil and needs to be considered when estimating soil carbon stocks. CONCLUSIONS In this study, we developed a novel method using image analysis to quantify the content and shape characteristics of coarse fragments of an Alfisol. We can conclude: • The amount of coarse fragments (coarse fragments, total count/mass, and total area/mass) varied with depth and showed peaks at 80 and 110 cm. The coarse glacial outwash layers and the mixing layer of coarse outwash and loess had a higher amount of coarse fragments, while the amount of coarse fragments content was lower in the loess and fine glacial outwash layers. • The variation of shape characteristics was lower when the parent material was loess or fine glacial outwash, but higher when the parent material was coarse outwash. The mean circularity, roundness, and solidity of the coarse fragments at the topsoil were larger than those at the 50–110 cm, indicating that rounder stones tend to move faster than irregular stones during the frost heaving. EURASIAN SOIL SCIENCE
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• The method provides a robust, fast and quantitative way to estimate the distribution of coarse fragments and the shape characteristics in a soil sample compared to the reference charts used to estimate coarse fragments counts and shapes. • The method can be applied elsewhere to help better understand soil formation and process, and improve soil classification by including more diagnostic features (e.g. shape characteristics of the coarse fragments). It can be also applied to map coarse fragments vertically and laterally in soil that may help to quantify root restrictive layers, manage drainage and aeration, and better estimate carbon and nitrogen stocks of soil. ACKNOWLEDGMENTS We would like to thank Dr. Yin Zhou for her assistance in the sample collection, and Richard Lee and Marina Steiner for their help in the sample preparation and image analysis.
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EURASIAN SOIL SCIENCE
Vol. 52
No. 8
2019