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Cracking in recent alluvial soils as related to easily determined soil properties
( ;1-~()I)I-~I~A ELSEVIER
Geoderma63 (1994) 289-298
Cracking in recent alluvial soils as related to easily determined soil properties N. Yassoglou a, C.S. Kosmas a, N. Moustakas a, E. Tzianis b, N.G. Danalatos a ~Laboratoryof Soilsand AgriculturalChemistry,AgriculturalUniversityof Athens, 75 lera Odos, Athens 11855, Greece ~FobaccoExperimentalStationof Agrinio,Ministryof AgricultureAgrinio30100, Greece) Received January 25, 1993; accepted after revisionOctober20, 1993
Abstract Cracking was studied in fifty soils located in two recent alluvial plains and it was related to soil parameters such as clay content, soil moisture content, bulk density and coefficient of linear extensibility (COLE). The total crack area per unit soil surface, the crack width and crack depth were measured by tracing them on transparent sheets. The rate of crack development was also studied in a soil immediately after irrigation. Results showed that crack width increased initially almost linearly with decreasing soil moisture content to a certain level and then increased in a declining rate until a maximum width was attained. Cracks appeared on the soil surface after water was removed from the large soil pores. The total crack area occupied a significant portion of the soil surface reaching a maximum value of 22.7%. Measurements of water flow into the cracks during irrigation were significant and in some soils exceeded the conductivity of the uncracked soil surface. An empirical relation was found, which calculates the total crack area per unit soil surface as a function of clay content, COLE and volumetric moisture content. The average crack width was linearly related to the total crack area measured on the soil surface. Furthermore, the crack depth in the plow layer was logarithmically related to clay content, COLE and moisture content.
1. Introduction Cracking soils occupy a considerable proportion of the agricultural land in the Mediterranean region. Alluvial soils usually contain high amounts of expanding clays, and soil layers at or near the surface undergo continual swelling and shrinking during wetting and drying cycles. Cracks are very common in periods of high evapotranspiration demands and 0016-7061/94/$07.00 © 1994ElsevierSciencePublishersB.V. All rights reserved SSDIO016-7061 (93)E0104-4
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may comprise a considerable part of the soil surface. They form a complicated network of interconnected channels, which can conduct water several meters away from an irrigated spot. Irrigation of a soil with continuous vertical cracks is subject to considerable loss of water below the root zone as well as to redistribution of soil materials and nutrients (Dekker and Bouma, 1984; Hasegawa and Sato, 1987; Van Stiphout et al., 1987). Kosmas et al. ( 1991 ) showed that a significant amount of the irrigation water may move, as bypass water, to the subsoil of swelling and shrinking soils without wetting the surface soil. However, in the same study was found that the amount of nitrates leached by the bypass water during a normal irrigation was rather small, ranging from 0.01 to 1.8% of the total nitrates present in the soil. The productivity of swelling and shrinking soils is usually very high. However, some physical properties of strongly swelling counterparts create unfavorable conditions for soil cultivation and plant growth: cracking during the drying period may damage the plant root system. Moreover, good tillage of these soils is possible only within a narrow moisture range. Shrinkage and drying are strongly dependent on the mobility of the soil phase and on the rate of moisture withdrawal (Stroosnijder, 1976). In a lysimeter study, Bronswijk ( 1991a, b) reported that shrinkage of clay progresses through three successive phases: structural shrinkage, isotropic normal shrinkage and isotropic residual shrinkage. The occurrence of normal and residual shrinkage could be predicted by changes in the soil moisture content and the shrinkage characteristics of soil aggregates. Haines (1923) and Keen (1931 ) had distinguished a fourth phase of shrinkage, i.e. the phase of zero shrinkage. During this phase, the soil particles had reached their densemost configuration, and any further decrease in the soil water content was not accompanied by any change in the volume of aggregates. McGarry and Malafant (1987) distinguished three shrinkage phases in unconfined units of soil when a clay soil dries: (a) a structural phase at high gravimetric water contents, where the volume change is less than the volume of water removed due to the drainage of large pores; (b) the normal phase, which lies in the drying sequence where the volume reduction is essentially equal to the loss of water; and (c) a residual zone at low gravimetric water contents, in which the volumetric reduction of the soil is smaller than the volume of water removed, implying an increase in the air volume. Most investigations of shrinkage and crack formation were conducted in the laboratory. Particularly, the relation between the changes in soil moisture content and volume has been investigated in several laboratory experiments using clay pastes, soil aggregates and soil cores of different sizes (Franzmeier and Ross, 1968; Reeve and Hall, 1978; Yule and Ritchie, 1980; Bronswijk, 1991a, b). Field data on soil shrinkage are scarce. The objective of the present work is to study the formation of cracks in the irrigated soils of recent alluvial plains and to develop such empirical formulas that relate crack development to soil properties easily determined and/or recorded in conventional soil survey reports. These relations could be used to develop pedotransfer functions (Bouma, 1989) relating the extend of cracking to easily determined soil parameters or that are given in soil maps. These functions could be used for developing efficient irrigation schemes maximizing water storage in cases that the cracks do not extend below the root zone (Van Stiphout et al., 1987), or for controlling cracking when they do. The soil properties investigated are:
N. Yassoglou et al. / Geoderma 63 (1994) 289-298
291
bulk density, clay content, coefficient of linear extensibility (COLE), and variations of moisture content as affected by irrigation.
2. Materials and methods Fifty soils in two recent alluvial plains of western Greece (32 soils in the Agrinio plain representing an area of about 10,000 ha, and 18 soils in the Lechena plain representing an area of 18,000 ha) were selected for this study. All soils contain high amounts of expanding clay minerals. The medium-textured soils are mainly well drained, whereas the fine-textured soils are poorly drained. They have an organic matter content that varies between 1.4 and 4.3%; however, the majority of them have low values, and a variable CaCO3 content (0.419.1%). The soils are classified as Typic Xerottuvents, Aquic Xerofluvents, Vertic Xerofluvents and Aeric-Vertic Fluvaquents (Soil Survey Staff, 1975). Most soils are cultivated with corn, vegetables and tobacco, and receive surface irrigation during the dry season. Another 14 alluvial soils with similar chemical and physical properties and soil management were chosen in two different alluvial plains (Alfios and Piros plains, western Greece) and were used to test the validity of the developed pedotransfer functions. The cracking of the soils was studied from late June (after the first irrigation) to the end of September. The size and the specific crack area, i.e. the surface area of the cracks per square meter of soil surface, were measured by tracing the cracks on a transparent sheet, and separating and weight the parts corresponding to the total crack and total surface area. In each of the fifty sampling sites, representing a certain mapping unit, cracks were measured in three 2.5 m by 0.5 m replicate plots. The replicate measurements were not significantly different and were averaged. The average depth and width along the cracks were measured in cross-sections dug in the plow layer. The estimation of the total crack volume was based on the observation that down to a certain depth from the soil surface, the cracks had an almost constant width. Below this depth, the width of the cracks decreased gradually. The crack volume in the upper section was estimated as the product of the specific crack area times depth of that section. The volume of the lower section was estimated as the product of the specific crack area times the crack depth of the lower section divided by 3. The bulk density and gravimetric field moisture content were determined in 300 cm 3 undisturbed soil cores, taken in triplicate from the plow layer, the latter extending down 37 to 45 cm from the soil surface, from each sampling site. The samples were dried at 105°C until constant weights. The degree of soil shrinkage was determined from the coefficient of linear extensibility (COLE) (Franzmeier and Ross, 1968). An amount of 30 g of a thoroughly mixed, airdried soil having passed through a 2 mm sieve was mixed with distilled water to almost saturation. The wet soil was placed in 10 cm long, 1 cm deep and 1 cm wide aluminum trays. The bottom of each tray was perforated to secure uniform drying. The bottom and the sides of the tray were covered with a thin film of petroleum gelly to prevent adhering of the soil to the metal surface and its consequent cracking. Subsequently, the soil was airdried, and the length of the dish and the dry rod were measured (Moustakas, 1990). Undisturbed soil samples were not used for COLE measurements because the soils were mainly friable due to the recent plowing and disking. Schafer and Singer (1976) found that
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COLE measured using soil pastes (rod method) was satisfactory correlated to COLE measured in natural undisturbed soil clods. The organic carbon content was measured according to the modified wet oxidation procedure of the Walkley-Black (Nelson and Sommers, 1982). The particle size analysis was done with the Bouyoucos hydrometer method (Gee and Bauder, 1986). Inorganic carbonates were determined by measuring the volume of CO2 evolved in the reaction with excess of HC1 (Nelson, 1982). The maximum amount of water applied on the soil surface, which could be conducted to the subsoil through cracks, was estimated in a number of representative soils. A cylindrical block of soil with a height of 40 cm and a diameter of 35 cm was carved in situ. The sidewalls of the block of soil were encased in gypsum by pouring slurry into a mold. Quicksetting hydraulic cement was applied in a thin layer on the sidewalls of the block before pouring the gypsum slurry to avoid lateral flow of gypsum into the cracks (Bouma and Wosten, 1984). The intercrack spaces were sealed with gypsum and plastic spray. Then water was applied into the cracks with 4 cm of water ponded on the surface by using a closed top infiltrometer with a Mariotte device. The volume of applied water was then recorded as a function of time. Additionally, the hydraulic conductivity of the consolidated soil surface was measured with double ring infiltrometers.
3. Results and discussion
The measured data on crack characteristics and some of the related physical and chemical properties of the soils studied are summarized in Table 1. It can be seen that the coefficient of linear extensibility (COLE) varied from 0.04 (Aquic Xerofluvent) to 0.155 (Vertic Xerofluvent). It increased linearly with the clay content of the soils in both alluvial plains (R = 0.91 ). The average crack width ranged from zero (in a wet Aquic Xerofluvent) to 19.1 mm (Aeric-Vertic Fluvaquent), depending on both soil moisture and clay content. Similarly, the specific crack area ranged from 0 to 22.7%. In average values, Aeric-Vertic Fluvaquents had the highest clay content (44.8%) and the highest COLE (0.133) followed by the Vertic Xerofluvents ( clay = 42.9%, COLE = 0.122) (Table 1 ). The latter soils, being generally drier upon examination, exhibited the highest average crack width, depth, surface area and volume (Table 1). The crack area was generally greater at the end of the growing period of maize due to the lower soil moisture status which was associated with a higher bulk density of the soils. However, the cracks did not exceed deeper than the plow layer, which is the layer of maximum water extraction by the roots. Note that the maize roots were mainly concentrated in the plow layer, whereas the subsoil was wet (including the Vertic Xerofluvents) throughout the growing period, preventing the extension of cracks in it. The rate of crack development was investigated in a soil representing the most extensive mapping unit in the areas studied. The measurements of crack width, crack depth and moisture content were conducted hourly starting immediately after the offset of irrigation. Cracking of the soil surface started a few hours after irrigation and continued as the soil moisture content decreased. The crack width initially increased almost linearly with decreasing soil moisture content to a certain level. Thereafter, it increased at a declining rate until the maximum width was attained.
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Table 1 Range and mean values (in brackets) of physical and chemical properties, and crack characteristics of the studied soils. Property
xf
vxf
axf
avfa
Clay (%)
15.3-38.5 (25.9) 0.5-19.1 (8.8) 1.4-3.3 (2.3) 0.05-0.11 (0.076) 1.34-1.47 (1.39) 0.08-0.28 (0.198) 0.70-1.59 ( 1.01 ) 7.3-34.8 (24.3) 0.08-0.18 (0.125) 0.01-04 (0.02)
30.3-58.3 (42.9) 0.3-15.2 (7.8) 1.54.3 (3.0) 0.08-0.15 (0.012) 1.17-1.65 (1.36) 0.11-0.35 (0.213) 0.58-1.79 (1.20) 15.1-57.5 (38.4) 0.09-0.22 (0.172) 0.01-0.06 (0.036)
18.6-39.5 (31.6) 0.4-14.1 (4.8) 1.8-3.1 (2.5) 0.04-0.13 (0.094) 1.28-1.55 (1.40) 0.11-0.27 (0.195) 0.0-1.54 (0.91) 0.0-38.2 (26.3) 0.0-0.21 (0.133) 0.0-0.03 (0.022)
36.1-53.1 (44.8) 0.6-9.4 (4.5) 3.2~,. 1 (3.5) 0.11-0.15 (0.13) 1.30-1.52 (1.35) 0.19-0.28 (0.245) 0.40-0.91 (1.06) 19.2~-8.2 (33.8) 0.06-0.23 (0.161) 0.01-0.06 (0.032)
CaCO 3 (%) OM (%) COLE BD ( g/cm 3) MC (v/v) CW* (cm) CD* (cm) CA* (m2/m 2) CVOL*
(m3/m3)
MC = moisture content, CW = average crack width, CA = specific crack area, CVOL = crack volume. xf= Typic Xerofluvents, axf= Aquic xerofluvents, vxf= Vertic Xerofluvents, avfa = Aeric-Vertic Fluvaquents.
c~ack~
curve
liE .
CO
4 tl 1,1 III
2
0 0.0
. . . . . . . . . . . . . . . . . O. 1
0.2
?'77 0.3
0.4
' 0.5
Moisture content (v/v)
Fig. 1. Changes in average crack width in relation to soil moisture content of a clay loam soil.
T h e d i a g r a m in Fig. 1 s h o w s t h a t d u r i n g the structural p h a s e as d e f i n e d b y M c G a r r y a n d M a l a f a n t ( 1 9 8 7 ) that c o r r e s p o n d s to s u c t i o n v a l u e s 0 to - 10 k P a ( p F v a l u e s f r o m 0.1 to 2) w a s v e r y s h o r t a n d v i r t u a l l y n o c r a c k s a p p e a r e d in the soil. T h i s p h a s e lasted 4.5 h o u r s
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N. Yassoglou et al. / Geoderma 63 (1994) 289-298
after the offset of irrigation. Note that during these measurements the weather was rather dry with an average air temperature of 36.5°C and a relative humidity of 66.5%. During the normal shrinkage phase, the average crack width shows a significant and almost linear increase with decreasing soil moisture content. This indicates that, upon drying, the reduction of volume in the soil domains is maximum at moisture contents corresponding to suction ranges between - 10 and - 100 kPa (pF values from 2 to 3). A specific range should be expected for each particular soil depending on its clay content and mineralogy. During the residual shrinkage, the average increase in crack width was less affected by water loss than in the previous phase (namely the upper curvilinear part of the curve in Fig. 1). The soil moisture content ranged within values corresponding to suction ranges from - 100 to - 1600 kPa (pF values from 3.0 to 4.2) in this phase. Finally, the phase of zero shrinkage was reached when a further drop of the soil moisture content did not affect the average crack width. The curve of Fig. 1 can be described by the following equation: CW
=
CWma x X [
1 - (MC/MCs)"]
(1)
where CW is the average crack width (mm) at soil moisture content MC, CWmax is the maximum average crack width (equal to 7.5 mm for the study soil), MCs the soil moisture at saturation ( v / v ) , and a is a constant calculated for each soil unit. Low and Margheim (1979) showed that swelling pressure (P) is related to mass of clay (Mc) and mass of water (Mw) according to the empirical equation: I n ( P + 1) = a × M c / M w + l n ( B )
(2)
where a and B are constants. An analogous relation was used in this investigation to describe the specific crack area (CA) as a function of the clay content (C), the COLE, and the volumetric moisture content (MC) specified by the following equation: CA = 2.04 × COLE exp(C/ln MC)
lim CA --+0,
pF~2
(3)
lim CA - - > CAmax
pF~4.2
A fair agreement between measured values of specific crack area and those calculated with Eq. (3) can be seen in Fig. 2, indicating that the above empirical equation satisfactorily describes (R = 0.86) the cracking of the soils. Fig. 3 shows that the specific crack area was linearly proportional to the average crack width. Cracking was initiated after the first irrigation due to the subsidence of the soil caused by flooding. During the period between two irrigations, the crack width on the soil surface increased and was accompanied by an increase in the crack width in the subsoil. The crack width was almost constant with depth, at least in the upper 10 cm soil layer, and then decreased forming a wedge shaped space. Preliminary observations in the field indicated that the depth of the constant cracking width must be connected with the moisture content of the soil. The maximum crack width on the soil surface was finally reached late in September. By that time, a system of interconnected cracks had been developed extending below the plow layer into the subsoil. As mentioned earlier, the crack depth increased with diminishing soil moisture and it was
295
N. Yassoglou et al. / Geoderma 63 (1994) 289-298
Agrinio plain
0
0.25
G"
Y = -0.007
+
• Lechena plain 1.028
*
X
J
R=0.86. n=49
•
0.20
°
,e ° / ~ g " o
~o ~ ~ o
0.15
°
"10 0.10
"5
.e
0.05 0,05
0.10
0.15
0.20
0.25
Measured area (m21m 2)
Fig. 2. Measured and simulated values of specific crack area in the study soils.
o 2.0
Agrinio plain Y
=
7.13
• Lechena plain
.,x.- X
0
1.5
• •
A ~ &
"o '~
1.0
O0
¢,,I,
O
O
O
°
•
2o 0.5
0.0 ~ 0.00
0.05
i
i
i
O. 10
O. 15
0.20
J 0.25
Specific crack area (m2/m2soil)
Fig. 3. Relation between measured crack width and specific crack area in the study soils.
related to the amount of clay, the field moisture content and the type of clay in the plow layer. The logarithmical equation: CD = 93.4 X (COLEX C/MC) 0"56
(4)
was found to predict satisfactorily the crack depth (CD, in cm) from clay content, COLE, and soil moisture content (MC, v/v) in the upper 5-35 cm soil layer of all studied soils. This is illustrated in in Fig. 4. Cracking of the surface layer of the soil to a depth of 6-8 cm occurred normally a few hours after irrigation, due to evaporation and drainage of water through the large pores. Afterwards, the depth of the cracks increased slowly, following the moisture withdrawal by transpiration of the growing plants and evaporation. Eqs. (3) and (4) can be used to calculate the total crack volume in swelling soils. However, an additional equation describing the change of crack width with depth is required.
296
N. Yassoglou et al. / Geoderma 63 (1994) 289-298 o Agrinio plain
• Lechena plain
60
40
•
O •O
•
O
°,
o
20 .ill 0
~ f
10
/
0.00
O.56
o •
Y = 93.4 -),(- X R=0.85. n = 5 0
o O. 10
0.20
COLE x (clay
0.30
/
0.40
MC)
Fig. 4. The change in crack depth as a relation of COLE, soil moisture (%, v/v) and clay contents in the study soils. Table 2 Water movementthrough cracks and conductivityof soil surface in representative soils (cm h- i ). Soil
AKR AER T183 T194 T30
Water movementthrough cracks after
Conductivityof soil surface after
5 min
90 min
5 min
90 min
0.76 1.15 5.51 7.92 3.79
0.14 0.35 1.92 2.11 0.46
4.56 3.56 6.72 6.01 1.72
1.23 0.71 3.20 0.47 0.11
The results of this investigation suggest that the change of crack width with depth cannot be described by a single continuous function. It seems that different functions apply for the soil sections with constant and with gradually decreasing crack width, respectively. This task would require further investigation of the behavior of thin layers in swelling soils. The data obtained here demonstrate that a wide range (0.4-5.5 cm) of irrigation water can be stored in the volume occupied by the cracks in the onset of irrigation. The water storage can be calculated by Eqs. (3) and (4) to a particularly acceptable degree and used in developing surface irrigation schemes for the study soils. Upon flooding a cracked soil, water flows through the cracks move downwards through an unsaturated soil matrix ( "short circuiting" ) and when macropores are not draining, they fill with water which subsequently moves laterally within the soil matrix (Bouma and Wosten, 1984). The results summarized in Table 2 demonstrate that lateral flow, after 90 min, was significant and, in some soils, exceeded the conductivity of the consolidated soil surface (soils T194 and T30). In an effort to validate the developed pedotransfer functions, the specific crack area and the crack depth were measured in the 14 alluvial soils of western Greece and compared with the values predicted by Eqs. (3) and (4), respectively. Fig. 5 illustrates a fairly good
N. Yassoglou et al. / Geoderma 63 (1994) 289-298 Y
0.25
=
0.054
~=O.B1.
*
071
-~
5D=0028.
297
".
CV=~89%
•
n=14
/ /
0.20 E
0.15
0.10
. . . .
0.05
,
,
,
,
,
0.~0
0.05
i
,
,
,
,
0.15
i
,
,
,
0.20
,
J 0.25
Measured area (m2/m 2) 40 A E
'~
Y
=
8.6
R=0.89.
35
+
0.69
$0=3.9.
* CV=
X 15.5%
n=14
30
~ 25 0
~ 20 15
. . . . 15
'
. . . .
20
~ , 25
,
,
,
i
,
J
,
30
•
J
. . . .
35
, 40
Measured depth (ern) F i g . 5. P r e d i c t e d
and measured
values
of specific
crack
area (a)
and of crack depth
(b)
in different
soils.
agreement between measured and predicted values. An underprediction of the crack area and depth is, however, noticeable, with a standard deviation of 3.9 cm for the cracking depth (CV = 15.5 % ) and 0.028 m 2 m - z for the cracking area (CV = 18.9%). This discrepancy must be attributed mainly to the different mineralogical composition of the soils. Introduction of the clay content into Eqs. (3) and (4) explained another 11% of the remaining variation between calculated and measured crack area and crack depth. This improvement may deserve further investigation. Concluding, the developed pedotransfer functions reliably describe the cracking of soils by using soil properties available in soil survey reports, and can be used for planning efficient surface irrigation schemes under semi-arid climatic conditions.
Acknowledgement This work was partially financed by the EC project "WASTES" (Water and Solute Transport in European Soils) [Contract No. STEP-CT90-0032-C (DSCN) ].
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N. Yassoglou et al. / Geoderma 63 (1994) 289-298
References Bouma, J., 1989. Using soil survey data for quantitative land evaluation. In: B.A. Stewart (Editor), Advances in Soil Science, 9. Springer, New York, pp. 117-123. Bouma, J. and Wosten, J.H.M., 1984. Characterizing ponded infiltration in a dry cracked clay soil. J. Hydrol., 69: 297-304. Bronswijk, J.J., 1991a. Drying, cracking, and subsidence of a clay soil in a lysimeter. Soil Sci., 152: 92-99. Bronswijk, J.J., 1991b. Relation between vertical soil movements and water-content changes in cracking clays. Soil Sci. Soc. Am. J., 55: 1220-1226. Dekker, L.W. and Bouma, J., 1984. Nitrogen leaching during sprinkler irrigation of a Dutch clay soil. Agric. Water Manage., 8: 37-47. Franzmeier, D.P. and Ross, S.J., 1968. Soil swelling: Laboratory measurements and relation to other soil properties. Soil Sci. Soc. Am. J., 32: 573-577. Gee, G.W. and Bauder, J.W., 1986. Particle size analysis. In: A. Klute (Editor), Methods of Soil Analyses. Part 1.2nd ed. Agron. Monograph 9. ASA and SSSA, Madison, WI, pp. 383-411. Haines, W.B., 1923. The volume changes associated with variations of water content in soil. J. Agric. Sci. Camb., 13:296-311. Hasegawa, S. and Sato, T., 1987. Water uptake by roots in cracks and water movement in a clayey soil. Soil Sci., 143: 381-386. Kosmas, C., Moustakas, N., Kallianou, Ch. and Yassoglou, N., 1991. Cracking patterns, bypass flow and nitrate leaching in Greek irrigated soils. Geoderma, 49: 139-152. Keen, B.A., 1931. The Physical Properties of the Soil. Longmans, Green and Co., London. Low, P.F. and Margheim, J.F., 1979. The swelling of clay. I. Basic concepts and empirical equations. Soil Sci. Soc. Am. J., 43: 473-481. McGarry, D. and Malafant, K.W., 1987. The analysis of volume change in unconfined units of soil. Soil Sci. Soc. Am. J., 51: 290-297. Moustakas, N., 1990. Relationships of morphological and physicochemical properties of Vertisols under Greek climate conditions. Ph.D. thesis, Agricultural Univ. of Athens, 175 pp. Nelson, R.E., 1982. Carbonate and gypsum. In: A.L. Page et al. (Editors), Methods of Soil Analysis. Part 2.2nd ed. Agron. Monograph 9. ASA and SSSA, Madison, WI, pp. 181-192. Nelson, D.W. and Sommers, L.E., 1982. Total carbon, organic carbon, and organic matter. In: A.L. Page et al. (Editors), Methods of Soil Analysis. Part 2.2nd ed. Agron. Monograph 9. ASA and SSSA, Madison, WI, pp. 539-594. Reeve, M.J. and Hall, D.G.M., 1978. Shrinkage in clayey subsoils of contrasting structure. J. Soil Sci., 29: 315323. Schafer, W.M. and Singer, J., 1976. A new method measuring shrink-swell potential using soil pastes. Soil Sci. Soc. Am. J., 40: 805-806. Soil Survey Staff, 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office, Washington, DC. Stroosnijder, L., 1976. Infiltration and redistribution of water in cracking upon water entry into the soils. Soil Sci. Soc. Am. J., 40: 352-358. Yule, D.F. and Ritchie, J.T., 1980. Soil shrinkage relationships of Texas Vertisols: 1. Small cores. Soil Sci. Soc. Am. J., 44: 1285-1291. Van Stiphout, T.P.J., Van Lanen, H.A.J., Boersma, O.H. and Bouma, J., 1987. The effect of bypass flow and internal catchement of rain on water regime in a clay loam grassland soil. J. Hydrol., 95:1-11.
Geoderma63 (1994) 289-298
Cracking in recent alluvial soils as related to easily determined soil properties N. Yassoglou a, C.S. Kosmas a, N. Moustakas a, E. Tzianis b, N.G. Danalatos a ~Laboratoryof Soilsand AgriculturalChemistry,AgriculturalUniversityof Athens, 75 lera Odos, Athens 11855, Greece ~FobaccoExperimentalStationof Agrinio,Ministryof AgricultureAgrinio30100, Greece) Received January 25, 1993; accepted after revisionOctober20, 1993
Abstract Cracking was studied in fifty soils located in two recent alluvial plains and it was related to soil parameters such as clay content, soil moisture content, bulk density and coefficient of linear extensibility (COLE). The total crack area per unit soil surface, the crack width and crack depth were measured by tracing them on transparent sheets. The rate of crack development was also studied in a soil immediately after irrigation. Results showed that crack width increased initially almost linearly with decreasing soil moisture content to a certain level and then increased in a declining rate until a maximum width was attained. Cracks appeared on the soil surface after water was removed from the large soil pores. The total crack area occupied a significant portion of the soil surface reaching a maximum value of 22.7%. Measurements of water flow into the cracks during irrigation were significant and in some soils exceeded the conductivity of the uncracked soil surface. An empirical relation was found, which calculates the total crack area per unit soil surface as a function of clay content, COLE and volumetric moisture content. The average crack width was linearly related to the total crack area measured on the soil surface. Furthermore, the crack depth in the plow layer was logarithmically related to clay content, COLE and moisture content.
1. Introduction Cracking soils occupy a considerable proportion of the agricultural land in the Mediterranean region. Alluvial soils usually contain high amounts of expanding clays, and soil layers at or near the surface undergo continual swelling and shrinking during wetting and drying cycles. Cracks are very common in periods of high evapotranspiration demands and 0016-7061/94/$07.00 © 1994ElsevierSciencePublishersB.V. All rights reserved SSDIO016-7061 (93)E0104-4
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may comprise a considerable part of the soil surface. They form a complicated network of interconnected channels, which can conduct water several meters away from an irrigated spot. Irrigation of a soil with continuous vertical cracks is subject to considerable loss of water below the root zone as well as to redistribution of soil materials and nutrients (Dekker and Bouma, 1984; Hasegawa and Sato, 1987; Van Stiphout et al., 1987). Kosmas et al. ( 1991 ) showed that a significant amount of the irrigation water may move, as bypass water, to the subsoil of swelling and shrinking soils without wetting the surface soil. However, in the same study was found that the amount of nitrates leached by the bypass water during a normal irrigation was rather small, ranging from 0.01 to 1.8% of the total nitrates present in the soil. The productivity of swelling and shrinking soils is usually very high. However, some physical properties of strongly swelling counterparts create unfavorable conditions for soil cultivation and plant growth: cracking during the drying period may damage the plant root system. Moreover, good tillage of these soils is possible only within a narrow moisture range. Shrinkage and drying are strongly dependent on the mobility of the soil phase and on the rate of moisture withdrawal (Stroosnijder, 1976). In a lysimeter study, Bronswijk ( 1991a, b) reported that shrinkage of clay progresses through three successive phases: structural shrinkage, isotropic normal shrinkage and isotropic residual shrinkage. The occurrence of normal and residual shrinkage could be predicted by changes in the soil moisture content and the shrinkage characteristics of soil aggregates. Haines (1923) and Keen (1931 ) had distinguished a fourth phase of shrinkage, i.e. the phase of zero shrinkage. During this phase, the soil particles had reached their densemost configuration, and any further decrease in the soil water content was not accompanied by any change in the volume of aggregates. McGarry and Malafant (1987) distinguished three shrinkage phases in unconfined units of soil when a clay soil dries: (a) a structural phase at high gravimetric water contents, where the volume change is less than the volume of water removed due to the drainage of large pores; (b) the normal phase, which lies in the drying sequence where the volume reduction is essentially equal to the loss of water; and (c) a residual zone at low gravimetric water contents, in which the volumetric reduction of the soil is smaller than the volume of water removed, implying an increase in the air volume. Most investigations of shrinkage and crack formation were conducted in the laboratory. Particularly, the relation between the changes in soil moisture content and volume has been investigated in several laboratory experiments using clay pastes, soil aggregates and soil cores of different sizes (Franzmeier and Ross, 1968; Reeve and Hall, 1978; Yule and Ritchie, 1980; Bronswijk, 1991a, b). Field data on soil shrinkage are scarce. The objective of the present work is to study the formation of cracks in the irrigated soils of recent alluvial plains and to develop such empirical formulas that relate crack development to soil properties easily determined and/or recorded in conventional soil survey reports. These relations could be used to develop pedotransfer functions (Bouma, 1989) relating the extend of cracking to easily determined soil parameters or that are given in soil maps. These functions could be used for developing efficient irrigation schemes maximizing water storage in cases that the cracks do not extend below the root zone (Van Stiphout et al., 1987), or for controlling cracking when they do. The soil properties investigated are:
N. Yassoglou et al. / Geoderma 63 (1994) 289-298
291
bulk density, clay content, coefficient of linear extensibility (COLE), and variations of moisture content as affected by irrigation.
2. Materials and methods Fifty soils in two recent alluvial plains of western Greece (32 soils in the Agrinio plain representing an area of about 10,000 ha, and 18 soils in the Lechena plain representing an area of 18,000 ha) were selected for this study. All soils contain high amounts of expanding clay minerals. The medium-textured soils are mainly well drained, whereas the fine-textured soils are poorly drained. They have an organic matter content that varies between 1.4 and 4.3%; however, the majority of them have low values, and a variable CaCO3 content (0.419.1%). The soils are classified as Typic Xerottuvents, Aquic Xerofluvents, Vertic Xerofluvents and Aeric-Vertic Fluvaquents (Soil Survey Staff, 1975). Most soils are cultivated with corn, vegetables and tobacco, and receive surface irrigation during the dry season. Another 14 alluvial soils with similar chemical and physical properties and soil management were chosen in two different alluvial plains (Alfios and Piros plains, western Greece) and were used to test the validity of the developed pedotransfer functions. The cracking of the soils was studied from late June (after the first irrigation) to the end of September. The size and the specific crack area, i.e. the surface area of the cracks per square meter of soil surface, were measured by tracing the cracks on a transparent sheet, and separating and weight the parts corresponding to the total crack and total surface area. In each of the fifty sampling sites, representing a certain mapping unit, cracks were measured in three 2.5 m by 0.5 m replicate plots. The replicate measurements were not significantly different and were averaged. The average depth and width along the cracks were measured in cross-sections dug in the plow layer. The estimation of the total crack volume was based on the observation that down to a certain depth from the soil surface, the cracks had an almost constant width. Below this depth, the width of the cracks decreased gradually. The crack volume in the upper section was estimated as the product of the specific crack area times depth of that section. The volume of the lower section was estimated as the product of the specific crack area times the crack depth of the lower section divided by 3. The bulk density and gravimetric field moisture content were determined in 300 cm 3 undisturbed soil cores, taken in triplicate from the plow layer, the latter extending down 37 to 45 cm from the soil surface, from each sampling site. The samples were dried at 105°C until constant weights. The degree of soil shrinkage was determined from the coefficient of linear extensibility (COLE) (Franzmeier and Ross, 1968). An amount of 30 g of a thoroughly mixed, airdried soil having passed through a 2 mm sieve was mixed with distilled water to almost saturation. The wet soil was placed in 10 cm long, 1 cm deep and 1 cm wide aluminum trays. The bottom of each tray was perforated to secure uniform drying. The bottom and the sides of the tray were covered with a thin film of petroleum gelly to prevent adhering of the soil to the metal surface and its consequent cracking. Subsequently, the soil was airdried, and the length of the dish and the dry rod were measured (Moustakas, 1990). Undisturbed soil samples were not used for COLE measurements because the soils were mainly friable due to the recent plowing and disking. Schafer and Singer (1976) found that
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N. Yassoglou et al. / Geoderma 63 (1994) 289-298
COLE measured using soil pastes (rod method) was satisfactory correlated to COLE measured in natural undisturbed soil clods. The organic carbon content was measured according to the modified wet oxidation procedure of the Walkley-Black (Nelson and Sommers, 1982). The particle size analysis was done with the Bouyoucos hydrometer method (Gee and Bauder, 1986). Inorganic carbonates were determined by measuring the volume of CO2 evolved in the reaction with excess of HC1 (Nelson, 1982). The maximum amount of water applied on the soil surface, which could be conducted to the subsoil through cracks, was estimated in a number of representative soils. A cylindrical block of soil with a height of 40 cm and a diameter of 35 cm was carved in situ. The sidewalls of the block of soil were encased in gypsum by pouring slurry into a mold. Quicksetting hydraulic cement was applied in a thin layer on the sidewalls of the block before pouring the gypsum slurry to avoid lateral flow of gypsum into the cracks (Bouma and Wosten, 1984). The intercrack spaces were sealed with gypsum and plastic spray. Then water was applied into the cracks with 4 cm of water ponded on the surface by using a closed top infiltrometer with a Mariotte device. The volume of applied water was then recorded as a function of time. Additionally, the hydraulic conductivity of the consolidated soil surface was measured with double ring infiltrometers.
3. Results and discussion
The measured data on crack characteristics and some of the related physical and chemical properties of the soils studied are summarized in Table 1. It can be seen that the coefficient of linear extensibility (COLE) varied from 0.04 (Aquic Xerofluvent) to 0.155 (Vertic Xerofluvent). It increased linearly with the clay content of the soils in both alluvial plains (R = 0.91 ). The average crack width ranged from zero (in a wet Aquic Xerofluvent) to 19.1 mm (Aeric-Vertic Fluvaquent), depending on both soil moisture and clay content. Similarly, the specific crack area ranged from 0 to 22.7%. In average values, Aeric-Vertic Fluvaquents had the highest clay content (44.8%) and the highest COLE (0.133) followed by the Vertic Xerofluvents ( clay = 42.9%, COLE = 0.122) (Table 1 ). The latter soils, being generally drier upon examination, exhibited the highest average crack width, depth, surface area and volume (Table 1). The crack area was generally greater at the end of the growing period of maize due to the lower soil moisture status which was associated with a higher bulk density of the soils. However, the cracks did not exceed deeper than the plow layer, which is the layer of maximum water extraction by the roots. Note that the maize roots were mainly concentrated in the plow layer, whereas the subsoil was wet (including the Vertic Xerofluvents) throughout the growing period, preventing the extension of cracks in it. The rate of crack development was investigated in a soil representing the most extensive mapping unit in the areas studied. The measurements of crack width, crack depth and moisture content were conducted hourly starting immediately after the offset of irrigation. Cracking of the soil surface started a few hours after irrigation and continued as the soil moisture content decreased. The crack width initially increased almost linearly with decreasing soil moisture content to a certain level. Thereafter, it increased at a declining rate until the maximum width was attained.
293
N. Yassoglou et al./ Geoderma 63 (1994) 289-298
Table 1 Range and mean values (in brackets) of physical and chemical properties, and crack characteristics of the studied soils. Property
xf
vxf
axf
avfa
Clay (%)
15.3-38.5 (25.9) 0.5-19.1 (8.8) 1.4-3.3 (2.3) 0.05-0.11 (0.076) 1.34-1.47 (1.39) 0.08-0.28 (0.198) 0.70-1.59 ( 1.01 ) 7.3-34.8 (24.3) 0.08-0.18 (0.125) 0.01-04 (0.02)
30.3-58.3 (42.9) 0.3-15.2 (7.8) 1.54.3 (3.0) 0.08-0.15 (0.012) 1.17-1.65 (1.36) 0.11-0.35 (0.213) 0.58-1.79 (1.20) 15.1-57.5 (38.4) 0.09-0.22 (0.172) 0.01-0.06 (0.036)
18.6-39.5 (31.6) 0.4-14.1 (4.8) 1.8-3.1 (2.5) 0.04-0.13 (0.094) 1.28-1.55 (1.40) 0.11-0.27 (0.195) 0.0-1.54 (0.91) 0.0-38.2 (26.3) 0.0-0.21 (0.133) 0.0-0.03 (0.022)
36.1-53.1 (44.8) 0.6-9.4 (4.5) 3.2~,. 1 (3.5) 0.11-0.15 (0.13) 1.30-1.52 (1.35) 0.19-0.28 (0.245) 0.40-0.91 (1.06) 19.2~-8.2 (33.8) 0.06-0.23 (0.161) 0.01-0.06 (0.032)
CaCO 3 (%) OM (%) COLE BD ( g/cm 3) MC (v/v) CW* (cm) CD* (cm) CA* (m2/m 2) CVOL*
(m3/m3)
MC = moisture content, CW = average crack width, CA = specific crack area, CVOL = crack volume. xf= Typic Xerofluvents, axf= Aquic xerofluvents, vxf= Vertic Xerofluvents, avfa = Aeric-Vertic Fluvaquents.
c~ack~
curve
liE .
CO
4 tl 1,1 III
2
0 0.0
. . . . . . . . . . . . . . . . . O. 1
0.2
?'77 0.3
0.4
' 0.5
Moisture content (v/v)
Fig. 1. Changes in average crack width in relation to soil moisture content of a clay loam soil.
T h e d i a g r a m in Fig. 1 s h o w s t h a t d u r i n g the structural p h a s e as d e f i n e d b y M c G a r r y a n d M a l a f a n t ( 1 9 8 7 ) that c o r r e s p o n d s to s u c t i o n v a l u e s 0 to - 10 k P a ( p F v a l u e s f r o m 0.1 to 2) w a s v e r y s h o r t a n d v i r t u a l l y n o c r a c k s a p p e a r e d in the soil. T h i s p h a s e lasted 4.5 h o u r s
294
N. Yassoglou et al. / Geoderma 63 (1994) 289-298
after the offset of irrigation. Note that during these measurements the weather was rather dry with an average air temperature of 36.5°C and a relative humidity of 66.5%. During the normal shrinkage phase, the average crack width shows a significant and almost linear increase with decreasing soil moisture content. This indicates that, upon drying, the reduction of volume in the soil domains is maximum at moisture contents corresponding to suction ranges between - 10 and - 100 kPa (pF values from 2 to 3). A specific range should be expected for each particular soil depending on its clay content and mineralogy. During the residual shrinkage, the average increase in crack width was less affected by water loss than in the previous phase (namely the upper curvilinear part of the curve in Fig. 1). The soil moisture content ranged within values corresponding to suction ranges from - 100 to - 1600 kPa (pF values from 3.0 to 4.2) in this phase. Finally, the phase of zero shrinkage was reached when a further drop of the soil moisture content did not affect the average crack width. The curve of Fig. 1 can be described by the following equation: CW
=
CWma x X [
1 - (MC/MCs)"]
(1)
where CW is the average crack width (mm) at soil moisture content MC, CWmax is the maximum average crack width (equal to 7.5 mm for the study soil), MCs the soil moisture at saturation ( v / v ) , and a is a constant calculated for each soil unit. Low and Margheim (1979) showed that swelling pressure (P) is related to mass of clay (Mc) and mass of water (Mw) according to the empirical equation: I n ( P + 1) = a × M c / M w + l n ( B )
(2)
where a and B are constants. An analogous relation was used in this investigation to describe the specific crack area (CA) as a function of the clay content (C), the COLE, and the volumetric moisture content (MC) specified by the following equation: CA = 2.04 × COLE exp(C/ln MC)
lim CA --+0,
pF~2
(3)
lim CA - - > CAmax
pF~4.2
A fair agreement between measured values of specific crack area and those calculated with Eq. (3) can be seen in Fig. 2, indicating that the above empirical equation satisfactorily describes (R = 0.86) the cracking of the soils. Fig. 3 shows that the specific crack area was linearly proportional to the average crack width. Cracking was initiated after the first irrigation due to the subsidence of the soil caused by flooding. During the period between two irrigations, the crack width on the soil surface increased and was accompanied by an increase in the crack width in the subsoil. The crack width was almost constant with depth, at least in the upper 10 cm soil layer, and then decreased forming a wedge shaped space. Preliminary observations in the field indicated that the depth of the constant cracking width must be connected with the moisture content of the soil. The maximum crack width on the soil surface was finally reached late in September. By that time, a system of interconnected cracks had been developed extending below the plow layer into the subsoil. As mentioned earlier, the crack depth increased with diminishing soil moisture and it was
295
N. Yassoglou et al. / Geoderma 63 (1994) 289-298
Agrinio plain
0
0.25
G"
Y = -0.007
+
• Lechena plain 1.028
*
X
J
R=0.86. n=49
•
0.20
°
,e ° / ~ g " o
~o ~ ~ o
0.15
°
"10 0.10
"5
.e
0.05 0,05
0.10
0.15
0.20
0.25
Measured area (m21m 2)
Fig. 2. Measured and simulated values of specific crack area in the study soils.
o 2.0
Agrinio plain Y
=
7.13
• Lechena plain
.,x.- X
0
1.5
• •
A ~ &
"o '~
1.0
O0
¢,,I,
O
O
O
°
•
2o 0.5
0.0 ~ 0.00
0.05
i
i
i
O. 10
O. 15
0.20
J 0.25
Specific crack area (m2/m2soil)
Fig. 3. Relation between measured crack width and specific crack area in the study soils.
related to the amount of clay, the field moisture content and the type of clay in the plow layer. The logarithmical equation: CD = 93.4 X (COLEX C/MC) 0"56
(4)
was found to predict satisfactorily the crack depth (CD, in cm) from clay content, COLE, and soil moisture content (MC, v/v) in the upper 5-35 cm soil layer of all studied soils. This is illustrated in in Fig. 4. Cracking of the surface layer of the soil to a depth of 6-8 cm occurred normally a few hours after irrigation, due to evaporation and drainage of water through the large pores. Afterwards, the depth of the cracks increased slowly, following the moisture withdrawal by transpiration of the growing plants and evaporation. Eqs. (3) and (4) can be used to calculate the total crack volume in swelling soils. However, an additional equation describing the change of crack width with depth is required.
296
N. Yassoglou et al. / Geoderma 63 (1994) 289-298 o Agrinio plain
• Lechena plain
60
40
•
O •O
•
O
°,
o
20 .ill 0
~ f
10
/
0.00
O.56
o •
Y = 93.4 -),(- X R=0.85. n = 5 0
o O. 10
0.20
COLE x (clay
0.30
/
0.40
MC)
Fig. 4. The change in crack depth as a relation of COLE, soil moisture (%, v/v) and clay contents in the study soils. Table 2 Water movementthrough cracks and conductivityof soil surface in representative soils (cm h- i ). Soil
AKR AER T183 T194 T30
Water movementthrough cracks after
Conductivityof soil surface after
5 min
90 min
5 min
90 min
0.76 1.15 5.51 7.92 3.79
0.14 0.35 1.92 2.11 0.46
4.56 3.56 6.72 6.01 1.72
1.23 0.71 3.20 0.47 0.11
The results of this investigation suggest that the change of crack width with depth cannot be described by a single continuous function. It seems that different functions apply for the soil sections with constant and with gradually decreasing crack width, respectively. This task would require further investigation of the behavior of thin layers in swelling soils. The data obtained here demonstrate that a wide range (0.4-5.5 cm) of irrigation water can be stored in the volume occupied by the cracks in the onset of irrigation. The water storage can be calculated by Eqs. (3) and (4) to a particularly acceptable degree and used in developing surface irrigation schemes for the study soils. Upon flooding a cracked soil, water flows through the cracks move downwards through an unsaturated soil matrix ( "short circuiting" ) and when macropores are not draining, they fill with water which subsequently moves laterally within the soil matrix (Bouma and Wosten, 1984). The results summarized in Table 2 demonstrate that lateral flow, after 90 min, was significant and, in some soils, exceeded the conductivity of the consolidated soil surface (soils T194 and T30). In an effort to validate the developed pedotransfer functions, the specific crack area and the crack depth were measured in the 14 alluvial soils of western Greece and compared with the values predicted by Eqs. (3) and (4), respectively. Fig. 5 illustrates a fairly good
N. Yassoglou et al. / Geoderma 63 (1994) 289-298 Y
0.25
=
0.054
~=O.B1.
*
071
-~
5D=0028.
297
".
CV=~89%
•
n=14
/ /
0.20 E
0.15
0.10
. . . .
0.05
,
,
,
,
,
0.~0
0.05
i
,
,
,
,
0.15
i
,
,
,
0.20
,
J 0.25
Measured area (m2/m 2) 40 A E
'~
Y
=
8.6
R=0.89.
35
+
0.69
$0=3.9.
* CV=
X 15.5%
n=14
30
~ 25 0
~ 20 15
. . . . 15
'
. . . .
20
~ , 25
,
,
,
i
,
J
,
30
•
J
. . . .
35
, 40
Measured depth (ern) F i g . 5. P r e d i c t e d
and measured
values
of specific
crack
area (a)
and of crack depth
(b)
in different
soils.
agreement between measured and predicted values. An underprediction of the crack area and depth is, however, noticeable, with a standard deviation of 3.9 cm for the cracking depth (CV = 15.5 % ) and 0.028 m 2 m - z for the cracking area (CV = 18.9%). This discrepancy must be attributed mainly to the different mineralogical composition of the soils. Introduction of the clay content into Eqs. (3) and (4) explained another 11% of the remaining variation between calculated and measured crack area and crack depth. This improvement may deserve further investigation. Concluding, the developed pedotransfer functions reliably describe the cracking of soils by using soil properties available in soil survey reports, and can be used for planning efficient surface irrigation schemes under semi-arid climatic conditions.
Acknowledgement This work was partially financed by the EC project "WASTES" (Water and Solute Transport in European Soils) [Contract No. STEP-CT90-0032-C (DSCN) ].
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N. Yassoglou et al. / Geoderma 63 (1994) 289-298
References Bouma, J., 1989. Using soil survey data for quantitative land evaluation. In: B.A. Stewart (Editor), Advances in Soil Science, 9. Springer, New York, pp. 117-123. Bouma, J. and Wosten, J.H.M., 1984. Characterizing ponded infiltration in a dry cracked clay soil. J. Hydrol., 69: 297-304. Bronswijk, J.J., 1991a. Drying, cracking, and subsidence of a clay soil in a lysimeter. Soil Sci., 152: 92-99. Bronswijk, J.J., 1991b. Relation between vertical soil movements and water-content changes in cracking clays. Soil Sci. Soc. Am. J., 55: 1220-1226. Dekker, L.W. and Bouma, J., 1984. Nitrogen leaching during sprinkler irrigation of a Dutch clay soil. Agric. Water Manage., 8: 37-47. Franzmeier, D.P. and Ross, S.J., 1968. Soil swelling: Laboratory measurements and relation to other soil properties. Soil Sci. Soc. Am. J., 32: 573-577. Gee, G.W. and Bauder, J.W., 1986. Particle size analysis. In: A. Klute (Editor), Methods of Soil Analyses. Part 1.2nd ed. Agron. Monograph 9. ASA and SSSA, Madison, WI, pp. 383-411. Haines, W.B., 1923. The volume changes associated with variations of water content in soil. J. Agric. Sci. Camb., 13:296-311. Hasegawa, S. and Sato, T., 1987. Water uptake by roots in cracks and water movement in a clayey soil. Soil Sci., 143: 381-386. Kosmas, C., Moustakas, N., Kallianou, Ch. and Yassoglou, N., 1991. Cracking patterns, bypass flow and nitrate leaching in Greek irrigated soils. Geoderma, 49: 139-152. Keen, B.A., 1931. The Physical Properties of the Soil. Longmans, Green and Co., London. Low, P.F. and Margheim, J.F., 1979. The swelling of clay. I. Basic concepts and empirical equations. Soil Sci. Soc. Am. J., 43: 473-481. McGarry, D. and Malafant, K.W., 1987. The analysis of volume change in unconfined units of soil. Soil Sci. Soc. Am. J., 51: 290-297. Moustakas, N., 1990. Relationships of morphological and physicochemical properties of Vertisols under Greek climate conditions. Ph.D. thesis, Agricultural Univ. of Athens, 175 pp. Nelson, R.E., 1982. Carbonate and gypsum. In: A.L. Page et al. (Editors), Methods of Soil Analysis. Part 2.2nd ed. Agron. Monograph 9. ASA and SSSA, Madison, WI, pp. 181-192. Nelson, D.W. and Sommers, L.E., 1982. Total carbon, organic carbon, and organic matter. In: A.L. Page et al. (Editors), Methods of Soil Analysis. Part 2.2nd ed. Agron. Monograph 9. ASA and SSSA, Madison, WI, pp. 539-594. Reeve, M.J. and Hall, D.G.M., 1978. Shrinkage in clayey subsoils of contrasting structure. J. Soil Sci., 29: 315323. Schafer, W.M. and Singer, J., 1976. A new method measuring shrink-swell potential using soil pastes. Soil Sci. Soc. Am. J., 40: 805-806. Soil Survey Staff, 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office, Washington, DC. Stroosnijder, L., 1976. Infiltration and redistribution of water in cracking upon water entry into the soils. Soil Sci. Soc. Am. J., 40: 352-358. Yule, D.F. and Ritchie, J.T., 1980. Soil shrinkage relationships of Texas Vertisols: 1. Small cores. Soil Sci. Soc. Am. J., 44: 1285-1291. Van Stiphout, T.P.J., Van Lanen, H.A.J., Boersma, O.H. and Bouma, J., 1987. The effect of bypass flow and internal catchement of rain on water regime in a clay loam grassland soil. J. Hydrol., 95:1-11.