Comparison of aggregate stability indices for soil classification and assessment of soil management practices

SOIL T E C H N O L O G Y

vol. 2, i13-133

Cremlingen 1989 ]

C O M P A R I S O N OF A G G R E G A T E STABILITY INDICES F O R SOIL CLASSIFICATION A N D A S S E S S M E N T OF SOIL M A N A G E M E N T PRACTICES G . Chisci, P a l e r m o R Bazzoffi, F i r e n z e J.S.C. M b a g w u , N s u k k a

Summary

1

In view of soil structure analysis and land-use and management history evaluation, several mechanical indices of soil structure were tested on 13 italian soils. From the wet and dry sieving aggregate distributions, a pseudo-textural aggregation index (Ipta) and a mechanical aggregation index ( l m a ) ) were determined and compared with other soil structure stability indices based on single-sieve analysis. The single-sieve indices S and WSI were good correlated with Ipta and Ima, showing the possibility of substituting the time-consuming aggregate-size distribution determinations. All the soil structure indices were correlated with selected semi-permanent soil characteristics. Soil structural characteristics can be assessed with multiple-linear regression models using the semi-permanent soil matrix characteristics.

Primary soil particles, particularly clay, tend to cohere under the action of climatic, physical-chemical, biological and cultural factors to form secondary units called aggregates. The dynamics of their formation and their ability to resist breakdown by disruptive forces is responsible for maintaining a high soil mass permeability to water and air and for favouring penetration and expansion of plant roots. The grain size distribution of soil aggregates under different field conditions is related to the susceptibility of the soil to detachment by raindrop impact "splashability", to movement "transportability" by wind and water and to the scoring action of runoff "rillability". These three factors represent the major mechanical processes of soil erosion. The grain size distribution of soil particles is also responsible for the dimension and characteristics of pore space in the soil mass and assumes an important agronomic value for soil management in relation to soil workability, crustabitity and compactability.

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Introduction

Chisci, B a z z o ~ & Mbagwu

t t4

While it is relatively easy to measure the primary particle-size distribution (texture), aggregate-size distribution analysis is a different matter because the aggregation of soil particles is a more or less transient feature of the soil solid phase, widely variable from soil to soil in its extent, dynamics, causes, binding forces etc. There are four main purposes for mechanical analysis of soil structure : (a) to acquire information on the soil physical behavior in relation to climatic, mechanical, biological and agronomical factors; (b) to design soil management practices in accord with a particular soil system; (c) to classify different soils in relation to their physical-agronomical structural properties; (d) to evaluate the influence of soil structure on different soil management practices. Several laboratory methods for direct soil aggregation and stability determination were tested on 13 Italian soils, with the aim of evaluating: (a) the replicability of the different analyses of aggregate stability and indices; (b) the sensitivity of the several indices in detecting historic land use differences; (c) the relationship between the different indices; (d) the relationship between some semipermanent soil characteristics and indices of aggregate stability;

(e) the diagnostic value of different indices for classification and soil management purposes.

2

Consideration on soil structure analyses

The choice of artificially simulated conditions under which the aggregate-size distributions may be measured in the laboratory is largely dependent on many different causes, binding forces, etc., responsible for aggregate formation and stability in difl'erent soils, climates and land management situations. As a consequence, the choice of conditions under which the aggregate-size distributions have to be measured in the laboratory is largely determined by the purpose of the analysis. Due to the transient aspect of soil particles aggregation, any determination of aggregate-size distribution is also a determination of aggregate stability (KEMPER 1965). Although the disintegrating forces applied in the laboratory may attempt to simulate those found in the field, it cannot be said that the forces used in the laboratory successfully duplicate field conditions. Consequently, the relationship between aggregate-size distribution in the laboratory and soil aggregation state and dynamics in the field is largely empirical ( K E M P E R 1965). For diagnostic purposes, however, the establishment of threshold conditions for the disintegrating forces and the application of continuous or increasing forces to the same sample, may enable the laboratory investigator to measure aggregatesize distributions yielding useful information for comparative classification of different soils and for previsional application of soil management practicies (DE SOIL TECHNOLOGY--A cfmperating Juurnal of CATENA

Aggregate Stabih'ty Indices BOODT et al. 1961, CHISCI & SPALLACCI 1984). In 1953 a comparison of aggregate-size distribution values obtained by different laboratories on the same soil, using similar disruptive forces (i.e. an Yodertype procedure) showed low replicability (VAN BAVEL 1953). Since then, a continuous effort made toward standardization of the methods of soil sampling, storage and treatment for dry and wet-sieving analysis of aggregate stability has been attempted, to increase the replicability of the data (BLACK 1965, DE BOODT 1967, ANONIMOUS 1968, BURKE et al. 1986). In an attempt to overcome the timeconsuming analysis of aggregate-size distributions, many investigators have meastlred the stability of a chosen group of aggregates rather than a multiple-class aggregate size distribution. This was done on the following assumptions: a) that a much simpler procedure is involved in taking only one size aggregate fraction; b) that the results of the stability of such aggregates are often highly correlated with that of aggregate size distributions; c) that the ability of an adequately chosen class of aggregates to resist breakdown by continuing and/or increasing disruptive forces may enable the investigators to understand a variety of phenomena at field scale (BRIANT et al. 1948, STRIKLING 1951, SHALLER & S T O E K I N G E R 1953, KEMPER 1965, PANABOKKE & QUIRK 1957). For summarization of data, comparison of different soils and easier interpreSOIL "I'ECHNOLOGY--A cooperating Journal of CATENA

115 tation, several indices of aggregation and aggregate stability have been proposed by different Authors. In relation to grain size distribution analyses, for instance, the calculation of the mean-weight diameter of the distribution was widely used (VAN BAVEL 1953, DE LEEHNEER et al. 1959, DE BOODT et al. 1961). Dry and wet-sieving grain-size distributions have long since been recognized as a valuable method for direct laboratory mechanical analysis of soil structure (TIULIN 1928, YODER 1936, RUSSELL 1938). The above methods are widely used by soil scientists for direct soil structure assessment (KEMPER & CHEPIL 1965, BURKE et al. 1986). Other methods for a better evaluation of the transient aspect of soil aggregation and stability were to compare multiple and single-sieve analyses of aggregate stability under different simulated conditions (moisture content of the aggregates, input of hydromolecular and mechanical forces etc.) (HENIN et al. 1958, KEMPER 1965 etc.). Also the change of the mean-weight diameter of grain size distributions obtained under different moisture, hydrodinamic and/or mechanical forces of disruption was expected to produce suitable indices of aggregate stability, especially to evaluate different soil management histories on similar soils (DE LEEHNEER et al. 1959, DE BOODT et al. 1961). For comparison of different soils, it was found necessary to recur to aggregation and stability indices taking into account the primary particle-size distribution (texture) (ALDERFER & M E R K L E 1941, VAN BAVEL 1953, CHISCI & SPALLACCI 1984). Also taken into account in the single-sieve analysis of aggregate stability was the

116

Chisci, Bazzom & Mb~gwu

to the soil system in the laboratory, correction for the primary particles simulating the environmental forces of the same size-class of the aggreacting in the field. gates subject to standardized disruptive forces (HI~NIN et al. 1958, It was already emphasized that, for the MALQUORI &CECCONI 1962, KEMpeculiar transient character of soil strucPER & CHEPIL 1965, KEMPER ture, extent and stability of soil aggre1965,DE BOODT 1967). gates cannot be assessed independently. The data of different mechanical analNevertheless, the extent of soil aggregayses of soft structure have often been tion may be evaluated for specific threshsynthetized by calculating indices with old conditions of aggregate stability, as the following purposes: in the aggregate index devised by VAN (a) to summarize and combine the data BAVEL (1953). of different grain-size distributions; (b) to express the data in a comparable form for different soils; (c) to interpret the data of structure analysis for specific purposes and (d) to improve the interpretation of soil structure analysis carried out in the laboratory for field applications.

3

Materials and methods

3.1

Soils

The 13 soils used in this study were collected in four locations of North-Central Italy : a) Modena (Emilia Romagna);

b) Lamporecchio (Tuscany); Many indices have been devised by soil scientists for specific purposes, often c) Volterra(Tuscany) ; based on different analyses of soil structure (T1ULIN 1928, 1933, MIDDLEd) Cremona (Lombardy) TON 1930, RUSSELL 1938, ALDERand are classified following the ARSFER & M E R K L E 1941, RUSSELL USDA Soil Taxonomy into four groups: M.B. et al. 1947, VAN BAVEL 1949, SHALLER & STOCKINGER (a) Vertic Xerochrept; 1953~ G A R D N E R 1956, STIRK 1958, HI~NIN et at, 1958, FEODOROFF (b) Fluventic Xerochrept; 1960, DE BOODT et al. 1961, BRYAN (c) Typic Psammaquent; 1968, etc.). All the mechanical indices, however, (d) Aquic Xerofluvent. were used with the purpose to assess and standardize two main interdependent asUndisturbed soil samples were taken pects of the soil structural status: following standard procedures as re(a) the extent of aggregation of the ported by KEMPER and CHEPIL primary particles in the soil solid (1965) and BURKE et al. (1986). These soils are widely different in parphase; ent material, geology, vegetation and cli(b) the stability of such aggregates un- mate. Moreover, in each location, samder inputs of different forces applied ples having different land-use histories SOIl. T E C | I N O L O Q Y - - A cooperalilLg Journal of CATENA

Aggregate Stability Indices

11 7

Sample"

Classification

Location

Land use history

M24A0

VerticXerochrept

Modena

Tilled soil

M24A3

VerticXerochrept

Modena

Tilled soil with yearly addition of pig slurry since 1970

M53AS0

Fluventic Xerochrept

Modena

Tilled soil

M60AS3

Fluventic Xerochrept

Modena

Tilled soil with yearly addition of pig slurry since 1970

VI6TS70

VerticXerochrept

Volterra

Undisturbed land since 1970, covered by natural vegetation

V16SS70

Vertic Xerochrept

Volterra

Sub-soil from land undisturbed since 1970, covered by natural vegetation

V17TSP

VerticXerochrept

Volterra

Tilled soil since 1970

L22T

Typic Psammaquent

Lamporecchio Tilledsoil

L20A

Typic Psammaquent

Lamporecchio Tilledsoil with yearly addition of sawage-sludge since 1970

LI7T

Typic Psammaquent

Lamporecchio Tilledsoil

L16A

Typic Psammaquent

Lamporecchio Tilledsoil with yearly addition or sawage-sludge since 1970

C2T

Aquic Xerofluvent

Cremona

Tilled soil

C3M

Aquic Xerofluvent

Cremona

Tilled soil with yearly addition of cattle-slurry since 1970

* Year of sampling fall 1986 horizontal lines differentiate soil groups which are homogeneous in relation to the locality of origin and USDA classification Tab. 1: Soil sub-groups and land-use histories.

were collected to evaluate the sensitivity of several structure indices in detecting the effect of different soil m a n a g e m e n t practices on the same matrix soil (tab.l). Some standard physical and chemical characteristics considered of i m p o r t a n c e to soil structure assessment are reported in tab.2. The primary particle grain-size distrib u t i o n analysis of the collected soils was carried out taking into a c c o u n t the sepSOIL TECHNOLOGY

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a r a t i o n of primary soil particles < 4 m m into classes o f 4 - 2 mm, 2-1 mm, 1 0.5 mm, 0.5-0.25 ram, 0.25-0.02 m m , 0.02-0.002 ram, <0.002 mm. 3.2 3.2.1

Analysis of soil structure Dry-sieving analysis

For Dry-sieving analysis of soil aggregate-size distribution, the m a i n lines o f the m e t h o d by K E M P E R &

I 18

Chisc~ Bazzoff) dz Mb~gwu

Sand ~

M24AI3 M24A3 M53AS0 M60AS3 Vt6TST0 V 16SS7(~71 VI7TSP L22T L20A

L17T LI6A C2T C3M

12.2 13.[ 56.8 54.2 26.0 28.9 22.2 52.0 59.1 66.1 64.6 49.2 46.4

Silt

Clay

Texlure 21

O.C.

Liquid limit %

Plastic limit %

ASC 3/

SCW 41

C.F..C.5~

pt-I {H2 O)

IiI~1

%

%

41.4 40.l 25.1

46.4 46.8 18.t

C C SL

1.31

62.I2 61.05

31,01 26,66 16.01

60.24

39.95

31.12 31,12 23,17

56.79

1.55

27.19

40.47

25,06

20.41

25.39

53.16 50.07 53.45 36.79 42.27 33.72 43,83 4L29 42.85

26,31 22,79 24,95 21,34 27,59 23.42 27.81 25.89 27.85

34,45 19,98 24.85 15.72 16.0i 10.24 16,38 23.4 25,92

39.55 51.12 52.95 32.28 27.19 23,66 19.02 27.40 27,68

31.5 29,9 21.7 25.9 22.5 28.5 22.8 21.2 22.5 16.9 20.7 21.7 25,D

7.64 7.55 7.56 7.41 7.59 7.77 7.84 6.29 7.13 5.80 6.90 6.20 6.80

64.68 69.64 38.03 37.28 47.65

1,14

21.7

24.1

SL

1,53

30.6 34.1 34 28.3 24.9 19.3 22,1 29,9 36

37.4 37 43.8 I9.7 16.0 14.6 13.3 20.9 17.6

CL CL C SL SL SL SL SCL SCL

1.35 (k92 0.88 0.54 1.14 0.53 1.62 1.49 1.72

%

11 S a n d = 2--0.02 ram, Silt = 0.02--0.002 rUm, Clay ~ <0.002 m m

2) C = clay, SL = Sandy-loam, CL ~ Clay-loam, SCL = sarxdy-day-loam 31 ASC % = aggregated silt+clay (silt+clay in calgon-dispersed sample (SCC%) minus silt+clay in water-dispersed sample) 4i S C W % = silt+clay in water dispersed sample 51 CEC = cation exchange capacity in milliequi,~alelats per 1130g ol" soil 61 Iif ~ Fine aggregate instability index ~ SCW% / SCC% x 100 71 Soil samples were taken in the 0-20 cru layer, with. the exception of soil V16SS70 (20-40 cm)

Tab. 2: Some physical and chemical semi-permanent soil characteristics. C H E P [ L (19651 were followed: for each sample 1 Kg of soil was collected in the field in "dry-tilth" condition (78 % moisture content, w/w, for coarsetextured soils and 10-12% moisture content, w/w, ~br clay soils). After air-drying, each sample was broken down into smaller mechanical aggregates by the use of a rubber hammer. Each sample was thereafter sieved through a 4 mm mesh. The soil sample < 4 m m (s put on top of a nest of sieves o f 2, 1, 05, 0.25, 0.12, 0.05 m m diameter and the nest of sieves is put on a vibrating shaker tbr 10 rain. The mass o f oven-dried particles at 105 ° for 24 hrs on each sieve, that resisted breakdown,

is then determined. To compact the data of the dry-sieving grain-size distribution of the different soils the mean-weight diameter of each distribution is calculated (MWDd), as proposed by VAN BAVEL (19501:

M WDd = ~

(qbi"mi)

(11

i=1 where M fVDd

mi

3.2.2

= mean-weight diameter, mm = mean diameter of the size-fraction, mm = proportion of the total mass retained in ~he ith size-fraction Wet-sieving analysis

Part of the sample (sieved through the 4 m m mesh) is soaked with distilled water for i0 rain on the top o f a nest o f sieves of (D m m 2, 1, 0.5, 0.25. The nest of sieves is then vertically oscillated in water with a stroke of 4 cm, for a time period long enough to have a relatively constant mass of particles in each sieve. This method, introducing a differential input of hydromolecular and mechanical forces for each soil, was introduced on the assumption that the remaining semistable aggregates m a y define a pseudo-

SOIL T E C H N O L o G Y - - A coopetat~rtg Jourtla( cff CA't'ENCt

63.50

65.86 43.21 45.64 36.01 27.77 35.91 33,59

Aggregate Stability Indices

119

textural m i c r o - s t r u c t u r e resisting break down u n d e r n o r m a l external environmental and c u l t u r a l factors for a short to m e d i u m time p e r i o d in the field ( T O R R I & S F A L A N G A 1980, C H I S C I & SPALL A C C I 1984). The mass o f o v e n - d r i e d particles (105 ° for 24 hr) in each sieve that resisted b r e a k d o w n is then d e t e r m i n e d and M W D w is c a l c u l a t e d using eq.1. 3.2.3

Single-sieve analysis I (single energy input)

Ten grams o f I - 2 m m air-dried aggregates (M1) is p l a c e d on a 0.2 m m sieve and soaked in distilled w a t e r for 10 rain. This is followed by an helicoidal up and d o w n oscillation for 20 sec, obtained using the wet sieving a p p a r a t u s by F E O D O R O F F (1960), at 1 cycle per sec and a 4 cm s t r o k e (with the sample immersed in water). T h e m a t e r i a l retained on the sieve (M2) is oven-dried (105 ° for 24 hr) and weighed. The A g g r e g a t e Stability Index "S" is then calculated as follows: S = ( M 2 / M 1). 100

Single-sieve analysis lI (double energy input)

Three grams of 1-2 m m air-dried aggregates are p l a c e d in a 0.2 m m mesh small sieve-basket o f the M A L Q U O R I & C E C C O N I e q u i p m e n t (1962) and soaked for 10 m i n over a layer of wet SOIL TI~.CHNOLOGY--,.A ctmperating Journal nf CATENA

The mass o f soil aggregates r e t a i n e d after 5 min and a f t e r 60 min in the two distinct sieve-baskets are then oven-dried (105 ° for 24 hr) a n d weighed. The quantities o f soil retained by the sieving baskets are used to calculate the oven-dried mass o f particles which pass after 5 min (A) a n d after 60 rain (B). The " W a t e r Stability I n d e x " (WSI) ( M A L Q U O R I & C E C C O N I 1962) is then calculated as follows: wsl

= ( 1 - A / B ) . lO0

(3)

where WSI A

= Water Stability Index, % = the percentage of oven-dried mass (t05 ° for 24 hr) of 1-2 mm particles which pass through the 0.2 mm sieve-basket mesh after 5 min; = the percentage of oven-dried mass (105° for 24 hr) of 1-2 mm particles which pass through the 0.2 mm sieve-basket mesh after 60 rain.

3.2.5

Aggregated silt + clay and silt + clay primary-particles free in water

(2)

where = aggregate stability index, % S MI = the mass of 1-2 mm particles used for analysis, adjusted for standard 105° oven-dried conditions M2 = the mass of 1-2 mm of oven-dried (105° for 24 hr) particles retained on the 0.2 mm sieve after treatment, corrected for sand (2-0.2 mm q~)

3.2.4

blotter. The sieve-baskets are then immersed in a 500-cc beaker o f distilled water at r o o m t e m p e r a t u r e a n d s h a k e n at a rate of 60 r p m .

A g g r e g a t e s i l t + c l a y ( A S C % ) is m e a s u r e d by the pipette m e t h o d t a k i n g silt + clay in a calgon dispersed s a m p l e ( S C C % ) minus s i l t + c l a y in a water dispersed s a m ple ( S C W % ) . S t a n d a r d h y d r o d y n a m i c and mechanical forces as in soil texture analysis are a p p l i e d to b o t h analyses. The a m o u n t of s i l t + c l a y free in a water suspension ( S C W % ) is then calculated as follows: SCW%

= SCC%

-- ASC%

(4)

C'Msci, B a z z o f f i

120

3.2.6

where

Pseudo-textural aggregation

Ima M WDd

index Using the a b o v e - m e n t i o n e d concept of " t h r e s h o l d condition o f disruptive forces" and the m e t h o d o f c o m p a r i s o n for M W D o f different grain-size distributions e m p l o y e d by D E B O O D T et al. (1961), an a g g r e g a t i o n index is calculated as follows:

MWDt

X

3.2.8 IpCa =

where lpta MWDw MWDt X

The on the

M W D w -- M W D t X -- M WDt • 100

-- mean-weight diameter of the dry-sieving grains-size distribution, mm = mean-weight diameter of the primary particles grain-size distribution, mm = maximum average grain-size diameter of the particles in the Oven grain-size distribution, mm (in our analysis X = 3 ram)

Aggregate stability index

W h i l e b o t h I p t a and I m a a r e in t h e m selves aggregate s t a b i l i t y indices u n d e r specific t h r e s h o l d c o n d i t i o n s o f d i s r u p tive forces, the c o m p a r i s o n o f t h e two = pseudo-textural aggregation index, % grain-size d i s t r i b u t i o n s o b t a i n e d u n d e r = mean-weight diameter of the high and low i n p u t o f h y d r o d y n a m i c a l wet-sieving grain-size distribution, mm a n d / o r m e c h a n i c a l forces on a i r - d r i e d o r = mean-weight diameter of the primary w a t e r - s a t u r a t e d soil s a m p l e s m a y p r o particles grain-size distribution, mm = maximum average grain-size diameter vide further i n f o r m a t i o n s i m i l a r to t h e of the particles in the given grain-size C M W D aggregate stability index d e v e l distribution, mm (in our analysis X = o p e d by D E B O O D T et al. (1961) to in3 ram) fer on the c o r r e l a t i o n b e t w e e n s t r u c t u r e c o m p u t a t i o n of index [5] is based stability a n d yield o f w h e a t a n d m a n following assumptions: golds.

(b) the m a x i m u m range o f possible M W D w m u s t be X - M W D t so that such a r a n g e is a constant for each soil.

T h i s index is c a l c u l a t e d as f o l l o w s :

- M fVDt X -- M WDt .100

Ias =

M WDd

(6)

MWDd--MWDw X - M WDw

• 100

(7)

where [as M WDd MWDw

Mechanical aggregation index

This index is calculated on approximately the s a m e assumptions of I p t a but considering the dry-sieving d i s t r i b u t i o n :

Ima =

= mechanical aggregation index, %

(5)

(a) the M W D w can assume the maxim u m value o f M W D t ;

3.2.7

& Mbagwu

X

aggregate stability index, % mean-weight diameter of the dry-sieving grain-size distribution, mm = mean-weight diameter of the wet-sieving grain-size distribution, mm = maximum average grain-size diameter of the particles in the given grain-size distribution, mm (in our analysis X = 3 ram)

T h e s t a n d a r d i z a t i o n o f i n d e x [7] is b a s e d on the a s s u m p t i o n t h a t the m a x i m u m range o f v a r i a b i l i t y o f M W D d falls b e t w e e n X and M W D w .

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Aggrega te Stability Indices

121

Percent of primary particles of size class (ram) 1-0.5 0.5-0.25 0.25-0.02 0.02-0.002

Sample

4-2

2-1

<0.002

MWDt t)

Group A2)

M24A0 M29A3 V16TS70 V16SS70 V17TSP

0,00 0.12 0.08 0.00 0.08

0.09 0.06 0.12 0.13 0.08

0.21 0.38 0,34 0,20 0.27

1.33 1.36 0.53 0.57 0.68

10.56 11.19 24.93 28.01 21.09

41.40 40.10 36.60 34.10 34.00

46.40 46.80 37.40 37.00 43.80

0.028 0,031 0,048 0.048 0.039 mean of group A 0.039+_,0,004

Group B

L22T L20A M53AS0 M60AS3

0.10 0.06 0.02 0,04

0.08 0.11 0.12 0.10

0.58 0.41 0.49 0.28

3.79 2.39 0.93 1.15

47.46 56.i3 55.24 52.64

28.30 24.90 25.10 21.70

19.70 16.00 18.10 24. I0

0.087 0.096 0.086 0.085 mean of group B 0.089+0,003

Group C

L17T LI6A C2T C3M

0.22 0.41 2.77 1.10

0,08 0.21 1.96 0.94

0.54 1,05 (I.55 0.82

4.15 4.52 1.25 1.53

61.12 58.41 42.67 42.01

19.30 22.10 29.90 36.00

14.60 13.30 20.90 17.60

0.114 0.118 0.187 0.119 mean of group C 0.134+0.016

1) S.E. of MWDt for each soil = -20.016; C,V. within each group = 1.89% 2) A = line-textured soils; B = intermediate textured soils; C = coarse textured soils

Tab. 3: Textural grain-size distribution and mean-weight diameter.

MWDt 1) M24A0 M29A3 VI6TS70 V16SS70 VI7TSP L22T L20A M53ASO M60A53 L17T LI6A C2T C3M S.E. within soils (+) C.V. within soils 3)

0.027 0,032 0.047 0,047 0.041 0,090 0.094 0,087 0.083 0.112 0.122 0.182 0.120

A2) A B B B DE E CD C F G H G

MWDw 0.781 0.759 1.520 1.184 0.614 0.183 0.247 0,633 0.732 0.174 0.503 0.393 0.469

F EF H G CD A A CED EDF A CB B B

MWDd 1.959 1.836 1.714 1.418 1,720 1,585 1.406 1.289 1.500 1.102 1.206 1.501 1.678

H G F D GF EF DC BC DE A AB DE F

lpta 25.35 24.50 49.88 38.48 19.37 3.21 5.27 18.74 22.25 2.16 13.23 7.48 12.12

las G FG I H E AB BC E F A D C D

53.06 48.04 13.06 12.46 46.35 49.77 42.10 27.72 33.85 32.83 28.18 42.49 47.73

Ima G F A A EF GF E B D CD CB E F

64.97 60,70 56.46 46.46 56.39 51.31 44.98 41.29 49.05 34.23 36.91 47.21 54.29

S H G F C F E C B DE A A CD F

48.75 46.15 56.80 45.50 30.20 9.35 19.05 26.15 34.60 8.80 28.90 35,30 42.50

WSI H GH I GF D A B C E A CD E F

46.70 45.50 70.10 68.95 63.80 2.50 11.30 42.65 45.00 2.40 34.15 49.20 66.00

0.002

0,047

0.049

1.29

2.41

1,51

0.58

1.87

2,68

7.40

3.21

6.98

6.63

3.01

1.69

2.83

1) For the explanation of the symbols see the text 2) Soils with different letters are significantly different by using Student LSD, P = 0.05 3) Pooled coefficients of variation within soils (replicate sample analysis)

Tab. 4: Results o/" the mechanical structure analysis and aggregate stability indices.

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D D E E E A B CD D A C D E

122

3.2.9

Ch&ci, B a z z o N & Mbagwu

Fine-aggregates instability index

M W D t and MWDd. A relatively higher variability coefficient was observed tbr MWDw. The amount of silt+clay free in water While the calculated indices show a (SCW%) was used to calculate a finefairly good replicability, the higher variaggregate instability index as follows: ability introduced by M W D w is however reflected in the indices including MWDw SCW% Ii T = - • 100 (8) for their calculation. silt+clay in calgon % Moreover, the variability of the indices includes the permutation of the basic 4 Results and discussion data of replicated distribution analyses required for the calculation of indices by 4.1 Replieability of laboratory analyses equation (5), (6) and (7). and indices Single-sieve analyses show a consisThe textural mean-weight diameter tent replicability, similar to the variabil(MWDt) was calculated to summarize ity coefficient indicated as appropriate the grain-size distributions of the differ- for single-sieve aggregate stability analyent soils and reported in tab.3. The com- sis by K E M P E R (1965). On the whole, the replicability of the parison of the M W D t of the different soils shows that the 13 soils can be di- mechanical analyses and that of the invided into three main groups. The differ- dices obtained in laboratory replicated ences between the average group M W D t analyses seems satisfactory for the practical purposes of detecting significant difwere highly significant. The mean values of: M W D d ; ferences between the different soils and M W D w ; Ipta; Ima; Ias; S; WSI of repli- treatments in this work. cated analysis are shown in tab.4 for the Structure C.V.% different soil samples. indices betweensoils Generalmean Because Ipta,Ima,Ias are derived indices, their mean values were calculated lpta 37.54 18.63__+6.99 lma 7.70 50.96_+3.87 from all the possible combinations of the Ias 16.85 36.33-t-6.12 replications of the variables from which S 19.97 34.55±6.91 they derive. WSI 30.07 38.98___11.72 The data o f ASC% and SCW%, being considered ah-nost semi-permanent soil Tab. 5: Coefficient o f variation "becharacteristics, are shown in tab.2. In the tween" soils for the different indices of same tab.2 is then reported the average structural stability. values for Iif. One of the most important aspects of different physical-mechanical analyses 4.2 Sensitivity of the different indices and structure indices is their replicability. Only two replications were provided Another aspect that was taken into acin our laboratory tests. However, the count was the sensitivity of the different variability coefficients (c.v.%) resulting structure indices in detecting differences in the "within soil" variation in labora- between soils and treatments (BRYAN tory analyses (tab.4) are acceptable, for 1971). ( M I D D L E T O N 1930)

SOIL TECHNOLOGY

A cooperating Jourl|al of CATENA

Aggregate Stability Indices

Textural group

M W D t 11

A fine-textured B intermediate-textured C coarse-textured

0.039 0.089 0.134

S,E, o f means +

0,015

C 2} B A

I23

Ipta 31,52 12,37 8.75 4,73

[ma A B B

56.99 46.66 43.16 3.42

las A B B

34,59 38.36 37.81 7,01

lif A B B

62.27 41.04 33.32 2.97

S A B B

45.48 22.29 28.88 5.57

WSI A AB B

58.99 25.36 37.94 9.82

l) For symbols explanation see the text 2) Means followed by different capital letters are significantly different using LSD by Student's t, P - 0.05

Tab. 6: Comparison between mechanical indices of structural stabilityJor soils grouped

according to texture class!fication. A comparison of the variability coefficients considering the between soils and treatment variability is shown in tab. 5. It appears that Ipta is the most sensitive index in detecting soil and treatment differences, followed in the order by WSI > S > Ias > Ima. While the ARS-USDA Soil Taxonomy classified the 13 soil studied into four groups: (a)Vertic Xerochrept (b) Fluventic Xerochrept; (c) Typic Psammaquent and (d) Aquic Xerofluvent, a different grouping can be used considering the MWDt. While the Vettic Xerochrept (group A, that we will denominate fine-textured) shows a very low MWDt, a second group (group B, that we will denominate intermediatetextured) giving a significantly higher MWDt, contains two sub-groups of soils:Typic Psammaquent and Fluventic Xerochrept. Finally, a third group C (that we will denominate coarsetextured) shows higher MWDt, which is statistically different from group A and B. This group contains some Typic Psammaquent and Aquic Xerofluvent soils having similar M W D t ratings. SOIL TECHNOLOGY--AcooperatingJournal or CA'rENA

Considering the mechanical indices of structure, it is shown that a consistent pattern exists between the grouping of soils obtained following MWDt and such indices (tab.6). The values of the structure indices are generally higher for group A in comparison to groups B and C. More uncertainty is found in discriminating between groups B and C. While it is not possible to make a reliable evaluation of the correlations between structure indices and semi-permanent matrix soil characteristics, due to the limited number of soil samples within each group, the observation of the data point out some trends requiring vaIidation on a larger number of samples. For instance, it seems that O.C.% content may assume a relatively more important role in explaining Ipta in groups B and C. SCW%, which appears linked with Ias in group B and C, may indicate that the increase of silt+clay free in water in coarse-soil systems may favour weak aggregation of larger grain-size particles. Furthermore, L1 and Pl appears to be better correlated to different structure indices in group A than in group B and C. Generally speaking, it can be deduced from the data that structure indices prevision from semi-permanent characteristics of the soil may be consistently im-

A AB B

Chisci, BazzofB & Mbagwu

124

Ipta li Group I

G r o u p I1

G r o u p HI

Group IV

Group V

las

Ima

S

WS1

Iif

Vertic Xerochrept (Modena) M24A0 25.35 A 21 M29A3 24.50 A

53.06 48.04

A B

64.97 60.70

A B

48.75 46.15

A A

46.70 45.50

A A

64.68 69.64

Means

50.55

a

62.87

a

47.45

a

46.05

b

67.16

Fluventic Xerochrept [Modena) M53A50 18.74 B 27.72 M60AS3 22.25 A 33.85

t3 A

41.29 49.05

B A

26.15 34.60

B A

42.65 45.00

A A

38.03 37.28

Means

30.79

ab

45.17

bc

30.37

ab

43.83

b

37.66

Vertic Xernchrept (Volterra) VI6TST0 49.88 A 13.06 V t6SS7O 38.48 B 12.46 VI7TSP 19.37 C 46.35

13 B A

56.46 46.46 56.39

A B A

56.80 45.50 30,20

A B C

70.10 68.95 63.80

A A B

47.65 63.50 65.85

Means

b

53.10

ab

44.17

a

67.62

a

59.00

Typic Psammaquent (Lamporecchio) L22T 3.21 CB 49.77 L20A 5.27 B 42.10 LITT 2.16 C 32.83 LI6A 13.23 A 28.18

A B C C

51.31 44.98 34.23 36.91

A B C C

9.35 19.05 8.80 28.90

C B C A

2.50 11.30 2.40 34.15

C B C A

43.21 45.64 36.01 27.77

Means

38.22

ab

41.86

c

16.52

b

12.59

c

38.16

Aquic Xerofiuvent (Cremona) C2T 7.48 8 42.49 C3M 12.12 A 47.73

A A

47.21 54.29

B A

35.30 42.50

B A

49.20 66.00

B A

35.91 33.59

Means

ab

50.75

abc

38.90

a

57.60

ab

34.75

24.92

20.49

35.91

5.97

9.80

ab 31

abc

a

c

bc

23.96

45.11

Standard Error within groups

1.29

2.4l

1.51

0.58

1.87

--

Standard Error between groups

5.22

7.19

3.90

3.74

6.35

4.41

11 For symbol explanation see the text 21 Means followed by different capital letters are significantly different using LSD by Student's t, P = 0.05 3) Means followed by different small letters are significantly different using LSD by Student's t, P = 0.05

T a b . 7 : Comparison between mechanical indices of structural stability for soils grouped according to soil classification and location.

proved by grouping the different soils by following their M W D t ratings. As a result a better interpretation of the different mechanical indices of structure for multiple applications would be obtained. In tab.7, the classification of the different structure indices are arranged according to the location the samples were taken from and the standard soil clas-

sification of the A R S - U S D A Soil Taxonomy making five groupings. In each grouping, different land-use histories are considered. Tab.7 shows significant differences of structure indices between soil groups and within groups for different land-use histories.

SOIL TECI-INOLOGY-.--.Acooperating JournM of CATENA

a

b

a

b

Aggregate Stability Indices

125

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P = 0.69

~

M2gA3

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V:t6TS70 []

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~ ,/, 20 ~ 0 •~ ~ l ~ - [ S p

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a

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Fine aggregates instability index (I!/) vs. mechnaical aggregation index (Ima).

SOIL TECHNOLOOY--A cooperating Journul of CATENA

Chisci, BazzofB & Mbagwu

126 80

70

r = 0.86 ~

. c 3 M#v~6ss7°'~/" /

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ViE'S70

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_

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40

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S

Fig. 3: Single sieve aggregate stability index ( S ) vs. MALQUORI & CECCONI aggregate stability imtex (WSI). 4.3

Correlation between indices

Ipta and Ima indices are not correlated. This may mean that each index provides independent information on the physical-mechanical soil structure for multiple-interpretation. In relation to [as, while a non significant positive trend is observed with Ima, the Ias index is significantly and negatively correlated with Ipta, (fig.l). A significant correlation (fig.2) was found between Iif and Ima, indicating that the amount of silt+clay free in water present in the soil mass may increase the stability of mechanical dry aggregates. This hypothesis may be confirmed by the fact that intermediate and coarsetextured soils, having a lower Iif, show a lower Ima. Single-sieve indices S and WSI are

significantly correlated for the tested soils (fig.3). The two indices show higher values for the fine-textured soils and progressively lower values for intermediate-textured and coarsetextured soils. In fine-textured soils the presence of a larger mass of stable aggregates >0.2 mm may exert a consistently beneficial effect on soil mass physical properties. In coarse-textured soils, the larger fraction of sand primary particles (2-0.2 mm) may compensate for the instability of aggregates of the same class, fi'om a mechanical point o f view. Our data confirm the statement of K E M P E R (1965) and other Authors that single-sieve indices can substitute for the more cumbersome and time-consuming aggregate-size distribution analysis. Both S and WSI are in fact well correlated

SOIL TECHNOI.OGY--A cooperadng Journal of CATENA

Aggregate StabHity Indices

127

70

60 ~

~

7:[67£7

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Ipta Fig. 4: Pseudo-textural aggregation index (Ipta) vs. single sieve aggregate stability index ( S ). with Ipta and, more scattered, with [ma 4.4 Statistical multiple regression (fig.4 and 5, only for S). Also there is a models for indices prevision certain trend between Ima and S. However it seems that other factors have t o The different mechanical indices o f strucbe taken into account for prevision of ture tested in this work were correlated with selected physical and chemical semiIma beside S. It follows that the easy-to-measure permanent soil matrix characteristics usstability index S may possibly reduce ing step-wise procedure in multiple-linear the need to measure the cumbersome regression analysis. Multiple regressions that were found and time-consuming wet-sieving grainstatistically significant are shown in size distributions. Additional informatab.8. tion on Ima can eventually be obtained It appears from the data that all o f by the easier-to-measure fine aggregate instability index Iif, eliminating the need the structure indices can be fairly well into measure the dry-sieving grain-size dis- ferred on fl'om selected semi-permanent tribution, at least for the scope of soil soil matrix characteristics, at least for the classification and land-use histories com- range of soils that have been tested. Ipta is well related to O.C.% parison. (GRIEVE 1980, M O L O P E et al. 1985). Moreover, the positive relationship with LI probably implies that the increase

SOIL TECItNOLOGY--A co~peratiag Journal of CATENA

Chiscl; BazzoI~ & Mbagwu

128

60

n V~6TS70 50

P = 0.64

~

t42gA3Z

M24AO

" Vi6SS70 40 r° 1-

C2T, N S ~ ~

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Ima Fig. 5:

Mechanical aggregation index ( lma ) vs. single sieve aggregate stability index

(S).

Independent variables Dependent Variable

O.C.

LI

PI

SCW

CEC

Intercept

R2

lpta

9.14 (10.96)*

3.29 (1.04)

-5.19 (1.46)

-0.88 (0.58)

--

12.12

0.78

Ima

7.28 (4.11)

22.22

0.68

WSI

57.97 fl 1.89)

2.80 (0.53)

-9,12 (2.03)

79.63

0,83

28.14 (6.78)

1.57 (0.30)

-3.05 (1.16)

5.66

0.86

-19.63 (3.56)

-11.26

0.93

(0.23)

Iif

0.51 (0.12)

1.17

1.16 (0.52)

* S.E. of coefficients in brackets 8: Multiple linear regressions of structure stability indices vs. some soil matrix semi-permanent characteristics. Tab.

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Aggregate Stability Indices of the mass of stable-aggregate particles may reduce the binding forces between particles. This result is sustained by the data of DE PLOEY (1979, 1981) showing that L1 is an important parameter in evaluating soil erodibility (DE PLOEY and M U C H E R 1981, BRYAN & DE PLOEY 1983). It may be expected that an additional increase of Ipta would increase "splashability" of the soil surface material in clay soils. Moreover, being that the splashed material is largely formed by particles (both pseudo-textural or primary particles) of the silt-fine sand class, the transportability of the material by overland flow may be increased. On the other hand, Ipta is negatively correlated with Pl. This may be explained considering that the increase of pseudo-textural aggregates may reduce the active-surface forces. Ima is also well correlated with O.C.%. Another interesting correlation is that with SCW%, sustaining the hypothesis that fine colloidal particles (free in water) present in the soil mass may bind surrounding larger primary particles, although weakly. Iif is also well estimated by O.C.%, L1 and CEC. While its negative relationship with O.C.% and positive relationship with CEC and L1 are clearly explainable.

5

Discussion

With regard to Ipta, the high input of hydrodynamic and mechanical forces applied to the soil system by rapid wetting and shaking in water would lead to a consistent breakdown of weak aggregates, so that only the most strongly bound aggregates would eventually resist. The resisting aggregates, then, are SOIL TECHNOLOGY--A cooperating J aurnal of CAI ENA

129

supposed to be less transient in the field soil system for short to medium-time periods. It follows that such aggregates may be considered pseudo-primary particles in the field. The pseudo-textural aggregation index was found useful in discriminating the different physical behaviour of clay soils having similar texture , in relation to a wide range of agronomical properties such as workability, crustability, compactability and erodibility (CHISCI & SPALLACCI 1984, BAZZOFFI & CHISCI 1986, CHISCI 1988, in press). In relation to erodibility the pseudoprimary particles of the soil seems to play an important role in the mechanics of soil erosion. A high [pta value for a clay soil is correlated to a reduction of soil particle cohesiveness, facilitating splash detachment (TORRI & SFALANGA 1980, 1982) and rill formation (GOVERS 1985, CHISC[ et al. 1985, DE PLOEY et al. I984, T O R R I 1986; TORRI et al. 1987). Moreover, an increase in the amount of detached semi-stable aggregates in the range of optimal trasportability as shown in the HJULSTROM (1935) diagram an by ROMKENS et al. (1987) may be expected. CHISCI (1988, in press) has reported data confirming the relationship between soil loss by water erosion and Ipta for three different clay soils. Other hypotheses are possible in relation to the agronomic value of Ipta for field application. For instance, it can be hypothesized that if the MWDw for a clay soil assumes a value similar to that of the MWDt of a silty or loamy soil, the crustability of a clay soil may also be similar to that of the latter types. Moreover, the presence of a more consistent pore-space on the range of capillary

130

forces may increase available water for plants when Ipta is high in a clay soil. With regard to Ima, it was established that a relatively low input o f mechanical disruptive forces in dry soil conditions would conserve a certain amount of weak aggregates, especially significant for short-term physical processes in the field. Specifically, Ima was assumed as a valuable index for establishing the performance of seed-bed conditions and, eventually,for short term structural conditions due to tillage practices and root growth, producing a mechanical aggregation of soil particles. M W D d was found useful for explaining the effect of different tillage practices such as ploughing and minimum-tillage, and their effect on winter wheat growth and yield and with regard to water storage, runoff generation and erosion on a single rainstorm basis during the fall season (CHISC[ et at. 1988, in press). While there is not yet sustained experimental support to assess the practical value of Ima, its usefulness may be hypothesized for evaluating the comparative capacity of different soils to maintain a good seed bed performance in relation to water infiltration and storage, soil aeration, seed germination, seeding and root growth. Ipta and Ima are in themselves aggregate stability indices under different threshold conditions o f disruptive forces and moisture contents of the soil sample. It was interesting to observe whether the use o f another index taking into account wet and dry-sieving distributions would add further information along the lines o f C M W D o f DE BOODT et al. (1961). As a matter o f fact, it is our opinion that the significant correlation that was found by the above Authors between C M W D and wheat and mangolds yield may be an indicator o f seed-bed perfor-

Chisci, Bazzofli & Mbagwu

mance in relation to uniform seed germination and seedling growth. If so, it may be hypothesized that [ma and Ias calculations may yield similar information. Moreover, it can be hypothesized that Ipta and Ias may be negatively correlated when group of soils having similar MWDt are considered. This is because the increase of pseudo-textural aggregates in the soil mass may decrease the strength of the binding forces in larger dry aggregates, especially in clay soils. Considering these hypothesis, Ipta and eventually Ima measurements may be sufficient for a diagnostic assessment of the soil structure condition, while Ias does not actually yield further information as, indeed, is considering the structure of equations (5) (6) and (7). The single-sieve aggregate stability indices are less easily interpreted than the indices based on grain-size distribution analysis. S may indicate the persistence of small aggregates to a given arbitrary hydrodynamic and mechanical force. WSI, using a higher and lower input of energy, may result less influenced by the arbitrary choice o f disruptive forces in comparing different soils. The drawback of the latter method is the need for special equipment for wet-sieving analysis. Moreover, it is more cumbersome and time-consuming to carry out than S. Finally, the Iifindex (obtained under a consistent input of hydrodynamical and mechanical forces), is an indicator of the instability of the finer aggregates of the silt+clay class. This index may be important in evaluating microstructure instability, especially in clay soils, where the relative mass of silt+clay fraction is very consistent. Iif may also have value for coarser-textured soils, where silt+clay SOIL TECHNOLOGY

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Aggregate Stability Indices particles free in water m a y provide cem e n t i n g material to bind weak aggregates with coarser p r i m a r y particles.

6

Conclusion

Mechanical analysis of soil structure can produce useful i n f o r m a t i o n for a g r o n o m ical soil classification a n d guidelines for soil m a n a g e m e n t . On the other hand, we c o n c u r with the o p i n i o n of other investigators that more sophisticated methods of direct and indirect soil structure analysis m a y be necessary to study the dynamics o f soil aggregate disruption for inferring on shortterm evolution of the structural soil state. The m e a s u r e m e n t of grain-size distribution u n d e r a set of increasing a n d qualitatively different h y d r o d y n a m ical a n d / o r mechanical forces simulating threshold conditions in the field, seems the more informative a n d up-todate method for direct soil structure assessment. Simple single-sieve aggregate stability indices, like S, however, can substitute for the c u m b e r s o m e assessment a n d t i m e - c o n s u m i n g grain-size distribution analysis for soil classification a n d land-use histories comparison. In any case, it is f u n d a m e n t a l , for soil structure assessment in different soils, to measure a detailed p r i m a r y particles distribution. The data of this work have shown that, for diagnostic purposes, the availability of specific d a t a on s t a n d a r d semip e r m a n e n t soil characteristics, such as 0 . C . % , L1, P1, C E C a n d S C W % may be sufficient to infer on mechanical soil structure characteristics (Ipta a n d Ima) using statistical models. SOIL TECUNOL(.)GY--A cot~peraling ,Iournal of CATENA

13 t

References

ALDERFER, R.B. & MERKLE, F.G. (1941):Tbe measurement of structural stability and permeability and the influenceof soil treatment on the properties. Soil Sci. 51, 201-212. BAZZOFF1, P. & CHISCI, G. (1986): Effetto del passaggio dei macehinari agricoli e di difFerenti pratiche agronomiche su alcune caratteristiche fisiche di un suolo limo-argilloso(Typi,c Udorthent) del Mugello (Toscana). Ann. Ist. Sper. per 1o Studio e la Difesa deI Suolo, vol. XVII, Firenze, 41-56. BLACK, C.A. (ed.) (1965): Methods of soil analysis. Am. Soc. Agron., Madison Wis., USA. BRYAN, R.B., BENDEXEN, T.W. & SLATER, C.S, (1948): Measurement of the water stability of soils. Soil Sci. 65, 341-345. BRYAN, R.B. (1968): The development, use and efficiency of indices of erodibility. Geoderma 2, 5-26. BRYAN,R.B.(1971): The efficiency of aggregation indices in the comparison of some English and Canadian soils.J.Soil Sci,22,167-177. BRYAN, R.B. & DE PLOEY, J. (1983): Comparability of soil erosion measurements with different laboratory rainfall simulators.ln: J.Do Ploey(ed.).Rainfall simulation,runoff and soil erosion.CATENA SUPPLEMENT 8, 97-105. BURKE, W., GABRIELS, D. & BOUMA, d. (ed.) (1986): Soil structure assessment,Balkema. CHISCI, G, & SPALLACCI, P, (1984): Nutrient losses by leachingand runoff and possibilitiesof their control. 18th Colloquium of the Int.Potash Institute, Bern, 137-155. CHISCI, G,, SFALANGA, M, & TORRI, D. (1985): An experimental model for evaluating soil erosion on a single rainstorm basis. In : S.A. E1-Swaify and M.C. Moldenhauer (ed.). Soil erosion and Conservation. Soil Cons. Soc. of Amer., 558-566. CHISC1, G. (1988): Measures for runoff'and erosion control: a review of trials carried-out in the Appennines hilly area. SOIL TECHNOLOGY (in press). DE BOODT, M., DE LEEHNEER, L & KIRKHAM, D. (1961): Soil aggregate stability and crop yield. Soil Sci. 91,2, 138-146. DE BOODT, M. (ed.) (1967): West European Methods for soil structure determinations. Intern. Soc. Soil Sci. Comm. I West Europe Group. Pt. VI, 17-18, 36-38.

132

DE LEEHNEER, L & DE BOODT, M. (1959): Determination of aggregate stability by the change in the mean-weight diameter. Proc. of the Int. Symp. on Soil Structure, Ghent, Mededelingen van de Landhonwhogeschool 24, 290300. DE PLOEY, 3. (1979): A consistency index and the prediction of surface crusting on Belgium loamy soils. Colloquium: Erosion agricole des sols en milieu tgmperfi non Mediterraneen. H. Vogt and T. Vogt (ed.). Univ. L. Pasteur, Strasburg, 133-137. DE PLOEY, d. (1981): Crusting and timedependent rainwash mechanism in loamy soils. In: R.P.C. Morgan (ed.), Soil conservation. Wiley-interscience, Chichester, 134-154. DE PLOEY, J. & MUCHER, H.T. (1981): A c~nsistenc~ i~dex arid rain,casl~ mechanisms on Belgium loamy soils. In: Erosion and sediments transport measurement. IAHS pub., 423-43[. DE PLOEY, J. & POESEN, d. (1984): Aggregate stability, runoff generation and interrill erosion. In: K.S. Richards, R.R. Arnett and S. Ellis (ed.), Geomorphology and soils. Allen & Unwin, London. FI~ODOROFF, A. (1960): Evaluation de la stabilit6 structurale d'un sol (indice S). Nouvelles normes d'~mploi pour l'appareil ~ tamiser. Annals Agronomiques 11, 651-659. GARDNER, W.R. (1956): Representation of soil aggregate size distribution by a logarithmicnormal distribution. Soil Sci. Soc. Am. Proc. 20, 151-153. GRIEVE, I.C. (1980): The magnitude and significance of soit structural stability declines under cereal cropping. CATENA 7, 79-85. GOVERS, G. (1985): Selectivity and transport capacity of thin flows in relation to rill erosion. CATENA 12, 35-49. HI~NIN, S., IMONNIER, G. & CAMBOU, A. (1958): M~thod pour l'fitude de la stabilitfi structurale de sols. Ann. Agron. 1, 73-92. HJULSTROM, F. (1935): Studies of morphological activity of rivers as illustrated by the River Fyries. Bull. Geol. Inst. Univ. Uppsala 25, 221-527. KEMPER, W.D. (1965): Aggregate stability.In Methods of Soil Analysis. part 2. C.A.Black (ed.) Am. Soc. Agron., Madison, Wis., 5l 1-519. KEMPER, W.D. & CHEPIL, W.S. (1965): Size distribution of aggregates. In: Methods of Soil Analysis. part 2. C.A. Black (ed.) Am. Soc. Agron., Madison, Wis., 499-5t0.

Chisel, Bazzolt7 & Mbagwu

MALQUORI, A. & CECCONI, S. (1962): Determinazione seriale dell'indice di struttura del terreno. Agrochimica 6, 198-204. MIDDLETON, H.E. (1930): Properties of soils which influence soil erosion. U.S. Dep. Agr. Tech. Bull. 178. MOLOPE, M,B., PAGE, E.R. & GRIEVE, I.C. (I985): A comparison of soil aggregate stability tests using soils with contrasting cultivation histories. Soil Sci. Plant. Anal. 16, 315-322. POESEN, J. (1986): Detachability and cohesion. Inter. Workshop on Theoretical Geomorphological Processes. Aachen, Germany. POESEN, J., & TORRI, D. (1986): The effect of cup-size on splash detachment and transport measurement. Part.I. Field measurement. IGU-COMTAG. Int. Symp. on Geomorphic Processes in Environments with strong Seasomtl Contrasts, 4-11. Sept. 1986, Spain. PANABOKKE, C.R. & QUIRK, J.P. (1957): Effect of initial water content on stability of soil aggregates in water. Soil Sci. 83, 185-195. ROMKENS, M.J.M., PRASAD, S.N. & POISEN, J. (1987): Soil erodibility and properties. Trans. XIII Congr. of" the Int. Soc. of Soil Sci., Hamburg, 13-20 August 1986. RUSSELL, E.W. (1938): Measurement of pore space and crumb Oil aggregate structure of the soils. Third Conf. Cotton Growers Problems. Rothamsted Exp. Sta. RUSSELL, M.B. & FENG, C.L. (1947): Characterization of the stability of soil aggregates. Soil Sci. 63, 229-304. SHALLER, F.W. & STOCKINGER, K.R. (1953): A comparison of five methods of expressing aggregation data. Soil Sci. Soc. Am. Proc. 17, 310-313. STIRK, G.B. (1958): Expression of soil aggregate distribution. Soil Sci. 86, 133-136. STRICKLING, E. (1951): The effect of soybean on volume weight and water stability of soil aggregates, soil organic matter content and crop yield. Soil Sci. Soc. Am. Proc. 15, 30-34. TORRI, D. & SFALANGA, M. (1980). Stima del.t.'erodihilit~ dei suoli mediante simulazione di pioggia in laboratorio. Nota II. Preparazione dei campioni di suolo. Ann. Ist. Sper. Studio e Dif. Suolo, Firenze, Vol. X, 141-157. TORRI, D. & SFALANGA, M. (1982): Estimate of soil erodibility by means of simulated rain in the laboratory. Note III. Some aspects of soil erosion mechanics. Ann. Ist. Sper, Studio e Difesa Suolo, Firenze, vol. XllI, 75-90.

SOIL TECHNOLOGY--A cooperating Journal of CATENA

Aggregate Stability In dices

TORRI, D. (1986): Detachability and cohesioia. Int. Workshop on Theoretical Geomrphological processes. Aachen, Germany. TORRI, D., SFALANGA, M. & CHISCI, G. (1987): Treshold conditions for incipient rilling. In: R.B. Bryan (ed.), Rill erosion. CATENA SUPPLEMENT 8, 97-105. TIUL1N, A.F. (1928): Questions on soil structure: II. Aggregate analysis as a method for determining soil structure. Porto. Agr. Exp. St. Div. Agr. Chem., Report 2, 77-122. TIULIN, A.F. (1933): Certain considerations on the general soil structure and on methods for its determination. Soviet Sec. Int. Soil Sci. Soc. AI, 111-132. VAN BAYEL, C.H.M. (1949): Mean weight diameter oF soil aggregate as a statistical index of aggregation. Soil Sci. Soc. Am. Proc. 14, 20-23. VAN BAVEL, C.H.M. (1953): Report of the commettee on physical analysis 1951-53. Soil Sci. Soc. Am. Prec. 17, 416-418.

Addresses of authors: Prof. G. Chisci Istituto di Agronomia, University of Palermo Palermo, Italy Dr. P. Bazzofli

Istituto Sperimentale per 1o Studio e la Suolo, Firenze, Physics Section Firenze, Italy Dr. J.S.C. Mbagwu Department of Soil Science, University of Nigeria Nsukka, Nigeria

Authors contributed equally to the work.

SOIL TECHNOLOGY - A cooperating Journal of CATENA

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