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Chilean journal of agricultural research

versión On-line ISSN 0718-5839

Chilean J. Agric. Res. vol.76 no.1 Chillán mar. 2016

http://dx.doi.org/10.4067/S0718-58392016000100015 

RESEARCH

Direct measurement and prediction of bulk density on alluvial soils of central Chile

Manuel Casanova1*, Elizabeth Tapia1, Oscar Seguel1, and Osvaldo Salazar1

1Universidad de Chile, Facultad de Ciencias Agronómicas, Casilla 1004, Santiago, Chile. *Corresponding author (mcasanov@uchile.cl).


ABSTRACT

The significance of soil bulk density (ρ) as a key indicator of soil quality was examined in this study. Bulk density values obtained by direct methods (clod, cylinder, and excavation) with three sample sizes (small, medium, and large) were compared with those obtained by 10 published pedotransfer functions (PTFs) for two alluvial soils (a massive fine-textured Fluventic Haploxeroll and an aggregated, coarse-textured Fluventic Haploxerept) of central Chile. With the exception of small cylinders in fine-textured soil, there were nonsignificant differences between the methods and sample sizes assessed. On the coarse-textured soil, there were nonsignificant differences between the excavation and clod methods, but medium-sized cylinders differed from other cylinder sizes. In general, the clod technique tended to give higher values than the other methods. Using basic information (texture and organic matter/C content) from the existing PTFs for both sites, a better fit for coarse-textured than fine-textured soils was obtained. This indicates that it is necessary to define a set of locally calibrated PTFs that address the complexity of the soil resource throughout Chile.

Key words: Alluvial soils, organic carbon, pedotransfer function, texture.


INTRODUCTION

The variability in soil quality influences mainly biogeochemical cycling, biodiversity, and agricultural productivity. However, soil quality cannot be directly determined, but can be inferred by measuring soil physical, chemical and biological properties. Soil bulk density (ρb) describes the spatial arrangement of the solid particles that compose soil matrix, providing an indication of basic soil quality index (Chan, 2005). As a key state variable, p provides valuable information relating to porosity, compaction, and penetration resistance of soil (Horn et al., 2003). Linked to soil hydraulic properties, it is vital in predicting rainfall-runoff-infiltration-erosion relationships, heat and gas exchange, seedling emergence, root growth, and crop yield (Siegel-Issem et al., 2005; Assouline, 2011). In addition to physical and biological roles, it is also used to convert soil organic carbon (SOC) and other nutrients from content (e.g. g kg-1 soil) into stock (e.g. kg m-2) at any specified depth. However, the measurement technique used may have dramatic implications for calculating carbon mass in soils (Throop et al., 2012).

Bulk density is not an intrinsic soil property but depends on external conditions, with changes associated with a variety of factors and with various natural and anthropogenic processes (Zeng et al., 2013). It can also change as a consequence of root growth, rainfall or normal traffic (Drewry, 2006). Both wetting-drying and freezing-thawing cycles after tillage may also cause the p to increase because of natural soil reconsolidation (Assouline, 2011; Hu et al., 2012).

Soil bulk density is actually a function rather than a single value where only soil mass remains unaltered, but the water status of the sample must be stated when soil volume is measured (Grossman and Reinsch, 2002). While measuring the mass of a soil sample is simple and routine, measuring its volume generates a degree of uncertainty (Hartge and Ellies, 1999). The determination methods commonly used for determining soil volume have their particular limitations and their suitability for specific conditions. The choice of assessment method depends on the purpose of the measurement, the required accuracy and precision, the need for repeated measurements at the same location, costs, operator expertise, and equipment/time availability (Cresswell and Hamilton, 2002). Besides the disturbance of the soil structure caused by the particular method, other constraints in ρb assessment are the size and representativity of the sample. While several techniques for determination of ρb have been developed (Grossman and Reinsch, 2002), no single standard exists.

Recent technological advances have allowed the development of many new non-destructive methods, such X-ray computed tomography (Helliwell et al., 2013), thermo-time domain reflectometry sensors (Liu et al., 2008) and automated 3-dimensional laser scanning (Rossi et al., 2008) to determine ρb. However, acquiring and employing such new technology can be complex and very expensive and access to it may be limited.

In general, both direct (requiring removal and weighing of soil from a known or measured volume) and indirect (transmitting or scattering instruments with empirical calibration that does not involves soil removal) methods for measuring ρb are used. Although there is general agreement between the results of direct and indirect methods (Chan, 2005), larger differences among direct methods (cylinder core, clod, and excavation) have been reported (Timm et al., 2005).

Pedotransfer functions (PTFs) have been gaining widespread recognition for their ability to predict ρb using extractable available soil databases (Tranter et al., 2007; Al-Qinna and Jaber, 2013). At least four factors affect the performance of a PTF in simulations: the accuracy of basic soil data used as inputs in PTFs, the accuracy of PTF itself, specific features of the simulation model, and the output used as a functional criteria (Donatelli et al., 2004). Although preliminary studies showed that soil organic matter (SOM) has an important effect on ρb, it has since been observed that soil texture plays a major role in controlling ρb and SOM is a minor component (Rawls, 1983; Al-Qinna and Jaber, 2013). Moreover, more recent research (Hollis et al., 2012) reports that other physical and chemical soil properties are involved. Regardless of the methodology used to derive them, PTFs are developed based on specific databases. Thus it is important to evaluate how well PTFs perform when applied outside the range of the data used to derive them.

The aims of this work were to compare three direct methods for determining soil bulk density (core, clod, and excavation) using three sample sizes on two alluvial soils characterized by crystalline mineralogy, and compare the performance of 10 published PTFs with those three direct methods. No previous attempts have been made to test and compare the applicability of pb PTFs for Chilean soils.

MATERIALS AND METHODS

The two selected soils are located in Mediterranean central Chile, both with an alluvial origin. They are classified (Soil Survey Staff, 2014) as a Fluventic Haploxeroll (MPC, Mapocho soil series; 33°30'11" S, 70°49'36" W, 452 m a.s.l.) and a Fluventic Haploxerept (RLV, Rinconada Lo Vial soil series; 33°30'04" S, 70°49'39" W, 455 m a.s.l.) and, their cartographic units extend over 5700 and 2400 ha, respectively, in the Metropolitan Region, Chile. As deep soils without coarse fragments and developed on a gentle slope, differences between the soils depend on the geomorphic position, with coarse- and fine-textured pedons occurring in the higher (RLV) and lower (MPC) alluvial terraces, respectively. Moreover, while both soils have been used under conventional tillage during the last 14 yr, in contrast to the MPC profile (massive fine-textured soil), the soil structure in the RLV profile is well-developed.

The prevailing climate conditions are semi-arid, with a thermic soil temperature regime (mean annual temperature: 14.2 °C), a xeric soil moisture regime (annual mean precipitation: 270 mm) and large inter-annual variations in precipitation (Montecinos and Aceituno, 2003; Pizarro et al., 2012).

The field study was carried out in 30 m2 experimental plots (Figure 1) within a single mapping unit of each soil series, which were divided into 100 micro-plots (0.5 m x 0.5 m). Soil bulk density (ρb) was measured in situ with random sampling and 10 replicates, and below the plough layer (30 cm) produced by conventional tillage, using three direct methods (cylinder, clod, and excavation) as described by Sandoval et al. (2012). Additionally three sample sizes were tested, metal cylinders 5 cm in height (H) and with diameter (D) of 5.0, 7.3, and 10.0 cm were driven into the soil manually with a hammer and used to extract soil cores within the range 1/2 < H/D < 2 as suggested by Pansu et al. (2001). Natural soil clods (2.5, 3.0, and 4.0 cm diameter for RLV soil and 4.3, 4.7, and 5.2 cm diameter for MPC soil) were placed in a hair net and dipped into molten paraffin (0.8 g cm-3 density). Finally, ρb was determined in irregular semi-spherical holes with sizes excavated within the range suggested by Grossman and Reinsch (2002) and Brye et al. (2004) (3, 4, and 6 cm cavity depth/6, 8, and 12 cm upper diameter in MPC soil; 4, 7, and 8 cm cavity depth/7, 16, and 18 cm upper diameter in RLV soil). Each hole was lined with a thin impermeable and flexible plastic film (60 thickness) and filled carefully with water to estimate the volume.

Figure 1. Distribution in the experimental plots of in situ soil bulk density measurements applying three sampling methods (cylinder [Cy], excavation [Ex], and clod [Cl), three sample sizes (small [S], medium [M], and large [L]), and 10 pedotransfer functions (PTF).




Developing new PTFs is an arduous task, so it is sensible to utilize already developed functions. Most existing PTFs have been developed from a large dataset of measured values (N > 100) and are used for environmental modelling where measured ρb data are lacking. Here, 10 PTFs that require mainly fine soil particles (< 2 mm) and organic matter (OM) content were chosen from the literature (Table 1). For example, the function by Rawls (1983) has been successfully used for Australian (Tranter et al., 2007), European (Hollis et al., 2012), and tropical soils (Minasny and Hartemink, 2011). The advantage of using the Rawls relationship is that the mineral bulk density (pm) can be defined for each soil type, and the variation in soil C can be incorporated independently. Consequently, SOM content by loss-on-ignition at 400 °C (Sadzawka et al., 2004), soil particle size analysis (Bouyoucos hydrometer method), soil particle density (pycnometers), and soil water retention with conventional pressure plate techniques were determined (Sandoval et al., 2012) for randomly selected samples (10 replicates) extracted from each of our field plots (Table 2).

Table 1. The 10 pedotransfer functions used for determining soil bulk density in this study.



n: Sampled soils, ρb: soil bulk density (Mg m-3), OM/OC: gravimetric organic matter/C content (%), pOM: average organic matter bulk density (0.224 Mg rrr3),
Pm: bulk density of mineral soil fraction (Mg m3), 5, 5i, and C: gravimetric contents (%) of sand, silt and clay particles, respectively.


Table 2. Some basic properties (n = 3) of the well aggregated, coarse-textured Rinconada de Lo Vial (RLV) and massive, fine-textured Mapocho (MPC) soils.


S: Sand, C: clay, Si: silt; SOC: soil organic carbon; W33 and
W1500: gravimetric water contents of soil at 33 and 1500 kPa; ρp: soil particle density

The Shapiro-Wilk and Anderson-Darling tests (a = 5%), which are reported to be powerful statistic tools in studies similar to this (Razali and Wah, 2011) were used to check data normality. When data fail to satisfy one of these tests, an appropriate transformation must be applied. Finally, ANOVA and Tukey-Kramer tests allowed comparison of methods and sample sizes.

RESULTS AND DISCUSSION

Some relevant properties of selected soils are included in Table 2. As they have the same parent material, the two soils show similar particle density (pp). Besides, although other soil properties differed, as a result of soil management and conventional tillage, the observed OM content was low in both soils.

Bulk density (ρb) varies with the packing of the soil particles and wide range for a particular texture indicates that other factors (such as OM and compaction history) have an important influence on this property. Except volcanic soils and peaty soils, coarse-textured soils pack more closely with typical values higher than 1.4 Mg m-3, while fine-textured soils tend to bridge and cannot pack as tightly, giving values below 1.4 Mg m-3 (Chan, 2005).

For all methods assessed, in the coarse-textured soil (RLV) the variation in ρb values tended to decrease with increasing sample size (Figure 2), but increased variation in ρb values with increasing sample size was observed in the fine-textured soil (MPC). On the other hand, fine-textured soils present a higher (0.17 to 0.04 Mg m-3) dispersion in ρb values than for coarse-textured (0.09 to 0.03 Mg m-3) soil.

Figure 2. Box plots of soil bulk density values according to sampling methods (cylinder [Cy], excavation [Ex], and clod [Cl]), and sample sizes (small [S], medium [M], and large [L]) in fine-textured (left) and coarse-textured (right) soils of central Chile.



Although the coring process itself can cause friction and lead to soil compression (Hartge and Ellies, 1999; Page-Dumroese et al., 1999), the small-cylinder (SCy) method showed a remarkably high mean value of ρb on MPC soil, denoting extreme method-induced soil shattering and compaction. In contrast, expected values were found in the coarse-textured soil (RLV), with the large-clod (LCl) method giving the highest and SCy the lowest mean value (Figure 2). It is known that the clod procedure does not account for inter-aggregate pores or cracks and thus gives higher values than other direct procedures, but this was more apparent in the structured soil (RLV) than in the massive fine-textured soil (MPC). Another concern is that ρb values may be higher because sampling could be biased toward the collection of firmer, more compact clods capable of withstanding disturbance during transport and measurement (Naeth et al., 1991).

After ρb data normality verification, nonsignificant differences between methods and sample sizes assessed for the fine-textured soil (MPC) were detected, with the exception of SCy (Table 3). For the coarse-textured soil (RLV), there were nonsignificant differences between size-excavation and size-clod treatments, but medium-cylinder (MCy) method differed from other cylinder sizes. Moreover, the clod technique tended to differ from the other methods (Tables 3 and 4), with generally higher values.


Table 3. Absolute values of mean differences between sample methods and sizes, according to the Tukey-Kramer test, for a fine-textured soil in central Chile.



*Values higher than Tukey-Kramer least significant difference (0.1408) denote differences between paired data.
Methods (Cy: cylinder, Ex: excavation, Cl: clod) and sample sizes (S: small, M: medium, L: large).


Table 4. Absolute values of mean differences between sample methods and sizes, according to Tukey-Kramer test, for a coarse-textured soil in central Chile.



*Values higher than Tukey-Kramer least significant difference (0.0852) denote differences between paired data.
Methods (Cy: cylinder, Ex: excavation, Cl: clod) and sample sizes (S: small, M: medium, L: large).


It is conceivable that some of the variation in pp values observed between methods could have been caused by differences in sample location. However, as stated by several authors (Page-Dumroese et al., 1999), all methods differ in accuracy (success in estimating the true value of pb), precision (clustering of sample estimates about their own mean), and bias (the systematic distortion of estimates for the true value). In the present work, the excavation method gave less precision (higher variation) for both soils analyzed, which was attributed to non-uniformity in the original samples.

When applied to the coarse-textured RLV soil, a best fit of some PTFs to measured data was observed (Figure 3, right), but another five PTFs, corresponding to studies in tropical regions, clearly underestimated (negative mean errors) the ρb values (Figure 3, left). The Tukey-Kramer test (results not included) showed that all ρb values measured with the excavation and clod methods were nonsignificantly different to four existing PTFs (Eschner et al., 1957; Rawls, 1983; Dexter, 2004; Hollis et al., 2012) for cultivated soils and two other functions (Leonaviciute, 2000; Rawls et al., 2004), respectively. Besides, ρb values determined with small and large cylinders corresponded to the functions of Eschner et al. (1957) and Rawls (1983), while those determined with medium cylinders corresponded to three of the selected PTFs (Leonaviciute, 2000; Dexter, 2004; Rawls et al., 2004).

Figure 3. Observed and predicted soil bulk density relative to the 1:1 reference line by dataset of a coarse-textured soil of central Chile (sandy loam, Rinconada Lo Vial soil series) and different pedotransfer functions (for pedotransfer function codes see Table 1).



On the other hand, the selected PTFs showed large differences in performance when applied to the fine-textured MPC soil (Figure 4), where their poor aggregation was attributable to very limited predictive potential (R2) of published ρb PTFs, which poorly capture the soil property variability (Kaur et al., 2002). In fact, according to the Tukey-Kramer test, only ρb values measured with the MCy and those obtained with the Leonaviciute (2000) function did not show significant differences.

Figure 4. Observed and predicted soil bulk density relative to the 1:1 reference line by dataset for a fine-textured soil of central Chile (clay loam, Mapocho soil series) and different pedotransfer functions (for pedotransfer function codes see Table 1).



Thus, it is not clear whether a particular PTF performs better than others because of: (i) differences between the datasets used to derive PTFs, (ii) differences between the algorithms used in developing PTFs, or (iii) differences in the input attributes used. Several authors (Kaur et al., 2002; De Vos et al., 2005) have concluded that existing PTFs display large differences in performance and should be used with care, especially when applied in environments other than those in which they were calibrated. The anomalous behavior of the published PTFs used in this study suggests that the mineralogy of the validation soil samples may have been different from that of the soils from which the PTFs were developed. On the other hand, ρb is largely controlled by SOM in a non-linear relationship, while soil particles have a linear effect on this property (Al-Qinna and Jaber, 2013). Further studies are needed to incorporate soil structure as an input parameter to derive PTFs.

Pedo-transfer functions are useful solutions for investigation of different aspects of the physical quality of soils that do not have readily-available measured data, but this study demonstrated the poor performance of some published PTFs when applied to contrasting alluvial Chilean soils (Figure 5). This raises concerns that the predictive ability of even the better models may not be adequate and that it is necessary to define local PTFs that address the complexity of the soil resource throughout Chile.

Figure 5. Mea soil bulk density values (n = 10, ± standard deviation) obtained by 10 different pedotransfer functions on two soils (Rinconada Lo Vial [RLV ] and Mapocho [MPC]) of central Chile (for pedotransfer function codes see Table 1).



CONCLUSIONS


Only the aggregated, coarse-textured soil (RLV, Rinconada Lo Vial) studied here showed a tendency for decreased variation in soil bulk density (ρb) values obtained by three direct methods with increasing sample size. There were nonsignificant differences between excavation sizes or clod sizes, but medium cylinders differed from other cylinder sizes and the clod technique tended to differ from the excavation and cylinder methods.

Considering these alluvial soils of central Chile, only large clods are recommended for massive fine-textured soils and optionally the excavation method for structured fine-textured soils. However, if clods are prone to disturbance and difficult to handle, then care should be taken to ensure that the clods are representative of the soil horizon being sampled and that there has been no deformation during collection and transport. Cylinders (H/D =1 to 2) can be used in coarse-textured soils, but excavations must be preferred if soil material is non-cohesive or abundant coarse fragments (> 2 mm diameter particles) are present. Excavation is an appealing alternative because it allows flexibility in the size of soil samples to be collected.

A remarkably high mean value of ρb was observed on the massive, fine-textured soil (MPC, Mapocho) with small cylinders, denoting extreme method-induced soil compaction and the impact of conventional tillage at the site. However, no other significant differences between the methods and sample sizes assessed were detected.

Most the 10 pedotransfer functions (PTFs) applied to the aggregated, coarse-textured alluvial soil (RLV) gave only comparable estimates of ρb, whereas when applied to the fine-textured and unstructured soil (MPC) a greater number of them differed from each other. Thus, when ρb was estimated from the selected functions big uncertainty is observed.

The PTFs based only on predictors that are easy to measure (soil organic matter, clay, silt, sand in the soil matrix) show evident limitations when applied to Chilean soils, even within the range of validity for which they had been derived. This indicates that the performance of PTFs is influenced by other factors (geographical source of datasets used for their development, management and structure differences in measurement techniques, or other dependencies) that were not considered here. Finally, although priority must still be given to in situ methods for measuring ρb in studies at field level, it appears necessary to define a set of locally calibrated PTFs that address the complexity of the soil resource throughout Chile.

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Received: 25 June 2015.
Accepted: 16 October 2015.

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