Water-dispersible clay after wetting and drying cycles in four Brazilian oxisols

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Soil & Tillage Research 83 (2005) 260–269

Water-dispersible clay after wetting and drying cycles

in four Brazilian oxisols

Teogenes Senna de Oliveiraa,*, Liovando Marciano de Costab,Carlos Ernesto Schaeferb

aSoil Science Department, Federal University of Ceara, Ceara 60455-760, BrazilbSoil Science Department, Federal University of Vicosa, Minas Gerais 36571-000, Brazil

Received 21 July 2003; received in revised form 21 July 2004; accepted 5 August 2004

Abstract

Widespread intensive land use in the seasonal tropics can damage the physical stability of aggregates. Similar damage can be

expected from wetting and drying cycles causing aggregate fragmentation and, consequently, leading to an increase in their

specific area and exposure of internal electric charges. Thus, we hypothetised that the influence of wetting and drying cycles is

dependent on the mineralogical composition of oxisols (latosols) and it is higher in soils with low aggregate stability. A

greenhouse experiment was carried out to test this hypothesis in highly weathered soils from Brazil, all with variable-charge

clays and highly stable aggregates. Wetting and drying cycles were defined from the quantity of water available between field

capacity and the permanent wilting point. Soil columns were submitted to 0, 2, 6, 9, 12, 15 and 18 wetting and drying cycles.

After each number of wetting and drying defined physical and chemical properties were determined. Statistical analysis, such as

simple and multiple linear regression and Pearson’s correlation were performed, showing significantly correlated WDC contents

with wetting and drying cycles. The obtained results led to the conclusion that there was a close interdependence among

mineralogical composition, aggregate stability and WDC influenced by wetting and drying cycles. Soils of reduced aggregate

stability like kaolinitcs made them more susceptible to the action of wetting and drying on the WDC. Changes in the WDC with

wetting and drying cycles showed correlated with eletrochemical properties.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Clay dispersion; Latosols; Oxisols; CEC; Surface area; Wetting and drying cycles

1. Introduction

Soils are subject to seasonal variations of tem-

perature and moisture, changing their physical and

* Corresponding author.

E-mail address: [email protected] (T.S. de Oliveira).

0167-1987/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.still.2004.08.008

chemical properties. Natural variations in moisture

conditions can result from wetting and drying cycles,

periodically intensified by rain, condensation, capil-

larity, solar radiation, wind, amongst others (Utomo

and Dexter, 1982). Depending on their ‘‘wet’’ strength

and on the internal forces produced on wetting,

different soils show different responses to rapid

.

T.S. de Oliveira et al. / Soil & Tillage Research 83 (2005) 260–269 261

wetting (Payne, 1988). Soils under irrigation can also

develop a cyclical moisture variation, as the amount of

water available to plants is reduced by plant

consumption which is, in turn, a function of the

irrigation cycle. Intensity of wetting and drying cycles

will depend on the physiological characteristics of the

irrigated crop, tillage system, climate, and soil class.

Nutrient poor oxisols (latosols) are the dominant

soil class under intensive agriculture in Brazil,

especially under no-tillage management. Their agro-

nomic favourable microstructure, with natural stable

microaggregates, can be modified by crop manage-

ment, affecting the degree, class, and type of structure

(Ferreira et al., 1999). Under conventional soil

management and with increasing adoption of irriga-

tion, these structural changes can be enhanced, and

yet, have not been assessed in Brazilian latosols.

Most studies available in the literature are

concerned on effects of wetting and drying cycles

on the stability of aggregates in water with varying

experimental conditions. Several authors (e.g.

Bouyoucos, 1924; Woodburn, 1944; Nijahawan and

Olmstead, 1947; Harris et al., 1966; Rovira and

Greacen, 1957; Hofman, 1976; Horn and Dexter,

1989) reported increased soil aggregation with wetting

and drying, whereas others (e.g. Willis, 1955; Chepil

and Woodruff, 1963; Salih and Maulood, 1988)

observed decreasing soil aggregation. The dynamics

of soil aggregation is also contradictory: McHenry and

Russell (1943) and Utomo and Dexter (1982)

observed an initial increase in the structural stability

followed by a reduction, while Rovira and Greacen

(1957) and Dexter et al. (1984) reported a reduction

followed by an increase.

Detailed information on the physicochemical

mechanism of the effects of wetting and drying

patchy. If soil aggregation is indeed modified by

Table 1

Location, texture, kaolinite and gibbsite contents, moisture content and m

Soilsa Location Kaoliniteb

(g/kg)

Gibbsiteb

(g/kg)

Coarse sand

(g/kg)

Fine

(g/kg

Fe-RYL Vicosa–MG 814.0 80.0 310 110

RL Rio Paranaıba–MG 5.0 850.3 30 60

RYL Sao Gotardo–MG 54.0 918.0 250 110

YL Marataızes–ES 880.3 30.8 650 110

a Fe-RYL: Fe-rich red-yellow latosol; RL: red latosol; RYL: red-yellob Source: Ferreira et al. (1999).

moisture regime, as a climatic factor, changes are

expected to occur in flocculation or dispersion of the

colloidal fraction. Studies carried out by Kay and

Dexter (1990, 1992) showed the influence of moisture

variation and aggregation to water-dispersible clay.

None of the studies, however, considered resulting

changes in related physical soil attributes.

On the other hand, the application of DLVO theory

to the prediction of soil clay stability, as suggested by

Missana and Adell (2000) implies a high degree of

uncertainty. Moreover, the DVLO theory is not able to

take into account the contribuition of the pH-

dependent charge of clays to its stability behavior.

Thus, this theory is not particularly suitable to

predicting the stability of clayey aggregates from

tropical soils, which are dominated by pH-dependant

charged gibbsite and kaolinite.

The present study was carried out to assess the

influence of wetting and drying cycles on water-

dispersible clay, using aggregates from four typical

brazilian oxisols (latosols). The hypothesis tested is

that the influence of wetting and drying cycles is

dependent on the mineralogical composition of

oxisols (latosols) and it is higher in soils with low

aggregate stability.

2. Materials and methods

Aggregates of selected brazilian oxisols were used;

these oxisols resulting have varying kaolinitic and

gibbsitic nature (Table 1) in the clay fraction (Ferreira

et al., 1999).

The experiments were carried out in a greenhouse,

using PVC columns and 2.00–0.25 mm diameter soil

aggregates. A preliminary separation of 2.00–

0.25 mm diameter aggregates was carried out manu-

atrix potential of the studied oxisols

sand

)

Silt

(g/kg)

Clay

(g/kg)

Matrix potential (MPa)

�0.01 �0.033 �0.1 �0.5 �1.0 �1.5

60 520 29.04 27.90 24.25 22.90 21.80 20.52

150 760 35.78 34.32 31.05 29.39 29.39 26.78

80 560 21.22 19.43 18.73 17.40 17.00 15.45

10 230 9.94 8.80 7.81 7.30 6.85 6.30

w latosol; YL: yellow latosol.

T.S. de Oliveira et al. / Soil & Tillage Research 83 (2005) 260–269262

ally with gentle pressure after air drying and sieving

through a 4.72 mm mesh, avoiding fragmentation of

natural aggregates. This was done for A and B

horizons, separately. Subsequentially, 400 g of A

horizon and 400 g of B horizon aggregates were

placed in a set of 2.00 and 0.25 mm sieves, with

vibratory movement for 4 min sieving. A preliminary

trial was used to determine the vibration time for the

amount of soil material.

The experimental columns measured 7.5 cm in

diameter and 15 cm in height. These PVC columns

were filled with a 1:1 mixture of A and B horizons

aggregates, followed by 2 min shaking at a vibration

intensity seven times weaker than that used for the

initial sieving, allowing the natural settlement of the

soil material.

Wetting and drying cycles were determined using a

water availability factor of 0.70 (so that re-wetting

occurred when available water in the column reached a

minimum of 30% above the permanent wilting point).

Deionized water was added to the column surface onto

a filter paper with the same diameter as the PVC

column, to ensure uniformity during wetting and to

minimize aggregate movement and/or fragmentation

due to water impact.

The permanent wilting point and the field capacity

of the soils were established according to Bernardo

(1982). The moisture content and soil matrix potential

were obtained (Table 1) with the porous plate extractor

(Richards and Fireman, 1943). Water retention were

determined at the potentials of �0.01, �0.1, �0.5,

�1.0 and �1.5 MPa.

The number of wetting and drying cycles was 0, 3,

6, 9, 12, 15 and 18. Experimental columns were

divided in three separate rings to evaluate changes in

different depths. A randomized complete block design

with five replicates was used arranging treatments in

split plots design. The four soil classes were the plots

and the three separate rings of the experimental

column (superior, 1; middle, 2; lower, 3) were the

subplots.

The natural soil used in this study was not subjected

to any chemical pre-treatment. When the number of

wetting and drying cycles established for each

experiment was completed, the column containing

soil was moistened again, collecting the three

individual rings separatedly, followed by air-drying

and storage.

Water-dispersible clay (WDC) was determined

together with aggregate stability. Thirty grams of

aggregates and 100 mL of de-ionized water were

placed in a 200 mL flask closed with a rubber stopper

and shaken in a horizontal shaker with 200 oscilla-

tions/min for 3 h. The material was then transferred to

a 500 mL test tube through a 0.210 mm mesh sieve,

completing the volume with deionized water. The test

tube suspension was then shaken for 1 min using a

manual shacker, and then left to rest for 4 h, after

which, a 10 mL aliquot was removed at 5 cm depth for

WDC determination.

The aggregates retained in the 0.210 mm sieve and

those contained in the 500 mL test tube were

fractionated in separate classes by a set of sieves

with the following diameter: 2–1, 1–0.5, 0.5–0.25,

0.25–0.105 and 0.105–0.053 mm. After separation,

the aggregates were dried at 105 8C for 24 h, weighed

and corrected for moisture content.

pH in water (pH-H2O) and in 1 mol/L KCl (pH-

KCl) were determined in suspension, using a 1:2.5

ratio soil:solution, after resting of at least 1 h and

shaking the suspension before reading (Embrapa,

1997). Exchangeable Al3+ (Al), Ca2+ (Ca) and Mg2+

(Mg) were extracted with a 1 mol/L KCl solution at

the 1:20 ratio; Al3+ was determined by titrating with

0.25 mol/L NaOH solution and the Ca2+ and Mg2+ by

atomic absorption spectrophotometry according to

Defelipo and Ribeiro (1981). Available K and P were

extracted with 1:10 solution of 0.05 mol/L HCl and

0.0125 mol/L H2SO4 (Mehlich-1). K+ (K) was

determined by flame photometry and P by colori-

metry in the presence of ascorbic acid, according to

Defelipo and Ribeiro (1981). The acidity potential,

H+ + Al3+ (H + Al), was determined with a 0.5 mol/L

Ca(OAc)2 solution adjusted to pH 7.0 at the

proportion of 1:15 and titrated with 0.0606 mol/L

NaOH solution according to Embrapa (1997).

From these data we calculated total (T-CEC) and

effective cation exchange capacity (E-CEC), bases

sum (BS), saturation base (SB) and aluminum satura-

tion (AlS).

The Statistical Analyses System Package devel-

oped by the Federal University of Vicosa (SAEG) was

used for all statistical analysis. The data of each

experiment were subjected to individual variance

analyses, isolating the wetting and drying cycle factor.

A joint analysis of all seven experiments was also


carried out. When the F-test indicated significance

(P < 0.01) of the triple interaction, individualized

analyses were performed for each soil class and

column ring. Since the water-dispersible clay content

in the studies of the effects of wetting and drying

cycles is a quantitative factor, a regression analysis

was carried out using the orthogonal polynomial

technique, with a significance level of up to 5%.

A multiple linear regression was also used to

evaluate the relation between diameter classes of

water stable aggregates with the water-dispersible

clay contents. The Pearson’s simple linear correlation

was used to analyze the relationship between the

chemical properties and the water-dispersible clay

content.

3. Results and discussion

Results obtained from the statistical analyses

indicated that the WDC contents were significantly

varied with wetting and drying cycles using fitted

linear regression, although some determination

coefficients were not very high. This result was

further corroborated by the significance of the

regression coefficients (Fig. 1). The WDC percen-

tage data of the Fe-RYL and RYL soils showed a

wider variation compared with the YL and RL soils,

notably after six wetting and drying cycles. A

reduction followed by an increase of WDC was first

detected in the Fe-RYL. Similar phenomenon was

observed in the RYL, through it occurred only in the

top ring and with a less variation. Changes in the

WDC percentage due to wetting and drying cycles

within a narrower variation were also observed in the

YL and RL.

According to Oliveira et al. (1996), the influence

of wetting and drying cycles on the linear, quadratic

or cubic behavior of the WDC contents are related to

changes in aggregate stability in the 2–1, 1–0.5, 0.5–

0.25, 0.25–0.105 and 0.105–0.053 mm diameter

classes. In this study, the diameter class of aggregates

studied followed the linear behaviour. This has

allowed the grouping of the studied oxisols into the

following groups: (1) the YL and Fe-RYL, with lower

percentage of stable aggregates and (2) the RL and

RYL, with the opposite. The latter group included the

most gibbsitic oxisols, having a higher aggregate

stability compared with the kaolinitic oxisols (YL

and Fe-RYL). Considering that the mechanical

separation of aggregates equalized their structural

conditions, it is evident that the observed differences

are related to the soil mineralogical composition.

Gibbsite has been considered the most important

mineral accounting for aggregate stability in RL and

RYL (Ferreira et al., 1999) and for the preservation of

water-stable aggregates (Schaefer, 2001). Kaolinitic

oxisols have generally a lower proportion of water-

stable aggregates (Ferreira et al., 1999; Schaefer,

2001).

Effects of wetting and drying cycles on aggregate

stability may also result from the relative influence

that different aggegate sizes have upon each other.

Reduction in the percentage of aggregates of a given

class may be compensated by more intense fragmen-

tation of larger aggregates. As a result, increasing

percentage of stable aggregates may be observed

instead of an expected reduction. However, this

reasoning does not apply to the 2–1 mm diameter

class, which is the largest. An explanation for

increased percentage of stable aggregates accompa-

nied by a reduction of a given class is the selective

fragmentation of the least resistant ones. These

fragmented particles can be transported to lower parts

(rings), increasing the contribution of the 2–1 mm

diameter aggregates, as observed. However, these

aggregates do not resist the cumulative effects of

wetting and drying cycles, hence showing a reduction

in percentage of water stable aggregates. Smaller

aggregates were linearly distributed within the soil

column, confirming that increasing in 2–1 mm

diameter stable aggregates was related to aggregate

fragmentation in the upper ring. With increasing

wetting and drying cycles, the resistance of non-

fragmented aggregates remaining in column was

reduced, with lower proportion of stable aggregates. In

all soil classes the effects of wetting and drying cycles

were greater in the upper ring.

Aggregate fragmentation due to wetting is selec-

tive, preferably affecting the least resistant ones.

Harris et al. (1966) and Baver et al. (1972) have

attributed this process mainly to slaking, described as

the internal collapse caused by the escape of

compressed air trapped in the micropores, causing

aggregate instability. Downward movement of aggre-

gates may be related to advancing wetting front that


Fig. 1. Percentage of water-dispersible clay by the wetting and drying cycles for the studied oxisols.

rearranged the aggregates in the soil column increas-

ing soil density (Harris, 1971). The intensity of this

movement depends on arrangement, contact surface

and soil structure. Aggregate stability in water can also

be an influencing factor, as varying fragmentation may

lead to changing particle arrangement in the soil

column. Experimental conditions allowed an initial

water movement caused by a matrix potential

gradient, followed by water movement governed by

hydraulic gradient, when a saturated water flow


occurred in the soil column (Hillel, 1982). This may

have contributed to the downward transport of

aggregates and/or particles in the column, with

progressive wetting and drying cycles.

In the Fe-RYL and YL, which are both kaolinitic

oxisols with a considerable proportion of sand, the

observed performance was possibly affected by the

diluting effect of quartz grains to the aggregating

action of colloidal fractions. The reduced stability of

these kaolinitic aggregates made them more suscep-

tible to the action of wetting and drying.

The relationship between the WDC contents and

the aggregate stability at different diameter classes

may be inferred by the multiple linear regression

coefficients shown in Table 2. Significant positive

coefficients (P < 0.05 or 0.01) of the different

components of the multiple regression equations were

observed as YL, RL and RYL in all depths. This

relationship varies with aggregate class, and in some

cases only one class of a given depth was responsible

for the WDC behavior (for example, ring 2 of YL and

RYL).

The interaction between the solid surface and the

soil solution is one of the basic aspects of the

physicochemical soil reaction, colloidal dispersion

and flocculation phenomena (Costa et al., 1984).

Table 2

Coefficients of multiple linear regression equations between water-dispersib

1, 1–0.5, 0.5–0.25, 0.25–0.105 and 0.105–0.053 mm, for the soil classes

Soil Ring Regression coefficients/variables

a b1

(2.00–1.00 mm)

b2

(1.00–0.50 mm)

Fe-RYL 1 Y ¼ Y ¼ 30:10 – –

2 Y ¼ Y ¼ 23:02 ns ns

3 460.70* 36.86** �36.06**

RL 1 �0.88 – 0.15**

2 7.89 – –

3 Y ¼ Y ¼ 0:90 ns ns

RYL 1 95.24 �4.58* –

2 �52.47 – –

3 �67.99 – 5.66**

YL 1 57.05 – �3.22**

2 14.54 2.13** –

3 Y ¼ Y ¼ 19:54 – –

ns: not significant, P > 0.05.* P < 0.05.** P < 0.01.

According to these authors the specific surface and

electric charges of soil are the most important

properties controlling soil reactivity. These two

properties can be enhanced by aggregate fragmenta-

tion and the resulting dispersed clay. Changes in net

balance of electric charges certainly occurred, result-

ing in changes in the zero point of charge (ZPC) and in

the diffuse double layer, causing the observed

phenomena. Consequently, increasing ionic concen-

trations and organic matter content can also be

associated with the newly formed aggregates resulting

from fragmentation, as observed by Moura Filho and

Buol (1976) and Mendonca et al. (1991) when

working with oxisols with similar characteristics to

those in the present study.

Under the experimental conditions, it was not

possible to determine the ZPC with application of

wetting and drying cycles. However, the relationship

between WDC with eletrochemical properties can be

observed clearly by significant regression coefficients

(P < 0.01 and 0.05) between WDC with OC, Al,

H + Al, and T-CEC, in all soils. Variables such as pH-

H2O, pH-KCl, and P, also suggested this relationship

for some soils at different depths (Table 3).

The aggregate breakdown due to moisture varia-

tions, can yield increasing mineralization of the

le clay (WDC) and aggregate stability in water, at the size classes 2–

and rings studied

R2

b3

(0.50–0.25 mm)

b4

(0.25–0.105 mm)

b5

(0.105–0.053 mm)

– – – –

ns ns ns –

�25.87** – �18.51* 0.99

0.19** �1.10** 0.32** 0.99

– �0.97 – 0.70

ns ns ns –

�2.23* – – 0.71

– 11.31* – 0.77

– – – 0.74

– �10.84** – 0.83

– – – 0.80

– – – –


Table 3

Correlation coefficients between the water-dispersible clay contents (WDC) and the properties and/or physical and chemical characteristics of

the Fe-RYL, RL, RYL, and YL soils in the three studied rings, under the influence of the wetting and drying cycles

Variables Fe-RYL RL RYL YL

Ring 1 Ring 2 Ring 3 Ring 1 Ring 2 Ring 3 Ring 1 Ring 2 Ring 3 Ring 1 Ring 2 Ring 3

Aggregates (mm)

2–1 �0.25 ns 0.22 ns 0.32 ns 0.36 ns 0.50 ns 0.20 ns �0.51 ns 0.41 ns 0.35 ns 0.52 ns 0.89** 0.39 ns

1–0.50 �0.41 ns 0.03 ns �0.59 ns 0.77* 0.66 ns 0.49 ns �0.25 ns 0.63 ns 0.86** �0.69* 0.29 ns �0.22 ns

0.50–0.25 �0.26 ns 0.13 ns �0.44 ns 0.40 ns 0.25 ns 0.25 ns �0.30 ns �0.56 ns �0.23 ns �0.26 ns �0.49 ns �0.04 ns

0.25–0.105 �0.36 ns �0.23 ns �0.37 ns �0.28 ns �0.83** �0.52 ns 0.37* 0.88** 0.71* �0.56 ns �0.25 ns �0.14 ns

0.105–0.053 �0.43 ns �0.47 ns 0.15 ns 0.94** 0.64 ns 0.01 ns 0.08 ns 0.03 ns �0.78* 0.27 ns �0.03 ns �0.53 ns

CO �0.16 ns 0.51 ns �0.12 ns 0.21 ns 0.03 ns �0.22 ns �0.43 ns �0.16 ns �0.42 ns 0.56 ns �0.32 ns 0.10 ns

pH-H2O 0.85** 0.79* 0.83** �0.36 ns �0.82** �0.88** 0.59 ns 0.96** 0.99** 0.53 ns 0.14 ns 0.28 ns

pH-KCl 0.74* 0.49 ns 0.59 ns �0.41 ns �0.78* �0.75* 0.57 ns 0.90** 0.88** �0.08 ns 0.08 ns 0.67*

P 0.53 ns 0.17 ns 0.43 ns �0.00 ns 0.00 ns 0.00 ns 0.36 ns �0.14 ns �0.39 ns �0.04 ns 0.87** 0.64 ns

K �0.15 ns 0.35 ns 0.12 ns �0.15 ns �0.41 ns 0.04 ns �0.33 ns 0.73** �0.55 ns 0.57 ns 0.57 ns 0.33 ns

Al �0.71** �0.86** �0.82** 0.41 ns 0.80* 0.94** �0.34 ns �0.60 ns �0.52 ns 0.33 ns �0.47 ns �0.01 ns

Ca �0.15 ns 0.36 ns 0.26 ns �0.66 ns �0.63 ns �0.72* �0.55 ns �0.11 ns 0.13 ns 0.62 ns 0.19 ns 0.42 ns

Mg �0.18 ns 0.47 ns 0.70* 0.22 ns 0.15 ns �0.29 ns 0.41 ns 0.72** 0.02 ns �0.07 ns 0.01 ns �0.43 ns

H + Al �0.71** �0.79* �0.80* 0.60 ns 0.96** 0.84** �0.88** �0.57 ns �0.70* �0.07 ns �0.53 ns �0.42 ns

SB �0.20 ns 0.46 ns 0.38 ns 0.01 ns �0.46 ns �0.52 ns �0.41 ns 0.23 ns �0.02 ns 0.57 ns 0.25 ns 0.28 ns

E-CEC �0.47 ns �0.39 ns �0.33 ns 0.29 ns 0.03 ns 0.01 ns �0.47 ns �0.34 ns �0.34 ns 0.72* 0.15 ns 0.26 ns

T-CEC �0.70** �0.63 ns �0.72* 0.77* 0.96** 0.83** �0.90* �0.52 ns �0.71** 0.37 ns �0.38 ns �0.16 ns

BS 0.03 ns 0.61 ns 0.52 ns �0.10 ns �0.61 ns �0.61 ns �0.26 ns 0.44 ns 0.13 ns 0.54 ns 0.44 ns 0.37 ns

AlS �0.21 ns �0.88** �0.88** 0.17 ns 0.64 ns 0.78** �0.03 ns �0.60 ns �0.46 ns 0.34 ns �0.53 ns �0.11 ns

ns: not significant, P > 0.05.* P < 0.05** P < 0.01.

organic matter newly exposed to microbial growth, as

reported by Soulides and Allinson (1961). According

to Moura Filho and Buol (1976), the release of

nutrients contained in the inner oxisol microaggre-

gates should also be considered. Thus, electric charges

may be partly neutralized, increasing the ZPC and

reducing the double diffuse layer, causing clay floc-

culation due to van der Walls bond forces. According

to Soulides and Allinson (1961), the sequential drying

caused by moisture cycles may lead to a slow

reduction in the microbial flora, which is more active

and stimulated after the wetting, hence solubilizing the

previously immobilized nutrients.

Mendonca et al. (1991) comments on the greater

content of organic matter in aggregates and the release

of ions from the inner zones as the aggregates diameter

was reduced by fragmentation. Leaching of ions

released by dispersion (Dong et al., 1985) in the upper

part of the column (first ring) may also have

contributed to the reduction of the free electric

charges. Consequently, the ZPC in the upper parts

were greater due to the possible of charge neutraliza-

tion and reduction of the diffuse double layer

thickness, leading to flocculation of colloids.

Table 4 illustrates the significant (by the t-test at

P < 0.01 and 0.05) regression coefficients of the

WDC content and selected soil properties at different

depths (rings) resulting from wetting and drying

cycles. Variables such as E-CEC and T-CEC that

indicate the intensity of electric charges showed a

cubic relationship, at the upper ring, similarly to the

effect observed for WDC in RL, Fe-RYL and RYL

soils (Table 4). The increasing soil pH as shown by

the positive linear effects, for both ring and soil class,

also caused increasing WDC contents. Hence, there

is a relationship between WDC and ZPC in these

soils, associated with changes in aggregate size, as

previously discussed. A notably exception was the

RL where the linear effect of pH-H2O and pH-KCl

was negative at any depth. All other variables studied


Table 4

Regression coefficients between the water clay dispersed contents (WDC) and the properties and/or physicochemical characteristics of the Fe-

RYL, RL, RYL and YL soils in the three rings studied, under the influence of the wetting and drying cycles

Soil Variables Ring Regression coefficients/random R2

a b1 b2 b3

Fe-RYL pH-H2O 1 �1187.73 497.53** –50.53** – 0.95

pH-KCl 1 �6901.01 3625.15** –467.36** – 0.92

Al 1 751.92 �3703.55** 6342.13** �3590.00** 0.99

H + Al 1 �0.810 0.303** –26.90** – 0.87

T-CEC 1 4544.69 �20973.30** 3214.17** �163.45** 0.98

pH-H2O 2 �98.17 26.78** – – 0.63

Al 2 70.98 �64.77** – – 0.74

pH-H2O 3 �106.40 28.79** – – 0.70

Al 3 71.60 �65.85* – – 0.68

Mg 3 415.94 �8164.51** 48880.08** 88116.10** 0.99

H + Al 3 162.42 �23.85* – – 0.64

T-CEC 3 �32135.50 15103.90* –2348.43* 120.90* 0.88

AlS 3 �389.80 16.17** –0.15** – 0.99

RL T-CEC 1 �2789.28 1815.02* –393.43* 28.41 0.99

pH-H2O 2 5.23 �0.97* – – 0.64

pH-KCl 2 15.65 �3.45* – – 0.60

Al 2 �0.92 10.53* – – 0.64

H + Al 2 �6.73 1.70** – – 0.92

T-CEC 2 �46.20 17.59* �1.61* – 0.97

pH-H2O 3 5.55 �1.03** – – 0.71

pH-KCl 3 14.53 �3.18* – – 0.49

Al 3 �1.09 11.69** – – 0.88

H + Al 3 �5.45 1.43** – – 0.70

T-CEC 3 �7.08 1.69** – – 0.70

AlS 3 �0.59 0.04* – – 0.61

RYL H + Al 1 �486.84 248.07** –30.40** – 0.98

T-CEC 1 �354.94 177.14* –20.98* – 0.95

pH-H2O 2 �41.07 10.06** – – 0.91

pH-KCl 2 �169.70 40.72** – – 0.81

K 2 5190.78 �623.45* 24.69* �0.32* 0.91

pH-H2O 3 �42.56 10.57** – – 0.98

pH-KCl 3 �180.91 43.32** – – 0.77

H + Al 3 528.11 �214.45* 21.64* – 0.85

T-CEC 3 537.22 �207.22* 19.87* – 0.87

YL E-CEC 1 627.51 �1321.11* 947.89* �224.26 0.91

P 2 18.57 1.23** – * 0.81

ns: not significant, P > 0.05.* P < 0.05.** P < 0.01.

behaved similarly with WDC, illustrating the

interdependence between effects of the wetting and

drying cycles and aggregate stability. Their frag-

mentation and vertical dislocation, caused exposure

of electric charges, release of internal nutrients and

protected organic matter.

4. Summary and conclusions

A green-house experiment was carried out to test

the hypothesis that wetting and drying cycles lead to

aggregate fragmentation and, consequently, to an

increase in their specific area and exposure of internal


electric charges. The physicochemical behavior of

the soil is modified by these new conditions, with

changes in the water-dispersible clay. Significant

Pearson’s correlation coefficients (P < 0.01 and

0.05) were obtained between the WDC contents

and the variables that reflect possible changes in the

electric charge and specific surfaces, such as T-CEC

and E-CEC. The relationship between WDC and

each of these variables varied from simple linear, to

quadratic or cubic, when analyzed by regression

equations. The WDC content also showed this type of

association with other variables such as pH-KCl, pH-

H2O, Al, H + Al and P in one or more types of soil or

specific ring. The obtained results led to the

conclusion that there was a close interdependence

among mineralogical composition, aggregate stabi-

lity and WDC influenced by wetting and drying

cycles. Soils of reduced aggregate stability like

kaolinitcs made them more susceptible to the action

of wetting and drying on the WDC. Changes in the

WDC with wetting and drying cycles showed

correlated with eletrochemical properties.

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Documents

Water-dispersible clay after wetting and drying cycles in four Brazilian oxisols