changes of structure and tilth mellowing in a vertisol due to wet/dry cycles in the liquid and...

European Journul of Soil Science, September 1995,46,357-368

Changes of structure and tilth mellowing in a Vertisol due to wetldfy cycles in the liquid and vapour phases

J . HUSSEIN” & M . A . ADEY Department of Agricultural & Environmental Science, University of Newcastle upon Tyne, NEI 7RU, UK

Summary

Development of a fine tilth in Vertisols increases infiltration, plant-available water and ease of cultivation and produces a fine seed bed. The tilth-mellowing properties of a strongly self-mulching Vertisol from Zimbabwe were investigated by applying different types of wetting to a worked soil and examining macromorphological features, size, density, strength and friability of the resulting cloddaggregates, developed through successive wetldry cycles. Wetting regimes were chosen to simulate likely field conditions and included rapid flood-, slow and fast capillary-, simulated rainfall- and vapour-wetting. Tilth development was compared to that of field soils.

All wetting treatments in the lfquid phase resulted in decreases in aggregate density. Fast capillary wetting rapidly reduced size and strength of aggregates to below that of field soils whereas slow capillary wetting similarly rapidly decreased size but reduced strength more slowly. Flood wetting caused little change in size but aggregates showed a small decrease in strength. Rainfall wetting resulted in changes intermediate between these extremes. There was a significant linear relationship between strength and porosity of aggregates. For rainfall- and flood-wetting, friabilities were at a maximum after one wetldry cycle but subsequently decreased.

Vapour wetldry cycles reduced strength but not density of worked soils, implying changes in internal microstructure without measurable porosity change.

Hypotheses to explain these changes are put forward.

Introduction

The form of surface structure or mulch developed in a Vertisol is of crucial importance as it strongly influences infiltration and water-holding properties, ease of cultivation, seed-bed aeration and evaporation (Robert et al., 1987). A coarse surface structure is associated with rapid surface sealing and low infiltration rates whereas Vertisols with fine crumb structures have sustained infiltration, greater water intakes and require few tillage operations to produce a good seed bed (Hussein, 1994). Vertisols which naturally develop a fine loose granular/ crumb structure through wetting and drying are termed ‘self mulching’ (de Vos & Virgo, 1969) whereas Vertisols which express this ability to a limited extent, or not at all, are termed ‘weakly’ or ‘non-self mulching’ soils, respectively. The ability of a soil to ‘self mulch’ is therefore very desirable but, as yet, is not fully understood (Pillai-McGarry & Collis-George, 1990a). It depends not only on the properties of the soil itself (Grant & Blackmore, 1991; Wenke & Grant, 1994), but also on

Received 28 September 1994; revised version accepted 5 May 1995 *Present address: Department of Soil Science & Agricultural Engineering, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe.

the superimposed processes that affect the ability of a soil to self mulch, such as cultivation methods and wetting and drying cycles.

Tillage of Vertisols can be difficult in both wet and dry conditions. In dry conditions there is excessively high tractor tyre wear and high fuel consumption as well as considerable wheel slip. Large draught power is required because the soil has a very hard consistence. In wet conditions, the sticky clay adheres to tillage tools and tractors may be immobilized due to sinkage. The optimum moisture range for tillage is very narrow (Jewitt et al., 1979), occurring at or just below the plastic limit, thus decreasing the number of work days. Timeliness of tillage is therefore critical and self mulching clearly decreases the number of tillage operations required during such available work days. Tilth development is especially important when considering the plight of small- scale farmers who do not have access to tractors and who must therefore rely on hand cultivation or ox-drawn ploughs to prepare their land. Vertisols which self mulch or mellow under wet/dry cycles facilitate early land preparation which, in turn, allows farmers to plant their crops early in the rainy season and extend the growing season (Nyamudeza & Mandiringana, 1992).

0 1995 Blackwell Science Ltd. 357

358 J. Hussein & M. A. Adey

A strongly 'self-mulching' Chisumbanje Vertisol occurs extensively in the semi-arid south-east of Zimbabwe and is derived from basalt (Hussein, 1994). It is cultivated under irrigation and dryland farming (Hussein et al., 1992) and is the site of current IBSRAM trials (Nyamudeza & Mandirin- gana, 1992). Some 40000 ha (out of an area of 600000 ha) have been surveyed and identified as suitable for irrigation development (Atkins Land & Water Management, 1983). In the light of this it was felt desirable to examine the tilth- forming properties of this soil and, in particular, to investigate the mechanistic basis of wet/dry cycles for tilth mellowing.

Wetting and drying have a marked effect on soil structure at both microscopic ('l'essier et al., 1990) and macroscopic (Shiel et al., 1988) levels. Wetting causes changes due to expansion of the electrical double layer, differential swelling, air entrapment and compression and heat of wetting, as well as the mechanical action of raindrop impact. On drying, the particledaggregates reorientate with changing surface tension forces, dispersed material settles out of suspension and cementation may occur as dispersed material, salts or organic materials are laid down between coarser particles. Changes due to wetting are not entirely reversed on drying and this hysteresis leads to progressive development of structural units through sequential wet/dry cycles. This phenomenon is seen to varying degrees in most soils but is strongly expressed in Vertisols due to their large active clay content. The effect of wetting depends on the potential of the wetting solution (Gusli et al., 1994), the rate of wetting (Chan & Mullins, 1994), which will depend on both the potential of the wetting solution and the soil hydraulic conductivity as suggested by Tessier et al. (1990), initial and final water potentials (McKenzie & Dexter, 1985), mechanical energy input (as in rainfall) and chemical composition of the wetting agent (Mullins et al., 1990). Rates and types of drying may affect structure although Dexter et al. (1984~) found no significant differences in strengths of soils which were air- compared to oven-dried. The wetting processes that affect the dynamics of self mulching in the Chisumbanje Vertisol were studied through laboratory experiments using wetting regimes chosen to simulate likely field conditions from simulated rainfall- to capillary wetting.

Materials and methods Field sites, climate and agriculture

Chisumbanje (20" 48' S, 32" 14' E) lies in a semi-arid region of Zimbabwe with a mean annual rainfall of 580 mm (coefficient of variation 35%), most of which falls in the summer season from December to March (Hussein et al., 1992). Mean annual temperature is 22"C, with a mean daily maximum of 32°C during the rainy season (Department of Meteorological Services, 1978). Crops grown under irrigation on the Chisum- banje Estate include wheat (in winter) and cotton in summer.

Dryland subsistence farming by small-scale farmers occurs in surrounding areas. They grow cotton, maize, sunflowers and sorghum during summer, although in many years crops fail (Nyamudeza & Mandiringana, 1992) due to low or erratic rainfall.

Soils

The Chisumbanje soils are formed on Jurassic basalt and are located on a flat plain. They have a characteristic well- developed loose crumb structure up to 150 mm deep. The soil is classified as a very fine montmorillonitic, hyperthermic, typic pellustert in Soil Taxonomy (Soil Survey Staff, 1975) and as a Chisumbanje 3B.2 under the Zimbabwean soil classifica- tion system (Thompson & Purves, 1978). Bulk samples (75 kg) of air-dry surface soil (0-150 mm) were collected from the dryland farming site at Chisumbanje Experiment Station. This site had been cultivated for eight years under a rotation of maize, sorghum and cotton and had been ridged prior to sampling.

Assessment of field sur3pace structure and properties

After air drying, c . 60 kg of the bulk sample was sieved into the following fractions using 600- and 200-mm diameter sieves: >37.5, 26.5-37.5, 19.0-26.5, 13.2-19.0, 8.0-13.2, 4.0-8.0, 2.0-4.0, 1.0-2.QO.5-1.0, 0.21-0.5, ~ 0 . 2 1 mm, and weighed. Sieving invohed gentle rolling of small batches of soil around the sieve in order to minimize fracture of aggregates. These fractions were almost entirely aggregates (10 g kg-' of particles were > 2 mm, almost entirely 2-4 mm). From these data, arithmetic mean size grades (AMSG) were calculated as in Shiel et al. (1988), and geometric mean diameters (GMD) and log standard deviations (LSD) were calculated as in Gardner (1956). Aspect ratios (defined as 1 : dz/dl: d3/dl, where dl , d2, d3 are the largest, intermediate and smallest axes, respectively (Braunack et al., 1979)) of eight replicates from each of the different sizes > 8 mm were determined using digital callipers. Bulk densities of replicate aggregates in each size group >13.2 mm were determined using the wax-coating technique (Blake, 1965).

Tensile strength measurements of > 8-mm aggregates were determined using an unconfindd compression-testing apparatus as described in Dexter & Kroesbergen (1985) and modified by Hadas (1990). Briefly, the air-dry aggregates were placed over silica gel for 5 d and dried to moisture contents of c. 0.1 g g-' . Aggregates were then crushed between two parallel plates, with their longest axis perpendicular to the plates, and the load at failure determined. Failure was evident as either a sudden decrease in the crushing load or development of a crack through the aggregate along the longest axis, d. The tensile strength, S, of the aggregate was calculated from:

S = 0.711F/?rd2

where F is the load at failure.

0 1995 Blackwell Science Ltd, European Journal of Soil Science, 46,357-368

Structural change and tilth mellowing in a Vertisol 359

Chemical and textural properties were determined by conventional procedures as in Hussein et al. (1992).

Wet/dry cycles

The remainder of the field (unsieved) sample of the soil was wet to a moisture content about midway between the liquid and plastic limits (c. 0.70 g g-’), left to equilibrate overnight, then thoroughly worked on a glass plate with a spatula to produce a massive structure. This, therefore, represented the worst possible structural condition in which the soil could exist. Replicate portions (400 g) of worked soil were placed in aluminium retaining rings (150 mm in diameter, 50 mm high) having a taut porous nylon mesh base and shaped into flat discs (150 mm in diameter by 15 mm high). The soils were allowed to air dry to constant mass (7-12 d) in the laboratory (temperatures of 20-26°C). At the end of this drying cycle, samples consisted of a dense massive disc which had shrunk away from the sides of the container. These samples were then wetted by various methods.

Rapid flood wetting was achieved by immersion of the ring plus soil in a tray of distilled water, so that the soil surface was covered by 2 to 3 mm of water. Thus the soils were wetted from above, below (through the mesh) and around the sides. Additional samples were included in the programme in order that they could be progressively removed during the wetting process, surplus water removed, and their water content determined by weighing and drying. This allowed a rough estimate of wetting rate. The soils remained immersed in water for 24 h (sufficient to reach equilibrium as judged by weighing the wet soil) after which the water was drained from the tray, the samples weighed, and left to air dry in the laboratory. This sequence was repeated a further three times.

The series of experiments extended over many months during which time relative humidities varied from 48 to 75% (tensions of 40 MPa to 100 MPa). Similar experiments were conducted in both Zimbabwe and Newcastle upon Tyne with similar ultimate results. It is therefore considered that the effect of such differences in air-dry potential were negligible compared with the different wetting treatments.

At the end of each drying stage, three replicates were removed for analysis. The @/wet cycles are hereafter numbered and subscripted as 0, 1, 2, 3 and 4, with 0 being the first stage of drying after working (i.e. 0 wetting).

Capillary and simulated rainfall wetting were also investigated using the same procedure as above and substituting either a ‘fast’ or ‘slow’ capillary wetting (wetted from below, under tension) or simulated rainfall wetting instead of the flood wetting. For the fast capillary wetting, the ring and sample was placed on a porous kitchen fabric draped over a support with the ends immersed in distilled water in a tray. Slow capillary wetting was accomplished using a kaolin bath. The soils were wet using either technique to a tension of 70 mm water (0.7 kPa) for 24 h after which time equilibrium, as judged by

the mass of wet soil, had been reached. This tension was chosen as it represented roughly half the vertical height from a wetted irrigation furrow to the top of a typical ridge in an irrigated field.

For the rainfall wetting, a rainfall simulator was used (Metelerkamp & Boundford, 1970), as modified by Verboom (1991). Drops (distilled water), 3.1 mm in diameter, were produced from a reservoir having 1 .O-mm internal-diameter needles passing through the base. They struck the sample at 95% of their terminal velocity with an intensity of 3 2 f 3 mm h-’. Rainfall was applied until the samples showed 1-3 mm of ponding (c. 40 min) after which they were removed from the simulator and left to air dry.

At the end of each drying cycle various measurements were made. The air-dry mass was measured and the surface morphology described (size and shape of cloddaggregates and cracks, surface features such as sand wash and microstructure). Clearly, the units resulting from the initial drying were very much products of working the soil, while after subsequent wet/dry cycles additional structure-forming processes will have occurred. The terminology of Dexter (1988) is therefore used in which the term ‘clod‘ is used to describe a dense unit formed after a single drying of the worked soil and ‘aggregate’ to describe a unit formed after subsequent cycles. Aggregate-size distribution was measured using a nest of upright 200-mm diameter sieves (8, 4, 2, 1, 0.5, 0.21, 0.105, 0.053 mm) with gentle hand shaking as before for 1 min as for the field sample. Units larger than 8 mm were separated by hand into >16 and 8-16 mm sizes. Arithmetic mean size grades (AMSG) were calculated. Bulk densities of clods/ aggregates larger than c . 20 mm were measured using the wax- coating technique (Blake, 1965). Tensile-strength measurements on cloddaggregates larger than 8 mm were determined as above.

Vapour wet/dry cycles

Worked soil, as previously described, was formed into spheres (c. 10 g, diameter c. 18 mm). These were left to age harden (Dexter, 1988), in order to allow strength changes to occur uniformly in all samples, at 0.7 g g-’ in covered plastic trays for 8 d. They were then placed on a prewetted ceramic pressure plate and equilibrated at a tension of 1500 kPa in a pressure chamber for 8 d. The balls were removed and dried to a tension of 153 MPa over saturated MgC12 (relative humidity, 33%). This moisture tension was termed Do. Subsequently, samples were re-equilibrated to a tension of 3.8 MPa over a saturated solution of KzS04 (relative humidity, 97%). This was termed W1. Samples were subsequently cycled between these endpoints (D1, W2, DZ, W3, D3) for a total of three cycles. Equilibration generally took 8-10 weeks and was confirmed by weighing. The choice of tensions to delineate the start and end of each cycle was guided by the availability of appropriate salts and by the field variations of relative humidities at



Table 1. Salient textural and chemical data for Chisumbanje Vertisol (0-150 mm).

Plastic Liquid

/g kg-' Immol, kg-' CaCI2 ESP - /g g-' -

Coarse sand Medium sand Fine sand Silt Clay PH (0.5-2.0 mm) (0.2-0.5 mm) (0.02-0.2 mm) (0.002-0.02 mm) (<0.002 mm) CEC 1 : 5 limit limit

30 10 160 110 690 736 7.5 0.27 0.45 1.01

Chisumbanje which vary from a mean monthly maximum of 95% at 06.00 h in March to a mean monthly minimum of 29% at 14.00 h in September (Department of Meteorological Services, 1978). The desiccator was left in the laboratory at a temperature of 19-23°C and weighed at the same time each day to minimize errors due to any slight temperature fluctua- tions.

Five similar balls were made, dried to 1500 kPa, and tied around their diameter with cotton and then coated with Saran resin by immersing three times in solution (1 part Saran to 7 parts methyl ethyl ketone) (Blake & Hartge, 1986). These were used to measure volume and bulk density at each stage of the wet/dry cycles, being placed together with the uncoated balls in the desiccator. After equilibration at each endpoint, the Saran-coated balls were weighed and their volumes determined by suspension in water.

At the end of each drying (D) stage (over MgClz), eight uncoated balls were removed and their tensile strengths were measured using unconfined compression apparatus, as above. At the end of the last (third) cycle of drying, the volumes of the

Table 2. Aggregate size distribution of Chisumbanje field sample.

Size range Proportion of aggregates /mm /g kg-'

>37.5 26.5-37.5 19.0-26.5 13.2-19.0 8.0-13.2 4.0-8.0 2.0-4.0 1.0-2.0 0.5-1 .O

0.21-0.5 <0.21

49 39 45 61 60

120 133 175 149 153 16

Saran balls were measured and their moisture contents determined by oven drying.

Results and discussion

Field structure and properties

The soil has a large clay content and a correspondingly large cation-exchange capacity (CEC) and liquid limit (Table 1). About 70% of the clay fraction is smectite with some random smectitehllite interstratification. The sand-sized fraction is dominated by fine sand, probably aeolian in origin (Low et al., 1984). The soil is base saturated and the pH is in the alkaline range, although exchangeable sodium percentage (ESP) is low.

The dry aggregate-size distribution (Table 2) is skewed to the fine end with a predominance of aggregates between 0.21- 8.0 mm. The GMD value is 2.5 mm with a LSD of 0.16 mm though strictly the log diameter was not normally distributed. This equates to an AMSG of 6.0 mm. The GMD is similar to values found by Yule et al. (1976) for those finely structured Australian cracking clay soils which exhibited better seedling emergence.

The bulk densities of > 13.2-mm aggregates are about 1500 kg mP3 and do not vary with size (Table 3). Aspect ratios are similar for all >8-mm aggregates, with a mean of 1 : 0.79 : 0.63. These are similar to those reported by Braunack et al. (1979) and may be described by the general form 1 : x : x 2 . Such ratios place aggregates in the equant class (Bullock et al., 1985), thus confirming their blocky or spheroidal structure.

Tensile strength and friability of field soil

Tensile strengths of the different sizes of aggregates (Table 3) indicate a progressive increase in strength with decreasing size.

Table 3. Densities, aspect ratios and tensile strengths of the field soil. (Standard errors of the mean are in brackets).

Size of aggregate Densitya Tensile strength /mm lkg m-3 Aspect ratios lWa

>37.5 1458 (29) 1 : 0.72 : 0.61 (0.03 : 0.03) 51 (8) 26.5-37.5 1518 (51) 1 : 0.80 : 0.62 (0.03 : 0.02) 85 (19) 19.0-26.5 1523 (18) 1 : 0.85 : 0.66 (0.02 : 0.03) 95 (16) 13.2-19.0 1505 (81) 1 : 0.78 : 0.62 (0.03 : 0.03) 147 (23) 8.0-13.2 ndb 1 : 0.78 : 0.65 (0.03 : 0.03) 309 (84)

aAverage particle density = 2804 kg m-3. bnd =not determined as aggregates too small.



v) c -

I I I 44 mm Aggregate

8 12 mm 20 mm

Fig. 1. Relationship between In tensile strength (S/Wa) and In aggregate volume 1 (V/m3) for field sample. Linear regression -1 5 -1 4 -13 -1 2 -1 1 -1 0 -9 -8

line: In S=O.241-0.358 In V, r= -0.70***. In V

The strength values are in a similar range to those found by Chan ( 1 9 8 9 ~ ) for self-mulching clays in Australia.

Although ‘friability’ is frequently used in a qualitative sense to describe the extent to which a soil demonstrates ‘friable’ properties, attempts have been made to define this more quantitatively. In this context, friability ( k ) can be defined as the tendency of an unconfined soil to break down and crumble under stress into smaller fragments and may be represented by the variation of strength of aggregates with size (Utomo & Dexter, 1981). Friability may be estimated from the negative gradient of the relationship between the natural logarithms of strength versus volume, In S versus In V, for a range of aggregate sizes (Fig. 1 ) . The Chisumbanje soil falls into the ‘very friable’ range of Utomo & Dexter (1981) (0.25 < k < 0.40). This value is higher than those of Braunack et al. (1979) for self-mulching Australian Vertisols but lower than those of Chan ( 1 9 8 9 ~ ) .

Wet/dry cycles

Moisture contents and wetting rates. After air drying, moisture contents varied from about 0.1 1 to 0.16 g g-’, depending on the sample and relative humidity of the atmosphere. The moisture contents after wetting varied according to wetting method (Table 4) and the number of wetting cycles, tending to increase as the soil became more fragmented.

Although the capillary-wetted soils were both equilibrated at 70 mm tension, the fast-wet soils took up more water than the slow-wet soils. This is due to the changes induced at the macro-structural level by differential swelling during rapid

wetting. Thorburn et al. (1989) similarly found that, when wetting Australian Vertisols under a range of tensions <90 kPa, the fast-wetted soils held more water due to fluid retained in the more numerous planar voids created between small water-stable aggregates. They found that fast wetting increased water uptake over a wide range of tensions (to 38 MPa in one soil) and suggested that at large tensions (> 1500 Wa) extra water uptake in fast-wetted soils was more likely to be due to micro-structural changes in clay quasi- crystals. Coughlan (1984) also reported that, at 20 mm tension, fast-wetted aggregates of a Wac0 Vertisol retained more water (0.96 g g-’) than slow-wetted aggregates (0.70 g g-’).

Initial wetting rates, for each wetting regime during the first 5 min, were 3.33 g kg-‘ of oven-dry soil s-l for the flood-wet and 1.05, 0.38 and 0.12 g kg-’ s-l for the rainfall-, the fast capillary- and the slow capillary-wet soils, respectively. These rates are only approximate, as it was difficult to weigh the soils accurately during wetting, particularly in the case of the flood- and rainfall-wet soils where there was free water held in the retaining ring during uptake.

Table 4. Range of moisture contents after wetting.

Tension Moisture content range Type of wetting /kPa /g g-’ Flood 0.0 c. 1.02-1.11 Rainfall 0.0 C . 0.91 -1.02 Fast capillary 0.7 C. 0.60-0.70 Slow capillary 0.7 C . 0.50-0.60

0 1995 Blackwell Science Ltd, European Journal of Soil Science, 46, 357-368


Macromorphological features. The initial worked soil usually dried to one dense (c. 1800 kg m-3) disc of c. 120-125 mm diameter and 11-14 mm thickness. Many discs remained intact although some split across the centre as found by Pillai- McGarry & Collis-George ( 1 9 9 0 ~ ) in their work on Australian Vertisols. During successive wetldry cycles, discs progressively broke down to finer aggregates.

Repeated flood wetting resulted in little change in aggregate size with most remaining >50 mm in diameter. Aggregate shapes were usually rounded at the outside edge due to contact with the container. Internal fracture surfaces were initially angular and smooth but became progressively more subangular and rough with a microcrumb appearance. Capillary wetting caused a rapid decrease in aggregate size. The fast-wetted soils had many rough fracture edges with very small and discontinuous microcracks in random directions. In contrast, the slowly-wetted aggregates were very angular with clean fractured faces and polished surfaces and showed fewer, but larger, planar microcracks than the fast-wetted aggregates. The cracks were usually parallel to or perpendicular to the direction of wetting. Simulated rainfall produced similar results to flood wetting except that the aggregates were more porous with a microcrumb structure at the surfaces, similar to that of Pillai- McGarry & Collis-George (1990a, b) for their spray-wet self- mulching Vertisol.

Aggregate size distribution as affected by wet/dry cycles. Figure 2 shows the progressive reduction in AMSG with increasing numbers of wetldry cycles. In practice, the actual initial AMSG of the worked soil approached 120 mm (i.e. the size of the container in which the sample was prepared), although Fig. 2 shows 16 mm as this was the largest sieve used.

I I I

0 1 2 3 4

Number of wet/dry cycles

Fig. 2. Variation of aggregate-size distribution with number of wet/ dry cycles using different methods of wetting; 0 flood, 4 capillary fast, Capillary slow, 0 rainfall, in comparison to field sample. Barred lines indicate the pooled standard errors of the means.

Table 5. Densities of air dry cloddaggregates 20-60 mm (kg C3) obtained after wet/dry cycles using different wetting methods. (Standard errors of the mean are in brackets).

No. of weddry Fast Slow cycles Flood capillary capillary Rainfall

0 1850 (20) 1750 (10) 1750 (10) 1870 (30) 1 1530 (20) 1600 (40) 1650 (60) 1610 (30) 2 1350 (30) nd“ nd 1460 (30) 4 1340 (20) nd nd 1270 (40)

“nd = not determined as aggregates too small.

Flood wetting resulted in marginal decreases in AMSG, rainfall wetting in a slow decrease and capillary wetting in large decreases. Rainfall wetting produced an AMSG of 14.3 mm after four cycles. Both types of capillary wetting resulted in a similar pattern with an initial rapid decrease in AMSG that moderated after two cycles. The final AMSGs were 4.1 and 4.6 mm for the fast- and slow-wetted treatments, respectively. These values compare fairly closely with the field AMSG of 6.0 mm and with Russell’s (1973, p. 506) suggestion of 1-5- mm aggregates producing the best seedbed. Flood wetting therefore showed the least change, indicating that rapid wetting of this nature does not easily regenerate a fine tilth. Coughlan (1984) reports a similar ranking of AMSG values for clay aggregates wet by immersion, spray and capillary wetting (7.4, 3.3 and 2.3 mm respectively).

Bulk densities. Although there are significant differences in the initial bulk densities of the capillary- versus flood- or rainfall-wetted samples (Table S), we do not consider that these are likely to have had an effect on behaviour during subsequent wetldry cycles. Initial tensile strengths (cycle 0) are not ranked according to bulk density and, after cycle 1, bulk densities are ranked in a different order.

The initially large densities (Table 5 ) rapidly decreased in all soils with the greatest decrease (32%) being shown in the rainfall-wetted soils after four cycles. The densities approached those of field aggregates (Table 3) between the first and second cycles of the flood and rainfall wetting but subsequently continued to decrease. Hadas (1990) reported small (but statistically non-significant) changes in the density of natural aggregates of an Israeli clay soil after 132 mm of natural rainfall and subsequent air drying.

Tensile strength variations with wet/dry cycles. The tensile strengths of those aggregates remaining in the 8- 16-mm size range after each drying cycle are reported in Fig. 3. As expected, the initial worked soils had very large tensile strengths (640-847 @a). Strengths reduced considerably as the soils went through wetldry cycles, although the reductions varied with wetting method. Rapid capillary wetting produced a 90% reduction in tensile strength (down to 77 kPa) after only



I

0 1 2 3 4

Number of wetldry cycles

Fig. 3. Relationship between the tensile strengths of 8-16-mm aggregatesklods and the number of successive wetldry cycles using different methods of wetting. Legend as in Fig. 2. Barred lines indicate & pooled standard errors of the mean.

one cycle. Rainfall wetting also caused a considerable reduction in strength but the changes were slower than the fast capillary-wet and required four cycles to achieve a 92% reduction, down to 49 kPa. In contrast, the slow capillary-wet sample showed a much smaller, and more gradual, decrease in strength to 307 kPa after four cycles, with the flood wetting being intermediate and reducing to 178 kPa after four cycles.

Tensile strengths for air-dry (tension c. 153 MPa) natural VertisoVclay aggregates from various sources (Braunack et al., 1979; Chan, 1989,) range between 181 and 493 kPa. The values obtained for the Chisumbanje soil are usually within this range after the first cycle. From the second to the fourth cycle the values fall below this range for the fast capillary-wet and rainfall-wet soils.

The strengths of the aggregates rapidly reached an asymptote after one to two cycles (Fig. 3), in contrast to the aggregate size which continued to decrease through the four cycles (Fig. 2). The macrostructure therefore changed slowly whilst the microstructure changed rapidly, indicating a different hierarchical order of structural change.

However, the degree and rate of wetting, and the soil type, will affect the approach to equilibrium in strength measurements. Dexter et al. (1984b) noted linear decreases in strength of two worked loam soils taken through four wet/ dry cycles (slow capillary wetting to 300 mm tension) but suggested that the curves could level off in an exponential fashion as the number of cycles increased. This was supported by later work (McKenzie & Dexter, 1985) which demonstrated that mellowing decreased exponentially for a Wiesenboden soil, to stabilize after five to seven cycles at about 65% of the strength of the original worked soil. In all

800

600

400

200

0 0.30 0.35 0.40 0.45 0.50 0.55 0 60

Porosity, e/m3m-3

Fig. 4. Tensile strength versus porosity of soils from Chisumbanje and different literature sources: Chisumbanje laboratory 0 (remoulded & wet/dry cycles, 20-50 mm) and field (natural aggregates > 8 mm); Dexter et al. (1984a,b) polder soils 0 (oven dry, natural aggregates, 8-9.6 mm) and + (oven dry, worked soil); Chan (19896) hardsetting Australian soils A (dried { 100 MPa], natural aggregates, 6.5-9.5 mm); Hadas (1990) Israeli sandy loam + and clay x (air dry, natural aggregates > 2 mm). (-) Regression line for Chisumbanje data only, see text for details. For calculation of porosities when not stated in the literature an average particle density of 2650 kg m-3 was assumed.

cases the strength of the Chisumbanje soil stabilized after two cycles, at relatively lower values than reported by Dexter and colleagues. By contrast, the strength of field aggregates of a silty red-brown earth are reported to progressively decrease through, perhaps, in excess of 18 wetting events (Kay & Dexter, 1992).

Clearly, the density of clods or aggregates decreases with successive wet/dry cycles (Table 5). The decrease in c lod aggregate strength probably reflects a combination of the effect of increases in the size of intra-aggregate pores and the total number of pores. Both result in increases in clodaggregate porosity. Figure 4 illustrates the relationship between clod or aggregate strength and porosity for a number of soils. Except for the hardsetting soils of Chan (1989b), which were consistently weaker than the other soils, all follow a similar pattern notwithstanding their range of textures, moisture contents and some being ‘remoulded’ as opposed to ‘natural’ aggregates. The Chisumbanje data can be described by the regression equation:

S = 1709 - 3255~ , 3 = 0.685***

where S is the tensile strength in kPa and E is the clod aggregate porosity in m3 m-3.



Table 6. Parameters from linear correlation equations for In strength (In S) versus In volume (In V ) plots of tensile-strength measurements: constants, friabilities and correlation coefficients (r) .

History of soil Constant Friability (k ) P

Field 0.241 0.36 -0.70***

Worked, Cycle 0 1.871 0.33 -0.30* Flood, Cycle 1 - 6,284 0.86 -0.70** Flood, Cycle 4 3.882 0.08 0.08 ns

Capillary fast, Cycle 1 -3.610 0.56 -0.47 ns Capillary fast, Cycle 4 Aggregates too small

Capillary slow, Cycle 1 -0.871 0.5 1 -0.45 ns Capillary slow, Cycle 4

Rainfall, Cycle 4 - 1.209 0.36 -0.38 ns

Aggregates too small

Rainfall, Cycle 1 -0.399 0.41 -0.58*

%=not significant; *; **; ***=significant at the 5%. 1% and 0.1% probability levels respectively.

The complete data set can be described by the regression equation:

S = 1974 - 3920~ , 3 = 0.364***

which improves if the proportion of clay (c/g kg-’) in each soil is included:

S = 1993 - 4391e +0.504c, r2 = 0.511***.

Friability as affected by wet/dry cycles. Strength-volume relationships were plotted for the different cycles and linear regression equations of the form In S =constant - k In V (Table 6 ) were calculated. The results for the worked soils (cycle 0) were combined from the four wetting experiments (flood, capillary fast and slow, and rainfall) to give one equation for all the worked samples. For the tensile-strength measurements, small clods of appropriate size were broken by hand from the disc and presumed to have separated at natural surfaces of weakness. For flood- and rainfall-wetted soils after one cycle, some pieces were also broken by hand from the large aggregates. Grant et al. (1990) found no differences between the fracture surfaces resulting from the fracture of soil clods on crushing by hand or between two parallel plates. We therefore considered that the method employed here to obtain clods for tensile-strength measurements resulted in the separa- tion of soil fragments at natural incipient fracture surfaces.

The friability of worked soil, cycle 0, was 0.33 indicating that it was not homogeneous. This may have been due to inhomogeneities remaining even after working the soil (Dexter et al. (1984b) had to work soil at a water content 30% above the liquid limit to attain a friability of zero) and/or inhomogeneities developing during the drying phase. Inevitably, drying was largely one dimensional and from the upper surface of the sample.

The statistical significance of the relationship between In S and In V decreased with number of wetldry cycles. This may

have reflected an increasing variability between samples but might also be a consequence of the progressively fewer available samples on which these measurements could be made. It would seem that friabilities decreased with number of wetldry cycles with a marked decrease for the flood-wetted soil. Most of the soils would be classed as very friable (0.25 < kc0.40) or mechanically unstable (k20.40) (Utomo & Dexter, 1981). In the fast and slow capillary-wetted soils (cycle 4) the aggregates had broken down to such small sizes that it was not possible to get a good spread of In V values for calculating k.

Figure 5a compares the tensile strengths after one cycle using different wetting regimes. The fast capillary wetting dramatically decreased strength relative to slow capillary wetting, although the k were similar (0.56 and 0.51, respectively). The differences in capillary-wetting rate have probably given rise to differences in internal microstructure of the aggregates.

12mm 16mm 22 mm 30mm 44mm 61 mm aggregate I I I I , I diameter

-14 -13 -12 -11 -10 -9

In V

Cycle 0

-- -+

‘1 cy<<-y-->; 3 22 mm aggregate

diameter -14 -1 3 -1 2

In v Fig. 5. (a) Linear regression lines for In tensile strength (S) versus In aggregate volume ( V ) after one wetldry cycle using different wetting regimes and for field soil. S is expressed in kPa and V in m3. The negative of the gradients of the lines represent the friability (k) values. (b) Linear regression lines for In tensile strength (S) variation with In aggregate volume (V) for different numbers of rainfall wetldry cycles in comparison to the field soil. S and Vunits as above. The negative of the gradients of the lines represent the friability (k) values.



The rainfall- and flood-wetted soils were intermediate in strength. The rainfall-wetted soil approached that of the natural field aggregates (Fig. 5b) after one cycle, whereas the strengths of soils which had undergone four cycles were much smaller. By contrast, Hadas (1990) found that, when an Israeli clay soil received 132 mm of rainfall, there was an increase in strength of 10-mm aggregates from c. 350 kPa to c. 450 kPa. The highest k-values (Table 6) were obtained after the first flood cycle. This agrees with Utomo & Dexter (1981) who found that a worked Umbrae loam soil developed the highest k after one cycle and thereafter k decreased slightly with successive cycles. However, although k was greater after one cycle, the characteristic strengths of 10-mm aggregates were still three times those of field aggregates of the same diameter. Clearly, other forces must be acting in the field in conjunction with wet/ dry cycles to determine strength characteristics (Kay & Dexter, 1992) and, although wetting and drying cycles act to decrease strength in unconfined soils, other forces such as cementation, overburden and compaction pressures may act in an opposing manner.

It becomes apparent from these data that not only the gradient of the In S : In V line is needed to characterize the strength variations of soils but also some measure of the intercept is required to compare absolute strengths for any given aggregate size. This may be obtained from the values of the constants in the regression equations (Table 6) which are the extrapolated strengths of 1-m3 volumes of soil.

Wet/dry cycles in the vapourphase. Surface soils are likely to undergo wet/dry cycles in the vapour phase and these may be important in the semi-arid tropics where there are large diurnal and seasonal changes in temperature and relative humidity but

750

650

m n $ 550 5-

E

2 350

450 - In 0 - .-

F

250

150 I I 4

Do WI DI W2 ‘32 W3 ‘A Number of vapour wetldry cycles

0.49

0.47

0.45

0.43 6 E

0.41

0.39 g 0.37

0

. c ._ u)

n

0.35

0.33

Fig. 6. Tensile strength (0) and porosity (+) changes through successive wet/dry cycles. D refers to the end of the dry cycle (tension 153 MPa), W refers to the end of the wet cycle (tension 3.8 MPa). Tensile strength measured only at 153 MPa. Barred lines indicate

standard errors of the means; these were negligible for porosity at D, and D3.

Table 7. Changes in soil-water tension, moisture content and volume through vapour wetldry cycles (standard errors of the mean).

Stage in wet/dry Soil tension Moisture content Volume cycle IMPa /g g-‘ /cm3

Initial worked soil nda 0.722 (nd) nd 153 0.154 (0.012) 3.42 (0.04) DO

W1 3.8 0.252 (0.004) 3.87 (0.04) D1 153 0.125 (0.001) 3.28 (0.05)

w2 3.8 0.253 (0.008) 3.81 (0.01) D2 153 0.134 (0,001) 3.33 (0.01)

w3 3.8 0.231 (0.004) 3.74 (0.02) D3 153 0.126 (0.001) 3.30 (0.02)

Oven dry 0.0 (nd) 3.20 (0.02)

‘Not determined.

little rainfall. In the vapour experiment, moisture contents v d e d between c. 0.25 and 0.13 g g-’ from the ‘wet’ to ‘dry’ endpoints of the cycle (Table 7) while ball volumes varied cyclically from 3.8 to 3.3 cm3. Sphere porosities varied cyclically (c. 0.35 to c. 0.45 m3 m-3) with a progressive decrease in porosities at the ‘wet’ end (tension 3.8 MPa) of a cycle but no trend apparent at the ‘dry’ end (153 MPa). Tensile strength (Fig. 6) measured at Do (610 kPa) for the worked spheres was similar to that of the worked soils in Fig. 3 (640- 780 kPa). The strengths of the spheres subsequently taken through successive vapour cycles (D1-D3) decreased, perhaps to a plateau at DZ, and were slightly higher than the slow capillary-wet soils (cycles 1-3), although the capillary-wet samples started from a higher initial worked strength of 780 kPa. This indicated a progressive change in internal microstructure through rearrangement of micropores with no net measurable change in total porosity. Either the increase in porosity was small (< 0.005 m3 mP3) or the arrangement of pores differed such that the strength of the soil after vapour wetting and drying was reduced.

Clearly, mellowing has occurred during this sequence, i.e. the strength of the soil matrix has been reduced on wetting from 153 to 3.8 MPa tension. This has important implications in the field as such soils that go through a fallow period are likely to mellow due to vapour wet/dry cycles even in the absence of rainfall, thus facilitating subsequent seedbed preparation. In these experiments, one complete cycle took 6 months to achieve, which is similar to the time scale for soils left to mellow through the dry winter season (April- September), although diurnal cycles would be superimposed on top of this.

Hypotheses to explain structural changes induced by wetting As a clod rapidly wets, air entrapment, differential swelling, etc., generate stresses which may cause the aggregate to



Table 8. Size distribution of fractions after various wetting treatments. Size fractionlpm

Wetting < 2 2-20 20-50 50-210 210-1000 >lo00 Sample treatment Ig kg-’

Field aggregates, 4-8 mm flood 40 90 50 620 130 70

slow capillary 40 0 0 160 400 400

Worked soil flood 50 160 100 270 380 40 fast capillary 0 60 20 110 180 630 slow capillary 20 20 10 60 90 800

fast capillary 40 60 70 240 490 100

disintegrate. Stresses are likely to be greater in flood wetting than in capillary wetting not only due to the greater differential swelling/air entrapment and consequent slaking (Chan & Mullins, 1994) but also due to the lower electrolyte concentration produced, which in turn leads to greater swelling and potential for dispersion (Kay & Dexter, 1990).

The observed decreases in strength result from ‘mellowing’ of the soil, defined by McKenzie & Dexter (1985) as partial slaking (through wetting) such that compacted soil weakens but does not fragment completely. Mellowing was shown by Dexter el af. (19846) to commence on wetting from oven dry to a ‘critical mellowing’ tension of c . 250 mm (2.5 kPa) for polder soils and to increase with decreasing soil moisture tension until a point was reached when the soil completely slaked. Kemper et al. (1975) reported that strengths (as measured by modulus of rupture) of aggregates from two loam soils greatly increased when immersion- compared to capillary-wetted. Indeed, strength increased on further wetting by immersion of samples previously capillary wetted. They postulated that the absence of an air-water interface and surface-tension forces on immersion wetting allowed considerable disintegration and subsequent melding together of aggregates on drying. Under conditions of rapid capillary wetting, these forces did not allow such disintegration of soil structure on wetting and strengths of subsequent dry aggregates were smaller. In contrast, McKenzie & Dexter (1985) found no change in degree of mellowing of a Vertisol when wetted from 2000 MPa to various tensions from 0- 18 kPa.

Although the critical mellowing tension is therefore the minimum tension (highest potential) to which the soil must be wetted in order to result in mellowing, it may not be well defined for a Vertisol. This is supported by the observed mellowing during vapour wetting and drying between 153 MPa and 3.8 MPa.

Changes in porosity and strength during wet/dry cycles are likely to reflect relative degrees of internal cracking of the matrix, slaking and dispersion. To assess the last two, samples of air-dry field and worked soil were either flood- or capillary-wetted and the resulting size fractions determined (Table 8).

Clearly very little clay disperses, even under flooding, but progressively more slaking occurs as one goes from slow capillary- to flood-wetting. In particular, flood wetting results in release of more fragments throughout the 2-1000 pm range including fragments of 2-20 pm. Slaking is consistently greater in field aggregates.

The general pattern observed, in which tensile strength progressively decreased with cycles, is consistent with this as it suggests that mellowing predominates over cementation by dispersed clay or dissolved cementing materials (Kay & Dexter, 1992), even under simulated rainfall. However, differing extents of slaking and dispersion may explain the contrasting changes in aggregate-size distribution and strength under the different wetting systems.

During flood wetting, some individual particles of clay and silt may disperse and larger groups of particles slake from the main soil mass but retain their coherence. Surface-tension forces decrease to zero as there is free water surrounding the slaked material. Such material may now move freely as water moves in and around the disaggregating soil. As the soil subsequently dries, some of the slaked material may be pulled back into fissures and will realign or repack by surface tension and bonding forces as the water drains from the pores and cracks. The resultant dried mass of soil may therefore remain as a large cohesive unit. Infilling was seen to a limited extent after rainfall wetting and to a lesser extent after flood wetting. Under capillary wetting, surface-tension forces hold slaked material in position and they are also less liable to move as water fluxes will be lower. Less infilling occurs and the matrix is more aggregated. Flood- and rainfall-wetting resulted in the largest size grades and capillary-wetting the least.

Aggregate strength reflects the occurrence of internal flaws or cracks. The vapour-wetting experiments clearly indicate that changes may occur to the macrostructure that result in a decrease in strength but with no measurable change in total aggregate porosity. Slow capillary-wetting results in the least steep wetting fronts and stress on the matrix. Reflecting this, the matrix is likely to be less flawed and the aggregates have the greatest strength (Fig. 3). The consistent greater strength of the faster-wetted of the capillary samples indicates that there

0 1995 Blackwell Science Ltd, European Journul ofsoil Science, 46,357-368


must be considerable differences in internal microstructure/ cracks between the two capillary regimes with the fast-wetted sample presumably having more flaws. Although clod densities of capillary-wetted samples are not significantly different (Table 5) , the fast-wetted sample is less dense. Possibly fast wetting induces more numerous microcracks due to differential swelling thus decreasing the internal strength and, marginally, the density of the aggregates.

Under flood- and rainfall-wetting, presumably infilling and some clay dispersion (Table 8) result in enhanced strengths while fast capillary wetting results in the most flawed internal structure and the lowest strengths.

Conclusions

The rate and type of tilth development of this self-mulching Vertisol was found to be dependent on the mode of wetting. From an initial massive structure, flood wetting produced little change in size of aggregate whereas both slow and fast capillary wetting reduced aggregate size below that of field soils after four wetldry cycles, with rainfall producing an intermediate effect. Slow capillary wetting produced sharp, angular aggregates with polished fracture edges in contrast to rapid capillary wetting which formed aggregates with rough fractures and many microcracks. Tensile strengths of 8- 16-mm aggregates decreased exponentially with the number of wetldry cycles and strength reduction was in the order of fast capillary > rainfall > flood > slow capillary wetting. Fast capillary wetting was, therefore, the most effective method of mellowing the high strength and large clod size of the worked soil, whereas rapid flood wetting did not lead to regeneration of a fine structure, even in a strongly self-mulching soil, such as this. The production of unfavourable coarse, and dense, structure by flood wetting is exacerbated in non-mulching Vertisols (Hussein, 1994). Rainfall of medium intensity slowly decreases the strength of the worked soil and does not favour self mulching under these conditions. Light rainfall (c 5 mm d-’) has, however, been found to produce fine structure in the field on these soils (Hussein, 1994).

Friabilities were generally greatest after one wetldry cycle but decreased with successive cycles. Successive vapour wet/ dry cycles produced significant decreases in tensile strength of the worked soil but no measurable change in porosity. This suggests that there was a progressive rearrangement, and possibly interconnection of micropores, leading to a reduction in strength. Thus, even in the absence of rainfall or irrigation, this soil will slowly mellow in the field during the winterldry season, due to wetldry cycles driven by relative humidity changes. Although this does not result in self mulching itself, it does act towards the same end and makes subsequent tillage much easier. This in turn would allow more timely planting of crops by small-scale farmers to take advantage of early rains. It also suggests that Vertisols may be so active that they do not have a ‘critical mellowing tension’.

Changes in strength of fast versus slow capillary-wetted and vapour-wetted samples, notwithstanding an absence of change in porosity, indicates that changes in the arrangement of pores during wetldry cycles can result in mellowing.

In the field it is difficult to isolate individual wetting methods and most wetting events involve a combination of methods such as flood wetting along furrows accompanied by vertical and lateral capillary movement into the ridge. However, in order to promote self mulching, irrigation management strategies can be used to limit the amount of soil wet by flooding and increase the amount wet by capillarity. This would include the use of alternate furrow irrigation or broad ridges (Hussein & Adey, 1994). Irrigation methods such as the use of border strips should be avoided, if possible, as they promote widescale flooding with concomitant poor structure.

Acknowledgements

This research was kindly facilitated by the provision of a TCTD award to J.H. and a British Council ‘link’ between the two host departments. Soil was imported to the UK under import licence No. PHD 1 163/8 1.

References Atkins Land & Water Management. 1983. Chisurnbanje Zrrigation

Feasibility Study, Technical Annex 3: Soils and Land Classifica- tion. ADA, Harare, Zimbabwe.

Blake, G.R. 1965. Bulk density. In: Methods of Soil Analysis: Part I , Physical and Mineralogical Methods, 1st edn (ed. C.A. Black) pp. 374-390. American Society of Agronomy, Madison, WI.

Blake, G.R. & Hartge, K.M. 1986. Bulk density. In: Methods of Soil Analysis: Part I , Physical and Mineralogical Methods, 2nd edn (ed. A. Klute), pp. 363-376. American Society of Agronomy, Madison, WI.

Braunack, M.V., Hewitt, J.S. & Dexter, A.R. 1979. Brittle fracture of soil aggregates and the compaction of aggregate beds. Journal of Soil Science, 30, 653-667.

Bullock, P., Federoff, N., Jongerius, A., Stoops, G. & Tursina, T. 1985. Handbook for Soil Thin Section Description. Waine Research Publications, Wolverhampton.

Chan, K.Y. 1989a. Effect of tillage on aggregate strength and aggregation of vertisols. Soil & Tillage Research, 13, 163-175.

Chan, K.Y. 19896. Friability of a hardsetting soil under different tillage and land use practices. Soil & Tillage Research, 13,287-298.

Chan, K.J. & Mullins, C.E. 1994. Slaking characteristics of some Australian and British soils. European Journal of Soil Science, 45,

Coughlan, K.J. 1984. The structure of Vertisols. In: The Properties and Utilization of Cracking Clay Soils (eds J.W. McGarity, E.H. Hoult & H.B. So), Reviews in Rural Science, 5, 87-96. University of New England, Armidale, NSW, Australia.

Department of Meteorological Services 1978. Climatological Sum- maries, Climate Handbook Supplement No. 5 . Government Printer, Harare.

De Vos, J.H. & Virgo, K.J. 1969. Soil structure in Vertisols in the Blue Nile Plains, Sudan. Journal of Soil Science, 20, 189-205.

273-283.


368 J . Hussein & M. A. Adey

Dexter, A.R. 1988. Advances in characterisation of soil structure. Soil & Tillage Research, 11, 199-238.

Dexter, A.R. & Kroesbergen, B. 1985. Methodology for determina- tion of tensile strength of soil aggregates. Journal of Agricultural Engineering Research, 31, 139-147.

Dexter, A.R., Kroesbergen, B. & Kuipers, H. 1984~. Some mechanical properties of aggregates of top soils from the Ijsselmeer polders. 1. Undisturbed soil aggregates. Netherlands Journal of Agricultural Science, 32, 215 -227.

Dexter, A.R., Kroesbergen, B. & Kuipers, H. 1984b. Some mechanical properties of aggregates of top soils from the Ijsselmeer polders. 2. Remoulded soil aggregates and the effect of wetting and drying cycles. Netherlands Journal of Agricultural Science, 32,

Gardner, W.R. 1956. Presentation of soil aggregate size distribution by logarithmic normal distribution. Soil Science Society of America Proceedings, 20, 151-153.

Grant, C.D. & Blackmore, A.V. 1991. Self-mulching behaviour in clay soils: its definition and measurement. Australian Journal of Soil Research, 29, 155-173.

Grant, C.D., Dexter, A.R. & Huang, C. 1990. Roughness of soil fracture surfaces as a measure of soil microstructure. Journal of Soil Science, 41, 95-110.

Gusli, S., Cass, A., Macleod, D.A. & Blackwell, P.S. 1994. Structural collapse and strength of some Australian soils in relation to hardsetting: 11. Tensile strength of collapsed aggregates. European Journal of Soil Science, 45, 23-29.

Hadas, A. 1990. Directional strength in aggregates as affected by aggregate volume and by a wet/dry cycle. Journal of Soil Science,

Hussein, J. 1994. The dynamics of self mulching in a Vertisol from Zimbabwe. Ph.D. thesis, University of Newcastle upon Tyne.

Hussein, J. & Adey, M.A. 1994. Sustainable development of surface tilth in a Zimbabwe Vertisol through water management. In: Sustainable Land Management in the Tropics (eds J.K. Syers & D.L. Rimer) , pp. 120-131. CAB International, Wallingford.

Hussein, J., Adey, M.A. & Elwell, H.A. 1992. Imgation and dryland cultivation effects on the surface properties and erodibility of a Zimbabwe Vertisol. Soil Use and Management, 8, 97-103.

Jewitt, T.N., Law, R.D. & Virgo, K.J. 1979. Vertisol soils of the tropics and subtropics: their management and use. Outlook on Agriculture, 10, 33-40.

Kay, B.D. & Dexter, A.R. 1990. Influence of aggregate diameter, surface area and antecedent water content on the dispersibility of clay. Canadian Journal of Soil Science, 70, 655-671.

Kay, B.D. & Dexter, A.R. 1992. The influence of dispersible clay and wetting/drying cycles on the tensile strength of a red-brown earth. Australian Journal of Soil Research, 30, 297-310.

Kemper, W.D., Olsen, J.S. & Hodgdon, A. 1975. lmgation method as a determinant of large pore persistence and crust strength of cultivated soils. Soil Science Society of America Proceedings, 39,

Low, D., Gibson, W.P., Godium, R.G.H. & Virgo, K.J. 1984. Variations in characteristics of soils developed on a basalt plain in the Zimbabwe Lowveld. Zimbabwe Agricultural Journal, 81, 121-126.

215-227.

41, 85-93.

519-523.

McKenzie, B.M. & Dexter, A.R. 1985. Mellowing and anisotropy induced by wetting of moulded soil samples. Australian Journal of Soil Science, 23, 37-47.

Metelerkamp, H.R.R. & Boundford, L.P. 1970. A laboratory rainfall simulator. Rhodesian Agricultural Journal, 67, 162- 163.

Mullins, C.E., MacLeod, D.A., Northcote, K.H., Tisdall, J.M. & Young, I.M. 1990. Hardsetting soils: behaviour, occurrence and management. In: Advances in Soil Science, Vol. 11, Soil Degrada- tion (eds R. La1 & B.A. Stewart), pp. 27-108. Springer-Verlag, New York.

Nyamudeza, P. & Mandiringana, O.T. 1992. ZimbabwelIBSRAM rainwater management project on Vertisols. In: Report of the Annual Review Meeting on Africaland Management of Vertisols in Africa, Kenya (1991), pp. 157-173. IBSRAM, Bangkok, Thailand.

Pillai-McGarry, U.P.P. & Collis-George, N. 1990a. Laboratory simulation of the surface morphology of self-mulching and non self-mulching Vertisols. 1. Materials, methods and preliminary results. Australian Journal of Soil Research, 28, 129-139.

Pillai-McGarry, U.P.P. & Collis-George, N. 1990b. Laboratory simulation of the surface morphology of self-mulching and non self-mulching Vertisols. 11. Quantification of visual features. Australian Journal of Soil Research, 28, 141-152.

Probert, M.E., Fergus, I.F., McGarry, D., Thompson, C.H. & Russell, J.S. 1987. The Properties and Management of Vertisols. CAB International, Wallingford.

Russell, E.W. 1973. Soil Conditions and Plant Growth, 10th edn. Longman, London.

Shiel, R.S., Adey, M.A. & Lodder, M. 1988. The effect of successive wetJdry cycles on aggregate size distribution. Journal of Soil Science, 39, 71-79.

Soil Survey Staff. 1975. Soil Taxonomy, Agricultural Handbook 436. United States Department of Agriculture, Washington, DC.

Tessier, D., Beaumont, A. & Pedro, G. 1990. Influence of clay mineralogy and rewetting rate on clay microstructure. In: Soil Micromorphology: a Basic and Applied Science (ed. L.A. Doyles), pp. 115-121. Elsevier, Amsterdam.

Thompson, J.G. & Purves, W.D. 1978. A Guide to the Soils of Rhodesia, Technical Handbook No. 3. Rhodesia Agricultural Journal, Harare, Zimbabwe.

Thorburn, P.J., Gardner, E.A., Geritz, A.F. & Coughlan, K.J. 1989. The effect of wetting pre-treatment on the desorption moisture characteristic of Vertisols. Australian Journal of Soil Research, 27,

Utomo, W.H. & Dexter, A.R. 1981. Soil friability. Journal of Soil Science, 32, 203-213.

Verboom, W. 1991. The influence of soil surface settling and sealing on the water dynamics of three Zimbabwe topsoils. D.Phil. Thesis, University of Zimbabwe, Harare.

Wenke, J.F. & Grant, C.D. 1994. The indexing of self-mulching behaviour in soils. Australian Journal of Soil Research, 32, 20 1-2 1 1.

Yule, D.F., Coughlan, K.J. & Fox, W.E. 1976. Factors affecting seedbed properties of cracking clay soils of the Darling Downs. Australian Journal of Experimental Agriculture and Animal Husbandry, 16, 771-774.

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changes of structure and tilth mellowing in a vertisol due to wet/dry cycles in the liquid and...

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