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Published online 6 May 2005
Published in Soil Sci Soc Am J 69:864-871 (2005)
DOI: 10.2136/sssaj2003.0256
© 2005 Soil Science Society of America
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Soil & Water Management & Conservation

Mechanical Resilience of Degraded Soil Amended with Organic Matter

Bin Zhanga,*, Rainer Hornb and Paul D. Hallettc

a Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing, 210008, People's Republic of China
b Institute of Plant Nutrition and Soil Science, CAU, 24118 Kiel, Federal Republic of Germany
c Plant-Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA Scotland, UK

* Corresponding author (bzhang{at}issas.ac.cn)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Organic matter may help prevent the degradation of the soil pore structure by mechanical stresses, and consequently the impact of compaction and surface erosion. To obtain a mechanistic understanding of some of the processes involved, we amended an Ultisol with peat particles, at amendment rates of 0, 10, and 50 g kg–1, and subjected the peat-soil mixtures to different wet/dry cycles (1, 4, or 10). Mechanical stresses were imposed to the soil as direct shear, surface shear, and virgin compression tests. The objective was to determine the effect of increasing organic matter on the resistance and resilience of the soil pore structure to mechanical stresses, after wet/dry cycles. Peat amendment increased soil porosity, mainly in the fine pores (<6 µm), whereas an increased number of wet/dry cycles affected the coarse pore fraction (>50 µm). Soil shear strength decreased with a greater peat-amendment rate. For soil amended with 50 g kg–1 of peat, increasing the number of wet/dry cycles increased the compressibility index from 1.14 to 1.43 and the friction angle under shear by 6°. Although peat may decrease the resistance to compression, the initial soil pore structure and the resilience to this mechanical stress was greatly improved. Wetting and drying cycles improved the impact of peat on the mechanical resistance of the soil, but reduced resilience slightly. The results suggest that to amend with organic matter will improve recovery from vehicle traffic damage and improve water retention during dry periods, providing better conditions for plants and microbes.

Abbreviations: CD, consolidated-drained • RC, recompression curve • UD, unconsolidated-drained • VC, virgin compression curve • Vc/Rc, compression index


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
WATER EROSION and compaction of soil are major global threats, causing negative impacts to food production, environmental contamination, and sediment transport (Morgan, 1995; Ball et al., 1997). The susceptibility of soil to water erosion and compaction depends on the physical resistance and resilience to compression and shear stresses, which affect the stability and recovery of the soil pore structure (Horn and Baumgartl, 2000). Increasing soil organic matter may help prevent soil pore structure degradation by increasing the physical stability (Dexter, 1988), the mechanical resistance to compression (Gupta et al., 1987) and shear stresses (Zhang and Hartge, 1990), and the rebound or resilience of soils after the removal of a compressive stress (Dexter, 1988; Kay et al., 1994). Organic matter may also improve soil aggregation, resulting in a higher total porosity and wider pore-size distribution (Anderson et al., 1990; Caron et al., 1996), which determines the magnitude of soil mechanical resilience (McBride and Watson, 1990).

The physical structure of organic matter has been shown to provide a ‘spring’ against mechanical deformation and a matrix for higher water absorption (Gupta et al., 1987; Dexter, 1988). This increases the rebound or resilience of soils after the removal of a compressive stress and affects the mechanical resistance to shear stresses (Zhang and Hartge, 1990). These mechanical processes help provide a quantitative understanding of why the physical stability and resilience of the soil pore structure is improved by organic matter amendment. Many studies have reported improved soil physical properties with organic matter amendment, but our knowledge on the underlying mechanical mechanisms that drive physical stabilization needs further research.

Compression and shear tests are geotechnical approaches (Craig, 1987, p. 233–243) that quantify soil resistance to mechanical stresses (Horn and Rostek, 2000). In the compression test, soil settlement under increasing mechanical stress describes the soil compressibility, which is the compression resistance. Soil recovery after the release of the stress is the compression resilience (Griffiths et al., 2004). Shear resistance is traditionally determined from the relationship between shear stress and strain in a direct shear test, using the Mohr–Coulomb equation to evaluate shear strength, cohesion, and internal friction (Craig, 1987, p. 233–243; Mitchell, 1993). These compression and shear tests describe the impact of agricultural machinery (O'Sullivan et al., 1999), but they are not sensitive to the much lower stresses that cause soil water erosion (Nearing and Bradford, 1985). A surface shear test developed by Zhang et al. (2001) mimics the low stress range generated by overland flow and raindrop impact, the causal agents of water erosion. Adopting geotechnical engineering approaches in controlled laboratory studies should help to describe interactions between organic matter and mechanical behavior, adding considerably to our understanding of the resistance and resilience of soil to mechanical stresses.

We hypothesize that organic matter serves as a mechanical spring in soil, lowering the resistance to applied mechanical stresses but improving the resilience once the stress is removed. With increasing number wet/dry cycles the organic matter may be incorporated into aggregates, reducing its effectiveness as a spring. A degraded soil with low organic C was homogenized to amend with the organic matter and repacked to provide repeatable and controlled test specimens. Particulate peat (<2 mm) was used as the organic matter because of its high resistance to biological decay. Changes to the soil pore structure from the applied mechanical stresses and the number of wet/dry cycles were evaluated from the water retention characteristics. A highly degraded Ultisol from southeastern China was studied so that the physical impacts of added organic matter to soil restoration could be assessed from the underlying mechanical processes.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Core Preparation
Surface soil (0–150 mm) was taken near the Experimental Station of Red Soil, Chinese Academy of Sciences, in Yingtan, Jiangxi Province, China (28°15' N, 166°55' E). The soil was a Typic Plinthudult according to the U.S. Taxonomy (Soil Survey Staff, 1998), developed from Quaternary red clay, and cultivated under peanut cropping for over 30 yr. It was kaolinitic, clayey and acidic, containing 206 g kg–1 sand, 336 g kg–1 silt, 458 g kg–1 clay, 4.6 pH (2:1 soil: water), 6.8 g kg–1 soil organic C, and 103.0 mmol (+) kg–1 cation exchange capacity. The land use caused severe soil degradation, with surface erosion being a major problem. More information on the soil properties can be found in Zhang and Horn (2001).

Highly decomposed peat (Floragard Vertriebs GmbH, Oldenburg, Germany) was used as the organic matter because it is resistant to biological decay during incubation. The peat was partially dried at 40°C in an oven to a water content of 200 g kg–1. The soil was air-dried to a water content of 46 g kg–1. Both the peat and the soil were crushed to pass through a 2-mm mesh. The peat was mixed with a spatula into soil to obtain peat contents of 0, 10, and 50 g kg–1 in soil, corresponding to the range of organic matter concentrations commonly found in degraded arable to grassland soils (Cambardella and Elliott, 1993). The peat-amended soils were packed to achieve a porosity that was similar to field conditions.

Soil cores were packed in 10-mm increments to reduce within core heterogeneity. Small cores (40 mm in diameter and 80 mm high) were used to determine soil water retention curves and pore-size distributions after each of wet/dry cycle treatment. Large cores (100 mm in diameter and 30 mm high) were used in compression and shear tests to evaluate soil structural recovery and resistances to compression and shear stresses. Compression and shear tests were not conducted on samples with 0 g kg–1 of added peat because of the time-constraints in using specialized equipment in a foreign laboratory. The influence of amendment was assessed by comparing differences between treatments amended with 10 and 50 g peat kg–1 soil.

The soil cores were subjected to 1, 4, or 10 wet/dry cycles. This involved slow wetting the soil cores from the bottom to –0.3-kPa water potential for 24 h in distilled water. One wet/dry cycle corresponds to a 12-h saturation (rapid wetting from the base in free water), 24 h free drainage at –30 kPa on a suction plate and 10 h of drying at 40°C: the temperature being the average soil temperature in summer. All samples were equilibrated to –30-kPa water potential before mechanical testing. Soil swelling during the first wetting cycle caused the soil to extend beyond the sample ring boundaries. Excess soil was removed with a knife, making sure to not smear the surface.

Measurement of Pore-Size Distribution
The tests were performed on the small soil cores prepared with the different peat amendment rate and wet/dry cycle treatments described previously. Total porosity was calculated from the bulk and particle densities, taking into account soil and peat particle densities of 2700 kg m–3 (Zhang and Horn, 2001) and 500 kg m–3, respectively (Zhang and Hartge, 1995). Therefore, the particle densities were 2590 kg m–3 for the 10-g kg–1 peat amended soil and 2210 kg m–3 for the 50-g kg–1 peat-amended soil. Changes in soil volume due to shrinkage were also measured and used when evaluating total porosity.

The pore-size distribution was calculated from the soil water retention curves according to the parallel capillary model (Kutilek and Nielsen, 1994). Water retention curves were determined by first saturating the cores for 3 d from the bottom on a sandbox, then draining at –3 kPa for 5 d, and finally moving to ceramic suction plates at –6 kPa for 7 d, –15 kPa for 10 d, –30 kPa for 14 d, and –50 kPa for 21 d. Soil pores, determined from water retention, were classified into fine <6-µm, medium 6- to 50-µm, and coarse >50-µm pore diameters.

Compression Test
Compression tests evaluated soil resistance to compression and soil structural recovery after the removal of compression (Craig, 1987, p. 233–243; O'Sullivan et al., 1999). More than 108 large soil cores were equilibrated after wet/dry cycle(s) treatments by saturating from the bottom for 12 h and draining at –30 kPa for 5 d on ceramic plates. Normal stresses of 20, 40, 70, 100, 200, and 400 kPa were applied to the soil cores. The applied stresses are consistent to the stress applied by different machines working in the field, ranging from a light seedbed roller to construction vehicles (Horn and Rostek, 2000). The soil cores were placed on porous metal plates for free drainage during the compression test. Three experimental replicates were used at each stress. Soil settlement over 3 h was recorded with linear strain transducers as the normal stresses were applied to the soil cores and over 1 h as the stresses were released. Soil water potential was recorded at 1-s intervals using pressure transducers connected to microtensiometers at the bottoms of the soil cores that were linked to a data logger. Soil settlement and soil water potential over time were used to calculate rebound height for the evaluation of soil structural recovery after the release of compression. The rebound height was the difference in soil settlement during the time when the stress was released and the time when the settlement was constant.

Compression curves were defined as void ratio against the logarithm of the applied compressive stress (Horn and Baumgartl, 2000). Void ratio, ei, corresponding to the ith applied stress was determined by

[1]
where {rho}s is the particle density of the soil and peat mixture, and {rho}bi is soil bulk density after a 3-h compression at the ith normal stress. The compression curves can be divided by the precompression stress into the less steep recompression curve and the steeper virgin compression curve (Horn and Baumgartl, 2000). The recompression curve represents the pore-controlled resistance to compression while the virgin compression curve represents the texture-controlled resistance to compression. The compressibility index (Vc/Rc) was calculated by the slopes of the virgin compression and recompression curves. Higher Vc/Rc values indicate less resilience to compression.

Shear Tests
Shear tests were performed on the soil surface using a novel approach (Zhang et al., 2001) and within soil using a conventional direct shear apparatus (Craig, 1987, p. 233–243; Mitchell, 1993). The surface shear test simulates a shearing process under a small applied-stress, for example, generated by overland flow and raindrop impact. Normal stresses of 0.2, 0.5, 0.8, 1, and 2 kPa were applied to the soil surface by a platen, with a piece of sandpaper (30 particles cm–2) adhered to the base. The shear stress at a given normal stress was defined as the minimum horizontal stress (horizontal force divided by acting area) at which shear displacement was recorded. Four experimental replicates were measured for each applied normal stress.

Direct shear tests were performed on the soil cores previously compacted to 20, 40, 70, 100, 200, and 400 kPa in the compression tests and on non-compacted cores. The soil cores were placed on porous metal plates to provide free drainage during the shear tests. Shear tests using the compacted soil cores were referred to as the consolidated-drained (CD) condition and shear tests using the non-compacted soil cores were referred to as unconsolidated-drained (UD) condition. The shear stress was applied at a speed of 0.2 mm min–1 using a stepper motor. Soil settlement, shear stress, and soil water potential were recorded using transducers. Three experimental replicates were measured at each applied stress.

Soil behavior was evaluated using the Mohr–Coulomb approach, where shear strength at the soil failure plane, {tau} as a function of applied normal stress, {sigma} was described as

[2]
The parameters, c and {phi}, were computed from the relationship between the recorded shear stress and the applied normal stress. In the surface shear test, c is the adhesion between sandpaper and the soil surface and {phi} is friction angle in surface soil. In the direct shear test, c is soil cohesion and {phi} is the friction angle within soil.

Statistical Analysis
Two-way analysis of variance (ANOVA) was performed for the treatment effects of either peat-amendment rate or wet/dry cycles on soil pore characteristics, rebound height, soil water potential, and shear stress with multiple comparisons as the arithmetic means were significantly different (StatSoft, 1995). The differences were assessed by least significant difference (LSD at P ≤ 0.05) tests.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Total Porosity and Pore-Size Distribution
The initial packing of the soil cores resulted in similar total porosities. The porosities of the smaller soil cores were 52.59 ± 0.37 (standard error) %, 52.83 ± 0.39%, 53.01 ± 1.36% for the amendment rates of 0, 10, and 50 g kg–1, respectively. In the larger soil cores, the porosities were 54.37 ± 0.39 and 54.81 ± 0.45% for the amendment rates 10 and 50 g kg–1, respectively. The soil cores swelled as a consequence of equilibration with water at the first wetting and the removal of the excess soil from the soil cores changed total porosity for 0 wet/dry cycles (Fig. 1). The total porosity in the soil with no peat changed little due to swelling, but it increased by almost 10% for the soil with the 50-g kg–1 peat amendment. Although 1 wet/dry cycle caused a statistically significant increase in total porosity for all soils (P < 0.001), the differences are only 1 to 2% (Fig. 1). With increasing wet/dry cycles, the total porosity remained unchanged.



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  		Fig. 1. Total porosity of the 0-, 10-, and 50-g kg–1 peat-amended soils after 0 (blank), 1 (slashed), 4 (crossed), and 10 (grayed) wet/dry cycles. The standard error is shown (n = 4).

 
The pore-size distributions derived from the soil water retention curves are presented in Fig. 2. The fraction of fine pores (<6 µm) increased with amendment rate while the fraction of coarse pores (>50 µm) decreased (P < 0.05) for all wet/dry cycles. For all the soils tested the fine pore fraction was largest at four wet/dry cycles and did not differ between 1 and 10 wet/dry cycles (P > 0.10). The coarse pore fraction increased as number of wet/dry cycles increased while the 6- to 50-µm pore fraction decreased. The increases in the coarse pore fraction diminished as the amendment rate increased.



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  		Fig. 2. Equivalent pore-size distribution of the 0-, 10-, and 50-g kg–1 peat-amended soils after 1 (blank), 4 (slashed), and 10 (crossed) wet/dry cycles. The standard error is shown (n = 4).

 
Resistance and Resilience to Compression Stress
Typical curves of soil settlement and soil water potential over time during the compression test are shown in Fig. 3 to illustrate the concept of soil mechanical resistance and resilience. Soil settlement and soil water potential increased as a stress was applied (resistance) and recovered as the stress was released (resilience).



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  		Fig. 3. Soil settlement (upper) and soil water potential (lower) of the 50-g kg–1 peat-amended soil after one wet/dry cycle after application and release of 200 kPa-applied stress in the compression test.

 
The compression curves, plotted as the void ratio against the applied stress, are shown in Fig. 4 and were used as a measure of soil mechanical resistance. As the initial void ratio was different between the two peat-amended soils, the void ratio at a given stress was higher for the 50-g kg–1 peat-amended soil. The compression curves had an inflection in the curves at about 100 kPa. The compression index, Vc/Rc increased with increasing number of wet/dry cycles and was lower in the 50-g kg–1 peat-amended soil than in the 10-g kg–1 peat-amended soil (Fig. 5). This result suggests that wet/dry cycles create more structural pores in the soil with the higher peat amendment.



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  		Fig. 4. Void ratio against the applied stress for the 10-g kg–1 (upper) and 50-g kg–1 (lower) peat-amended soils after 1 (square), 4 (circle), and 10 (triangle) wet/dry cycle(s) at –30-kPa water potential. The standard error is shown (n = 3). S.d. indicates significantly different between wet/dry cycles.

 


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  		Fig. 5. The compressibility index of the 10-g kg–1 (left) and 50-g kg–1 (right) peat-amended soils after different wetting and drying (wet/dry) cycles at –30-kPa water potential.

 
The rebound height was used as a measure of soil mechanical resilience. It increased with applied stress and amendment rate (Fig. 6); about 100% higher in the 50-g kg–1 peat-amended soil than in the 10-g kg–1 peat-amended soil. The rebound heights at four wet/dry cycles were higher than at 1 wet/dry cycle and 10 wet/dry cycles, with a greater effect at higher stresses (P < 0.01).



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  		Fig. 6. Rebound height after stress release for the 10-g kg–1 (left) and 50-g kg–1 (right) peat amended soils after 1 (blank), 4 (slashed), and 10 (crossed) wet/dry cycles. The standard error is shown (n = 3).

 
Stress application increased soil water potential (Fig. 3 and 7) and the magnitude of the increase was greater at higher applied stresses and peat amendment to soil (Fig. 7). An applied stress of 400 kPa caused positive pore water potentials for the 50-g kg–1 peat-amended soil. Stress release resulted in the water potential either recovering to its initial value for the applied stresses ≤70 kPa or decreasing to more negative values for the applied stresses ≥200 kPa (Fig. 7). This effect was more profound for the 10-g kg–1 peat-amended soil than for the 50-g–1 peat-amended soil. Wet/dry cycles affected the change in soil water potential only at the application and release of 400-kPa stress (P < 0.01), at which soil water potential was highest at four wet/dry cycles.



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  		Fig. 7. Pore water potentials of the 10-g kg–1 (upper) and 50-g kg–1 (lower) peat-amended soils before (blank) and after (crossed) application of compressions, and after removal of compression (crossed) as affected by 1 (left), 4 (middle), and 10 (right) wet/dry cycles. The standard error is shown (n = 3).

 
Resistance to Shear Stress
The stress-strain data fit Eq. [2] well, with R2 value > 0.98 in the surface shear tests and with R2 value from 0.93 to 0.99 in the direct shear tests. The fitted parameters c and {phi} are summarized in Table 1. In the surface shear test the cohesion, c was low and its practical significance to erosion stability is therefore minimal (Horn and Rostek, 2000), although its levels dropped slightly with increasing number of wet/dry cycles. The soil friction angle, {phi}, increased from 45.2° (1 wet/dry cycle) to 51.7° (10 wet/dry cycles) for the 10-g kg–1 peat amended soil and changed little for the 50-g kg–1 peat-amended soil.


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  		Table 1. Soil shear strength parameters of the 10- and 50-g kg–1 peat-amended soils in the surface shear test and direct shear test at the conditions of unconsolidated-drained (UD) and consolidated-drained (CD) as affected by number of wet/dry cycles.{dagger}

 
In the direct shear, test the soil friction angle, {phi}, did not vary significantly with number of wet/dry cycles and was reduced slightly as the amendment rate increased. The friction angles were higher under the CD condition than under the UD condition. The cohesion, c varied with wet/dry cycles and amendment rate, but no significant trend was identified. The cohesion of the 10-g kg–1 peat amended soil increased as the number of wet/dry cycles increased from 1 to 4 under both UD and CD condition. As the number of wet/dry cycles increased to 10, the cohesion decreased to the level at one wet/dry cycle under the UD condition and did not vary from that at four wet/dry cycles under the CD condition. The cohesion for the 50-g kg–1 amendment soil decreased under the UD condition, while it increased from one wet/dry cycle to four wet/dry cycles and decreased to the level at one wet/dry cycle as the number of wet/dry cycles increased to 10.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The effects of added organic matter and wet/dry cycles on soil aggregation and mechanical behavior are conceptualized in Fig. 8. Organic matter particles filled in the coarse pore fraction to produce a greater amount of fine pores and increased total porosity by swelling under wetting (Fig. 2). By acting as elastic springs, organic matter increased rebound (Fig. 6) and compression resilience (Fig. 5), indicated by the smaller plastic dashpot and larger elastic spring in Fig. 8. The wetting and drying cycles provides dispersed clays that may coat on humified organic matter or particulate organic matter by binding and adsorption (Dexter, 1988), causing the compression index and hence resilience to diminish. With a low amendment of organic matter the coarse pores increased with increasing number of wet/dry cycles, but this was buffered with a high amendment of organic matter (Fig. 2).



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  		Fig. 8. A conceptual diagram of how peat amendment and wet/dry cycles influence soil structure and mechanical behavior. At the top of the figure, the lightly textured blocks represent soil aggregates and the thick lines peat. At the bottom of the figure, the size of the spring represents elastic deformation that is recoverable once the stress, {sigma} is removed. The size of the dashpot represents plastic deformation that is not recoverable when the stress is removed. Aggregation increases with number of wet/dry cycles, leading to coarser pores that are more easily compressed but not recovered on unloading. The peat particles fill in coarse pores and move apart soil aggregates, causing a higher total porosity and fine pore-size fraction. Peat acts as an elastic spring, but with number of wet/dry cycles the impact diminishes as some peat becomes physically protected in soil aggregates.

 
The compressibility index (Fig. 5) indicated that the amount of compression for an applied stress increased with peat amendment. One cause will be the greater initial porosity of the soil and the greater compressibility of peat. Zhang and Hartge (1995) and McBride and Watson (1990) also demonstrated that the amendment of soil with organic matter increased soil compressibility because of the higher initial porosity. The increase in rebound with higher peat amendment indicates that the resilience to compressive stresses is greater. A similar finding was reported by Griffiths et al. (2004) who studied soils amended with contaminated sewage sludge.

The mechanical resistance to shear stress was also influenced by peat amendment. The interparticle strength of peat is much lower than that of soil particles (Mitchell, 1993), causing decreased cohesion with higher peat amendment (Table 1). Cohesion appeared to diminish with increasing number of wet/dry cycles, but these results should be viewed cautiously as they are near to 0 Pa and are probably not detectable by the test.

There was little influence of wet/dry cycles on the friction angle in the direct shear tests, which is different from the findings of Zhang and Hartge (1995), who found major differences when the peat was incubated in soil over several months. This could be due to a lower shrink/swell potential in the highly kaolinitic Ultisol studied here in comparison with their study. The longer term incubation used by Zhang and Hartge (1995) could have also increased aggregation by wetting and drying cycles because decomposition of the peat may have produced biological exudates that bound soil particles (Chenu and Guérif, 1991). An increase in friction angle with increasing number of wet/dry cycles was only identified in the surface shear tests of the 10-g kg–1 amended soil. The drying intensity may be higher on the soil surface than within the soil, causing some aggregation that is only detectable at the very low stresses imposed by the surface shear tests. The friction angles were higher under the CD condition than under the UD condition as the soil samples had been compressed and the number of contact points increased.

The surface shear tests indicated that organic matter incorporation, simulated here by adding peat, would have little influence on erosion through particle detachment by rainwater as suggested by Rachman et al. (2003). However, this result should not be taken out of context, as organic matter in natural soil can be a nucleus for soil aggregation as microbial exudates on its decomposing surface encapsulate and bind soil particles, decreasing erosion susceptibility (Dexter, 1988). Other tests such as aggregate stability (Le Bissonnais, 1996; Levy and Mamedov, 2002) and particle detachment by raindrop impact (Rachman et al., 2003), are needed to properly assess erosion susceptibility.

A mechanical analysis of soil physical behavior provides valuable information that quantifies its resistance and resilience to physical stresses (Gupta et al., 1987). Our work shows that adding peat to a soil low in organic matter improves its physical properties. The increase in the amount and stability of total porosity and pores <50 µm should provide a better biophysical environment for roots and microbes (Ball et al., 1997).

Fundamental information has been gained about the physical impacts of an organic amendment on soil mechanical behavior. Organic matter can act as a mechanical spring in the soil, allowing it to recover from external stresses like surface traffic and overburden pressure. By using peat as the organic amendment and conducting the study over a relatively short time, we were able to deduce the physical impacts with biological decay minimized. Zhang and Hartge (1995) reported greater impacts on mechanical behavior if the peat was incubated in the soil, presumably because decomposition produces exudates that bind soil particles. Future research should measure mechanical behavior after various incubation times of organic matter in soil.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Organic matter amendment by peat improved the resistance and resilience to mechanical stress of a severely degraded Ultisol. It also increased soil porosity, mainly in the fine pore fraction <6 µm and reduced the formation of coarse pores (>50 µm) through wet/dry cycles. The compressibility of the soil increased with added peat, but this was compensated by a greater physical resilience to compaction. Peat particles may act as springs in soil, thereby increasing rebound after the release of stresses. This made the soil easier to deform mechanically, but far more resilient once the mechanical stress was removed.

The mechanical tests used in this study, combined with wet/dry cycles, quantified the potential benefits of organic matter in soil. Many studies have shown that a greater proportion of fine pores caused by organic matter improves water retention during dry periods, providing better conditions for plants and microbes. A more unique finding of this work is that the mechanical resilience of these pores is also improved by organic matter. As a result, the soil has a better capacity to recover from the myriad of mechanical stresses imposed under arable systems, including vehicle traffic to the weight of overburden soil.


   ACKNOWLEDGMENTS
 
The research funding from the National Foundation of Sciences in China (NSFC) and the Research Fellowship from the Alexander von Humboldt Foundation to Dr. B. Zhang are acknowledged. The Scottish Crop Research Institute is Grant-Aided by the Scottish Executive, Environment and Rural Affairs Department. Dr. M. Wander and three other anonymous rewires are acknowledged for the helpful comments to improve the paper.

Received for publication September 29, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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