Tensile Strength and Friability of an Alffsol Under Agricultural

163

Reis et al. ScientiaPhysical Agricola parameters of an Alﬁ sol

Tensile strength and friability of an Alﬁ sol under agricultural management systems

Diony Alves Reis1*, Cláudia Liane Rodrigues de Lima1, Eloy Antonio Pauletto1, Patrícia Bianca Dupont1, Clenio Nailto Pillon2

1Federal University of Pelotas/FAEM – Dept. of Soils, s/n, ABSTRACT: Management systems may infl uence the structural quality of soils. Tensile strength C.P. 354 – 96010-900 – Pelotas, RS – Brazil. (TS) and friability (F) are indicators of soil structural quality. The aim of this study was to evaluate 2Embrapa Temperate Agriculture, Rod. BR 392, km 78, C.P. the TS and F of an Alfi sol under different management systems. The treatments were as follows: 403 – 96010-971 – Pelotas, RS – Brazil. (i) soil under conventional system with growing maize after tobacco cultivation, (ii) soil under con- *Corresponding author ventional system with growing maize after use as pasture, (iii) soil under natural pasture, and (iv) a natural area with predominance of spontaneous vegetation. TS and F were evaluated at depths Edited by: José Miguel Reichert/Luís Reynaldo Ferracciú of 0.00-0.05 and 0.05-0.10 m. The water content of soil aggregates, soil particle-size distribu- Alleoni tion, total organic carbon, carbon in the coarse fraction and carbon associated with minerals were also determined. The increase in clay content and soil organic carbon infl uences the values of TS. The lowest TS was for the soil under maize cultivation after tobacco in the conventional system. Soil under natural area in the 0.05-0.10 m layer was classifi ed as slightly friable, while other systems were classifi ed as friable. Evaluations of the structural quality of soils under man- Received March 04, 2013 agement systems can be performed using TS. However, F was not effi cient in detecting changes Accepted October 29, 2013 between the different management systems.

Introduction interactions are factors that affect the aggregation and stability of soil aggregates. Furthermore, the greater the The tensile strength (TS) of aggregates is defi ned concentration of poorly crystalline Fe oxides, conversely, as the force per unit area required to cause the disrup- the greater the impact of crystalline Fe forms on the TS tion of aggregates, being the most useful measure of of aggregates and soil F (Ley et al., 1993; Imhoff et al., individual resistance of soil aggregates consisting of an 2002). Thus, the objective of this study was to evaluate indicator sensitive to the structural soil condition, that the effect of agricultural management systems on the refl ects the effects of natural factors, as well as land use tensile strength and friability of an Alfi sol. and management (Dexter and Kroesbergen, 1985; Dex- ter and Watts, 2000). Materials and Methods

The distribution of TS could be utilized as an index

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of soil friability (Utomo and Dexter, 1981). The friabil- located

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ity (F) of the soil can be deﬁ ned as the tendency of a 31º25’S; 52º15’W) in

soil mass to break into smaller sizes of aggregates under the state of

application of stress or load. As a result of its hetero- , the climate (mesothermic humid),

' ' (

geneous nature, TS responds to the weakness planes or with ' mean

¢ © ¤ ¥ ¢ #) * fault zones between aggregates in this way and it can be of The study was carried out on eroded estimated by the coefﬁ cient of variation of TS (Watts and granite and granitic sediments from the Rio Grande do Dexter, 1998). Sul Shield. The relief varies from wavy to smooth wavy F is synonymous with soil quality and is indica- and all soils are shallow. The soil is a sandy loamy tex- tive of the soil structure being classiﬁ ed by these authors tured Hapludalf (USDA, 2012). The region where the from measurements obtained by the volume of aggre- city of Turuçu is located is composed of pioneer vegeta- gates method from sandy loam soils that produces F tion characterized by shrub-herbaceous species, typi- values smaller than soils with different textures (Utomo cal of the lagoon, with forest formations and alluvial and Dexter, 1981). The volume of aggregates method es- lowland, typical of the deciduous and semi-deciduous timates F values ( F’ ) as being the slope of the straight forests, with natural pastures as the primary land use

line that relates the logarithm of TS to the logarithm of (IBGE, 2004).

¡ ¢ ¤ ¢ © ¤ ¢ ¤ £ ¨ ¢ ¢ £ ¥ ¦ ¢ " ¢ ¤ © £ § £ (

the sample size (aggregate volume). This method produc- ' : i)

¤ ¢ ¥ £ ¤ ¡ ¢ ¦ £ + ¨ $ ¨ ¢ ¦ , § ¤ ¨ ¡ © ¨ £ - . - $

es F values smaller than those determined from the coef- '

( ( ﬁ cient of variation method ( F). Chan et al. (1999) found ' Zea mays (L.) Nicotiana

a mean ratio of F/F’ of ~ 2 for clayey soils. Imhoff et al. alata (Link et Otto) his area has been cultivated an-

£ ¥ ¢ £ £ £ £ ¤ © ¨ , ¢ §

(2002) used the classiﬁ cation of F proposed by Utomo nually with a ( ( Fragaria

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and Dexter (1981) multiplied by 2 to deﬁ ne classes of F vesca (L.) ' ; (ii) soil

¥ ¦ ¢ " ¢ ¤ © £ § £ ¤ ¢ © ¢ ¥ ¤ " © ¤ © ¤ ¢ © £ ¤ ¥ ¢ '

for an Oxisol. The F classiﬁ cation proposed by Imhoff et '

/ 0 - $ ¥ £ ¢ ¦ © ¤ ¢ © £ ¤ ¤ ¡ ¤ § § ¢ © £ $ 1 & & 2 , ¢ ¤ ¡ ¢ £ ¤ § ¢ ©

al. (2002) was used for the F values in this work. .

¥ ¤ " © ¤ ¨ ¤ ¡ © ¢ 3 £ ¥ ¦ ¢ © ¤ ¥ © © £ ¤ ¥ ¢ / 0 -

Textural class, clay mineralogy and dispersion of ' (iii)

£ ¢ ¦ © © ¤ ¢ © £ ¤ ¤ ¡ ¤ § § ¢ © £ $ ¨ ¤ ¡ ¥ ¤ £ ¤ © ¢ these, content of clay and silt, organic matter and their ¥

Sci. Agric. v.71, n.2, p.163-168, March/April 2014 164

Reis et al. Physical parameters of an Alﬁ sol

4 5 6 7 4 ? @A 6 5 4 5 C A ? 5 4 5 A A G A 4 5

:; < : = > 9 B : 9 D 8 9 = 9 E 8 F = = : 9

iv) 8 9 , gregates (Chan et al., 1999). On the assumption that ag-

A AC A 6 4 A A@

= : F : B =

herein called natural area (NAT), without soil tillage. gregate density is constant, the DD of each

H M R

4 @ A 4 4 ? A 6 5 I G 4 5 6 7 4 4 4 5 6 e A A@ 7

8 9 :; < 8 8 B ; : E J K K L J K NK O = 9 8 f = = 8 Z = NL L V W

> aggregate D) was calculated in

R Q R

4 5 7 C S 7 C 5 A G A 5 G 4 5 6

9 P = : NL L V ; 9 8 9 D 8 9 :; NK T J K T NK B : > : > = O; = >

P :

6 @A ? AC A @ A 4 A@ 4 5 6

< : = > O 8 = : F ;U V D 9 B = > ; U 8 9 D K WK K XK WK Y K WK Y XK WNK

A 4 6 4 A 7 @A ? C 4 A @ @A 4 A 5

B < : = > = > : 9 D 8 > 9 F ; 8 : Z ;: = 8 T D 9 E = = B = 8 T

Q R H

6 A ? A A 4 A @ A@A C A 5 4 S 5 G 5 C 5

9 = > 8 [ V W > 8 ; U 8 < > 9 8 = : : = 9 9 X

= <

e 4 G G @AG 4 A 7 A @A A 4 A @ 4 G A 6 4 A A @ 6 A @ 4 5 A @ 5 G 6 A ? C @ ? 4 5 6 C 5 6 A @ A 6 A \ 5 A

B g = < > = > F : B = 9 D = : 9 = > 9 9 = : = > 9 D 9 8 9 8 : = > 9

8 : is mm), M is

M h

A 4 A 5 6 6 4 4 G G @AG 4 A 7 A 4 A @ 4 G A 5 ] A 5 C A 6 4 5 4 G A A 5

= > B 8 8 9 D = > : :F : E ; = = > F 9 8 = : E P U B B = 8 W

B Soil blocks were wrapped g), and

4 4 G G @A G 4 A 5 A ? ? 4 5 7

8 8 9 D = 8 : = > 9 E ; = : 9

in plastic ﬁ lm immediately after removal to maintain B g).

@ 4 4 4 A 4 A 6 A A 6 ? @ ? A 6

P : ; : = U < 8 ; 8 9 8 = : B = P U = > B = > 9 9 9 8 P U

the natural soil water content and the integrity of the :

4 4 5 6 e A A@ 7 4 C C @ 6 5 G A A j 4 5

= = 8 Z = NL L V 9 : = 9 = > E = : 9 g

samples until their arrival at the laboratory. They were f

A @ ? 4 5 A

: ; 8 9 D

carefully and manually fragmented along = >

4 @ 6 @ A 6 4 A@ A 4 S 5 A 4 5 6

: : W = O 8 8 < 60 soil aggregates were se-

lected per depth in the four areas studied, totaling 480

@ 4 5 G 5 G @ 5 6 4 A A @

: 9 B _ = 9 L B B : : B = W

soil aggregates samples, D

` i

4 C 4 G G @A G 4 A 4 A G A 6 4 5 6 A A A C A 6 4 A A@ A@A A @ 4 l m

> 8 9 :; = < 8 < : > = > DD = : F : B = > : 8 = > 8 9 :; D : P : ; : = U k

< the standard deviation of mea-

4 4 5 A 6 A 4 @ A C 4 A @ 4 G A A A G 6 4 5 6 A 4 A @ 4 G A A A 4 @A 6 4 A

8 9 P = : P U = > : = > B = : F 9 D = > > : > = O < : = > = > F 9 D = > B 8 E F ;E 8 9 D

< sured values of TS, Y

H n H

A 5 G A 4 C 4 G G @A G 4 A 5 4 4 G G @AG 4 A 4 5 6 5 A 5 A@ @A ? C 4 A A AC 5 6

; = > 9 D > = W ;; = 8 O = > E B P 9 D ;: = 8 > 8 9

: .

A@ A j 4 5 @A ? @A A 5 A 4 5 6 4 @ 6 A @@ @ A C A

= B 9 D = > : 8 E = : 9 8 = 8 = > 8 = 9 D = > 9 D X

The indirect tension test was carried out using 9

M i

5 A 4 @ 6 A I 4 @ 4 5 @ 4 C 4 A A 6 A@A 4 A 6 5 A 4 A AC @ 5 C 4 C 4 @ C A 5

;: B 9 ; L a a 9 D F : = : 9 W : P : ; : = U ; 8 8 8 E 8 < P 8 9 = > F ; X = 9 : = E = 9 O : =

an ; at a constant

i p

7 ? @ ? A 6 A 4 7 5

–1 A

8 9 D 9 9 8 P U B > 9 DD = ; J K K J V 9 =

speed of 4 mm s until the aggregate failed i.e., until E dimensionless) . :

@ 4 A 7 @ 4 A 7 @ 4 A 7 q

D : P ; D : P ; : P ;

the formation of a continuous tensile crack which ran D 0.1), slightly 0.1-0.2), 0.2-0.5),

` R

4 C @ 4 A 7 4 5 6 AC 4 5 C 4 5 4 A 7

> D : P ; B > : ;;U E 8 = P ; K W V

approximately between the polar diameters W soil ag- very 0.5-0.8) > .

? 4 C A 6 5 A 4 A ? 5 4

; : = > B 9 8 = 8 = P ; 9 8 : = : 9 8 gregate < and loaded Disturbed samples were obtained after passing progressively, across a diameter, between a ﬁ xed lower through a sieve (mesh of 2 mm) for the physical fraction- plate and an upper parallel mobile plate that was assem- ation of soil organic matter according to Cambardella bled to accept an electronic load cell of 20 kg capacity. and Elliott (1992). The concentration of total organic car- The electrical output was recorded by a data acquisition bon (TOC) and carbon of the coarse fraction (CCF) in the

system. After each test, the aggregates of each set were soil mass were quantiﬁ ed by dry oxidation in an elemen- W oven dried at 105 b c to calculate the soil water content tal analyzer. The carbon associated with minerals (CAM) The gravimetric water content and particle size was obtained by difference between TOC and CCF. fraction analysis (Embrapa, 1997) are presented in Table Data were tested for normality using Shapiro-Wilk 1 and Table 2 respectively. The TS (kPa) was calculated statistics. Analysis of variance (Anova) and Duncan test in accordance with the expression formulated by Dexter (p ≤ 0.05) were performed per treatment and per layers. and Kroesbergen (1985): Regression analysis was carried out on the values of TS and TOC, CCF and CAM. These statistical tests were performed using the SAS software (Statistical Analyses

System Institute, version 5, 1999).

A@A A ? 4 @ 4 A A@ 4 A ? @ ? @ 5 4 C 5 4 5

< > = > B = K W Y d _ : 8 = > 9 = : 9 ; : = U 9 8 = =

9 , P

4 ? ? A 6 @C A 7 4 5 6 e A A AC A 6 4 A A@ 7

;: D 9 DD = : F : B = B B at failure N), = > ). Table 2 – Particle size fraction of an Hapludalf under several The equation was elucidated from the theory of loading management systems. spherical samples of linearly elastic material. Because of Particle-size distribution (g kg–1 ) its simplicity it has been used to estimate the TS of ag- Treatment 1 Soil textural class Clay Silt Sand 0.00 - 0.05 m Table 1 – Gravimetric water content of analyzed aggregates of an MT 95 205 699 Sandy loam Hapludalf under several management systems. MNP 133 211 655 Sandy loam Gravimetric water content (kg kg–1 ) NP 152 238 609 Sandy loam Treatment 1 0.00 - 0.05 m 0.05 - 0.10 m NAT 191 296 512 Sandy loam MT 0.55 a 0.64 a 0.05 - 0.10 m MNP 0.24 b 0.19 b MT 92 208 699 Sandy loam NP 0.40 ab 0.34 ab MNP 135 218 646 Sandy loam NAT 0.49 ab 0.52 ab NP 151 243 605 Sandy loam 1MT: soil under maize cultivation after tobacco in conventional system; MNP: NAT 164 244 591 Sandy loam soil under maize cultivation after natural pasture in conventional system; NP: 1MT: soil under maize cultivation after tobacco in conventional system; MNP: natural pasture; NAT: natural area. Numbers followed by the same letters in the soil under maize cultivation after natural pasture in conventional system; NP: column are not different by Duncan test (p ≤ 0.05). natural pasture; NAT: natural area.

Sci. Agric. v.71, n.2, p.163-168, March/April 2014 165

Reis et al. Physical parameters of an Alﬁ sol

~ x s } u s { { v t r z s | w } s x z v ~ z ~ } t s

Results and Discussion v t

v r s r { ~ x { x { r } r v { t

t u v w s u r x s t y r z r { | z { s r } w s } w } r s r z s v t } t v u { w } t

Statistical moments for r s (TS),

w { x y } t v ~ ~ } x { t { t r z } r r z s v t s x } w x } ~ r v { t { r z s u { v w ~ w } } t u v w r z } u }

r } (TOC), carbon of the coarse fraction

s x u r x { t y v t r s x } ~ r v { t v r z r z s { x y } t v ~ x } ~ r v { t

(CCF) and carbon associated with minerals (CAM) are

r z s u { v w } t r z } r r z s s s ~ r u { u v w r } t ~ w } ~ { t r s t r { t r z s

presented in Table 3. The results of the coefﬁ cient of {

s s t { t r z s ~ { t r s t r { { x y } t v ~ } r r s x variation (CV) for tensile strength are lower than the re- Czarnes et al. (2000) sults of Bartoli et al. (1992) that found CV values rang- reported that tensile strength was affected by organic ing from 40 to 73% for Oxisols. Imhoff et al. (2002) and exudates, rooting and plant growth. Accordingly, reduc- Blanco-Canqui et al. (2005) have reported log-normal tions in organic carbon caused by cropping practices, led distribution, while Dexter and Watts (2000) reported to changes in the values of tensile strength of aggregates, normal distribution, corroborating the ﬁ ndings of this making them less resistant to management processes.

study. Agricultural soils exhibit little stability against

v w r x s } r s t r u u s v t ~ { t s t r v { t } w u u r s u x s s } w s

{ applied tensile stresses. Nevertheless, tensile linkages

~ s w s s w u { u { v w { x y } t v ~ } r r s x } } ~ r r z } r ~ } t s } r r x v |

x s in soils are the most effective factors in soil resistance

r s r { } x v } r v { t u { u z { v t y r z s v { x r } t ~ s { { x y } t { |

against cultivation operations, execution of horizontal

v t s x } w v t r s x } ~ r v { t u { x } y y x s y } r v { t } t u { v w u r x ~ r x s } w s

load (in tensile processes), and vertical load (in displace-

z s x s ~ r v { t { } t } w s u v t |

ment processes) under ﬁ eld conditions (Chancellor,

| w } s x u { x r z s } t r x s } r s t r u ~ } t

r { 1994). Soils which possess aggregates with greater TS are

s y s t s x } w w } r r x v r s r { r z s r v s { u s { ~ { t s t r v { t } w r v w w } y s

more resistant to mechanical dispersion when subjected

w { v t y w u z } x x { v t y t w r z s r x s } r s t r x s u w r s v t } t to soil preparation.

Table 3 – Statistic moments of total organic carbon (TOC), carbon of the coarse fraction (CCF), carbon associated with minerals (CAM) and tensile strength (TS) of an Hapludalf under several management systems. TOC CCF CAM TS TOC CCF CAM TS ------g dm–3 ------kPa ------g dm–3 ------kPa MT 1 0.00 - 0.05 m 0.05 - 0.10 m Mean 9.7 4.03 5.67 111.15 11.04 3.85 7.19 131.07 Standard deviation 1.48 1.15 1.67 34.67 4.69 1.12 4.22 24.08 Coeffi cient of variation (%) 15.29 28.65 29.5 31.2 42.51 29.21 58.65 18.37 Minimum value 7.61 3.18 2.01 59.17 7.22 3.02 2.9 85.71 Maximum value 12.8 7.29 8.34 155.33 22.47 6.5 17.42 167.53 MNP2 0.00 - 0.05 m 0.05 - 0.10 m Mean 15.95 3.95 12 154.15 14.36 3.64 10.72 131.3 Standard deviation 2.31 0.52 2.07 48.7 1.17 0.58 1.13 38.6 Coeffi cient of variation (%) 14.51 13.04 17.3 31.59 8.16 15.95 10.56 29.4 Minimum value 12.55 3.19 8.96 108.36 11.93 3.04 8.43 94.58 Maximum value 20.23 4.66 15.65 270.35 16 4.8 12.76 218.02 NP 3 0.00 - 0.05 m 0.05 - 0.10 m Mean 29.27 5.75 23.52 166.21 19.35 4.59 14.76 155.91 Standard deviation 2.51 2.42 2.11 46.18 2.1 2.73 3.28 61.09 Coeffi cient of variation (%) 8.59 42.06 8.96 27.78 10.87 59.5 22.25 39.18 Minimum value 24.82 3.43 19.2 110.95 16.87 2.84 7.71 88.76 Maximum value 33.01 10.09 27.18 246.86 21.88 11.7 18.85 316.21 NAT 4 0.00 - 0.05 m 0.05- 0.10 m Mean 31.33 7.75 23.58 169.36 19.89 3.91 15.98 165.68 Standard deviation 6.78 2.16 5.38 45.59 6.88 0.56 6.42 55.3 Coeffi cient of variation (%) 21.63 27.82 22.84 26.92 34.58 14.31 40.16 33.38 Minimum value 22.42 5.72 15.86 114.28 11.79 3.14 8.63 91.53 Maximum value 43.2 13.42 32.94 281.81 31.96 4.95 27.5 261.84 1MT: soil under maize cultivation after tobacco in conventional system; 2MNP: soil under maize cultivation after natural pasture in conventional system: 3NP: natural pasture; 4NAT: natural area.

Sci. Agric. v.71, n.2, p.163-168, March/April 2014 166

Reis et al. Physical parameters of an Alﬁ sol

Table 4 – Concentration of total organic carbon (TOC), carbon of the coarse fraction (CCF) and carbon associated with minerals (CAM) of an Hapludalf under several management systems. Treatment 1 TOC CCF CAM ------g dm–3 ------0.00 - 0.05 m MT 9.70 c 4.03 b 5.67 c MNP 15.95 b 3.95 b 12.00 b NP 29.27 a 5.75 b 23.52 a NAT 31.33 a 7.75 a 23.58 a 0.05 - 0.10 m MT 11.04 b 3.85 a 7.19 c MNP 14.36 b 3.64 a 10.72 bc NP 19.35 a 4.59 a 14.76 ab NAT 19.89 a 3.91 a 15.98 a 1MT: soil under maize cultivation after tobacco in conventional system; MNP: soil under maize cultivation after natural pasture in conventional system: NP: natural pasture; NAT: natural area. Numbers followed by the same letters in the column are not different by Duncan test (p ≤ 0.05).

Correlations between total organic carbon carbon of the coarse fraction and carbon associated with minerals suggest increases in TS present a signiﬁ cant linear

relationship (p < 0.01) (Figure 1). Similar results were

¢ £ £ ¤ ¥ ¤ ¦ ¡ ¥ ¦ ¦ § ¨

obtained by Blanco-Canqui et al. (2005). ¡

¢ ¡ ¤ ª « ¢ ¥ ¥ ¬ § ¢ ¡ £ ª ¢ ® ª ¦ ¡ ¥ ¬ ¡ ¥ ¯ ¤ ¥ ª ¦ ¡ ¥ ¬ ¡ ¥ ¢ ¡ £ ° ¦ ¦ § ® ª § ® ¯ ¥ ¢ ¨

¤ ¡ ¬ ¤ § ¦ ¡ ¦ ± ¤ £ ¬ ¯ ¢ ¯ ¦ ² ¢ ³ ¬ ¢ ° ¦ ¯ ¤ ¥ ¤ ³ ¬ ¬ ´ ´ ¬ ª ¥ ¦ ¡ µ ¶ · « ² ¦ ´ ´ ¬ ¥ ¢ ¸

¹ º º ¹ »

¸ The relationships of TS and other soil properties, such as organic carbon, have not been well resolved due to the many factors involved, e.g. soil texture, porosity, water content (Munkholm and Kay, 2002). TS is more strongly related to TOC and CAM than CCF which can be attributed to the formation of stable bonds between soil organic matter and the metal ions and minerals from the soil, that are signifi cantly affected by soil management. There are two opposing effects of soil organic matter on TS: an increase in the number and strength of linkages between the particles and the effect of dilution that would reduce the bulk density or increase the porosity of aggregates (Zhang, 1994). The increase in soil organic matter results in higher porosity of the aggregate which reduces the number of bonds between particles, in which case the strength of these links is not increased, and there is a consequent reduction in TS. Figure 1 – Correlation between tensile strength (TS) and total The predominance of one or another mechanism organic carbon (TOC) (A), carbon associated with minerals (CAM) will determine the direction of correlation between TS of (B), carbon of the coarse fraction (CCF) (C) of an Hapludalf under aggregates and soil organic matter, as verifi ed in studies several management systems. by Rahimi et al. (2000). Zhang (1994) found that there is reduction in TS with increasing organic matter due to the increase the porosity of aggregates. The degree of humidifi cation also infl uences TS, demonstrating that to the content of TOC, which in general was also high the more humidifi ed the soil organic matter becomes, in the surface layer. The MT and MNP systems had the the less its effect in reducing TS (Zhang, 1994). lowest TS in the two soil layers (Table 5), probably due The highest average values of TS were obtained to the land use history, in line with the common fact that in the 0.00 to 0.05 m layer. This fact can be attributed management systems promote changes in soil physical

Sci. Agric. v.71, n.2, p.163-168, March/April 2014 167

Reis et al. Physical parameters of an Alﬁ sol

ÆÊ Ã Ó Å ÆÃ¿ Æ Ë ¾ Ç Ï ¾ Ô Ô ÂÃ Ô ¾ Æ Ã Ì ÆÅ ÌÍ ÏÏÃ Â Ë¿ ÆÃ¿ ÌÃ Ï Â ¾ ÄÆÍ ÂË¿ Ô

Table 5 – Average tensile strength (TS) and friability (F) values of two Å , when

Ë¿ ËÐ ¾ Ç ÏÅ ÂÄ Ã Ë Ì ¾ Ó Ó ÇËÃ À ËÆ ÌÍ ÌÃ Ë ÌË¿ ¾ Ó Ó ÂÅ Ó Â Ë ¾ Æ Ã

layers of an Hapludalf under several management systems. Ð , which is why

Ð Ã ÄÊ ¾ ¿ Ë ã ÃÀ ¾ Ô ÂËÄÍ ÇÆÍ Â ¾ Ç Ä ÂÅ Ó Ì Ã Æ ¾ Ç Å ¿ Î

Treatment 1 TS, KPa F Soil classiﬁ cation as the friability 2 Ë¿ Macks ., 1996).

Ã ÂË¿ Ô ÆÊ Ã Ê Ë ÌÆÅ ÂËÄ ¾ Ç Í ÌÃ È ¾ ÌÃ À Å ¿ ÆÊ Ã Ä Ç ¾ Ì Ì Ë ê Ä ¾ Æ Ë Å ¿ Ó ÂÅ Ó Å ÌÃ À

0.00 - 0.05 m ÌËÀ

É å Ð Å Ê ÏÏ ÃÆ ¾ Ç Ý Ù Ú Ú Ù Ü ØËÆ ¾ Ó Ó Ã ¾ Â Ì ÆÊ ¾ Æ ÏÅ Â ÆÊ Ã ØÆÊ Ã

MT 111.15 b 0.27 ± 0.05 Friable È Hapludalf

Ç ¾ Ì Ì Ë ê Ä ¾ ÆËÅ ¿ À Å Ã Ì ¿ Å Æ Ó ÂÅ á ËÀ Ã Ã á ËÀ Ã ¿ Ä Ã Å Ï Ä Ê ¾ ¿ Ô Ã Ì Ä ¾ Í ÌÃ À È É

MNP 154.15 a 0.28 ± 0.06 Friable Ä

ËÇ Ð ¾ ¿ ¾ Ô ÃÐ Ã¿ Æ Ý NP 166.21 a 0.32 ± 0.06 Friable ÌÅ NAT 169.36 a 0.31 ± 0.06 Friable 0.05 - 0.10 m Acknowledgments MT 131.07 a 0.33 ± 0.07 Friable MNP 131.30 a 0.39 ± 0.08 Friable To the National Council for Scientiﬁ c and Techno- NP 155.91 a 0.29 ± 0.06 Friable logical Development (CNPq), for a scholarship grant, su- NAT 165.68 a 0.18 ± 0.04 Slightly friable plied to the fourth author and to the agricultural farmers 1MT: soil under maize cultivation after tobacco in conventional system; MNP: of Turuçu, in the state of Rio Grande do Sul , Brazil. soil under maize cultivation after natural pasture in conventional system: NP: natural pasture; NAT: natural area; 2 Based on Imhoff et al. (2002). Same References letters in the same column means no difference between values (Duncan test p ≤ 0.05). Bartoli, F.; Burtin, G.; Guérif, J. 1992. Inﬂ uence of organic matter on aggregate in Oxisols rich in gibbsite or in goethite. II. Clay

dispersion, aggregate strength and water-stability. Geoderma

½ ¾ ¿ À Á ¾ ÂÃ Ä Å ¿ ÆÂÅ ÇÇÃ À È É ÆÊ Ã À Ë ÌÆÂËÈ Í Î

quality and TOC. ¼ 54: 259-274.

¿ Å Ï Ð ËÄ ÂÅ Ä Â ¾ Ä Ñ Ì ¾ ¿ À Ò Ã ¾ Ñ Ó Å Ë¿ Æ Ì Ò ËÆÊ Ë¿ ÆÊ Ã ÌÅ ËÇ ¾ Ô Ô ÂÃÔ ¾ ÆÃ Ì

ÆËÅ Blanco-Canqui, H.; Lal, R.; Owens, L.B.; Post, W.M.; Izaurralde,

ÕÖ

× ÆÃÂ ØÙ Ú Ú Û Ü Ý ¼ Ê Ã ÂÃ ÌÍ ÇÆ ÌË¿ Þ ¼ ¾ ¿ À Þ ß à Ð ¾ É ¾ Ç ÌÅ È Ã ¾ ÌÌÅ ÄË Î

Ã R.C. 2005. Mechanical properties and organic carbon of soil

ÆÃ À Ò ËÆÊ á ¾ Â Ë ¾ ÆËÅ ¿ Ì Ë¿ Ë¿ ÆÃ ¿ ÌËÆÉ ¾ ¿ À Ï Â Ã â Í Ã¿ Ä É Å Ï Ò ÃÆÆË¿ Ô ¾ ¿ À

¾ aggregates in the Northern Appalachians. Soil Science Society

ÂÉ Ë¿ Ô ÄÉ Ä ÇÃ Ì Å Ï ÌÅ ËÇ ÂÃ ÌÍ ÇÆË¿ Ô Ë¿ Ð Ã ÄÊ ¾ ¿ Ë Ä ¾ ÇÇÉ Ò Ã ¾ Ñ Ã Â ¾ Ô Ô ÂÃ Î

À of America Journal 69: 1472-1481.

¾ ÆÃ Ì Ø ¾ ¿ À ÆÊ ¾ Æ ØÍ ¿ À Ã Â ¾ Ó Ó ÇËÄ ¾ ÆËÅ ¿ Å Ï ¾ ÇÅ ¾ À ØÆÊ Ã É À Å ¿ Å Æ Ó ÂÅ À Í Ä Ã

Ô Cambardella, C.A.; Elliott, E.T. 1992. Particulate soil organic

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