the dynamics of soil aggregate formation and the effect on soil physical properties

SOIL TEC HNOLOGY vol. 3, p. 113-129 Cremlingen 1990 ]

THE D Y N A M I C S OF SOIL A G G R E G A T E F O R M A T I O N

A N D THE EFFECT O N SOIL PHYSICAL P R O P E R T I E S

H. Semmel, R. Horn, U. Hell, Kiel A.R. Dexter, Glen Osmond,

E. D. Sehulze, Bayreuth

Summary

Homogenized loess from the Negev Desert, Israel, was used to fill 48 lysimeters, of which 42 were each planted with a single almond tree. Wetting and drying cycles started after an initial watering to field capacity. Three groupings of lysimeters were formed according to the water suction at which the soil was irrigated thus initiating a new wetting and drying cycle. Water suctions of 31 kPa, 63 kPa, and 1500 kPa respectively were chosen. After up to 20 cycles the trees were harvested and the physical properties of single aggregates were determined. Mea- sured parameters were aggregate size distribution, bulk density, tensile strength, penetrometer resistance, water retention curves, aspect ratios, spatial salt concentration, spatial particle size distribution and saturated water conductivity.

It was found that drying intensity had a highly significant effect on physical properties. More intensive drying before irrigation decreased tensile strength and penetrometer resistance and also re-

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suited in a finer aggregate size distribution and in higher contents of plant available water. Bulk density seemed to decrease again after having reached a maximum value. Aspect ratios and saturated water conductivity were not affected significantly by drying intensities. Higher electrical conductivity (salt concentration) and a slight accumulation of clay was found in the outer part of aggregates.

Six lysimeters contained loess without a tree. The soil in these lysimeters showed much less strength than aggregates formed under similar conditions, but influenced by plant roots. Mecha- nisms are proposed to explain these ob- servations.

1 Introduction

Physical properties of homogenized soil will be changed by swelling and shrinking that leads to an aggregation of the soil material. Generally this results in a heterogenisation of the pore system ( G U N Z E L M A N N & H O RN 1985) and in a region of stability with time. The latter effect is known variously as "age hardening", "curing", "strength

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114 Semmel, Horn, Hell, Dexter & Schulze

regain" or " thixotropy" (BLAKE & G I L M A N 1970, U T O M O & DEXTER 1981, K E M P E R & ROSENAU 1984, K E M P E R et al. 1987, H O R N & DEX- T E R 1989).

There are a number of different mechanisms involved in the age hardening process. I f the soil is kept at a con- stant water content, the most important are rearrangement of soil particles into positions o f minimum free energy and chemical cementation at the points of contact of mineral particles. These are most efficient at medium water contents around the plastic limit and may act separately or together.

Another mechanism is based on organic matter, especially root exudates and soil microorganisms that produce extracellular polysaccharides which are ca- pable of forming and stabilizing soil aggregates ( C H E S H I R E 1979, CHANEY & SWIFT 1986a, b). Also, if soil water content changes, the effective water stresses on drying can pull soil particles into a denser configuration.

In contrast to these bonding mechanisms, the main disruptive forces are caused by wetting the aggregates during watering. They are larger if the water content prior to watering is lower. These forces include differential swelling and escape of entrapped compressed air ( K E M P E R & R O S E N A U 1984).

With respect to plant growth soil strength is important for root penetration of aggregates. Critical values that may inhibit root growth are given at about 2.5 MPa (TAYLOR & BUR- N E T T 1964). For the measurement of penetration resistance blunt penetrometer probes have been widely used. The maximum value of penetrometer pressure increases with the aggregate sizes for small aggregates, but becomes indepen-

dent of it for diameters in excess of 15-25 times the diameter of the penetrometer probe (MISRA et al. 1986).

In the literature there are different opinions about possible relationships between bulk density, soil strength, and aggregate size. H O RN & D E X T E R (1989) found increasing tensile strength with time, independent of bulk density but D E X T E R et al. (1984a, b) found a cor- relation between both parameters. Fur- thermore the tensile strength of soil aggregates has been found to be a function of aggregate size in such a way that the larger the aggregates are, the smaller is the mean tensile strength (BRAUNACK et al. 1979).

Further processes of importance in aggregate formation may be mass fluxes and diffusion of salts as well as the move- ment of clay particles, induced by hydraulic gradients.

Evidence of the mechanisms of aggregate formation can be gained from the shape o f aggregates, which can be ex- pressed as "aspect ratios". This is the ratio of the longest : intermediate : shortest axis length, x : y : z. Few data have been reported on aspect ratios up to date in the literature, but those that have are all close to 1 : q: q2, where q = (0.5) 1/3 -- 0.794 (DEXTER 1985).

Thus, various physical parameters of soil aggregates have received consider- able attention in literature. Most of the papers, however, focus on more a single parameter rather than on interactions. In addition, little attention has been given to the origin of samples in the sense of their formation or to the formation of the soil aggregates under different conditions.

In the present study special attention is given to aggregate formation out of homogenized loess under clearly defined

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Soil Aggregate Formation 115

Physical properties:

grain size distribution (%) (diameter in/zm) <2 2-6,3 6.3-20 20-63 63-200 200-630 630-1000

15 13.8 14.6 35.4 14.3 4.6 2.3

specific density (g/era3): 2.66

bulk density (g/cm 3) at the beginning of the experiment: 1.34

plastic limit: 18 (%)

liquid limit: 24 (%)

Chemical properties:

pH(H20): 8.1

Electrical conductivity Ec (pS/cm) : 400

org. C (%): 0.4

C/N ~ 9

CaCO3 (%): 47

Sodium adsorption ratio 3.3 mg/1

Tab. l : Physical and chemical characterisation of the desert loess (Avdat/Israel).

conditions. These show the influence of soil/water conditions and plant growth on aggregation. In this paper a number of different parameters have been used to quantify the induced aggregation and also to give special attention to the in- teraction between different properties.

2 Material and methods

2.1 Site and climate

The experiments were done at the Av- dat Experimental Farm, Israel. The site is situated in the central highlands of the Negev Desert (30 ° 47' North, 30 ° 46 t East, 560 m altitude). The climate is arid with an annual average rainfall of 83 mm, pan evaporation of around

2600 mm and annual average temperature of 18.1°C. Further details of the site and environment are given in EVENARI et al. (1982).

2.2 Soil and plants

Soil for filling the 48 small lysimeters was taken from a nearby wadi. The wadi soil is classified under the FAO system as a Calcic Fluvisol, and under the USDA 7 th Approximation as a Torrifiu- vent. The parent material is a calcare- ous loess, eroded from the hills around. Some main physical and chemical properties are listed in tab. 1.

Each lysimeter had a total volume of 931. On the bot tom there was a drainage system of gravel and sand, connected

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to the top of the lysimeter by an alu- minium pipe. Before filling the lysimeters all the soil was homogenized by passing through a 1 mm screen whilst dry. Af- ter filling with soil, 42 lysimeters were planted with small almond trees (Prunus dulcis (Miller) D.A. Webb var. ne plus ultra) grown from seed at Avdat Farm. Only healthy, well developed, trees were chosen. The other six lysimeters were not planted with trees.

The first watering to field capacity (a matric potential o f 6 kPa) was done from the bot tom. Later waterings all took place from the top of the lysimeters. Wa- ter suction measurements up to 70 kPa were taken with tensiometers. For drier soil, a neutron probe with special calibra- tion was used (Depth Moisture Gauge, model D G M 33, Ronley Electronics Ltd., Israel). Water loss by evaporation from the planted soil was reduced by a gravel layer on top of the soil.

2.3 Treatments

The experiments took place on level open ground with no constructions such as fences etc. For simplicity in the irrigation scheduling, the different treatments were not randomised spatially. However, they were organized at right angles to the pre- vailing wind so that one treatment would not affect another.

The 42 lysimeters planted with trees were divided into three groups each of 14. After the first watering to field capacity the groups recieved different treatments as follows:

I. The soil dried out to a water suction of 31 kPa (pF 2.5), mainly by transpiration of the plant and then was watered again to field capacity. After 4, 6, 10, 15 and 20 cycles of

wetting and drying, two trees were harvested and the soil in these two lysimeters was sampled.

II. The soil dried out to a water suction at 63 kPa (pF 2.8) and then was watered again to field capacity. Two trees were harvested and soil samples were taken after 4, 7, 10, 15 and 20 cycles of wetting and drying.

IIl. The soil dried out to a water suction of 1500 kPa (pF 4.2) and then was watered again to field capacity. Two trees were harvested and soil samples were taken each time after 2, 3, 4, 5, 6 and 7 cycles of wetting and drying. These took much more time than with the other treatments.

IV. Additionally, there were six lysimeters without a tree to demonstrate the influence of the plant on aggregate properties. These lysimeters were divided into three pairs. The three pairs were watered in the same fashion as the three treatments described above. Two of them were added to each one of the treatments described before. In this way a destinction of water use between the evaporation and the plant transpiration was possible. It showed that water loss mainly was induced by the plants' transpiratory demand. This caused slower drying of lysimeters without trees. At the end o f the vegetation period these lysimeters were sampled (after 2-6 wetting and drying cycles) in the same way as the other lysimeters.

Sampling itself always took place in the same way. A soil volume of about 10 1 was taken out of the sample part of the lysimeters. In general this samples contained only a minor volume



of fine earth between macro-aggregates. For later study undamaged aggregates of about 10-30 mm in diameter were chosen. Big aggregates were never divided into smaller ones.

Most of the properties described below were measured in a laboratory close to the field site. Some measurements were done at the University of Bayreuth, FRG.

2.4 Aggregate size distribution

An air dried soil sub sample of about 2 1 by volume was passed through 12, 8, 5, and 2 mm screens. After drying at 105°C the four fractions of aggregates (2-5 mm, 5-8 mm, 8-12 mm, >12 mm) were weighed and their proportion in the whole mass of aggregates larger 2 mm in diameter was calculated. Between 45 and 85% of the total soil samples were aggregates larger than 2 mm.

2.5 Aggregate bulk density

The aggregate bulk density was determined for 20-30 aggregates as follows f rom each of the 38 sampled lysimeters. Each dried aggregate was weighed and then dipped into molten paraffin wax to seal the aggregate surface. The thickness o f the paraffin coat on the aggregates was vey small and the volume change of the total aggregate was negligible. The volume was determined from the displace- ment in water.

2.6 Water retention curves of the aggregates

Water retention curves were calculated f rom the volumetric water contents at 3 kPa, 6 kPa, 10 kPa, 30 kPa and 1500 kPa water suction. Water extrac- tion to these water suctions took place on

porous ceramic plates in pressure pots. From every sampled lysimeter and for every water suction at least 20 single aggregates were measured (at least 10 at 1500 kPa). All aggregates were used for one measurement only.

The water retention data were fitted to the equation o f VAN G E N U C H T E N (1980).

2.7 Saturated water conductivity (Ks)

Aggregates were covered with paraffin wax to seal the surface and were then saturated with ethanol. Holes at the top and the bottom of the aggregate permit- ted flow of ethanol. Ethanol was used instead of water to prevent destruction of the aggregate structure. A capillary tube full of ethanol was connected to the upper hole. This produced a gradi- ent across the sample causing ethanol to flow out of the lower hole. The head of ethanol in the capillary tube was measured at various times as in a standard falling head permeability test. The data were corrected for the different viscosi- ties of water and ethanol. Usually there were at least 6 aggregates measured for each lysimeter. Each measurement was repeated and an average value was calculated. The total number of aggregates measured was 144.

2.8 Aspect ratios

Aspect ratios, the dimensionless ratio of the longest : intermediate : shortest axis length (x : y : z), being rectangular to each other were calculated as 1 : y/x : z/x from each lysimeter. Between 45 and 72 aggregates of >8 mm in diameter were measured. The total number of aggregates measured was 1368.

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2.9 Particle size distribution within aggregates

In order to study a possible segregation of the particle size distribution within the aggregates, separate measurements were performed for the outer and the inner parts of aggregates.

For each lysimeter, 20 aggregates of about 10-30 mm in diameter were pealed with a scalpel in such a way that the aggregates were partit ioned into an outer and an inner part having the same total mass of about 25 g. The removed layer had a thickness of about 1-2 mm.

2.10 Salt concentration

To characterize salt concentration the electrical conductivity was measured. This was done separately for the outer parts and for inner parts of aggregates. Partitioning took place in the same way as described above for the particle sizes.

The electrical conductivity was measured in the 2.5 : 1 water extract of the homogenized Soil ( S C H L I C H T I N G & BL UME 1966).

2.11 Tensile strength of aggregates

Aggregate tensile strength of air-dried aggregates was measured by an indirect (crushing) test. The apparatus used was similar to the one described by H O R N & D E X T E R (1989).

The tensile strength, y, was calculated from crushing force, f, the mean diameter, d, as follows:

y --- 1.86 x f / d 2

The coefficient, 1.86, has been determined for irregular aggregates from the tilled layer of a sandy loam (DEXTER 1986). When compared with more com- plicated methods there was no significant

reduction in the coefficients of variation of the tensile srength values below those obtained by using this simple equation (HORN & D E X T E R 1989).

Usually there were at least 20 aggregates measured for each lysimeter exam- ined. The total number of aggregates measured was 873.

2.12 Penetrometer resistance of aggregates

The penetrometer resistance was measured with a conical (60 semiangle), steel penetrometer probe of 2 mm in diameter. The aggregates were placed on an elec- tronic balance during penetration, and maximum penetration forces (when the penetrometer tip passed the center of the aggregate or cracked it) were read. The penetration forces were converted to aggregate penetration resistance by divid- ing them by the projected area o f the penetrometer cone. The water content of each aggregate was determined after penetration. Water contents were between 2 and 20% by volume.

Usually at least 25 aggregates were tested for each lysimeter. The total number of aggregates measured was 734.

3 Results

3.1 Aggregate size distribution

The change of the aggregate size distribution with number of drying events is shown in fig. 1 as an example only for treatment IL In all treatments, there is a trend towards smaller aggregate sizes with increasing number of drying cycles, whereby fewer wetting and drying cycles are needed in treatment III compared to II and/or I to reach a similar ditribution with an increased proportion of small ag-

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Soil Aggregate Forma lion 119

60

> - , f ro u c- tD

c r oJ

21::1

o

O

. . . . . . . . - _ . - - . _ . . _ . _ - . : ~ ~ - _ . - . - - - . - _ . . . . . .~ . . _ .

i I r i ~ I , r i i ~ i , i , I , I , I i i , I , I , I , I , ! , I i I I I i I i i L

0 5 i0 15 20

Number of drying cycles

Fig. 1" Change o f the aggregate size distribution with number o f drying cycles for treatment 1I = pF 1.8 - 2.8.

o = d i a m e t e r > 12 m m ; + = d i a m e t e r 8 - 1 2 m m ; A = d i a m e t e r 5 - 8 m m ; o = d i a m e t e r 2 - 5 m m

~ t . 6

t_)

En

In 1.5 t- O

.x

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L . 3

0

(D

+ +

I I t I r I , I ! ! , r ! i J ~ f ! I i I , I ! I I q ' I I I | I ~ I I I I ! ~ I ~

5 I0 15 2n

number of drying cycles

Fig. 2: Effect of drying intensity (o = pF 1.8 - pF 2.5; /x pF 1.8 - pF 2.8; ÷ = pF 1.8 - pF 4.2) and number of drying cycles on aggregate bulk density (g/cm3 ). Shown are real values as well as the corresponding mean values.



treatment number of : ~ SD :z SD n drying cycles

I 4 1 : 0.779 0.129 I 4 1 : 0.801 0.135 I 10 1 : 0.758 0.154 I 10 1 : 0.759 0.123 I 20 1 : 0.775 0.125 I 20 1 : 0.781 0.123

I 4-20 1 : 0.776 0.016

II 4 1 : 0.761 0.123 II 10 1 : 0.725 0.165 II 10 1 : 0.759 0.113 II 20 1 : 0.774 0.111 II 20 1 : 0.736 0.120

II 4-20 1 : 0.754 0.019

0.562 0.117 65 0.617 0.128 72 0.588 0.121 60 0.575 0.111 60 0.671 0.119 45 0.638 0.139 45

0.600 0.030 6

0.584 0.123 61 0.573 0.l l6 60 0.570 0.111 60 0.609 0.130 60 0.533 0.117 60

: 0.578 0.026 6

III 2 1 : 0.774 0.116 III 2 1 : 0.818 0.125 III 4 1 : 0.760 0.148 III 4 1 : 0.761 0.109 III 6 1 : 0.807 0.114 III 6 1 : 0.753 0.120

III 2-6 1 : 0.779 0.027

IV 6 1 : 0.759 0.110 IV 6 1 : 0.778 0.112 IV 4 1 : 0.781 0.133 IV 5 1 : 0.799 0.128 IV 3 1 : 0.747 0.133 IV 2 1 : 0.769 0.126

IV 2-6 1 : 0.772 0.018

0.613 0.099 62 0.614 0.116 62 0.583 0.120 60 0.609 0.109 60 0.589 0.131 60 0.580 0.106 60

0.598 0.016 6

0.604 0.129 45 0.618 0.115 45 0.573 0.119 45 0.584 0,134 45 0.554 0.102 56 0.579 0.101 55

: 0.585 0.023 6 total average 1 : 0.770 0.022 : 0.590 0.024 24

T a b . 2: Aspect ratios of aggregates in dependence o f drying intensity (treatment I - I I I ) and number of drying cycles n. SD = standard deviation.

g r e g a t e s . F r a g m e n t a t i o n p r e d o m i n a n t l y

t a k e s p l a c e by c r a c k i n g i n t he m i d d l e o f

t h e l o n g e s t p r i n c i p a l axis b e c a u s e t h i s

c a u s e s t h e h i g h e s t p o s s i b l e re l ie f o f t e n -

s ion ,

3.2 Aspect ratios

T h e a s p e c t r a t i o s o f a l l t r e a t m e n t s

w i t h a n d w i t h o u t t r ees a n d a f t e r d i f -

f e r e n t n u m b e r s o f cycles a re s h o w n in

t ab . 2, T h e a v e r a g e v a l u e o f all d a t a is

1 : 0 .77 : 0.59 a n d t h e r e is n o s i g n i f i c a n t

i n f l u e n c e o f d r y i n g i n t e n s i t y or n u m b e r

o f cyc les o n a s p e c t r a t ios .

3.3 Bulk density

E x p e r i m e n t a l r e su l t s o f a g g r e g a t e b u l k

d e n s i t y m e a s u r e m e n t s a re g i v e n in fig. 2.

I t s h o w s two m a i n c h a r a c t e r i s t i c s . F i r s t ,

a l l t r e a t m e n t s s h o w in i t i a l l y a s t e e p in-

c r ease i n b u l k d e n s i t y to a m a x i m u m

va lue , w h i c h a p p e a r s to b e f o l l o w e d b y

a s l i gh t d e c r e a s e t o w a r d s a n e q u i l i b r i u m

v a l u e a t h i g h e r n u m b e r s o f w e t t i n g a n d

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2 5 0

200 o..

i,.,a

,dz

150

cn I00 @

(n

5 0 F -

I I , I r I i f , ' r l i l r l r l l ! l l l l l l i l l ! , r , l l l , l l ! r r r

5 tO 15 20

a ) volL~netric water content 8V [ ~/0 ]

Fig. 3a: Effect of volumetric water content (Or,) on aggregate tensile strength o f planted lysimeters (n = 240 per treatment).

t r e a t m e n t I = p F 1.8 - 2 . 5 - - ; t r e a t m e n t I I = p F 1.8 - 2 .8 - - - ; t r e a t m e n t I I I = p F 1.8 - 4 . 2 . . . . .

250

200

"J 150 ¢...

(n lOO

50

0

b)

I 1 I l I I , l , I t I , I , I I I ! 1 t I , I I I I I t I ' I I l I I I I I ~ ' ' I

5 l 0 1 5 2 0

volL~netric water co~tent GV [ °/0 I

Fig. 3b: Effect of volumetric water content ( O r ) on aggregate tensile strength o./" unplanted lysimeters (n = 50 per treatment).

t r e a t m e n t I = p F 1.8 - 2 . 5 - - ; t r e a t n a e n t I I = p F 1.8 - 2 .8 - - - ; t r e a t m e n t I I I = p F 1.8 - 4 , 2 . . . . .

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drying cycles. Second, the change in bulk density and the maximum and "final" value depend on the drying intensity. The wetter the soil has been kept, the more pronounced are the bulk density changes at the same number of wetting and drying cycles.

3.4 Tensile strength

The changes of tensile strength with water content are shown in fig. 3a and b for treatments I - I I I with trees (a) or without a tree (b). For all treatments tensile strength decreases with increasing water content. However, if we com- pare the planted treatments I-III (a), the tensile strength was always highest for t reatment I at the same water content, but the slope of curve I was steepest. The number of wetting and drying cycles since homogenisat ion seems to affect tensile strength only during an initial phase of aggregate formation. Thus, the tensile strength becomes independent of the number of cycles after 4 (III) to 6 (I) cycles (not shown). The unplanted treatments show the same pattern of tensile strength with increasing water content, but the strength values as well as the slope of the curves are smaller compared to the planted ones. Furthermore, especially at very small water contents the tensile strength is smaller for treatment I compared to II.

3.5 Penetrometer resistance

The results for the penetrometer resistance are very similar to those of tensile strength if we look at the average relationship between water content and penetrometer resistance. With respect to the effect of the number of drying cycles on penetration resistance of single

aggregates, it initially increases with the first 4 to 6 drying cycles at the same water content, but the differences are more pronounced with increasing water suction range (treatments I-III , fig 4a).

Similar to tensile strength the unplanted treatments show the same pattern of behaviour. But the penetrometer resistance values as well as the slope of the curves are smaller compared to the planted ones (fig. 4b).

3.6 Water retention curves

Water retention curves of single aggregate, sampled at treatment II are shown in fig. 5a. Increasing number of drying cycles result in a lower total porosity and the increase of fine pores. Results of treatments I and III are similar to those that are shown.

However the increase of fine pores is the less pronounced, the higher the drying intensity has been (fig. 5b) (increase from treatment I to III). This affects the plant available water capacity that increases significantly from 25.1% (SD = 1.7, n = 10) for treatment I, to 27% (SD = 2.0, n = 10) for treatment II to 30.3% (SD = 1.6, n = 8) for treatment I I I . .

3.7 Saturated water conductivity (Ks)

Although the pore size distribution varies for the different treatments, the saturated hydraulic conductivity of the aggregates does not differ significantly between the treatments at the same number of drying cycles. However with increasing number of swelling and drying cycles the saturated hydraulic conductivity is reduced by up to 2 orders of magnitude, whereby the change seems to get smaller with increasing water suction range (treatment III) (tab. 3).

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6 -

a. 5 : r

u 4

m

"2 3 c-

.£ 2

eo 1 c-

g lb lg 2'0 volumetric water content 8v 1% )

Fig. 4a: Change of penetration resistance (MPa) with moisture content for planted lysimeters (n = 210 per treatment).

t r e a t m e n t I = p F 1.8 - 2 . 5 - - ; t r e a t m e n t I I = p F 1.8 - 2 . 8 - - - ; t r e a t m e n t I I I = p F 1.8 - 4 . 2 . . . . .

El 0.. I E

13 t - El

. m

u~

¢-.

.o

13.

5

3

0 5 10 15 20 volumetric water content 8v (%)

Fig. 4b: Change of penetration resistance (MPa) with moisture content for unplanted lysimeters (n = 35 per treatment).

t r e a t m e n t I = p F 1.8 - 2 . 5 - - ; t r e a t m e n t I I = p F 1.8 - 2 . 8 - - - ; t r e a t m e n t I I I = p F 1 .8 - 4 . 2 . . . . .



pF 5

4-

_

.

' ' ' 4 "0 20

volumetric content Ov (%)

60

Fig. 5a: Change of the water retention curve with number of drying cycles (4, 10, 20)for treatment II.

4 cyc les = - - ;

10 cycles . . . . ;

20 cyc les = -- o - - o --

pF 5

4

3

.

\

i

20 40 60

volumetric water content Ov (%1

Fig. 5b: Change of the water retention curve with drying intensity.

t r e a t m e n t I = p F 1.8 - 2.5 - - ( a f t e r 6

cycles) ;

t r e a t m e n t I I = p F 1 . 8 - 2 . 8 - - - ( a f t e r 7

cyc les ) ;

t r e a t m e n t I I I = p F 1.8 - 4 . 2 - o - - o - - ( a f t e r

6 cycles)

3.8 P a r t i c l e s i z e d i s t r ibut ion wi th in

a g g r e g a t e s

With increasing number of swelling and drying cycles not only the bulk density of single aggregates has been changed but also the grain size distribution inside those newly formed aggregates is altered (fig. 6). The outer skin (ca. 1-2 mm) of aggregates (10-30 mm in diameter) always has a higher clay content than

the inner part, while silt is increased in the aggregate center. But the different treatments don't vary significantly.

3.9 S a l t concentra t ion

Separate measurements of the electrical conductivity were obtained for outer and inner parts of aggregates (fig. 7). The outer skin of the aggregates had always a higher electrical conductivity due to the



n u m b e r of t rea tment drying cycles Ks cm/see S D

I p F 1.8 - 2.5

I I p F 1.8 - 2.8

I I I pF 1.8 - 4.2

4 8.38 x 10 -3 5.5 x 10 .3

6 1.72 x 10 -3 1.47 x 10 .3 I0 2.2 x 10 -4 1.9 x 10 .4 20 1.27 x 10 -4 8.5 x 10 -5

4 4.4 x 10 -3 2 x 10 -3

7 4.7 x 10 -4 1.2 x 10.4 10 5.4 x 10 .4 5.5 × 10 -5

20 1.68 x 10 .4 1.1 x 10 -5

2 3.1 x 10 -3 1.66 × 10 -3 4 1.0 x 10 .3 7.5 x 10 -4 5 6.0 x 10 -4 6.2 x 10 .4 6 2.77 x 10 4 2.6 x 10 4

Tab. 3: Saturated hydraulic conductivity Ks (cm/sec) of single aggregates as a function of moisture suction range and number of drying cycles.

salt enrichment because of water evaporation at the outer edge of the aggregates, while the inner part of the aggregates is depleted. This is true even though, with irrigation salt will be transported towards the center of the aggregate. With increasing number of drying cycles the salt concentration is also enhanced, but this increase is the more pronounced the higher the drying intensity during the cycles (increase from treatment I to III).

4 Discussion

Continued swelling and shrinking cycles of homogeneous soil material results in the well known heterogenisation of the pore size distribution by the formation of inter- and intraaggregate pores. Fur- thermore, the strength of structured soil and newly formed single aggregates is also significantly influenced by the total number of swelling and drying cycles, the maximum water suction range and

plant-soil interactions.

Strength increase can be caused by water menisci forces as well as by the mechanisms of age hardening. The for- mer are able to pull mineral particles together to a higher aggregate bulk density, the more intensive soil is drying out. In case, that single mineral particles can be rearranged to greater units by water menisci forces and by hydraulic gradients and thereby create a greater number of contact points, those aggregates have greater cohesive forces which determine bulk density as well as soil stability parameters (HORN 1976).

However, if more intensive drying has caused smaller bulk density and soil stability compared to the wetter treatments I and II, the reduced possibility of particle rearrangement in the less water filled and finer pores and the smaller number of wetting and drying cycles have to be considered as main reasons as well as the more pronounced disruptive forces

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18-

16- El u

1L,-

12-

18-

"~ 16- ~ J

C

1L,-

12-

38-

O o 32-

30-

t l I

I I I I

I t I I

S I0 IS 20 Number of drying cycles

Fig. 6: Clay and silt content ( % ) at the outer skin ( o - o) and in the inner part ( A - - H ) of aggregates (diameter 10-30 ram).

during swelling by hydraulic gradients. Thus, even if the age hardening effect may be more effective during longer time periods o f drying, the range of opti- mal water content or water saturation for particle rearrangement is rather small and therefore only available for a mechanism of stabilisation.

Salts, humic acids and soil colloids are transported in water films because of hydraulic gradients to the contact points increasing the effective stresses at each point. Consequently the single contact points can become more cemented

and therefore stronger, even though the bulk density can be decreasing slightly as swelling and shrinking continues (DEX- TER et al. 1988).

The latter effect can also be verified by the higher tensile strength of aggregates from planted lysimeters compared with those from the unplanted ones. Even if we assume that root growth results in a slight compaction of soil aggregates caused by volume expansion of roots (DEXTER 1987) or as a result of more intensive drying of the soil adhering to roots (GUID1 et al. 1985), the net el-

SOIL TECHNOLOGY--A cooperating Journal of CA'I'ENA


E 2000

bO

1500

L~

C 0

u 1000

._ L.

"6

-~ 500

o S;- -° ooO >O

- i

I I I I

5 I0 15 20

number of drying cycles

Fig. 7: Electrical conductivity Ec (gS/cm) at the outer and inner part of aggregates in dependence of the water suction range and number of drying cycles.

treatment I = - - outer, - - - inner part; treatment [I = o o o outer, o - o inner part; treatment I I I= AA~ outer, A -- A inner part

fect of root volume expansion on bulk density is negligible small as roots rarely occupy more than 2% of the total soil volume.

A difference (except for treatment 2) in tensile strength and penetration resistance but not in bulk density can only be explained if root exudates such as polysaccharides and organic substances produced in the rhizosphere are considered. Aggregate stability can be increased either directly by polysaccharides etc. or after soil microorganisms have metabolized them. A third possibility may be the increased growth of bacteria colonies. Stabilisation by bacteria colonies, their extracellular products of metabolism and by root exudates mainly

take place at contact points of mineral particles (CHESHIRE 1979, GOSS & REID 1979, R E I D & GOSS 1980, 1981). That root exudates can be produced in large quantities (up to more the half of the root biomass) has been shown by M A R T I N (1977).

The rearrangement of particles in single aggregates can also be derived f rom the change in the pore size distribution of single aggregates. The wetter the soil has been kept the more pronounced is the increase in fine pores and the decrease in plant available water and air capacity.

Furthermore, the decrease of the saturated hydraulic conductivity (Ks) of single aggregates with increasing number

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of wetting and drying cycles can also be correlated with the particle rearrangement. Although the aggregate bulk density is highest for treatment I, the saturated hydraulic conductivity is not significantly smaller compared with treatment II and III. Thus, a higher pore continuity balances the smaller porosity. Such a phenomenon is often described for soils under different tillage treatments (EHLERS 1982).

The process of increasing soil tensile strength and penetration resistance with increasing aggregate bulk density mainly occurs during the first few wetting and drying cycles. Lateron, because of the more completed particle rearrangement, the porosity increases even if the tensile strength is further enhanced. Thus only after many wetting and drying cycles, the "normal" bulk density (according to HEINONEN 1977) may be obtained but this value depends on the hydraulic his- tory of the site. These changes in soil structure and strength will have implica- tions for root growth and on the acces- sibility of nutrients and available water for plant uptake (HORN 1987).

Acknowledgements

The authors wish to thank the Jakob Blaustein Institute for Desert Research of the Ben Gurion University of the Negev for their help and assistance with the experiment at Avdat. The work was supported by the German Re- search Foundation (DFG), Special Re- search Program 137 at the University of Bayreuth, FRG.

References

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BRAUNACK, M.V., HEW1TT, J.S. & DEXTER, A.R. (1979): Brittle fracture of soil aggregates and the compaction of aggregate beds. J. Soil Sci. 30, 653-667.

CHANEY, K. & SWIFT, R.S. (1986): Studies on aggregate stability. I. Reformation of soil aggregates. J. Soil Sci. 37, 329-335.

CHANEY, K. & SWIFT, R.S. (1986b): Studies on aggregate stability. II. The effect of humic substances on the stability of reformed soil aggregates. J. Soil Sci. 37, 337-343.

CHESHIRE, M.V. (1979): Nature and origins of carbohydrates in soils. Academic Press, London.

DEXTER, A.R. (1985): Shapes of aggregates from tilled layers of some Dutch and Australian soils. Geoderma 35, 91-107.

DEXTER, A.R. (1986): Strength of soil aggregates and of aggregate beds. In: Drescher, J., Horn, R. & de Boodt, M. (eds.), Impact of Water and External Forces on Soil Structure. CATENA SUPPLEMENT 11, 35-52.

DEXTER, A.R. (1987): Compression of soil around roots. Plant and Soil 97, 401--406.

DEXTER, A.R., KROESBERGEN, B. & KUIPERS, H. (1984a): Some mechanical properties of aggregates of top soils from the Js- selmeer polders. I. Undisturbed soil aggregates. Neth. J. Agric. Sci. 32, 205-214.

DEXTER, A.R., KROESBERGEN, B. & KUIPERS, H. (1984b): Some mechanical properties of aggregates of top soils from the Js- selmeer polders. IL Remoulded soil aggregates and the effects of wetting and drying cycles. Neth. J. Agric. Sci. 32, 215-227.

DEXTER, A.R., HORN, R. & KEMPER, W.D. (1988): Two mechanisms for age hardening of soil. J. Soil Sci. 39, 163-175.

EHLERS, W. (1982): Die Bedeutung des Bo- dengef'tiges Ftir dasd Plfanzenwachstum bei moderner Landbewirtschaftung. Mitt. Dtsch. Bodenkdl. Ges. 34, 115-128.

EVENARI, M., SHANAN, L. & TADMOR, N. (1982): The Negev: The Challenge of a Desert. 2 'zd ed., Harvard Univ. Press.

GOSS, M.J. & REID, &B. (1979): Influence of perennial ryegrass roots on aggregate stability. Agric. Res. Council Letcombe Laboratory Ann. Report 1978, 24-25.

GUIDI, G., POGGIO, G. & PETRUZELLI, G. (1985): The porosity of soil aggregates from bulk soil and from soil adhering to roots. Plant and Soil 87, 311-314.

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GUNZELMANN, M. & HORN, R. (1985): Wasserhaushaltsuntersuchungen an natf.irlich gelagerten Bodenaggregaten. Mitt. Dtsch. Bo- denkdl. Ges. 42, 239-245.

HEINONEN, R. (1977): Towards "normal" soil bulk density. Soil Sci. Soe. Ant. J. 41, 1214- 1215.

HORN, R. (1976): Festigkeits[inderungen infolge yon Aggregierungsprozessen eines mesozoischen Tones. Dissertation, TU Hannover.

HORN, R. (1987): Die Bedeutung der Ag- gregierung f'dr die Nghrstoffsorption im Boden. Z. f. Pflanzenern. u. Bodenkde. 150, 13-16.

HORN, R. & DEXTER~ A.R. (1989): Dynamics of aggregation in an irrigated desert loess. Soil Tillage Res. 13, 253-266.

KEMPER, W.D. & ROSENAU, R. (1984): Soil cohesion as affected by time and water content. Soil Sci. Soc. Am. J. 48, 1001-1006.

KEMPER, W.D., ROSENAU, R. & DEXTER, A.R. (1987): Cohesion development in disrupted soils as affected by clay and organic matter content and temperature. Soil Sci. Soc. Am. J. 51, 860-867.

MARTIN, J.K. (1977). Factors influencing the loss of organic carbon from wheat roots. Soil Biol. and Biochem. 9, 1-7.

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SNYDER, V.A. & MILLER, R.D. (1985): Tensile strength of unsaturated soils. Soil Sci. Soc. Am. J. 49, 58-65.

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Addresses of authors: H. Semmel R. Horn U. Hell Institut fiir PflanzenernRhrung und Bodenkunde Olshausenstr. 40 D-2300 Kiel 1, FRG A.R. Dexter Wake Agricultural Research Institute University of Adelaide Glen Osmond South Australia 5064, Australia E.D. Schulze Department of Plant Ecology P.O. Box 101251 D-8580 Bayreuth, FRG

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the dynamics of soil aggregate formation and the effect on soil physical properties

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