| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Institute of Soil, Water, and Environmental Sciences, Agricultural Research Organization (ARO), The Volcani Center, P.O. Box 6, Bet-Dagan 50-250, Israel
* Corresponding author (vwguy{at}agri.gov.il)
 |
ABSTRACT |
|---|
 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
Abbreviations: HEMC, high energy moisture characteristic • MC, moisture curve • ESP, exchangeable Na percentage • VDP, volume of drainable pores
|
INTRODUCTION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
The uncertainty clouding the effects of aggregate stability on soil susceptibility to seal formation may stem from the fact that aggregate breakdown by water may result from a variety of physical and physicochemical mechanisms. Four main mechanisms have been identified: (i) slaking, that is, breakdown caused by compression of entrapped air during fast wetting (Panabokke and Quirk, 1957); (ii) breakdown by differential swelling during fast wetting (Kheyrabi and Monnier, 1968); (iii) breakdown by impact of raindrops (McIntyre, 1958); and (iv) physicochemical dispersion because of osmotic stress upon wetting with low electrolyte water (Emerson, 1967). These mechanisms differ in the type of energy involved in aggregate disruption. For instance, swelling can overcome attractive pressures in the magnitude of megapascals (Rengasamy and Olsson, 1991) while slaking and impact of raindrops can overcome attractive pressures in the range of kilopascals only (Rengasamy and Sumner, 1998). In addition, the various mechanisms may differ in the size distribution of the disrupted products (Farres, 1980; Chan and Mullins, 1994), and in type of soil properties affecting the mechanism (Le Bissonnais, 1996). Regarding the latter, slaking may be affected by porosity and internal cohesion while physicochemical dispersion will be affected by clay mineralogy, ionic composition, and concentration.
There are numerous methods for determining aggregate stability. Common methods include wet sieving (Kemper and Koch, 1966; Kemper and Rosenau, 1986), the drop test technique (Farres, 1980), and application of ultrasonic energy (North, 1976). Recently, Le Bissonnais (1990)(1996) proposed to use wet sieving in alcohol together with three treatments (i.e., fast wetting, slow wetting, and shaking after prewetting with a nonpolar liquid) as a measure of aggregate stability. Different processes dominate in the breakdown of the aggregates in the various stability tests (Loch, 1994). Thus not surprising it has been reported that use of different methods for determining aggregate stability has resulted in different rankings of soils studied (Amezketa et al., 1996; Le Bissonnais and Arrouays, 1997).
Among the many methods known to have been used for assessing aggregate stability, yet not a very common one, is the HEMC method (Childs, 1940; Pierson and Mulla, 1989). In this method, the wetting process is accurately controlled and energy of hydration and entrapped air are the only forces responsible for breaking down of aggregates. Aggregate stability is inferred from changes in pore-size distribution following wetting. Levy and Miller (1997) used the HEMC method and obtained significant correlations between aggregate stability indices and infiltration rate and runoff for numerous predominantly kaolinitic southeastern U.S. soils.
Recently, some studies have clearly demonstrated the predominant role of aggregate wetting rate in determining soil susceptibility to sealing and runoff (Le Bissonnais and Singer, 1992; Levy et al., 1997; Mamedov et al., 2001). Furthermore, Mamedov et al. (2001) have shown that for clay soils (>40% clay), slow wetting of aggregates (2 mm h-1) almost prevented seal formation and runoff. This indicated that in these soils, impact energy of raindrop was not enough to disintegrate aggregates and lead to soil sealing. The observations of the aforementioned studies may explain, at least partially, the reason for the correlation between aggregate stability indices obtained using the HEMC method and infiltration and runoff data for southeastern U.S. soils (Levy and Miller, 1997). This correlation could be ascribed to the fact that the same mechanism (i.e., rate of aggregate wetting) largely controlled the parameters studied in the two investigations (aggregate stability vs. infiltration rate and runoff).
The objectives of the current study were: (i) to evaluate aggregate stability of several cultivated predominantly smectitic Israeli soils varying in clay content (80–690 g kg-1), and level of sodicity (0.9 < ESP < 10) using the HEMC method, and (ii) to test the suitability of the aggregate stability parameters derived from the HEMC method for assessing soil susceptibility to seal formation and runoff. Data for seal formation and runoff were obtained for the same soils from the rainfall simulation studies of Mamedov et al. (2001).
|
MATERIALS AND METHODS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
5), and high-sodicity level (ESP 10), all together 18 samples. Divergence in ESP level within a soil type were because of differences in water quality used for irrigation (fresh water or treated effluent, corresponding to low and medium sodicity, respectively) or because of soil leveling that was done in the 1960s (high sodicity). In addition, six more samples with low ESP were taken from different fields to obtain a wide range of soils varying in their clay content. The samples (totaling 24) were brought to the laboratory, air-dried and sieved to <2 mm. Selected physical and chemical properties of the soils, determined by standard analytical methods (Klute, 1986; Page et al., 1986), are presented in Table 1.
|
Briefly, aggregates are wetted either slowly or rapidly in a controlled manner, and a moisture content (MC) curve at high energies (i.e., energies up to 500 mm H2O tension) is performed. An index of aggregate stability is obtained by quantifying differences in MC curves for fast and slow wetting (Fig. 1a)
. For a given wetting rate, a structural index is defined as the ratio of volume of drainable pores (VDP) to modal suction (Collis-George and Figueroa, 1984). Modal suction corresponds to matric potential (
, J kg-1) at the peak of the specific water capacity curve (d
/d), where is the water content (kg kg-1) (Fig. 1b). The VDP is defined as the area under the specific water capacity curve and above the dotted baseline (Fig. 1b). The dotted baseline represents rate of water loss because of aggregate shrinkage rather than pore emptying (Collis-George and Figueroa, 1984). The ratio of fast/slow structural indices, termed stability ratio, is used to compare stability of aggregates on a relative scale of zero to one.
|
5-mm thick bed. The fritted disk had a nominal maximum pore size of 20 to 40 µm. Saturation of the fritted disc was ensured prior to placing aggregates in the funnel. The funnel was connected from its bottom via a tubing to a peristaltic pump, which was then used to wet the aggregates in the funnel either fast (100 mm h-1) or slowly (2 mm h-1). At the end of wetting, aggregates were covered by standing water to ensure saturation. Distilled water (electrical conductivity of 0.004 mS cm-1) was used for wetting the aggregates in the funnel.
A MC curve, at a matric potential range of 0 to -5.0 J kg-1, was obtained using a hanging water column, whereby height of the meniscus in the pipette was decreased in increments of 0.1 to 0.2 J kg-1 thereby increasing the suction applied. Volume of water that drained from the aggregates at each matric potential was recorded after a 2 min of equilibrium period and corresponding water content of the aggregates was calculated. Preliminary studies showed that under our experimental conditions, no additional change in volume of drainage was noted at equilibrium time >2 min. Each treatment was duplicated. Coefficient of variation between replicates of water content () was <6%.
Data Analysis
To accurately calculate VDP and modal suction, modeling of MC curves was carried out with the following seven-parameter modified van Genuchten model (Pierson and Mulla, 1989):
 | [1] |
s and r are pseudo saturated and residual gravimetric water contents, respectively; 
and n control location and steepness of the S-shape inflection of the MC curve, respectively; and A, B, and C are the quadratic terms added by Pierson and Mulla (1989) to improve fitting of the model to the MC curve. The term pseudo was added to saturated and residual water contents owing to modification of the original van Genuchten model (van Genuchten, 1980). Values of s and r can no longer be physically interpreted in terms of saturated and residual water contents (Pierson and Mulla, 1989).
Specific water capacity curve (d/d), needed for obtaining the value of modal suction, was computed by differentiating Eq. [1] with respect to matric potential, and had the explicit form:
 | [2] |
The VDP, that is, the area under the specific water capacity curve and above the soil shrinkage line (Fig. 1b), was calculated by subtracting the terms for pore shrinkage (2A + B) from Eq. [2], and analytically integrating the reminder of that equation.
Seal Formation and Runoff Data
Mamedov et al. (2001) compared the effects of rate of wetting on sealing for the six soils, each at three different ESP levels, as used in the current study, using a drip-type laboratory rainfall simulator. In their study, the samples in the rainfall simulator were wetted from the bottom, at different rates (2, 8, or 64 mm h-1) using a peristaltic pump, prior to being exposed to 60 mm of distilled water rain. During each storm, water infiltrating through the soil was collected intermittently (every 4 min), in graduated cylinders placed underneath a special outlet at the bottom of the tray, and its volume was recorded. Runoff was collected continuously throughout the simulated rainfall event. Seal formation was quantitatively characterized by: (i) the measured near steady state infiltration rate at the end of the storm, termed final infiltration rate, and (ii) total volume of runoff obtained from the storm.
|
RESULTS AND DISCUSSION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
|
Changes in VDP ratio and stability ratio of soils with low ESP (
2.5) as a function of organic matter content are presented in Fig. 2a
. Regression analyses of the two aggregate stability indices vs. organic matter content using different functions (linear, logarithmic, hyperbolic), yielded low coefficients of determination (R2 < 0.5). Similarly, Coughlan and Loch (1984) and Goldberg et al. (1988), who studied aggregate stability of samples from semi-arid and arid-zone soils, also reported low coefficient of determinations between their aggregate stability data and organic matter content. Conversely, other studies (e.g., Kemper and Koch, 1966; Le Bissonnais and Arrouays, 1997) reported of a strong link between aggregate stability and soil organic matter. Note that in the latter two studies organic matter content in many of the soils studied was >3%. Absence of a strong relation between aggregate stability and organic matter content found in our study and those of Coughlan and Loch (1984) and Goldberg et al. (1988) could be ascribed to the fact that in semi-arid and arid soils, organic matter content is low, usually <2%. Apparently, under conditions where the range of organic matter content in the soils is limited, other soil properties (e.g., clay content, Fe oxides) may have a greater impact on aggregate stability, and hence, overshadow effects of organic matter.
|
The strong links between aggregate stability and clay content and organic matter are well documented (e.g., a recent review of Kay and Angers, 1999). However, the variations in reported relations between aggregate stability and clay or organic matter content suggested that aggregate stability cannot be inferred solely from clay content or organic matter content. Thus, assessment of aggregate stability can be obtained only from the knowledge of a range of soil properties (clay, organic matter, and oxides content) which are all linked by their representation of binding processes in the soil.
Stability ratios for samples with medium (3.7–6.6) and high (7.5–10.2) ESP levels were calculated as the ratio of structural index obtained from fast wetting to structural index obtained from slow wetting for a given ESP of the same soil. The results showed that the soils studied can be divided into two groups. The first group included the loamy sand and loam soils (8 and 22.5% clay), where stability ratio was similar for all ESP levels (Fig. 3)
. Furthermore, stability ratio in the loamy sand and loam was significantly lower than that in the other soils (Table 2 and Fig. 3). These observation may suggest that in soils with unstable aggregates, conditions favoring clay dispersion, for example, increased sodicity, are of minor importance in determining aggregate stability. In the second group that included the sandy clay and the three clay soils (clay content 
38%), stability ratios for high ESP were significantly lower than those for the other ESP levels (Fig. 3). Stability ratios for medium ESP were similar to or lower than those for low ESP (0.9–25). However, a significant negative linear relation was noted between stability ratio of the clay soils and ESP (Fig. 4)
. Decrease in aggregate stability with increase in sodicity in the second group soils was attributed to presence of substantial amounts of clay in these soils. Increase in sodicity makes soil clay more dispersive (Oster et al., 1980), consequently at higher sodicity levels, soil clay became less effective as a cementing agent in aggregates.
|
|
5 and ESP >10 caused no reduction and a significant decrease, respectively, in aggregate stability of loam as well as sandy clay soils. Conversely, Coughlan and Loch (1984) who studied 12 clay soils from Australia (0.4 < ESP < 26) and Goldberg et al. (1988) who studied 34 arid zone soils (0.2 < ESP < 19.6) did not observe any correlation between aggregate stability and ESP. The inconsistency in reported effects of sodicity on aggregate stability may stem from the difference in methods used in determining aggregate stability in the various studies; HEMC (current study), drop technique (Shainberg et al., 1992), immersion wetting (Coughlan and Loch, 1984), and wet sieving (Goldberg et al., 1988). The different methods use different forces in destroying the aggregates, which may emphasis different soil properties that affect aggregate stability.
Aggregate Stability, Seal Formation, and Runoff
Correlation analyses between stability ratios, final infiltration rate, and runoff data reported by Mamedov et al. (2001) for the same soils used in our study (Table 3) are given in Table 4. It is evident that no correlation existed between final infiltration rate or runoff obtained after fast wetting of the soil, and stability ratio (Table 4). This observation held true for each individual ESP level as well as for the case where data from the various ESP levels were pooled together. Unlike fast wetting, when slow wetting was used significant correlation coefficients (0.65 < R < 0.76) were noted between stability ratio, infiltration, and runoff data from the various ESP levels pooled together (Table 4). Moreover, for ESP levels of <2.5 and 5, the aforementioned correlation analysis resulted in higher correlation coefficients (0.70 < R < 0.85). On the other hand, correlation coefficients for ESP 10 data were low (Table 4).
|
|
These results of the correlation analyses indicated that aggregate stability as determined by the HEMC method was not always effective in predicting soil susceptibility to seal formation. The HEMC aggregate stability indices performed better under the conditions where (i) only raindrop impact was responsible for aggregate disintegration in the seal formation process, and (ii) contribution of physicochemical clay dispersion to seal development was mild (i.e., low-to-moderate levels of sodicity).
These finding were in partial disagreement with our initial hypothesis that determining aggregate stability in a method in which the destructive force used to break aggregates was a controlled wetting process (e.g., HEMC method) will correlate well with seal formation indices. Levy and Miller (1997) reported that aggregate stability indices obtained with the HEMC method correlated well with seal formation indices for southeastern U.S. soils. Our study showed that aggregate stability of the predominantly smectitic Israeli soils was lower than that of the predominantly kaolinitic southeastern U.S. soils. Furthermore, Stern et al. (1991) showed that smectitic soils were more sensitive to seal formation than kaolinitic soils. It is postulated that the weaker stability of the smectitic Israeli soils compared with kaolinitic soils, could explain the inability of the HEMC aggregate stability indices to accurately predict soil sensitivity to seal formation under the aforementioned specific conditions.
|
SUMMARY AND CONCLUSIONS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
10). Aggregate stability determined with the HEMC method could serve as a predictor for soil susceptibility for seal formation only under the following specific conditions: (i) aggregate disintegration in the course of seal formation was mainly due to impact energy of rain drops, and (ii) the contribution of physicochemical clay dispersion to the sealing process was not severe (ESP < 6).
For practical purposes, it is expected that in environments where aggregate wetting is slow (e.g., rainstorms are of low intensity [5–10 mm h-1] or aggregates have high water content), and soil ESP does not exceed 6, use of the HEMC method for determining aggregate stability will yield stability indices that can satisfactorily predict soil susceptibility to seal formation, and runoff generation.
|
ACKNOWLEDGMENTS |
|---|
|
NOTES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
Received for publication May 2, 2001.
|
REFERENCES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  A. I. Mamedov, S. Beckmann, C. Huang, and G. J. Levy Aggregate Stability as Affected by Polyacrylamide Molecular Weight, Soil Texture, and Water Quality Soil Sci. Soc. Am. J., October 29, 2007; 71(6): 1909 - 1918. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. K. Bhardwaj, I. Shainberg, D. Goldstein, D. N. Warrington, and G. J.Levy Water Retention and Hydraulic Conductivity of Cross-Linked Polyacrylamides in Sandy Soils Soil Sci. Soc. Am. J., March 12, 2007; 71(2): 406 - 412. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. M. Ruiz-Vera and L. Wu Influence of Sodicity, Clay Mineralogy, Prewetting Rate, and Their Interaction on Aggregate Stability Soil Sci. Soc. Am. J., September 20, 2006; 70(6): 1825 - 1833. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. I. Mamedov, C. Huang, and G. J. Levy Antecedent Moisture Content and Aging Duration Effects on Seal Formation and Erosion in Smectitic Soils Soil Sci. Soc. Am. J., March 29, 2006; 70(3): 832 - 843. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. Zhang, R. Horn, and P. D. Hallett Mechanical Resilience of Degraded Soil Amended with Organic Matter Soil Sci. Soc. Am. J., May 6, 2005; 69(3): 864 - 871. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. J. Levy, D. Goldstein, and A. I. Mamedov Saturated Hydraulic Conductivity of Semiarid Soils: Combined Effects of Salinity, Sodicity, and Rate of Wetting Soil Sci. Soc. Am. J., April 11, 2005; 69(3): 653 - 662. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
