Aggregate coalescence in irrigated cracking clays constrains crop yields, yet little is known about it or howit can be managed. A measure of coalescence is introduced to separate the effects of natural aggregate-beddensification from those of age-hardening; this measure, �, comprises a ratio of the net change in (tensileor penetrometer) strength, Y, that occurs in relation to the corresponding net change in dry bulk density, �b,as follows: � = �Y/��b. A laboratory study was conducted to illustrate the variation in � for a virgin andcultivated cracking clay exposed to 16 weekly cycles of wetting and draining. Penetrometer resistance andtensile strength at –100 kPa, plus bulk density and other physical and chemical properties, were measuredthroughout the experiment. The cultivated soil rapidly became denser and stronger, it developed largeraggregates, and its water-uptake rate in the air-dry state was significantly greater than that for the virginsoil. The � values suggested that age-hardening processes constituted a greater component of coalescencein the cultivated soil than it did in the virgin one, and this was thought to be mediated by the large differencesin the content and composition of organic matter in the two soils.
C. D. Grant , D. A. Angers, R. S. Murray, M. H. Chanti gny, U. HasanahSR99073Soil aggr egate coalescenceC. D. Gr antet al.
Introduction
When unsaturated aggregates are subjected to internal or external stresses they undergovarying degrees of viscous deformation at contact points and coalesce into larger, strongerunits (Keller 1970). Terms such as aggregate welding (Bresson and Boiffin 1990; Kwaadand Mücher 1994; Bresson and Moran 1995), sintering, or aggregate coalescence(Ghezzehei and Or 2000) describe this soil hardening process, which may or may notinvolve densification (Or 1996), slaking, dispersion, cementing (Cockroft and Olsson2000), or ageing (Utomo and Dexter 1981). A common consequence of high strength incoalesced soils, even at water contents near field capacity, is restricted root growth anddiminished crop productivity (Cockroft and Martin 1981; Masle and Passioura 1987;Cockroft and Olsson 2000).
Ghezzehei and Or (2000) presented a model using viscous deformation to describe theconditions necessary for aggregate coalescence to begin and end, when a major componentof soil hardening results from volume reduction. However, in cases where soil hardeningoccurs without significant volume change (e.g. Utomo and Dexter 1981), other models ofcoalescence need to be considered. For example, soil strength can increase due to age-hardening of soil aggregates, which results from (Dexter et al. 1988): (i) an increase in thenumber of interparticle bonds per unit volume, and/or (ii) an increase in the strength ofindividual bonds. Age-hardening is largely independent of compaction pressure (Dexteret al. 1988), but also occurs during compaction. An understanding of the relativecontributions that volume change and age-hardening processes make to coalescence in
permanent horticultural beds may provide clues on how to manage them better forhorticultural production.
Assuming as a first approximation that soil strength increases linearly with bulk density(Greacen 1981), any change that occurs in strength, Y, can be normalised using theaccompanying change in bulk density that occurs, �b, to produce a strength/density ratio, �:
� = �Y/��b [1]
A change in � indicates the relative importance of ageing versus densification during thatperiod for any increase in soil strength that occurs. For example, if �� remained constantover time any increases in strength could be accounted for by concomitant increases in bulkdensity, which need not involve age-hardening. If, on the other hand, �� increased withtime, increases in strength could be attributed to age-hardening processes beyond thosecaused by natural densification.
The purpose of this work was to evaluate the relative contributions that age-hardeningand densification make in causing aggregate coalescence in a cracking clay undersimulated conditions typical of those occurring in a single growing season. In this paper,various measures (including �) are used to illustrate densification and age-hardening in avirgin and cultivated cracking clay soil exposed to cycles of wetting and draining.
Materials and methods
The soil used in this experiment was a self-mulching, heavy grey clay (65% <2 �m; Cornella Clay,described by Skene and Harford 1964) obtained from adjacent locations on a property near Rochester insouth-eastern Australia. One of the two locations had been under intensive cropping for more than 50 years(‘cultivated’ soil) where, during the period 1988–98, there were 4 cereal crops, 4 maize crops, and 2 tomatocrops, interspersed with perennial pastures. The cultivated soil also received 3 applications of gypsum(2 t/ha) and cow manure (1 t/ha), plus N and P fertilisers during this time. The second location was 5 maway from the cultivated soil, under native vegetation along a fence-row, and is believed to have never beencultivated (‘virgin’ soil). Some chemical characteristics appear in Table 1.
Soil was taken from the top 10 cm with a spade, brought to the laboratory, and spray-wetted from theair-dry state (using an aspirator) to bring the water content to approximately 90% of the plastic limit(0.9 × �PL � 0.243 kg/kg). The soil were stored at 4°C for 48 h in this moist state to promote maximumaggregation and friability (Ojeniyi and Dexter 1979), so that when they were subsequently passed througha sieve (13 mm), they produced a relatively uniform, stable size distribution of aggregates at the start of theexperiments. Subsequent measures of the dry aggregate sizes from the soil cores confirmed their similarinitial size distributions. Because both soils had an exchangeable sodium percentage (ESP) ~ 5 (Table 1),measurements of spontaneous dispersible clay were made. Samples of the moist, sieved soils were placedin beakers, brought slowly into contact with sufficient distilled water to completely immerse them, and wereallowed to sit undisturbed for 24 h. The amount of dispersed clay was found to be <1% of the total mass ofoven-dry soil immersed, which indicated both soils had relatively low dispersibility.
Samples of the sieved soils (50 ± 0.20 g, moist) were poured into 120 stainless-steel cylindrical rings(5 cm inside diam., 5 cm high) with a thin, nylon mesh attached to one end. Initial (time zero) measurementsof dry bulk density were made on 20 of the 120 cores by measuring the average height of the soil surfacebelow the top of the cylindrical rings (5 times on each core using a depth gauge); oven-dry soil masses were
Table 1. Elemental analyses for virgin and cultivated Cornella Clay, Rochester, VicAnalyses performed by CSIRO Analytical Chemistry Unit, Adelaide, South Australia
Soil NH4 NO3 Total P Exchangeable cations at pH 7 ESPname (KCl extractable) (ICP) Ca Mg Na K Total
calculated from the soil water content. The 20 initial cores were dried (40°C) for 7 days and their aggregatesize distributions determined (see below).
The remaining 100 cores were subjected to up to 16 weekly wetting and draining (not drying) cycles asfollows. Wetting was done under a small suction by placing the cores in a tray filled to 1 cm with distilledwater for 8 h. All the (near-saturated) cores were placed on ceramic plates in sealed pressure chambers at awater potential of –100 kPa for 6 days in a controlled temperature room (20°C). After specific samples wereremoved for analyses (see below) the remaining samples were brought back to near-saturation on the platesand put back into the sealed pressure chambers for the next draining cycle. During the first 2 weeks, whenmicrobiological activity was presumed to be greatest, the pressure chambers were opened and flushed withair twice each week to replenish oxygen supplies because the head-space to soil ratio in the chambers wasonly 10:1.
Twenty cores (10 virgin, 10 cultivated) were removed from the pressure plates at 1, 2, 4, 8, and 16 weeks,and the following measurements made for both soils (summarised in Table 2). The wet mass and volume(for water content and bulk density) were measured to evaluate the extent of volume change during eachwetting/draining cycle. Two measures of the moist soil strength were collected. Firstly, the mean penetrationresistance at a depth of 10 mm below the soil surface was measured using a 1-mm-diameter recessed-shaftcone (Whiteley and Dexter 1981) advanced at 1 mm/min in 5 different locations on 2 of 10 cores. Secondlythe tensile strength (indirect tension test, Dexter 1988) was measured by sliding a soil core from its ring andmeasuring the strength between 2 flat, parallel plates at failure. Structural collapse of dry aggregates oftenoccurs during the application of irrigation water, so the rate of uptake of water by the soil cores (dried at40°C for 6 days beforehand) was measured gravimetrically over a period of 1.5 h by placing the cores onporosity #3 sintered-glass funnels at a constant water supply potential of –0.7 kPa.
The degree of coalescence was assessed visually by examining the fracture surfaces of the cores onwhich tensile strength measurements were taken. Samples were placed under a binocular light microscope(18 × magnification) with the fracture surface facing upward. The microscope was fitted with an SLRcamera, and the photographic images were examined to estimate the degree (%) of welding betweenaggregates at contact points (Cockroft and Olsson 2000). After visual examination, these samples weredried at 40°C for 7 days and their dry aggregate size distributions measured using a nest of sieves (13, 10,7, 5, 4, 2, 1, 0.5, 0.25, and 0.053 mm); data were used to calculate the meanweight diameter (van Bavel1949).
The samples used to measure penetration resistance were dried at 20°C for 2 weeks and passed througha 6-mm sieve to characterise the nature of the organic matter in the soils. For the separation of particulateorganic matter (POM, >50 �m) and micro-organic matter (MOM, <50 �m), 25 g of soil and 100 mLdeionised water were shaken with glass beads for 12 h (reciprocating shaker, 180 cycles/min). The mixturewas washed through a 50-µm sieve with deionised water. The 2 fractions (>50 �m and <50 �m) were driedat 50°C and weighed, and their total C and N contents determined as described below for the whole soil. Aportion of the 6-mm sieved soil was ground to pass a 0.5-mm sieve for the following determinations. Totalsoil C and N contents were determined by dry combustion (LECO CNS-Analyser, LECO Corp., St. Joseph,
Table 2. Measurements taken on groups of ten soil cores
Number of Properties measured Matric potential/ Details of measurementssamples on each sample moisture status
Water content Mass10 of 10 Dry bulk density –100 kPa Vertical strain
2 of 10 Shear strength –100 kPa Soil resistance to 1 mm cone penetrometerOrganic matter Air-dried, 20°C Various fractions (see text)
6 of 10 Tensile strength –100 kPa Modified Brazilian testCoalescence –100 kPa Visual examination under light microscope
(×18 magnification, photos)Aggregate size
distributionDried, 40°C Mass of aggregates on nest of sieves shaken
30 s
2 of 10 Maximum water uptake rate
Dried, 40°C Mass water taken up by dry soil as function oftime, Ψm= –0.7 kPa
MI). For total carbohydrate analysis, 1–2 g soil was first hydrolysed with 8 mL 12 M H2SO4 for 2 h at 20°C.The mixture was then diluted to a concentration 0.5 M H2SO4 and incubated at 85°C for 24 h. The extractswere centrifuged and neutralised with 0.5 M NaOH. Carbohydrate composition of the extracts wasdetermined by liquid anion-exchange chromatography with pulsed amperometric detection (Martens andFrankenberger 1991). The relative abundance of fungal and bacterial biomass was evaluated from theglucosamine and muramic acid contents (Chantigny et al. 1997). Soil lipid content was measured byextraction with a mixture of chloroform and water (Chae 1993).
Results and discussion
Visual comparisons of the tensile fracture surfaces for the virgin and cultivated soils at 1,4, and 16 weeks (Fig. 1) indicate that the cultivated aggregates coalesced significantlyduring the first wetting and draining cycle and continued to weld together with time,whereas the virgin aggregates maintained their integrity much longer. Cockroft andOlsson’s (2000) measure of the percentage coalescence under the light-microscope (Fig. 2)roughly quantifies these visual differences, which are confirmed in the different aggregate
Fig. 1. Tensile fracture surfaces of moist soil cores of virgin Cornella Clay (left side) andcultivated Cornella Clay (right side) after (a) 1 week, (b) 4 weeks, and (c) 16 weeks(magnification × 18).
Soil aggregate coalescence 569
size distributions shown as a function of time (Fig. 3). The cultivated soil developedaggregates with a meanweight diameter nearly an order of magnitude larger than those ofthe virgin soil after only 1 wetting/draining cycle.
Fig. 2. Percent coalescence measured using Cockroft and Olsson’s (2000) method on photographs oftensile fracture surfaces.
Fig. 3. Meanweight diameter (MWD) of air-dry aggregates as a function of time with weeklywetting/draining for virgin and cultivated Cornella Clay.
570 C. D. Grant et al.
The dry bulk densities of both soils were initially low and while they increased todifferent degrees during the first week, no further significant changes occurred thereafter(Table 3). The cultivated soil maintained the greater bulk density throughout theexperimental period. Importantly, tensile strength and penetrometer resistance bothincreased with time for both soils (Table 3), and the tensile strength of the cultivated soilincreased significantly faster than that of the virgin soil (Fig. 4). The relatively constantratio of penetrometer resistances shown in Fig. 4 indicates that while the cultivated soil wasalways stronger than the virgin soil; its resistance to penetration did not appear to increaseany faster with time. Penetration resistance is, of course, highly sensitive to water content,
Table 3. Dry bulk density, tensile strength, penetrometer resistance, and water content of soil cores measured at a matric potential of –100 kPa for virgin and cultivated soil (mean ± s.d.)
Time Dry bulk density Tensile strength Penetrometer Water content(weeks) (Mg/m3) (kPa) resistance (kPa) (kg/kg)
A Tensile strength of an assembly of loose aggregates assumed to be zero when poured into the rings.n.d., not determined.
Fig. 4. Cultivated:virgin ratios of soil strengths as a function of time with weekly wetting/draining (S =tensile strength; Q = penetrometer resistance).
Soil aggregate coalescence 571
which was always greater in the cultivated soil (Fig. 5); the differences in penetrometerstrength shown in Fig. 4 were thus less pronounced than they would otherwise have been atthe same water content.
Fig. 6. Coalescence, �tensile, as a function of time, using net changes in tensile strength and bulkdensity for cultivated and virgin Cornella Clay.
Fig. 5. Resistance to penetration of moist soil (matric potential –100 kPa) as a function of water contentfor virgin and cultivated soils.
572 C. D. Grant et al.
By taking into account the changes in bulk density that took place, to distinguishpacking-effects from age-hardening effects according to Eqn 1 (Figs 6 and 7), the followingtwo results were obtained: Firstly, the rate at which coalescence increased with time(measured using tensile strengths, �tensile) was significantly greater for the cultivated thanfor the virgin soil (Fig. 6); this suggests age-hardening as well as densification took placein the cultivated soil. By comparison, the slope of �virgin line was not significantly differentfrom zero, indicating that any increases in strength in the virgin soil could be largelyaccounted for by the small increases in bulk density; age-hardening was presumably unableto occur in this soil. Similarly, coalescence as measured using the penetrometer strengths,χpenetrom, showed that the cultivated soil invariably coalesced to a greater extent than didthe virgin soil (Fig. 7). The fact that the initial �penetrom value for both soils was negativeis perhaps less important than their comparative values, and a detailed interpretation of theabsolute magnitudes of the various �penetrom values is not warranted with this small data set.
An explanation for the various differences in coalescence observed above may lie in theway the dry aggregates of the different soils responded to water when applied at near-zeromatric potentials. Figure 8 shows the maximum rate at which water was taken up by the air-dry aggregates in soil cores exposed to water on a sintered-glass funnel. The cultivated soilaggregates exhibited maximal water absorption rates that were 10 times greater (and morevariable) than those for the virgin aggregates. Hydrophobic compounds, such as lipids,reduce water uptake by porous media and these were 3 times more concentrated in thevirgin soil (Table 4). Particulate organic matter and plant debris also exhibit hydrophobicproperties (e.g. McGhie and Posner 1980; Chenu et al. 2000), and these were also moreplentiful in the virgin soil. Indeed, the content of particulate organic C (>50 �m) was 10times greater in the virgin soil (Table 4), and the C/N ratio of the organic matter was alsogreater, indicating more undecomposed plant debris in the virgin soil. The lower ratios of
Fig. 7. Coalescence, �penetrom, as a function of time, using net changes in penetrometer resistance andbulk density for cultivated and virgin Cornella Clay.
Soil aggregate coalescence 573
(galactose + mannose) to (arabinose + xylose) in the virgin soil also indicate it containedrelatively more sugars of plant origin (Cheshire 1979). In addition to showing hydrophobicproperties, coarse organic debris can block pores against water uptake and act as a physicalbarrier to prevent inter-aggregate contact (Caron et al. 1996).
Organic matter also increases intra-aggregate cohesion, and this reduces deformationduring wetting. Deformation is known to be a primary factor leading to aggregatecoalescence (Bresson and Boiffin 1990). Carbohydrates in particular can increase aggregatecohesion (Chenu and Guérif 1991), and the neutral sugar analysis (Table 4) indicates that thevirgin soil had considerably more carbohydrates than the cultivated soil. Furthermore, thegreater ratio of glucosamine to muramic acid in the virgin soil indicates that fungi were moreabundant than bacteria (Amelung et al. 1999), and fungi are known to stabilise
Fig. 8. Maximum rate of water-uptake by air-dry soil aggregates for virgin and cultivated Cornella Clayover a period of 16 weeks with weekly wetting/draining.
Total neutral sugars Galact.+mann. Glucosamine Muramic acid G/M (g C kg) arabin.+xyl. (mg C kg) (mg C kg) ratio
Table 4. Characteristics of the organic matter fraction of the two soilsPOM, particulate organic matter; MOM, microbial organic matter; G/M, glucosamine/muramic acid
Soil Total C Total N C/N POM C MOM C Lipids(g C/kg) (g C/kg) ratio (g C/kg) (g C/kg) (g lipids/kg soil)
macroaggregates (Tisdall and Oades 1982), thus increasing their resistance to deformationand welding. It should be noted that the total and fractional amounts of C did not changeduring the 16-week experiment, so the data presented in Table 4 are the averages for allsamples. The greater quantities of all types of organic matter in the virgin soil thus acted inconcert to minimise coalescence in this soil according to the mechanisms described above.
Conclusions
The �-values introduced here, particularly those based on net changes in tensile strengthand dry bulk density, indicate that coalescence associated with age-hardening (above thatcaused by natural densification) occurred in the cultivated soil but not in the virgin soil.Organic matter clearly played an important role in reducing both water uptake rates andage-hardening processes. The fact that age-hardening did not get underway in the virgin soilduring this 16-week experiment supports the common observation (Cockroft and Olsson2000) that coalescence is minimal during the first growing season after virgin soils arebrought into cultivation. Once coalescence begins, however (as found in the cultivated soil),it is an increasingly progressive phenomenon, which suggests that management practicesmight best be directed toward preventing its initial occurrence.
Acknowledgments
The authors thank the Faculty of Agricultural & Natural Resource Sciences of AdelaideUniversity, for awarding DAA a Visiting Scientist Travel Grant to conduct this study inAustralia. We would also like to thank Dr B. Cockroft for visiting numerous field sites withus and arranging the collection of the soils for this study from the property of Mr Bill Smith,Rochester, Victoria. Thanks are extended to Mr Patrice Jolicoeur for his contribution to theorganic matter analyses.
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Manuscript received 28 June 1999, accepted 27 October 2000
