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The role of moisture content in the stability of soil aggregates from a temperate silty soil to raindrop impact
CATENA
Vol. 11, 313-320
Braunschweig 1984
THE ROLE OF MOISTURE CONTENT IN THE STABILITY OF SOIL AGGREGATES FROM A TEMPERATE SILTY SOIL TO RAINDROP IMPACr S.M. Cousen & P.J. Farres, Portsmouth SUMMARY Aggregate stability to drop impact is of fundamental importance to many aspects of the soil system, in particular rainsplash erosion. The role of moisture content is considered by the use of a single drop simulator on soil aggregates of known moisture content selected from the Hamble series. Results indicate a negative logarithmic relationship between time to breakdown and specific moisture content. In addition the variability of the results drastically declines with increasing moisture content. The results are explained in terms ofdistributio9 of moisture within the aggregates prior to experimentation.
1. INTRODUCTION • The role ofindividual soil aggregate stability as an important control on the magnitude of soil loss by erosion and soil crust formation has long been appreciated (BRYAN 1977, 1973, 1971, FARRES 1978a). A knowledge of the behaviour of soil under the erosive influence of raindrops is imperative for planning sound soil water management stategies and conservation programmes (CHANDRA & DE 1978). However, the relative significance of the variables controlling this rainsplash erosion of soils remain, as yet not fully understood. A soil aggregate may be defined as a group of primary particles intimately bound such that they form secondary units (BAVER, GARDNER & GARDNER 1972). Soil aggregates are naturally occurring and are formed as a result of a number of gradual processes operating in the soil environment (CHESTERS et al. 1957, DUCHAUFOUR 1982). One of their most important properties is perhaps aggregate strength which along with aggregate stability is the major determinant of soil quality for plant growth and nutrition (RUSSELL 1973). Aggregate strength and stability also determine to a large extent, the resistance of soil to compaction and dispersion and thus its erodibility (LUK 1979). The mechanisms of soil particle flocculation and aggregation are complex and are more fully discussed elsewhere (MORTLAND 1970, MARTIN et al. 1950, CHESTERS et al. 1957). It is apparent however, that clay content, organic matter type and amount, cation/anion status and soil water energy conditions all play their part in the mechanisms controlling aggregate stability. One way in which the stability of an individual aggregate may be assessed is by the use of a single drop rainfall simulator (GRIEVE 1979, PALMER 1962), the simplest form of which was used by McCALLA (1944). This method has since become an integral part of soil aggregate stability studies. Basically, the number of drops produced from a partially open burrette needed to bring about aggregate breakdown, are counted. Similar but more refined apparatus and techniques have subsequently been recorded by LOW (1954), BRUCE-OKINE & LAL (1975) and FARRES (1980). One parameter that has been shown to be of crucial importance and has been investigated ISSN 0341 - 8162 c~ Copgoght 1984 bg CATENA VERLAG. D- 3302 Cremhngen- Destedt, W. Oermang
314
COUSEN & FARRES
using the single drop simulator is the initial or antecedent moisture status of the aggregate. As yet no actual connection between specific moisture content and aggregate stability has been revealed since most of the work in this area has demonstrated the link indirectly, by prewetting and leaving for various lengths of time (LOW 1954, IMESON & JUNGERIUS 1976, BERGSMA & VALENZUELA 1981). This series of experiments attempts to redress the balance by trying to establish a relationship between the moisture content of an aggregate and the time taken to breakdown under simulated rainfall. This has been done with a view to establishing optimum moisture conditions for future aggregate stability studies and perhaps increasing our understanding of the basic mechanisms involved in aggregate breakdown.
2.
METHODOLOGY
The single drop simulator used in this series of experiments (Fig. 1), in which a constant head of water feeds a drop forming nozzle, is developed from ideas proposed by PALMER (1962), BRUCE-OKINE & LAL (1975) and GRIEVE (1979). The head of water is maintained at a constant level using a perisaltic pump that conveys water from one reservoir to another, via an overflow system. The drip rate is controlled using a series of small damps and a tap, a drop height of two metres is used. The size of the drops produced is 3.5 mm diameter and their rate is 100 drops per minute. This is similar to that used by several other investigators (LOW 1959, McCALLA 1954, IMESON & JUNGERIUS 1976, BERGSMA & VALENZUELA 1981). Aggregates forming in the top soil of the Hamble series as developed on the West Sussex coastal plain U.IC are used in these experiments (HODGSON 1967, FARRES 1978b). This soil is ideal for our purposes because it has uniform textural and chemical properties (Table 1). Bulk samples were borken down by hand and spread out on trays to air dry for one month before being sieved. Once sieved aggregates from the 4.75-8.00 mm fraction were weighed. Five groups consisting of 30 individual aggregates from the weight range 0.23-0.33 g which is typical of this fraction, are selected. Four of the five groups are subjected to prewetting by adding distilled water drop-wise from a microsyringe. The volume of water added is calculated as a percentage of the weight of the respective aggregate such that group 1 receives 0% moisture, group 2, 10% and so on by weight. All wetted aggregates are placed in a desiccator for twenty four hours prior to experimentation. Each aggregate is placed on an aggregate stand (Fig. 1) and subjected to impacts of distilled water (pH 7) until breakdown. This is taken to be the point at which all the soil material has passed through the underlying 3 mm hole or when 90% of this opening is considered clear of material. The time taken for this to happen and the number of drops are recorded. The data produced enables the quantifications of the relationship between moisture content, original weight and time to breakdown.
3.
OBSERVATIONS
The data are considered in an exploratory framework (ERIKSON & NOSANCHUK 1982), first in their five moisture content groups by looking at 'box-dot' plots to describe the nature of the distributions (Fig. 2). From these the first observation that can be made is that there are a few 'outliers'. This phenomenon is not uncommon when using this method of aggregate stability assessment, and in the past has been generally regarded as pertaining to
MOISTURE CONTENT AND AGGREGATESTABILITY
315
Reservoir[
f
l
/
c,amO
l
| tap Droppingnozzle
I "
~u'~ "'c
rPre~t~,Vl~uerb~to r nce
.
Sg r~gate stand
~EEnlarg~l ~..~__~ hydrophobicfilm ~,~,-e,~ Wire frame
Fig. 1: Single drop rainfall simulator and aggregate stand for soil aggregate stability assessment. Tab. 1: BASICPROPERTIESOFTHESOIL
% by Wt
Particle size (Wentworth Scale) C.Sand M.Sand ESand 8 30 35
Silt 17
Chemical Properties (me / 100 gm.) Ca Na CEC 10 0.18 16.9
Clay 10
Organic Loss on ignition 4.3O/o
Clays present ///(Kaolinite)/(Montmorilionite)
316
COUSEN& FARRES
i
0 -
+
I
I
m l
10
I
+
!
I
J
o=
E 20
-
8
-i + i I
,5 E 3O
-
I
i+i I
m 1
-JrJ
40
-
iI + i ~i
median
J quartlles x
outhers
Fig. 2: Box-dot plots showing form of the distributions. rainstable aggregates (IMESON & JUNGERIUS 1976, BERGSMA & VALENZUELA 1981). A second and far more important characteristic shown is the change in variability of the data; high variation for low moisture content, low variation for high moisture content. The form of the relationship is well described by the model; L o g Q = b0 + b iC + e Where
(1)
Q is the quartile deviation of time to breakdown C is percentage content bobi are estimated regression coefficients e is the random error
The exact estimated form of the model for this particilar soil type is the logarithmic type function given in (2) Log Q = 0.13 + 0.0027C
(2)
Where percentage explanation is 78% adjusted for degrees of freedom. The model shows that at low moisture content the time to breakdown is highly variable between each aggregate, but that this variability declines rapidly with increase in moisture content. The 'box dot' plot also shows that median time to breakdown mirrors the logarithmic decline in the spread of data, being high for low moisture content and low for high moisture content. This observation confirms findings reported in previous literature. The relationship between time to breakdown and initial moisture content, as expressed as a percentage by weight can be considered in a little more detail by examining mean, or median time to breakdown as a function of moisture content (Fig. 3a). The generalizations thus made however, are vast and much of the information available becomes redundant: hence it is desirable to find an analysis that will employ all the observations. Graphical exploration of the data using Tukeys ladders of powers (ERIKSON & NOSANCHUK 1982) reveals the most efficient data transformations to represent the
MOISTURE CONTENT AND AGGREGATE STABILITY
317
(a)
o~ 2.30
o
~ 2.00
JE .J 1.70
o
2;
,b
In,tlal moisture content
Fig. 3a:
Generalized relationship of time to breakdown and initial moisture content.
(b) 0.7_~ 3 3
A I-
log 10 (T) = 0.005-0 019 (C)
~5~~s 7
2 3
"/
2
2
4
o.o3
o
. 4
o -0,7
-
2
~
8~---~.~. 3 7 2
78 "-----4
2 5 7 ~ 9 4 2
-1.4 -
1'o
2'o
~o
&
=
Initial r n o l s t u r e c o n t e n t
Fig.3b:
Complete relationship of time to breakdown as a function of initial moisture content (Direct from M]N]TAB output).
relationship to be Log]0 time against moisture content. A temptation exists to use ordinary least squares regression, but the 'box dot' plots referred to earlier, indicate that the data is highly heteroskedastic and that any model fitted using traditional methods may not produce the best unbiased estimators of the true population coefficients. A robust line fit yields after only three iterations of the algorithm a solution; Log10 (T) = 0.005 - 0.019C Where
(3)
T is time to breakdown C is percentage moisture content
The analysis was undertaken using the interactive statistical package MINITAB (RYAN, JOINER & RYAN 1981). The so called 'catch all plot' (CHATTERJEE & PRICE 1977) produces a well defined model with no need for a further transformation, however it is
318
COUSEN & FARRES
essential to note that we are still dealing with a logarithmic type model (Fig. 3b). Now considering aggregate weight, it may be possible to get an indication of the processes involved. As all the soil aggregates used are of similar dimensions differences in weight give a measure, be it very generalized, of pore size structure. High weights for a constant diameter aggregate imply high solid component in a given volume and hence small pore spaces. On the other hand, low weights for the small size aggregate indicates larger pores. It is realized this is an over simplification but it may well help in the explanation of the processes taking place. Examining the relationship between time to breakdown and initial weight of aggregates prior to adding water it is possible to detect an emergent pattern (Fig. 4). The lines are not true least squares regression lines but are obtained using the previously mentioned robust line fitting algorithm to obviate any problems that may be due to 'outliers' in the data set. Again this analysis was achieved using the statistical computer package MINITAB, and more specifically the RLINE command found in the exploratory Data Analysis section of this package. The resistant lines become parallel to the X axis with increasing moisture content. Thus it follows that only at very low moisture contents is the weight of the aggregate at all significant in predicting time to breakdown (i.e. estimated regression coefficients are significantly different from zero). The possible implication of this and the other results will be discussed later. What should be noted here is that for the only significant relationship (i.e. moisture content groups 0 and 10%) the form is negative; as weight increases so time to breakdown decreases, for this particular soil.
Significant regression c o - e f t 200
%
~
( 0 . 0 5 level)
Inslgnificant regression co-eft ( 0 . 0 5 Initial moisture content of aggregate
level)
o% 100-
10%
30%~
s
~
~
20% --
--
~.'--=" ~ - -
--
- - ~
0
o!25
02 Imtml
_40% •
"
o'.3
weight of aggregate
Fig. 4: Robust lines for the relationship of time to breakdown and initial weight of aggregate for different moisture content groups.
4.
PROCESS IMPLICATIONS
Breakdown occurs when the input force exceeds the internal resistance of the aggregate. This condition is reached sooner, and with much lower variation when the moisture content is
MOISTURECONTENTAND AGGREGATESTABILITY
319
high. It should perhaps be stated at this point that 40% moisture content represents very nearly the maximum possible addition using the technique adopted here, before spontaneous breakdown occurs. As expected, time to breakdown decreases with increasing moisture content. This is because the aggregate has had time to absorb moisture deep into its structure, so increasing the length of the 'water bridges' between particles, and allowing the exchange of cations, clay mineral expansion etc. This helps to reduce the total internal resistance to breakdown. The relationship is non-linear. One can conceive of soil aggregates as being composed of smaller units (microaggregates) with each mineral grain being held in a fine matrix of clay/organic matter complexes and sesquioxides. A characteristic of these micro-aggregates is that they possess very small void spaces (FARRES 1978b). These in turn duster together via a fine matrix. Between some of the facets of the micro-aggregates are found the larger internal pores of the total aggregate. During the twenty four hour period prior to our experimentation, water has time to move along micro-hydraulic gradients into the heart of the aggregate, distributing itself uniformly throughout the structure filling the smallest pores first, and then gradually the larger ones (CHILDS 1969). It might be assumed that it is within the micro-pore structure that the major variability between chemical and physical properties in the aggregate is to be found. At low moisture content only part of the micro-structure has taken up water and therefore there is a greater variability of time to breakdown at low levels of moisture content. However, as moisture content increases, more of the micropores in all aggregates within that group become filled, they become affected by the physical and chemical mechanisms previously mentioned and these r~duce their total internal strength. Therefore, with increased moisture content the basic resistance of all the aggregates are on one hand reduced, and on the other becoming increasingly similar in magnitude. The result is lower, yet far less variable time to breakdown. This explanation is further substantiated if the time to breakdown as a function for each moisture content group is considered. If weight is considered as a surrogate measure for pore volume within the aggregate, accepting the above hypothesis it may be expected that any relationship between weight and time to breakdown would be significant only at low moisture contents. This is in fact demonstrated for the 0% and 10% moisture content, and the slope of the relationship reaches zero at approximately 25% moisture by weight (i.e. no significant relationship).
5.
CONCLUSIONS
Moisture content of the soil aggregates has been shown to be important indirectly by other authors who consider different lengths of prewetting time before experimentation. What has been shown here is the form of the relationship between specific moisture content and time to breakdown. The relationship is non-linear and the spread of data declines significantly about the central value for each moisture percentage class. This may be regarded as simply being a function of the soil water distribution within the aggregate becoming more uniform with increasing moisture content. The implications of these results is that in any single drop stability test on soil aggregates careful controls are required to ensure, as nearas possible, identical antecedant conditions between experiments.
320
COUSEN & FARRES
REFERENCES
BAVER, L.D., GARDNER, W.H. & GARDNER, W.R. (1972): Soil Physics. J. Wiley, Chichester. BERGSMA, E. & VALENZUELA, C.R. (1981): Drop testing aggregate stability of some soils near Merida - Spain. Earth surfaces Processes V6, 309-318. BRUCE-OKINE, E. & LAL, 1L (1975): Soil erodibility as determined by raindrop technique. Soil Sci. Vl19 (2), 149-157. BRYAN, R. (1971): The efficiency of aggregation indices in the comparison of some English and Candian soils. Journal Soil Sci. V22, 166-178. BRYAN, IL (1973): Surface crust formed under simulated rainfall on Canadian soils. Consiglio Nazional delle Ricerche. Pisa Conference 2. BRYAN, R (1977): Assessment of soil erodibility: New approaches in erosion. Research techniques erodibility and sediment delivery. Ed. T. Toy, Geo Abstracts Ltd., Norwich. CHANDRA, S. De S.I( (1978): A simple apparatus to measure relative erodibility of soils. Soil Sci. V152 (2), 115-121. CHATERJEE, S. & PRICE, B. (1977): Regression analysis by example. J. Wiley. CHESTERS, C. et al. (1957): Soil aggregation in relation to various soil constituents. Proc. Soil Sci. Soc. Am. V21,272-277. CHILDS, E.C. (1969): An introduction to the physics of soil water phenomena. J. Wiley, Chichester. DUCHAUFOUR, P. (1982): Pedology. Translated by T.R Paton. George Allen & Unwin, London. ERIKSON, B.H. & NONSANCHUK, T.A. (1982): Understanding data: An introduction ot exploratory and confirmatory data analysis for students in social sciences. Open University Press, London. FARRES, P.J. (1978a): The role of time and aggregate size in the crusting process. Earth Surface Processes V3, 243-254. FARRES, P.J. (1978b): The development of digital simulation of the rainsplash process. Unpublished PhD thesis. University of Reading. FARRES, P.J. (1980): Some observations on the stability of soil aggregates to raindrop impact. CATENA 7, 2/3, 223-231. GRIEVE, I.C. (1979): Soil aggregate stability tests for geomorphologists. I.B.G., B.G.RG. Tech. Bull. No. 25. HODGSON, J.M. (1967): Soils of the West Sussex Coastal Plain. Soil Survey of Great Britain Bulletin 3. IMESON, A.C. & JUNGERIUS, P.D. (1976): Aggregate stability and colluviation in the Luxembourg Ardennes: an experimental and micromorphological study. Earth Surface Processes. VI, 259-271. LOW, A.G. (1954): The study of soil structure in the field and the laboratory. Journal of Soil Sci. Vol. 5(1), 57-78. LUK, S.H. (1979): Effect of soil properties on erosion by wash and splash. Earth Surface Processes. V4, 241-255. McCALLA, J.P. et al. (1944): Water drop method for determining stability of soil structure. Soil Sci, V58, 117-121. MARTIN, J.P. et al. (1955): Soil aggregation. Adv. in Agron VT, 1-34. MORTLAND, M.M. (1970): Clay organic complexes and interactions. Adv. in Agron. V22, 75-177. RUSSELL, E.W. (1973): Soil conditions and plant growth. Longmans, London. RYAN, T.A. Jr., JOINER, B.L. & RYAN, B.F. (1981): MINITAB reference manual. Stats. Dept. Penn State University. PALMER, RS. (1962): An apparatus for forming waterdrops. U.S.D.A., ARS. Res. Report No. 3.
Addresses of authors: Ms. S.M. Cousen, Research Assistant, Department of Geography, Portsmouth Polytechnic, Lion Terrace Portsmouth, Hampshire, England Dr. P.J. Farres, Senior Lecturer, Department of Geography, Portsmouth Polytechnic, Lion Terrace Portsmouth, Hampshire, England.
Vol. 11, 313-320
Braunschweig 1984
THE ROLE OF MOISTURE CONTENT IN THE STABILITY OF SOIL AGGREGATES FROM A TEMPERATE SILTY SOIL TO RAINDROP IMPACr S.M. Cousen & P.J. Farres, Portsmouth SUMMARY Aggregate stability to drop impact is of fundamental importance to many aspects of the soil system, in particular rainsplash erosion. The role of moisture content is considered by the use of a single drop simulator on soil aggregates of known moisture content selected from the Hamble series. Results indicate a negative logarithmic relationship between time to breakdown and specific moisture content. In addition the variability of the results drastically declines with increasing moisture content. The results are explained in terms ofdistributio9 of moisture within the aggregates prior to experimentation.
1. INTRODUCTION • The role ofindividual soil aggregate stability as an important control on the magnitude of soil loss by erosion and soil crust formation has long been appreciated (BRYAN 1977, 1973, 1971, FARRES 1978a). A knowledge of the behaviour of soil under the erosive influence of raindrops is imperative for planning sound soil water management stategies and conservation programmes (CHANDRA & DE 1978). However, the relative significance of the variables controlling this rainsplash erosion of soils remain, as yet not fully understood. A soil aggregate may be defined as a group of primary particles intimately bound such that they form secondary units (BAVER, GARDNER & GARDNER 1972). Soil aggregates are naturally occurring and are formed as a result of a number of gradual processes operating in the soil environment (CHESTERS et al. 1957, DUCHAUFOUR 1982). One of their most important properties is perhaps aggregate strength which along with aggregate stability is the major determinant of soil quality for plant growth and nutrition (RUSSELL 1973). Aggregate strength and stability also determine to a large extent, the resistance of soil to compaction and dispersion and thus its erodibility (LUK 1979). The mechanisms of soil particle flocculation and aggregation are complex and are more fully discussed elsewhere (MORTLAND 1970, MARTIN et al. 1950, CHESTERS et al. 1957). It is apparent however, that clay content, organic matter type and amount, cation/anion status and soil water energy conditions all play their part in the mechanisms controlling aggregate stability. One way in which the stability of an individual aggregate may be assessed is by the use of a single drop rainfall simulator (GRIEVE 1979, PALMER 1962), the simplest form of which was used by McCALLA (1944). This method has since become an integral part of soil aggregate stability studies. Basically, the number of drops produced from a partially open burrette needed to bring about aggregate breakdown, are counted. Similar but more refined apparatus and techniques have subsequently been recorded by LOW (1954), BRUCE-OKINE & LAL (1975) and FARRES (1980). One parameter that has been shown to be of crucial importance and has been investigated ISSN 0341 - 8162 c~ Copgoght 1984 bg CATENA VERLAG. D- 3302 Cremhngen- Destedt, W. Oermang
314
COUSEN & FARRES
using the single drop simulator is the initial or antecedent moisture status of the aggregate. As yet no actual connection between specific moisture content and aggregate stability has been revealed since most of the work in this area has demonstrated the link indirectly, by prewetting and leaving for various lengths of time (LOW 1954, IMESON & JUNGERIUS 1976, BERGSMA & VALENZUELA 1981). This series of experiments attempts to redress the balance by trying to establish a relationship between the moisture content of an aggregate and the time taken to breakdown under simulated rainfall. This has been done with a view to establishing optimum moisture conditions for future aggregate stability studies and perhaps increasing our understanding of the basic mechanisms involved in aggregate breakdown.
2.
METHODOLOGY
The single drop simulator used in this series of experiments (Fig. 1), in which a constant head of water feeds a drop forming nozzle, is developed from ideas proposed by PALMER (1962), BRUCE-OKINE & LAL (1975) and GRIEVE (1979). The head of water is maintained at a constant level using a perisaltic pump that conveys water from one reservoir to another, via an overflow system. The drip rate is controlled using a series of small damps and a tap, a drop height of two metres is used. The size of the drops produced is 3.5 mm diameter and their rate is 100 drops per minute. This is similar to that used by several other investigators (LOW 1959, McCALLA 1954, IMESON & JUNGERIUS 1976, BERGSMA & VALENZUELA 1981). Aggregates forming in the top soil of the Hamble series as developed on the West Sussex coastal plain U.IC are used in these experiments (HODGSON 1967, FARRES 1978b). This soil is ideal for our purposes because it has uniform textural and chemical properties (Table 1). Bulk samples were borken down by hand and spread out on trays to air dry for one month before being sieved. Once sieved aggregates from the 4.75-8.00 mm fraction were weighed. Five groups consisting of 30 individual aggregates from the weight range 0.23-0.33 g which is typical of this fraction, are selected. Four of the five groups are subjected to prewetting by adding distilled water drop-wise from a microsyringe. The volume of water added is calculated as a percentage of the weight of the respective aggregate such that group 1 receives 0% moisture, group 2, 10% and so on by weight. All wetted aggregates are placed in a desiccator for twenty four hours prior to experimentation. Each aggregate is placed on an aggregate stand (Fig. 1) and subjected to impacts of distilled water (pH 7) until breakdown. This is taken to be the point at which all the soil material has passed through the underlying 3 mm hole or when 90% of this opening is considered clear of material. The time taken for this to happen and the number of drops are recorded. The data produced enables the quantifications of the relationship between moisture content, original weight and time to breakdown.
3.
OBSERVATIONS
The data are considered in an exploratory framework (ERIKSON & NOSANCHUK 1982), first in their five moisture content groups by looking at 'box-dot' plots to describe the nature of the distributions (Fig. 2). From these the first observation that can be made is that there are a few 'outliers'. This phenomenon is not uncommon when using this method of aggregate stability assessment, and in the past has been generally regarded as pertaining to
MOISTURE CONTENT AND AGGREGATESTABILITY
315
Reservoir[
f
l
/
c,amO
l
| tap Droppingnozzle
I "
~u'~ "'c
rPre~t~,Vl~uerb~to r nce
.
Sg r~gate stand
~EEnlarg~l ~..~__~ hydrophobicfilm ~,~,-e,~ Wire frame
Fig. 1: Single drop rainfall simulator and aggregate stand for soil aggregate stability assessment. Tab. 1: BASICPROPERTIESOFTHESOIL
% by Wt
Particle size (Wentworth Scale) C.Sand M.Sand ESand 8 30 35
Silt 17
Chemical Properties (me / 100 gm.) Ca Na CEC 10 0.18 16.9
Clay 10
Organic Loss on ignition 4.3O/o
Clays present ///(Kaolinite)/(Montmorilionite)
316
COUSEN& FARRES
i
0 -
+
I
I
m l
10
I
+
!
I
J
o=
E 20
-
8
-i + i I
,5 E 3O
-
I
i+i I
m 1
-JrJ
40
-
iI + i ~i
median
J quartlles x
outhers
Fig. 2: Box-dot plots showing form of the distributions. rainstable aggregates (IMESON & JUNGERIUS 1976, BERGSMA & VALENZUELA 1981). A second and far more important characteristic shown is the change in variability of the data; high variation for low moisture content, low variation for high moisture content. The form of the relationship is well described by the model; L o g Q = b0 + b iC + e Where
(1)
Q is the quartile deviation of time to breakdown C is percentage content bobi are estimated regression coefficients e is the random error
The exact estimated form of the model for this particilar soil type is the logarithmic type function given in (2) Log Q = 0.13 + 0.0027C
(2)
Where percentage explanation is 78% adjusted for degrees of freedom. The model shows that at low moisture content the time to breakdown is highly variable between each aggregate, but that this variability declines rapidly with increase in moisture content. The 'box dot' plot also shows that median time to breakdown mirrors the logarithmic decline in the spread of data, being high for low moisture content and low for high moisture content. This observation confirms findings reported in previous literature. The relationship between time to breakdown and initial moisture content, as expressed as a percentage by weight can be considered in a little more detail by examining mean, or median time to breakdown as a function of moisture content (Fig. 3a). The generalizations thus made however, are vast and much of the information available becomes redundant: hence it is desirable to find an analysis that will employ all the observations. Graphical exploration of the data using Tukeys ladders of powers (ERIKSON & NOSANCHUK 1982) reveals the most efficient data transformations to represent the
MOISTURE CONTENT AND AGGREGATE STABILITY
317
(a)
o~ 2.30
o
~ 2.00
JE .J 1.70
o
2;
,b
In,tlal moisture content
Fig. 3a:
Generalized relationship of time to breakdown and initial moisture content.
(b) 0.7_~ 3 3
A I-
log 10 (T) = 0.005-0 019 (C)
~5~~s 7
2 3
"/
2
2
4
o.o3
o
. 4
o -0,7
-
2
~
8~---~.~. 3 7 2
78 "-----4
2 5 7 ~ 9 4 2
-1.4 -
1'o
2'o
~o
&
=
Initial r n o l s t u r e c o n t e n t
Fig.3b:
Complete relationship of time to breakdown as a function of initial moisture content (Direct from M]N]TAB output).
relationship to be Log]0 time against moisture content. A temptation exists to use ordinary least squares regression, but the 'box dot' plots referred to earlier, indicate that the data is highly heteroskedastic and that any model fitted using traditional methods may not produce the best unbiased estimators of the true population coefficients. A robust line fit yields after only three iterations of the algorithm a solution; Log10 (T) = 0.005 - 0.019C Where
(3)
T is time to breakdown C is percentage moisture content
The analysis was undertaken using the interactive statistical package MINITAB (RYAN, JOINER & RYAN 1981). The so called 'catch all plot' (CHATTERJEE & PRICE 1977) produces a well defined model with no need for a further transformation, however it is
318
COUSEN & FARRES
essential to note that we are still dealing with a logarithmic type model (Fig. 3b). Now considering aggregate weight, it may be possible to get an indication of the processes involved. As all the soil aggregates used are of similar dimensions differences in weight give a measure, be it very generalized, of pore size structure. High weights for a constant diameter aggregate imply high solid component in a given volume and hence small pore spaces. On the other hand, low weights for the small size aggregate indicates larger pores. It is realized this is an over simplification but it may well help in the explanation of the processes taking place. Examining the relationship between time to breakdown and initial weight of aggregates prior to adding water it is possible to detect an emergent pattern (Fig. 4). The lines are not true least squares regression lines but are obtained using the previously mentioned robust line fitting algorithm to obviate any problems that may be due to 'outliers' in the data set. Again this analysis was achieved using the statistical computer package MINITAB, and more specifically the RLINE command found in the exploratory Data Analysis section of this package. The resistant lines become parallel to the X axis with increasing moisture content. Thus it follows that only at very low moisture contents is the weight of the aggregate at all significant in predicting time to breakdown (i.e. estimated regression coefficients are significantly different from zero). The possible implication of this and the other results will be discussed later. What should be noted here is that for the only significant relationship (i.e. moisture content groups 0 and 10%) the form is negative; as weight increases so time to breakdown decreases, for this particular soil.
Significant regression c o - e f t 200
%
~
( 0 . 0 5 level)
Inslgnificant regression co-eft ( 0 . 0 5 Initial moisture content of aggregate
level)
o% 100-
10%
30%~
s
~
~
20% --
--
~.'--=" ~ - -
--
- - ~
0
o!25
02 Imtml
_40% •
"
o'.3
weight of aggregate
Fig. 4: Robust lines for the relationship of time to breakdown and initial weight of aggregate for different moisture content groups.
4.
PROCESS IMPLICATIONS
Breakdown occurs when the input force exceeds the internal resistance of the aggregate. This condition is reached sooner, and with much lower variation when the moisture content is
MOISTURECONTENTAND AGGREGATESTABILITY
319
high. It should perhaps be stated at this point that 40% moisture content represents very nearly the maximum possible addition using the technique adopted here, before spontaneous breakdown occurs. As expected, time to breakdown decreases with increasing moisture content. This is because the aggregate has had time to absorb moisture deep into its structure, so increasing the length of the 'water bridges' between particles, and allowing the exchange of cations, clay mineral expansion etc. This helps to reduce the total internal resistance to breakdown. The relationship is non-linear. One can conceive of soil aggregates as being composed of smaller units (microaggregates) with each mineral grain being held in a fine matrix of clay/organic matter complexes and sesquioxides. A characteristic of these micro-aggregates is that they possess very small void spaces (FARRES 1978b). These in turn duster together via a fine matrix. Between some of the facets of the micro-aggregates are found the larger internal pores of the total aggregate. During the twenty four hour period prior to our experimentation, water has time to move along micro-hydraulic gradients into the heart of the aggregate, distributing itself uniformly throughout the structure filling the smallest pores first, and then gradually the larger ones (CHILDS 1969). It might be assumed that it is within the micro-pore structure that the major variability between chemical and physical properties in the aggregate is to be found. At low moisture content only part of the micro-structure has taken up water and therefore there is a greater variability of time to breakdown at low levels of moisture content. However, as moisture content increases, more of the micropores in all aggregates within that group become filled, they become affected by the physical and chemical mechanisms previously mentioned and these r~duce their total internal strength. Therefore, with increased moisture content the basic resistance of all the aggregates are on one hand reduced, and on the other becoming increasingly similar in magnitude. The result is lower, yet far less variable time to breakdown. This explanation is further substantiated if the time to breakdown as a function for each moisture content group is considered. If weight is considered as a surrogate measure for pore volume within the aggregate, accepting the above hypothesis it may be expected that any relationship between weight and time to breakdown would be significant only at low moisture contents. This is in fact demonstrated for the 0% and 10% moisture content, and the slope of the relationship reaches zero at approximately 25% moisture by weight (i.e. no significant relationship).
5.
CONCLUSIONS
Moisture content of the soil aggregates has been shown to be important indirectly by other authors who consider different lengths of prewetting time before experimentation. What has been shown here is the form of the relationship between specific moisture content and time to breakdown. The relationship is non-linear and the spread of data declines significantly about the central value for each moisture percentage class. This may be regarded as simply being a function of the soil water distribution within the aggregate becoming more uniform with increasing moisture content. The implications of these results is that in any single drop stability test on soil aggregates careful controls are required to ensure, as nearas possible, identical antecedant conditions between experiments.
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REFERENCES
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Addresses of authors: Ms. S.M. Cousen, Research Assistant, Department of Geography, Portsmouth Polytechnic, Lion Terrace Portsmouth, Hampshire, England Dr. P.J. Farres, Senior Lecturer, Department of Geography, Portsmouth Polytechnic, Lion Terrace Portsmouth, Hampshire, England.