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A study on cracking in some Vertisols of the Sudan
Geoderma - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
A STUDYON CRACKING IN SOME VERTISOLS OF THE SUDAN
A. ZEINEL ABEDINE and GLENN H. ROBINSON F.A.O. Regional Office, Bangkok (Thailand) (Received May 23, 1969) (Resubmitted June 26, 1970) ABSTRACT Zein el Abedine, A. and Robinson, G.H., 1971. A study on cracking in some Vertisols of the Sudan. Geoderma, 5: 229-241. A procedure for measuring soil cracks in certain vertisols of the Sudan w a s developed. This included the use of plate cut like a bilateralwith a base 12 c m long and a head angle of 22°30 '. The two sides of the bilateral were graduated to read the width of a crack. The depth was measured by flexible graduated metal probes. M e a s u r e ments were made along a straight line andthe frequency and volume of cracks determined. It was found that the widths of the cracks are affected by the length of drying period, the kind of soil and by the clay content, irrigation reduces the volume of cracks in the Gezira cotton-growing area. The vol~me of cracks under natural
vegetation was not altered by kind of vegetation. However, grass increases their volume in the surface layers and trees increases it at the deep layers. Thus the total volume of cracks in the upper layers of all soils studied is much affected by soil history and vegetation.
I N T R O D U C TION
The soils of the e a s t e r n part of the central clay plain of the Sudan a r e mostly Vertisols. T h e s e Vertisols were studied in r e p r e s e n t a t i v e p a r t s of the clay plain as f o l l o w s : the C,e z i r a a r e a which lies between the W h i t e Nile and Blue Nile, t h e Link Canal a r e a which extends in a broad s t r e t c h east of the Blue Nile, and the Gash delta which lies some 300 km east of the Gezira (see F i g . l ) . Of the distinguishing c h a r a c t e r i s t i c s of the Vertisol o r d e r of soils the p r e s e n c e of c r a c k s , c l a y content of 30% or more, slickensides and parallelepiped s t r u c t u r a l a g g r e g a t e s a r e definitive (Soil Survey Staff, 1967). The c r a c k s d e v e l o p when an expanding type of clay dries. The w i d t h and depth of c r a c k i n g a r e associated with the degree of desiccation, the amount and type of c l a y , and the amount and frequency of rainfall. The desiccation is also o f t e n related to the type and density of vegetation. Cracking i n f l u e n c e s soil development and soil c h a r a c t e r i s t i c s to s u c h a d e g r e e that it is c o n s i d e r e d at the highest level of soil classification. Some of the a s s o c i a t e d effects of soil. cracking include the formation of gilgai m i c r o - r e l i e f , s l i c k e n s i d e s , a churning o r mixing of the upper p a r t of the soil profile, a n d in some instances i n c r e a s i n g the amount and d e p t h of w a t e r penetration. Geoderma, 5 (1971)
229
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36°
34 °
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O |
50uth •
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I
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Fig.1. E a s t e r n p a r t of the central Clay Plain, Sudan. In areas with a thick well-developed surface mulch and wide cracks, the mulch will fall into the cracks. During the next rain the expansion of the soil plus the mulch due to wetting will be such that it causes the soft to heave, thus causing gilgai surface relief. This process churns and m i x e s the soil material to approximately the depth of cracking, thus often preventing the development of a textural B horizon. The churning and mixing tend to incorporate organic matter and may help in maintaining a better structure. In addition, the expansion with wetting causes slip planes, and, with the m o v e m e n t of soil, forms slickensides and parallelepiped structural aggregates. The penetration of water through the cracks in the Gezira area w a s estimated by J. Coulter (personal communication, 1967) to be 2.5 inches (63 ram) of water, when the cracks develop to their m a x i m u m at the e n d of the dry season. In a study on follow irrigation in Egypt, El Shal (1954) found that filling the cracks developed in a clay soil in middle Egypt required 75 m m of water more than the amount needed to raise the moisture to the field capacity. Within the G e z i r a a r e a the thickness of the s u r f a c e mulch is r e l a t i v e l y small, 1-5 ram, and gilgai relief is not well developed. However, e v i d e n c e of c r a c k i n g can be o b s e r v e d in pits to depths of m o r e than 1 m by the p r e s e n c e of wedge shaped intrusions of brown m a t e r i a l f r o m the s u r f a c e into a m o r e g r a y m a t e r i a l of subsoil and substratum. It has been o b s e r v e d by many s c i e n t i s t s (Greene, 1928; Finck and Ochtman, 1981; Soil S u r v e y Staff, 1968) that t h e r e often is a dark gray horizon underlying the b r o w n e r s u r f a c e soils, the d e p t h of which ranging between 40 cm andF m o r e t h a n 100 cm. In a r e a s w h e r e this dark g r a y horizon o c c u r s nearithe s u r f a c e , the c r a c k i n g and c h u r n i n g often break and mix the l a y e r to a point that it is n e a r l y d e s t r o y e d with only f r a g m e n t s remaining. Where the dark g r a y horizon is d e e p e r it b e c o m e s m o r e prominent and consequently m o r e
continuous. In profiles where this horizon is below 90 cm, there is little o r no mixing with the b r o w n e r soil material other than that which has fallen into the deeper c r a c k s . This phenomenon has been used by Zein el Abedine et al. (1971) to e s t i m a t e the age of the central G e z i r a Vertisols. A considerable p a r t of the Gezira a r e a is, at present, i r r i g a t e d w h i c h changes the natural m o i s t u r e balance in the soil, and consequently the pattern of cracking. F u r t h e r m o r e , in the common p r a c t i c e of m a n a g e m e n t of these soils, a y e a r of fallowing is inserted in the crop rotation as a beneficial m e a s u r e f o r fertility conservation and a helpful one for water penetration in the soil through the developed c r a c k s . The Gash delta V e r t i s o l s are brown clays of alluvial fan formation and a r e believed to be younger than the Gezira and Link Canal soils. They r e c e i v e less than 300 m m of precipitation and a r e cultivated by a rotation in which one i r r i g a t i o n by flooding takes place e v e r y three years. The usual p r o c e d u r e is to flood one third of the a r e a one y e a r and fallow it f o r two y e a r s . The flood w a t e r is distributed by canals and controlled by d y k e s which enable the w a t e r to stand on the land to a period of about three w e e k s . When the water is lost by evaporation and penetration to the deep layers, the soil b e c o m e s r e a d y for cultivation of certain c r o p s with no further irrigation. Such p e r i o d i c a l flooding may affect the cracking pattern of the soils. The Link Canal V e r t i s o l s are dark g r a y i s h brown clays all through the profiles. The c l i m a t e is m o r e humid than in the G e z i r a with r a i n fall of 500-600 m m p e r year. The a r e a is one of the proposed a r e a s for a g r i c u l t u r a l development, but is still under natural conditions. Neither the c h a n g e s in the cracking of the G e z i r a soils due to the change of m o i s t u r e balance, nor the amounts and depths of water penetration through the c r a c k s have been s e r i o u s l y investigated. In this study, the depth, volume, moisture conditions and certain other f a c t o r s relative to c r a c k s in the aforementioned soils of the central Clay Plain of the Sudan a r e evaluated.
MATERIALS AND METHODS The investigations were c a r r i e d out using 9 soil profiles: 2 profiles, GTO3 and GTO4, in c e n t r a l Gezira, 2 profiles, GTO5 and GTO6 in south Gezira, 1 profile, GRF in central Gezira f r o m a plot in the Gezira R e s e a r c h F a r m that is n o n - i r r i g a t e d and left under natural conditions, 2 profiles~ GHATO5 and GHBTO7, in the Gash delta area, and 2 profiles, O4H, and O4MH, in the Link Canal area. The f i r s t 4 profiles r e p r e s e n t irrigated soils in Gezira, and t h e r e s t a r e under natural conditions, bearing in m i n d that the Gash delta p r o f i l e s a r e artificially flooded once e v e r y three y e a r s . Moisture d e t e r m i n a t i o n s were c a r r i e d out at the time of crack m e a s u r i n g and a r e r e p o r t e d f o r t h r e e of the nine profiles (Table HI). The periods t h a t elapsed between the l a s t irrigation or rain and the m e a s u r e m e n t s of c r a c k s a r e given in Table I f o r all profiles. Measuring the depths of c r a c k s was c a r r i e d out using thin flexible graduated metal p r o b e s with pointed smooth noses. Deep c r a c k s were m e a s u r e d with a 150 c m long probe of a d i a m e t e r of 3 ram, and shallow c r a c k s w e r e m e a s u r e d by a 50 cm long one with a 1.5 mm diameter. The probe was gently pushed down vertically into the c r a c k until it stops, then it was moved few c e n t i m e t e r s back and forth along the c r a c k and
TABLE I C rops, last irrigation dates, time of sampling, and period of fallowing of p r o f i l e s under investigation Area
Profile no.
Crop and conditions before last irrigation or rain
Date of last irrigation or rain
Date of sampling
P e r i o d of drying months
Central Gezira
(GTO3 ~GTO4 ~GRF
cotton fallow fallow
Jan. 1968 rain Oct. 1967 rain Oct. 1 9 6 7
May 1968 May 1968 June 1968
4 7 8
South Gezira
/GTO5 (GTO6
wheat wheat
Feb. 1968 Feb. 1968
May 1968 May 1968
3 3
Link Canal
(O4H ~O4MH
natural forest
rain Oct. 1967 rain Oct. 1 9 6 7
Apr. 1968 6 Apr. 1968 6
Gash delta
fGHATO5 ~.GHBTO7
castor fallow
flood Aug. 1967 Feb. 1968 rain Aug. 1967 Feb. 1968
6 6
p u s h e d a g a i n , s o that the n o s e c h a n g e d i t s r e s t i n g point in the b o t t o m of t h e c r a c k . By doing t h i s , the p r o b e , in m o s t c a s e s , e n t e r e d a f u r t h e r d e p t h . B y r e p e a t i n g t h i s m o v e m e n t the f i n a l m e a s u r a b l e depth of the c r a c k w a s r e a c h e d and r e c o r d e d . In c e n t r a l G e z i r a p r o f i l e s the r e s u l t s w e r e c h e c k e d a n d found in a g r e e m e n t with v i s u a l m e a s u r e m e n t s i n f r e s h l y dug p i t s . The w i d t h s of c r a c k s at the s o i l s u r f a c e l e v e l w e r e m e a s u r e d b y a s p e c i a l h a r d m a t e r i a l p l a t e cut like a b i l a t e r a l with a b a s e 12 c m l o n g a n d a head a n g l e of 2 2 0 3 0 ' . The two s i d e s of the b i l a t e r a l a r e a g r a d u a t e d t o r e a d the width of a c r a c k w h e r e the two s i d e s of the b i l a t e r a l wedge r e s t on t h e s i d e s of the c r a c k s . This m o d e l w a s u s e d b e c a u s e the u p p e r m o s t e d g e s of the c r a c k s w e r e u s u a l l y d i s t o r t e d o r s l i g h t l y w i d e n e d due to the f a l l i n g of s u r f a c e m u l c h i n t o the c r a c k s . To m e a s u r e the c r a c k s in the f i e l d a s t a r t i n g point A was c h o s e n f r o m w h i c h a tape 20 m long was s t r e t c h e d in a c o n v e n t i o n a l d i r e c t i o n A B . T h e s t r a i g h t l i n e AB c r o s s e d a n u m b e r of c r a c k s that w e r e m e a s u r e d f o r d e p t h s a n d w i d t h s , a n d f o r d i s t a n c e s f r o m the point A. T h e widths r e c o r d e d w e r e t h o s e f a l l i n g on t h e l i n e AB. A n o t h e r l i n e AC, l a t e r a l to AB, was m a r k e d by the tape, f o r t h e s a m e d i s t a n c e of 20 m, s t a r t i n g f r o m point A, a n d t h e s a m e m e a s u r e s w e r e m a d e of the c r a c k s c r o s s e d by AC. Although t h e r e c o r d e d w i d t h s i n t h i s method a r e not the c r o s s - s e c t i o n a l o n e s a n d d e p e n d upon the a n g l e s b e t w e e n the c r a c k s a n d the l i n e s AB and AC, yet t h e r e s u l t s r e p r e s e n t the w i d t h s in a v e r t i c a l c r o s s - s e c t i o n a l o n g the l i n e s AB a n d AC, which are used a s b a s e s for calculating their volume. T h e a v e r a g e s p a c i n g of c r a c k s a l o n g a s t r a i g h t l i n e can be o b t a i n e d by d i v i d i n g the t o t a l length of m e a s u r e m e n t s by the n u m b e r of c r a c k s . The s t e p s t o c a l c u l a t e the l i n e a r s h r i n k a g e of the s o i l f r o m t h e s u r f a c e down to the b o t t o m of the d e e p e s t c r a c k , a n d the f i n a l c o m p u t a t i o n o f the v o l u m e of c r a c k s , a r e b a s e d on the a s s u m p t i o n that the c r o s s - s e c t i o n of a c r a c k i s a b i l a t e r a l the b a s e of which i s the width of the c r a c k at t h e s u r f a c e , and its h e i g h t is the depth. 232
Geoderma, 5 (1971)
These steps a r e as follows: (a) The c r a c k s m e a s u r e s a r e a r r a n g e d in a descending 0 r d e r of magnitude according to their depths. (b) Using a suitable scale, they a r e drawn in additive o r d e r on an X Y g r a p h , b a s e s on X, the deepest next to Y, a n d t h e heights a r e related t o Y axis. (c) A final accumulative curve is obtained representing the linear shrinkage of the soil in relation to depth. The scale on X is corrected to read percentage of linear shrinkage (Fig.2A). Linear shrinkage
~ 30
2
a
% °
-~-
--
F
Fig.2. A. D i a g r a m of producing linear shrinkage curve. B. Diagram of volume calculation of soil bulk and c r a c k s . (d) The total volume of soil bulk without c r a c k s can be calculated f o r any unit square a r e a by using the frustum of p y r a m i d equation of volume: v = 1/3 h (A 1 + A 2 + VA 1 • A 2) w h e r e u = volume of f r u s t u m of pyramid; A1 = a r e a of upper base; A2 = a r e a of bottom base; h = height. (e) The total volume, ABCDEFG (Fig.2B), is calculated by dividing it into segmental l a y e r s of known height: SO, S1, $2... (f) Each l a y e r has an upper base and a bottom one, the side lengths of which can be defined f r o m the curve of linear shrinkage, and by using the f r u s t u m of p y r a m i d equation, its volume is calculated. (g) The total bulk volume of the l a y e r s is the total shrunken volume of the soil without c r a c k s . (h) By s u b t r a c t i n g this volume f r o m the initial geometric volume, abcdE~G, of the unit a r e a , f r o m the s u r f a c e to the deepest crack, the total volume of c r a c k s is obtained. T h r e e c o m m e n t s m u s t be made here concerning the results obtained by this method. (1) It is r e a l i z e d that the linear shrinkage is related, in this method, to the dimensions of the soil at the time of m e a s u r e m e n t and is e x p r e s s e d as p e r c e n t a g e of them. (2) The ultimate l o w e r end of any c r a c k will be so narrow that it will not allow the probe to t o u c h its final end, thus, the figures obtained a r e of l e s s magnitude than the actual depths. The e r r o r will be within 5%, which i s the ratio of the probe d i a m e t e r to the a v e r a g e width of the crack at the s u r f a c e , i.e., 1.5:30 and 3:60 ram. Geoderma, 5 (1971)
23J
(3) In e x t r e m e c a s e s of d e s i c c a t i o n of soil, the c r o s s - s e c t i o n o f a c r a c k , e s p e c i a l l y a deep one, will not be a b i l a t e r a l . The s i d e s of t h e c r a c k at the top l a y e r s , in e x t r e m e d r y n e s s , a r e n e a r e r to being p a r a l l e l t o e a c h other than being the s i d e s of a b i l a t e r a l . Thus this m e t h o d would be a l w a y s l i a b l e to give r e s u l t s of t o t a l m e a s u r e s l e s s than the a c t u a l v a l u e s of the c r a c k depths, and, c o n s e q u e n t l y s l i g h t l y s m a l l e r v o l u m e . But we b e l i e v e that such r e s u l t s a r e s a t i s f a c t o r i l y c o m p a r a b l e to e a c h other.
RESULTS AND DISC USSION The clay c o n t e n t s of the s o i l s of the four a r e a s a r e not v e r y w i d e l y different. Clay c o n t e n t s a r e 51-58% in c e n t r a l G e z i r a , 60-64% in s o u t h G e z i r a , 71-76% in Link Canal and 55--67% in Gash d e l t a ' . E x c h a n g e a b l e sodium p e r c e n t a g e s in a l l l a y e r s of Link Canal and Gash delta p r o f i l e s a r e 3-6%. In the G e z i r a p r o f i l e s ESP is 4-8% in the s u r f a c e l a y e r s i n c r e a s i n g to 20-30% in the d e e p e r l a y e r s . The d i s c u s s i o n of r e s u l t s is p r e s e n t e d in four p a r t s : c r a c k d e p t h s , widths, spacing, a n d total volume.
Crack depths The data summarized in Table II present an overall picture of the differences in crack depths among the nine sites, as well as the frequencies of occurrence percent of the depths. The first point noted in the results, in spite of the different drying periods which m a y change the order if equalized, is the general adverse correlation between the average crack depths and the amount of precipitation. The descending order of magnitude in crack depths runs as: C a s h delta (non-flooded) > central Gezira (non irrigated) > Link Canal ~ the irrigated soils of Gezira. The descending order of rainfall or irrigation is: about 1,500 m m in irrigated Gezira, 600 m m in Link Canal, 300-400 in Gezira, and less than 300 m m in Gash delta. The amount of water u s e d for flooding profile G H A T O 5 Gash delta is not known, but for reasons discussed in a later part the average depth of this profile is equal to GI-IBTO7 which received natural precipitation only. The m a x i m u m depth reached in the irrigated Gezira area does not exceed 65 cm, while in the non-irrigated one the cracks reached 135 cm. It is also noticed that although the m a x i m u m depths in the G e z i r a irrigated soils range between 45 and 65 cm, yet, there are differences amongst the peaks of occurrence. Profile G T O 3 has 73% of the total number of cracks between 16 and 35 c m with an average of 26 cm. T h i s shallow cracking is due to the fact that the site was occupied by cotton plants and the stalks were still standing furnishing a cover that reduces evaporation. In the s a m e time the absorption of moisture by the m a t u r e d plants was naturally low. Both conditions check losses by evaporation and consequently reducing the total shrinking and cracking. 1 Data obtained by Soil Survey Laboratory Staff, Wad Medani, Sudan. 234
Geoderma, 5 (1971)
TABLE
lI
Frequencies of occurrence percent, and average depths of cracks in the Gezira, Link Canal and Gash delta soils Range
Gezira
(cm)
irrigated
Link Cannal natural natural condition cond.
Gash delta flooded every 3 years
GTO3
GTO4
GTO5
GTO6
GRF
O4H
O4MH
G H A T 5 GHBT7
12 46 27 15
5 17 38 29
4 9 23 31
2 8 41 44
1 24 21 11
10 30 18 14
3 14 24 17
11 16 11 8
4 8 8 16
46-55 56-65 66-75 76-85 86--95
. . .
i0 1 . . .
27 6
5 . . .
8 6 9 6 7
13 10 5 --
25 9 5 3 --
5 4 15 15 7
28 6 3 9 8
96--105
.
1
-
--
3
4
106-115 ~>115
. .
. .
4 2
--
---
4 1
3 3
51
42
36
60
60
416 16--25 26-35 36-45
Profile
aver. cm
26
.
. . .
. . . 34
. . . 39
35
P r o f i l e GTO4, o r i g i n a l l y fallowed a f t e r the r a i n f a l l of 1967, h a s 84% of the c r a c k s b e t w e e n 16 a n d 45 cm with a n a v e r a g e of 34 cm. P r o f i l e s GTO5 a n d GTO6 have m o r e than 80% of t h e i r c r a c k s b e t w e e n 26 and 55 c r n , with a v e r a g e s of 39 a n d 35 c m , r e s p e c t i v e l y . It is n o t i c e d that although t h e l a s t two p r o f i l e s w e r e u n d e r wheat, which had b e e n i r r i g a t e d only 3 m o n t h s p r i o r to s a m p l i n g , y e t t h e y have about the s a m e p a t t e r n of c r a c k i n g a s GTO4 which w a s f a l l o w e d f o r 7 months. T h i s i s due to the fact that w h e a t c r o p , w h i c h m a y be c o n s i d e r e d as a d e n s e g r o w i n g g r a s s , would r a p i d l y c o n s u m e the m o i s t u r e f r o m the top l a y e r s . A f t e r h a r v e s t i n g wheat the s o i l was e x p o s e d to the a t m o s p h e r i c e l e m e n t s that e n h a n c e m o i s t u r e l o s s e s b y e v a p o r a t i o n . T h e s e c o n d i t i o n s b r i n g the u p p e r p a r t of the p r o f i l e n e a r e r t o the c o n d i t i o n s of the l o n g f a l l o w e d p r o f i l e GTO4 and the n o n - i r r i g a t e d c o n d i t i o n s of p r o f i l e G R F . The m o i s t u r e c o n t e n t s d e t e r m i n e d in p r o f i l e s GTO4, GTO5 and G R F c o n f i r m t h i s s t a t e m e n t ( T a b l e III). T h e n o n - i r r i g a t e d p r o f i l e G R F which r e p r e s e n t s the s t a t e of c r a c k i n g u n d e r n a t u r a l c o n d i t i o n s , h a s a d i f f e r e n t p a t t e r n . The peak of c r a c k d e p t h s a p p e a r s at the s h a l l o w r a n g e of 16 to 25 c m , followed by a g r a d u a l d e c r e a s e t o w a r d s d e e p e r l e v e l s u n t i l 135 cm. As the "dark g r a y h o r i z o n a p p e a r s i n t h i s p r o f i l e a t 65-120 c m , t h u s , at l e a s t 40% of the c r a c k s t a p it and v e r y few of t h e m cut t h r o u g h i t s e n t i r e t h i c k n e s s . T h e s e f i n d i n g s a r e in a g r e e m e n t with t h e m o r p h o l o g i c a l f e a t u r e s o b s e r v e d by the Soil S u r v e y Staff, w h e r e t h e top b r o w n and the d a r k g r a y m a t e r i a l s m i x o r f o r m wedge s h a p e d i n t r u s i o n s of b r o w n into the d a r k g r a y m a t e r i a l of the s u b s o i l . T h e two p r o f i l e s i t e s of the Link C a n a l a r e a a r e s l i g h t l y d i f f e r e n t i n that s i t e O4H i s u n d e r g r a s s and s c a n t y t r e e s , and s i t e O4MH i s u n d e r trees and scanty grass.
Geoderma, 5 (1971)
235
TABLE III Moisture contents at time of cracks measuring in certain profiles (GTO4, GTO5 and GRF ) in Gezira GTO4
GTO5
GRF 1
depth (cm)
percent,
depth (cm)
percent,
depth (cm)
percent.
0-1 1-30 30--40 40-70 70-105 105-140
6.0 7.9 11.0 15.5 19.2 24.0
0-2 2-10 10--35 35-67 67-102
7.5 8.0 13.0 18.0 24.5
0--20 20-40 40-60 60-80 80-100 100-120
6.3 10.5 16.9 20.3 21.6 22.1
1 With kind permission of H. Farbrother, Cotton Research Corporation, Wad Medani, Sudan. The effect of n a t u r a l v e g e t a t i o n e x p r e s s e s i t s e l f in the d e p t h s o f c r a c k s . P r o f i l e O4H, u n d e r g r a s s , has m o r e than 60% of i t s c r a c k s b e t w e e n 16 and 45 cm d e p t h , w h e r e a s p r o f i l e O4MH, u n d e r t r e e s , has 75% o f t h e c r a c k s between 26 a n d 55 cm. It is evident that g r a s s d e v e l o p s s h a l l o w c r a c k i n g and t r e e s d e p l e t e s o i l m o i s t u r e to a g r e a t e r d e g r e e and f r o m deeper layers. The two s i t e s of the Gash d e l t a d i f f e r in t h e i r h i s t o r y . Site G H A T O 5 was flooded 6 m o n t h s p r i o r to c r a c k m e a s u r i n g and s t a l k s of c a s t o r p l a n t s w e r e s t i l l s t a n d i n g . S i t e GHBTO7 has not b e e n flooded and r e c e i v e d o n l y the r a i n s of 1967. A l t h o u g h both p r o f i l e s have t h e s a m e a v e r a g e d e p t h , yet, the peak of o c c u r r e n c e a p p e a r s at the shallow depth of 26 cm in G H A T O S , and a p p e a r s b e t w e e n the depths 46-55 in GHATO7 that r e c e i v e d r a i n f a l l only. The s u r f a c e l a y e r of GHATO5 must have s w e l l e d to a m a x i m u m u n d e r flooding and when i t d r i e d it c r a c k e d m o r e than GHBTOT. In c o n c l u s i o n i t i s s t a t e d that the a v e r a g e depth of c r a c k s i s i n v e r s e l y r e l a t e d to the a m o u n t of p r e c i p i t a t i o n or i r r i g a t i o n . The f r e q u e n c i e s of o c c u r r e n c e of c r a c k depths a r e affected to a c o n s i d e r a b l e extent b y t h e kind and d e n s i t y o f v e g e t a t i o n and h i s t o r y of the soil.
Crack widths The data of c r a c k widths do not show g r e a t d i f f e r e n c e s a m o n g t h e d i f f e r e n t p r o f i l e s a n d a r e a s a s those data of c r a c k depths. The r e a s o n , of c o u r s e , is that t h e p l a i n o r s u r f a c e s h r i n k i n g a l m o s t c o m p l e t e l y s t o p s , at i t s m a x i m u m , w h e n t h e m o i s t u r e r e a c h e s the h y g r o s c o p i c r a n g e . T h i s i s c o n t r a r y to the d e p t h s of c r a c k s which will i n c r e a s e a s long a s d e p l e t i o n of m o i s t u r e c o n t i n u e s a d v a n c i n g f r o m the s u r f a c e downwards to t h e d e e p l a y e r s with no l i m i t a t i o n e x c e p t the t i m e f a c t o r and t h e deptn of w e t l a y e r s . The d a t a of c r a c k widths i s p r e s e n t e d in T a b l e IV f o r 9 p r o f i l e s , a s f r e q u e n c i e s of o c c u r r e n c e p e r c e n t and p r o f i l e a v e r a g e s . It is noted f r o m the d a t a of the G e z i r a p r o f i l e s , that the p e a k s of o c c u r r e n c e of c r a c k widths a r e m o r e or l e s s r e l a t e d to the p e r i o d o f fallowing. The p e a k of 28% a p p e a r s in the n o n - i r r i g a t e d p r o f i t e s i t e G R F 236
Geoderma, 5 (1971}
TABLE IV F r e q u e n c i e s of o c c u r r e n c e p e r c e n t and a v e r a g e c r a c k widths of the s o i l s of G e z i r a , Link C a n a l and Gash d e l t a a r e a s
Crack width (cm)
<2.6 2.6--3.5 3.6-4.5 4.6-5.5 5.6--6.5 6.6--7.5 7.5
Average width of profile (cm)
Gezira
Link Canal
Gash delta
GTO3 GTO4 GTO5 GTO6 GRF
(%)
(%)
(%)
(%)
(%)
O4H O4MH
(%)
(%)
GHATO5 GHBTO7
(%)
(%)
8 27 38 19 4 4 --
9 23 32 24 7 4 1
18 42 27 11 2 --
14 51 24 11 ---
9 25 22 28 13 2 1
6 22 31 16 6 13 6
2 3 37 27 15 7 9
30 30 19 4 17 --
22 31 19 19 6 3
3.9
4.2
3.4
3.4
4.2
5.1
4.8
3.4
3.7
at 4.5-5.5 cm width. T h i s s i t e has the longest p e r i o d u n d e r d r y i n g conditions. The p e a k s of 38 and 32% in s i t e s GTO3 and GTO4, that w e r e fallowed f o r 4 and 7 m o n t h s r e s p e c t i v e l y , a p p e a r at c r a c k width 3.6-4.5 c m . Sites GTO5 and G T O 6 , that w e r e fallowed f o r 3-4 months have t h e i r p e a k s of 42 and 51% r e s p e c t i v e l y at 2.6-3.5 cm width. The a v e r a g e s of c r a c k widths in G e z i r a a r e a l s o r e l a t e d to the p e r i o d s of fallowing. The d e s c e n d i n g o r d e r of length of fallowing is 8, 7, 4 and 3 months and c o r r e s p o n d s with the d e s c e n d i n g o r d e r of c r a c k widths of 4 . 2 , 4.2, 3.9 and 3.4 cm r e s p e c t i v e l y . The effect of v e g e t a t i o n in the Link Canal a r e a is dominant. In s i t e O4H, u n d e r g r a s s , t h e a v e r a g e c r a c k width i s 5.1 cm, while in s i t e O4MI-I, u n d e r t r e e s , the a v e r a g e is 4.8 cm. The data i n d i c a t e a g r e a t e r s u r f a c e d r y n e s s in the s i t e w i t h dominant g r a s s vegetation. The m e a s u r e s of the Gash d e l t a indicate that the s i t e that was f l o o d e d 7 months p r i o r to c r a c k m e a s u r i n g has an a v e r a g e width of 3.4 cm, w h i l e the s i t e that was u n d e r r a i n only has an a v e r a g e of 3.7 cm. It is a l s o n o t i c e d that the f r e q u e n c i e s of o c c u r r e n c e in the f o r m e r s i t e a r e h i g h e r than t h e l a t t e r at the r a n g e s of n a r r o w c r a c k s . In c o m p a r i s o n with o t h e r s i t e s u n d e r n a t u r a l conditions, t h e Gash d e l t a p r o f i l e has the n a r r o w e s t a v e r a g e w i d t h of c r a c k s owing to i t s l o w e r c l a y content. The effect of c l a y content on the width of c r a c k s can be t r a c e d o n l y by c o m p a r i n g G a s h d e l t a with Link Canal s o i l s . The Gash d e l t a s o i l s c o n t a i n 55-67% c l a y and t h e i r a v e r a g e width of c r a c k s i s 3.4-3.7 c m . The Link Canal s o i l s have 7 1 - 7 6 % c l a y and t h e i r a v e r a g e width of c r a c k s is 4.8-5.1 cm. In c o n c l u s i o n it i s c l e a r t h a t c r a c k widths a r e affected by the l e n g t h of d r y i n g p e r i o d , by t h e h i s t o r y of the soil, and by the c l a y content. The d i f f e r e n c e s in the c r a c k widths a r e r a t h e r l i m i t e d to a n a r r o w r a n g e of variability.
Geoderma, 5 (1971)
237
Crack spaci~zg Although t h e g e n e r a l shape of soil b l o c k s e n c i r c l e d by c r a c k s i s a polygon, and a m e a s u r e m e n t of s p a c i n g a l o n g a s t r a i g h t l i n e will n o t n e c e s s a r i l y m e a s u r e the a r b i t r a r y c e n t r e l i n e of the p o l y g o n s , y e t , t h e d a t a help in p r e s e n t i n g an a p p r o x i m a t e s u r v e y of the a v e r a g e s p a c i n g b e t w e e n c r a c k s on a l i n e a r b a s i s . The r e s u l t s a r e g i v e n in T a b l e V f o r all s i t e s . The d a t a a r e p r e s e n t e d a s a v e r a g e s p a c i n g in c e n t i m e t e r s and a s f r e q u e n c i e s of o c c u r r e n c e of spacing intervals. TABLE V Average spacings and frequencies of occurrence percent of spaces between cracks in Gezira, Link Canal and Gash delta soils Spaces
Gezira
Link Canal
Gash delta
GTO3 GTO4 GTO5 GTO6 GRF
O4H O 4 M H
GHATO5
GHBTO7
<21 21-40 41--60
20 40 32
14 41 19
22 29 20
5 30 35
39 53 7
19 26 21
31 35 18
17 49 22
6 19 41
61-80 81-100
4 4
8 6
20 7
19 ii
1 -
31 3
8 6
8 4
19 9
2
-
-
-
2
-
6
45
54
28
51
39
39
62
(cm)
> I 0 0
Average spacing of profile (cm)
-
39
12
52
The c o m p a r i s o n a m o n g s t a r e a s and i n d i v i d u a l p r o f i l e s p r e s e n t s o m e p o i n t s of i n t e r e s t . The i r r i g a t e d s o i l s of G e z i r a and G a s h d e l t a (flooded) c r a c k a t a v e r a g e s p a c i n g b e t w e e n 39 and 54 cm, with p e a k s of o c c u r r e n c e a t 21-60 cm spacing. T h e G e z i r a soil u n d e r n a t u r a l ( n o n - i r r i g a t e d ) c o n d i t i o n s s h o w s a m u c h n a r r o w e r s p a c i n g with an a v e r a g e of 28 cm. T h i s i n d i c a t e s t h a t e x t r e m e d e s i c c a t i o n , in the G e z i r a , i n c r e a s e s b r e a k - d o w n and f i s s u r i n g of the b i g g e r s u r f a c e b l o c k s . The L i n k C a n a l s o i l s show d i f f e r e n t a v e r a g e s . The s o i l u n d e r d o m i n a n t g r a s s , O4H, c r a c k s at an a v e r a g e s p a c i n g of 51 c m , w h i l e that u n d e r t r e e s , O4MH, h a s an a v e r a g e of 39 cm. T h i s i s r e n d e r e d to the m e c h a n i c a l b i n d i n g effect of the s h a l l o w root s y s t e m s of g r a s s w h i c h act a s i n f o r c e m e n t f o r t h e s u r f a c e b l o c k t h u s r e d u c i n g the f i s s u r i n g of t h e i r s t r u c t u r e . It c a n b e c o n c l u d e d that the s p a c i n g of c r a c k s i s i n f l u e n c e d b y t h e h i s t o r y of the s o i l a n d by the n a t u r e of the v e g e t a t i v e c o v e r .
L i n e a r shrinkage and total volume of c r a c k s T h e l i n e a r s h r i n k a g e c u r v e s of the 9 s i t e s u n d e r i n v e s t i g a t i o n a r e p r e s e n t e d in F i g . 3 , c a l c u l a t e d a s p e r c e n t of e x i s t i n g d i m e n s i o n s a t t h e t i m e of m e a s u r i n g . 238 Geoderma, 5 (1971)
Linear shrinkageY%) 00
40
.
•
5
I0
15
,,
~+o1[I! ,
~ so~ Itz~
GT03 04 H . GT04 ....... 04MH . . . . GT05 ...............GT06 .-,.-,_~GHBT07 ~GRF . . . . GHATO5
Fig.3. L i n e a r shrinkage of soils: GTO3 and GTO4 in central Gezira, GTO5 and GTO6 in south Gezira, GRF permanent fallow in central G e z i r a , O4H and O4MH in Link Canal, and GHATO5 and GHBTO7 in Gash delta. It is noticed that although the surface shrinkage in the G e z i r a i r r i g a t e d soils is between 7 and 10% , yet the shrinkage d e c r e a s e s to l e s s than 0.7 cm at a depth of 50 cm f r o m the surface. The G e z i r a soil under natural conditions, GRF, has the g r e a t e s t value of shrinkage of 15% in t h e s u r f a c e , d e c r e a s i n g to about 3% at 50 cm depth, and continues to d e c r e a s e m o r e slowly to 0% at the depth 130 cm. Thus i r r i g a t i o n has reduced the shrinkage to about half that under natural conditions. The Link Canal soils have s m a l l e r shrinkage than Gezira soils u n d e r natural conditions in both surface and deep l a y e r s . The shrinking in the Link Canal soils ends at about 70 cm depth. This difference is mainly due to the difference in rainfall and partly to that of the drying period, as L i n k Canal a r e a r e c e i v e s about 600 mm against about 300 m m in Gezira, while the drying periods a r e 6 and 8 months respectively. The Gash delta soils have a different pattern due to their lower c l a y contents and higher permeability. The surface shrinkage is r a t h e r s m a l l , between 5 and 9%, but the shrinking d e c r e a s e s at a much slower rate than all o t h e r profiles to r e a c h the deepest level of all, between 130 and 150 c m . The deep c r a c k i n g of the soils of Gash delta is r e n d e r e d to their c o m p a r a tively higher p e r m e a b i l i t y which helps to drain them m o r e efficiently than the other a r e a s , thus reducing the moisture contents in the deep l a y e r s a n d shortening the t i m e of drying. The d i f f e r e n c e s observed in linear shrinkage a r e emphasized in the total volume of c r a c k s . The data of t o t a l crack volumes a r e p r e s e n t e d in Table VI. The r e s u l t s a r e p r e s e n t e d so that c r a c k volumes a r e given for each 10 cm l a y e r of the profile down to the deepest crack. The r e s u l t s a r e c a l c u l a t e d as cubic m e t r e s p e r h e c t a r e , m i l l i m e t r e s of water, and cubic m e t e r s of w a t e r p e r feddan*. The total v o l u m e of c r a c k s in the i r r i g a t e d a r e a of the Gezira r a n g e s between 222 m 3 / h a and 313 m3/ha, equivalent to 22.2 mm and 31.3 mm o f w a t e r respectively. In the site GRF of n o n - i r r i g a t e d soil, the c r a c k s volume r e a c h e s 867 m 3 / h a , equivalent to 86.7 m m of water, i.e., about 3--4 t i m e s the volume of the i r r i g a t e d soils. *Feddan = 4,200 m 2.
TABLE
VI
Volumes of cracks in Gezira, hectare per 10 cm layer Layers
Link Canal,
and Gash delta soils in metre
Gezira
cube
Link Canal
Gash Delta
per
(cm) GTO3
GTO4
GTO5
GTO6
GRF
O4H
O4MH
GHATO5
GHBTO7
0-10 10-20 20-30 30-40 40--50 50--60 60-70 70-80 80-90 90-100 100-110 110-120 120-130
157 85 33 10 2 . . . . . . .
140 90 48 19 6 2
130 94 54 25 8 2 . . . . . . .
102 70 36 12 2 -
247 187 131 92 69 52 38 24 14 8 5
184 122 74 40 20 8 2 -
172 125 82 46 23 9 4 1 --
159 125 92 71 56 42 29 18 7 4 3 2 1
105 83 64 47 32 22 15 10 7 4 2 1
Total m3/ha
287
305
313
222
867
450
462
609
392
m3/feddan
121
128
131
93
364
189
194
256
165
28.7
. . . . .
. . . . . . .
. .
30.5
31.3
. .
. .
22.2
. .
86.7
45,0
46.2
60.9
39.2
Concerning the question of water penetration in the soil profile, it will be noticed that only between 1.2 m m and 3.5 m m of water do penetrate the soil profile to the depths below 30 c m from the surface in the irrigated soils of Gezira. The site under natural conditions is much better this way, as 30 m m of water penetrate below 30 cm. Thus, in this respect, the beneficial effect of fallowing the land for one year in the c o m m o n rotation applied in Gezira, becomes questionable from the point of view of water penetration in the profile. The two Link Canal profiles, O 4 H and O 4 M H , have almost the s a m e volume of cracks, 450 and 462 m3/ha, equivalent to 45.0 and 46.2 r n m of water respectively. Though the total volume is about the same in both sites, yet, it should be noticed that in O4H, under grass, the volume of cracks below 30 c m depth is only 70 m3/ha, while in O 4 M H , under trees, the volume below the same depth is 83 m 3/ha. This indicates m o r e shrinking in the deep layer under trees than under grass. The two Gash delta sites have appreciably different volumea of cracks, the one previously flooded and cultivated with castor crop, G H A T O S , has 609 m3/ha, and the other one that received rain only, GIIBTO7, has 392 rn3/ha..This difference is due to two factors: (a) the greater depletion of moisture from the surface by the cRstor plants; and (b) the flooding for 3 weeks of G H A T O 5 allowed the soil to swell more freely than the scanty rain received by GIIBTO7, and consequently the shrinking of the former will be greater than the latter. The volume of cracks in the top 30 c m in the site under castor plants is 376 m3/ha 'against 255 m3/ha in the site with rain only.
240
Geoderma,
5 (1971}
In c o n c l u s i o n it i s c o n s i d e r e d that i r r i g a t i o n r e d u c e s the v o l u m e of c r a c k s in the G e z i r a to one t h i r d o r one f o r t h of the v o l u m e u n d e r n a t u r a l c o n d i t i o n s . The v o l u m e of c r a c k s in Link C a n a l a r e a i s not affected by t h e k i n d of v e g e t a t i o n a l t h o u g h g r a s s i n c r e a s e s t h e i r v o l u m e i n the s u r f a c e l a y e r s , a n d t r e e s i n c r e a s e s it at the deep l a y e r s . T h e s o i l s of Gash d e l t a c r a c k to g r e a t e r d e p t h s than i n a l l o t h e r a r e a s , b u t the c r a c k s a r e n a r r o w . T h e t o t a l v o l u m e of c r a c k s in the u p p e r l a y e r s i s m u c h affected by soil history and vegetation.
REFERENCES El Shal, M.J., 1954. Water Requirements of Main Crops in Menoufia District. T h e s i s , Faculty of Engineering, Cairo University, Cairo, U.A.R., 62 pp. Finck, A. and Ochtman, L.H.J., 1961. Problems of soil evaluation in the Sudan. J. Soil Sci., 12, 87-95. Greene, H., 1928. Soil profile in the eastern Gezira. J. Agr. Sci., 18: 518-530. Soil Survey Staff, 1968. Reports on the Gezira soils. Division of Soil Survey, M i n i s t r y of Agriculture, Wad Medani, Sudan. Soil Survey Staff, 1967. Soil Classification, a Comprehensive System, Supplement t o 7th Approximation. 1967. U.S. Dept. of Agriculture, 207 pp. Zein el Abedine, A., Robinson,. G.H. and Commissaris, A., 1971. Approximate age of the Vertisols of Gezira, central Clay Plain, Sudan. Soil Sci., in p r e s s .
Geoderma, 5 (1971)
241
A STUDYON CRACKING IN SOME VERTISOLS OF THE SUDAN
A. ZEINEL ABEDINE and GLENN H. ROBINSON F.A.O. Regional Office, Bangkok (Thailand) (Received May 23, 1969) (Resubmitted June 26, 1970) ABSTRACT Zein el Abedine, A. and Robinson, G.H., 1971. A study on cracking in some Vertisols of the Sudan. Geoderma, 5: 229-241. A procedure for measuring soil cracks in certain vertisols of the Sudan w a s developed. This included the use of plate cut like a bilateralwith a base 12 c m long and a head angle of 22°30 '. The two sides of the bilateral were graduated to read the width of a crack. The depth was measured by flexible graduated metal probes. M e a s u r e ments were made along a straight line andthe frequency and volume of cracks determined. It was found that the widths of the cracks are affected by the length of drying period, the kind of soil and by the clay content, irrigation reduces the volume of cracks in the Gezira cotton-growing area. The vol~me of cracks under natural
vegetation was not altered by kind of vegetation. However, grass increases their volume in the surface layers and trees increases it at the deep layers. Thus the total volume of cracks in the upper layers of all soils studied is much affected by soil history and vegetation.
I N T R O D U C TION
The soils of the e a s t e r n part of the central clay plain of the Sudan a r e mostly Vertisols. T h e s e Vertisols were studied in r e p r e s e n t a t i v e p a r t s of the clay plain as f o l l o w s : the C,e z i r a a r e a which lies between the W h i t e Nile and Blue Nile, t h e Link Canal a r e a which extends in a broad s t r e t c h east of the Blue Nile, and the Gash delta which lies some 300 km east of the Gezira (see F i g . l ) . Of the distinguishing c h a r a c t e r i s t i c s of the Vertisol o r d e r of soils the p r e s e n c e of c r a c k s , c l a y content of 30% or more, slickensides and parallelepiped s t r u c t u r a l a g g r e g a t e s a r e definitive (Soil Survey Staff, 1967). The c r a c k s d e v e l o p when an expanding type of clay dries. The w i d t h and depth of c r a c k i n g a r e associated with the degree of desiccation, the amount and type of c l a y , and the amount and frequency of rainfall. The desiccation is also o f t e n related to the type and density of vegetation. Cracking i n f l u e n c e s soil development and soil c h a r a c t e r i s t i c s to s u c h a d e g r e e that it is c o n s i d e r e d at the highest level of soil classification. Some of the a s s o c i a t e d effects of soil. cracking include the formation of gilgai m i c r o - r e l i e f , s l i c k e n s i d e s , a churning o r mixing of the upper p a r t of the soil profile, a n d in some instances i n c r e a s i n g the amount and d e p t h of w a t e r penetration. Geoderma, 5 (1971)
229
32o
36°
34 °
)'
m
#I I
ASSALA~ f"
t
Centrat
O |
50uth •
~l
.t2•
MEDANI
I
~t ~'~
,os.,,Es ~r~
34 e
I
:..-~
~':" "
36°
Fig.1. E a s t e r n p a r t of the central Clay Plain, Sudan. In areas with a thick well-developed surface mulch and wide cracks, the mulch will fall into the cracks. During the next rain the expansion of the soil plus the mulch due to wetting will be such that it causes the soft to heave, thus causing gilgai surface relief. This process churns and m i x e s the soil material to approximately the depth of cracking, thus often preventing the development of a textural B horizon. The churning and mixing tend to incorporate organic matter and may help in maintaining a better structure. In addition, the expansion with wetting causes slip planes, and, with the m o v e m e n t of soil, forms slickensides and parallelepiped structural aggregates. The penetration of water through the cracks in the Gezira area w a s estimated by J. Coulter (personal communication, 1967) to be 2.5 inches (63 ram) of water, when the cracks develop to their m a x i m u m at the e n d of the dry season. In a study on follow irrigation in Egypt, El Shal (1954) found that filling the cracks developed in a clay soil in middle Egypt required 75 m m of water more than the amount needed to raise the moisture to the field capacity. Within the G e z i r a a r e a the thickness of the s u r f a c e mulch is r e l a t i v e l y small, 1-5 ram, and gilgai relief is not well developed. However, e v i d e n c e of c r a c k i n g can be o b s e r v e d in pits to depths of m o r e than 1 m by the p r e s e n c e of wedge shaped intrusions of brown m a t e r i a l f r o m the s u r f a c e into a m o r e g r a y m a t e r i a l of subsoil and substratum. It has been o b s e r v e d by many s c i e n t i s t s (Greene, 1928; Finck and Ochtman, 1981; Soil S u r v e y Staff, 1968) that t h e r e often is a dark gray horizon underlying the b r o w n e r s u r f a c e soils, the d e p t h of which ranging between 40 cm andF m o r e t h a n 100 cm. In a r e a s w h e r e this dark g r a y horizon o c c u r s nearithe s u r f a c e , the c r a c k i n g and c h u r n i n g often break and mix the l a y e r to a point that it is n e a r l y d e s t r o y e d with only f r a g m e n t s remaining. Where the dark g r a y horizon is d e e p e r it b e c o m e s m o r e prominent and consequently m o r e
continuous. In profiles where this horizon is below 90 cm, there is little o r no mixing with the b r o w n e r soil material other than that which has fallen into the deeper c r a c k s . This phenomenon has been used by Zein el Abedine et al. (1971) to e s t i m a t e the age of the central G e z i r a Vertisols. A considerable p a r t of the Gezira a r e a is, at present, i r r i g a t e d w h i c h changes the natural m o i s t u r e balance in the soil, and consequently the pattern of cracking. F u r t h e r m o r e , in the common p r a c t i c e of m a n a g e m e n t of these soils, a y e a r of fallowing is inserted in the crop rotation as a beneficial m e a s u r e f o r fertility conservation and a helpful one for water penetration in the soil through the developed c r a c k s . The Gash delta V e r t i s o l s are brown clays of alluvial fan formation and a r e believed to be younger than the Gezira and Link Canal soils. They r e c e i v e less than 300 m m of precipitation and a r e cultivated by a rotation in which one i r r i g a t i o n by flooding takes place e v e r y three years. The usual p r o c e d u r e is to flood one third of the a r e a one y e a r and fallow it f o r two y e a r s . The flood w a t e r is distributed by canals and controlled by d y k e s which enable the w a t e r to stand on the land to a period of about three w e e k s . When the water is lost by evaporation and penetration to the deep layers, the soil b e c o m e s r e a d y for cultivation of certain c r o p s with no further irrigation. Such p e r i o d i c a l flooding may affect the cracking pattern of the soils. The Link Canal V e r t i s o l s are dark g r a y i s h brown clays all through the profiles. The c l i m a t e is m o r e humid than in the G e z i r a with r a i n fall of 500-600 m m p e r year. The a r e a is one of the proposed a r e a s for a g r i c u l t u r a l development, but is still under natural conditions. Neither the c h a n g e s in the cracking of the G e z i r a soils due to the change of m o i s t u r e balance, nor the amounts and depths of water penetration through the c r a c k s have been s e r i o u s l y investigated. In this study, the depth, volume, moisture conditions and certain other f a c t o r s relative to c r a c k s in the aforementioned soils of the central Clay Plain of the Sudan a r e evaluated.
MATERIALS AND METHODS The investigations were c a r r i e d out using 9 soil profiles: 2 profiles, GTO3 and GTO4, in c e n t r a l Gezira, 2 profiles, GTO5 and GTO6 in south Gezira, 1 profile, GRF in central Gezira f r o m a plot in the Gezira R e s e a r c h F a r m that is n o n - i r r i g a t e d and left under natural conditions, 2 profiles~ GHATO5 and GHBTO7, in the Gash delta area, and 2 profiles, O4H, and O4MH, in the Link Canal area. The f i r s t 4 profiles r e p r e s e n t irrigated soils in Gezira, and t h e r e s t a r e under natural conditions, bearing in m i n d that the Gash delta p r o f i l e s a r e artificially flooded once e v e r y three y e a r s . Moisture d e t e r m i n a t i o n s were c a r r i e d out at the time of crack m e a s u r i n g and a r e r e p o r t e d f o r t h r e e of the nine profiles (Table HI). The periods t h a t elapsed between the l a s t irrigation or rain and the m e a s u r e m e n t s of c r a c k s a r e given in Table I f o r all profiles. Measuring the depths of c r a c k s was c a r r i e d out using thin flexible graduated metal p r o b e s with pointed smooth noses. Deep c r a c k s were m e a s u r e d with a 150 c m long probe of a d i a m e t e r of 3 ram, and shallow c r a c k s w e r e m e a s u r e d by a 50 cm long one with a 1.5 mm diameter. The probe was gently pushed down vertically into the c r a c k until it stops, then it was moved few c e n t i m e t e r s back and forth along the c r a c k and
TABLE I C rops, last irrigation dates, time of sampling, and period of fallowing of p r o f i l e s under investigation Area
Profile no.
Crop and conditions before last irrigation or rain
Date of last irrigation or rain
Date of sampling
P e r i o d of drying months
Central Gezira
(GTO3 ~GTO4 ~GRF
cotton fallow fallow
Jan. 1968 rain Oct. 1967 rain Oct. 1 9 6 7
May 1968 May 1968 June 1968
4 7 8
South Gezira
/GTO5 (GTO6
wheat wheat
Feb. 1968 Feb. 1968
May 1968 May 1968
3 3
Link Canal
(O4H ~O4MH
natural forest
rain Oct. 1967 rain Oct. 1 9 6 7
Apr. 1968 6 Apr. 1968 6
Gash delta
fGHATO5 ~.GHBTO7
castor fallow
flood Aug. 1967 Feb. 1968 rain Aug. 1967 Feb. 1968
6 6
p u s h e d a g a i n , s o that the n o s e c h a n g e d i t s r e s t i n g point in the b o t t o m of t h e c r a c k . By doing t h i s , the p r o b e , in m o s t c a s e s , e n t e r e d a f u r t h e r d e p t h . B y r e p e a t i n g t h i s m o v e m e n t the f i n a l m e a s u r a b l e depth of the c r a c k w a s r e a c h e d and r e c o r d e d . In c e n t r a l G e z i r a p r o f i l e s the r e s u l t s w e r e c h e c k e d a n d found in a g r e e m e n t with v i s u a l m e a s u r e m e n t s i n f r e s h l y dug p i t s . The w i d t h s of c r a c k s at the s o i l s u r f a c e l e v e l w e r e m e a s u r e d b y a s p e c i a l h a r d m a t e r i a l p l a t e cut like a b i l a t e r a l with a b a s e 12 c m l o n g a n d a head a n g l e of 2 2 0 3 0 ' . The two s i d e s of the b i l a t e r a l a r e a g r a d u a t e d t o r e a d the width of a c r a c k w h e r e the two s i d e s of the b i l a t e r a l wedge r e s t on t h e s i d e s of the c r a c k s . This m o d e l w a s u s e d b e c a u s e the u p p e r m o s t e d g e s of the c r a c k s w e r e u s u a l l y d i s t o r t e d o r s l i g h t l y w i d e n e d due to the f a l l i n g of s u r f a c e m u l c h i n t o the c r a c k s . To m e a s u r e the c r a c k s in the f i e l d a s t a r t i n g point A was c h o s e n f r o m w h i c h a tape 20 m long was s t r e t c h e d in a c o n v e n t i o n a l d i r e c t i o n A B . T h e s t r a i g h t l i n e AB c r o s s e d a n u m b e r of c r a c k s that w e r e m e a s u r e d f o r d e p t h s a n d w i d t h s , a n d f o r d i s t a n c e s f r o m the point A. T h e widths r e c o r d e d w e r e t h o s e f a l l i n g on t h e l i n e AB. A n o t h e r l i n e AC, l a t e r a l to AB, was m a r k e d by the tape, f o r t h e s a m e d i s t a n c e of 20 m, s t a r t i n g f r o m point A, a n d t h e s a m e m e a s u r e s w e r e m a d e of the c r a c k s c r o s s e d by AC. Although t h e r e c o r d e d w i d t h s i n t h i s method a r e not the c r o s s - s e c t i o n a l o n e s a n d d e p e n d upon the a n g l e s b e t w e e n the c r a c k s a n d the l i n e s AB and AC, yet t h e r e s u l t s r e p r e s e n t the w i d t h s in a v e r t i c a l c r o s s - s e c t i o n a l o n g the l i n e s AB a n d AC, which are used a s b a s e s for calculating their volume. T h e a v e r a g e s p a c i n g of c r a c k s a l o n g a s t r a i g h t l i n e can be o b t a i n e d by d i v i d i n g the t o t a l length of m e a s u r e m e n t s by the n u m b e r of c r a c k s . The s t e p s t o c a l c u l a t e the l i n e a r s h r i n k a g e of the s o i l f r o m t h e s u r f a c e down to the b o t t o m of the d e e p e s t c r a c k , a n d the f i n a l c o m p u t a t i o n o f the v o l u m e of c r a c k s , a r e b a s e d on the a s s u m p t i o n that the c r o s s - s e c t i o n of a c r a c k i s a b i l a t e r a l the b a s e of which i s the width of the c r a c k at t h e s u r f a c e , and its h e i g h t is the depth. 232
Geoderma, 5 (1971)
These steps a r e as follows: (a) The c r a c k s m e a s u r e s a r e a r r a n g e d in a descending 0 r d e r of magnitude according to their depths. (b) Using a suitable scale, they a r e drawn in additive o r d e r on an X Y g r a p h , b a s e s on X, the deepest next to Y, a n d t h e heights a r e related t o Y axis. (c) A final accumulative curve is obtained representing the linear shrinkage of the soil in relation to depth. The scale on X is corrected to read percentage of linear shrinkage (Fig.2A). Linear shrinkage
~ 30
2
a
% °
-~-
--
F
Fig.2. A. D i a g r a m of producing linear shrinkage curve. B. Diagram of volume calculation of soil bulk and c r a c k s . (d) The total volume of soil bulk without c r a c k s can be calculated f o r any unit square a r e a by using the frustum of p y r a m i d equation of volume: v = 1/3 h (A 1 + A 2 + VA 1 • A 2) w h e r e u = volume of f r u s t u m of pyramid; A1 = a r e a of upper base; A2 = a r e a of bottom base; h = height. (e) The total volume, ABCDEFG (Fig.2B), is calculated by dividing it into segmental l a y e r s of known height: SO, S1, $2... (f) Each l a y e r has an upper base and a bottom one, the side lengths of which can be defined f r o m the curve of linear shrinkage, and by using the f r u s t u m of p y r a m i d equation, its volume is calculated. (g) The total bulk volume of the l a y e r s is the total shrunken volume of the soil without c r a c k s . (h) By s u b t r a c t i n g this volume f r o m the initial geometric volume, abcdE~G, of the unit a r e a , f r o m the s u r f a c e to the deepest crack, the total volume of c r a c k s is obtained. T h r e e c o m m e n t s m u s t be made here concerning the results obtained by this method. (1) It is r e a l i z e d that the linear shrinkage is related, in this method, to the dimensions of the soil at the time of m e a s u r e m e n t and is e x p r e s s e d as p e r c e n t a g e of them. (2) The ultimate l o w e r end of any c r a c k will be so narrow that it will not allow the probe to t o u c h its final end, thus, the figures obtained a r e of l e s s magnitude than the actual depths. The e r r o r will be within 5%, which i s the ratio of the probe d i a m e t e r to the a v e r a g e width of the crack at the s u r f a c e , i.e., 1.5:30 and 3:60 ram. Geoderma, 5 (1971)
23J
(3) In e x t r e m e c a s e s of d e s i c c a t i o n of soil, the c r o s s - s e c t i o n o f a c r a c k , e s p e c i a l l y a deep one, will not be a b i l a t e r a l . The s i d e s of t h e c r a c k at the top l a y e r s , in e x t r e m e d r y n e s s , a r e n e a r e r to being p a r a l l e l t o e a c h other than being the s i d e s of a b i l a t e r a l . Thus this m e t h o d would be a l w a y s l i a b l e to give r e s u l t s of t o t a l m e a s u r e s l e s s than the a c t u a l v a l u e s of the c r a c k depths, and, c o n s e q u e n t l y s l i g h t l y s m a l l e r v o l u m e . But we b e l i e v e that such r e s u l t s a r e s a t i s f a c t o r i l y c o m p a r a b l e to e a c h other.
RESULTS AND DISC USSION The clay c o n t e n t s of the s o i l s of the four a r e a s a r e not v e r y w i d e l y different. Clay c o n t e n t s a r e 51-58% in c e n t r a l G e z i r a , 60-64% in s o u t h G e z i r a , 71-76% in Link Canal and 55--67% in Gash d e l t a ' . E x c h a n g e a b l e sodium p e r c e n t a g e s in a l l l a y e r s of Link Canal and Gash delta p r o f i l e s a r e 3-6%. In the G e z i r a p r o f i l e s ESP is 4-8% in the s u r f a c e l a y e r s i n c r e a s i n g to 20-30% in the d e e p e r l a y e r s . The d i s c u s s i o n of r e s u l t s is p r e s e n t e d in four p a r t s : c r a c k d e p t h s , widths, spacing, a n d total volume.
Crack depths The data summarized in Table II present an overall picture of the differences in crack depths among the nine sites, as well as the frequencies of occurrence percent of the depths. The first point noted in the results, in spite of the different drying periods which m a y change the order if equalized, is the general adverse correlation between the average crack depths and the amount of precipitation. The descending order of magnitude in crack depths runs as: C a s h delta (non-flooded) > central Gezira (non irrigated) > Link Canal ~ the irrigated soils of Gezira. The descending order of rainfall or irrigation is: about 1,500 m m in irrigated Gezira, 600 m m in Link Canal, 300-400 in Gezira, and less than 300 m m in Gash delta. The amount of water u s e d for flooding profile G H A T O 5 Gash delta is not known, but for reasons discussed in a later part the average depth of this profile is equal to GI-IBTO7 which received natural precipitation only. The m a x i m u m depth reached in the irrigated Gezira area does not exceed 65 cm, while in the non-irrigated one the cracks reached 135 cm. It is also noticed that although the m a x i m u m depths in the G e z i r a irrigated soils range between 45 and 65 cm, yet, there are differences amongst the peaks of occurrence. Profile G T O 3 has 73% of the total number of cracks between 16 and 35 c m with an average of 26 cm. T h i s shallow cracking is due to the fact that the site was occupied by cotton plants and the stalks were still standing furnishing a cover that reduces evaporation. In the s a m e time the absorption of moisture by the m a t u r e d plants was naturally low. Both conditions check losses by evaporation and consequently reducing the total shrinking and cracking. 1 Data obtained by Soil Survey Laboratory Staff, Wad Medani, Sudan. 234
Geoderma, 5 (1971)
TABLE
lI
Frequencies of occurrence percent, and average depths of cracks in the Gezira, Link Canal and Gash delta soils Range
Gezira
(cm)
irrigated
Link Cannal natural natural condition cond.
Gash delta flooded every 3 years
GTO3
GTO4
GTO5
GTO6
GRF
O4H
O4MH
G H A T 5 GHBT7
12 46 27 15
5 17 38 29
4 9 23 31
2 8 41 44
1 24 21 11
10 30 18 14
3 14 24 17
11 16 11 8
4 8 8 16
46-55 56-65 66-75 76-85 86--95
. . .
i0 1 . . .
27 6
5 . . .
8 6 9 6 7
13 10 5 --
25 9 5 3 --
5 4 15 15 7
28 6 3 9 8
96--105
.
1
-
--
3
4
106-115 ~>115
. .
. .
4 2
--
---
4 1
3 3
51
42
36
60
60
416 16--25 26-35 36-45
Profile
aver. cm
26
.
. . .
. . . 34
. . . 39
35
P r o f i l e GTO4, o r i g i n a l l y fallowed a f t e r the r a i n f a l l of 1967, h a s 84% of the c r a c k s b e t w e e n 16 a n d 45 cm with a n a v e r a g e of 34 cm. P r o f i l e s GTO5 a n d GTO6 have m o r e than 80% of t h e i r c r a c k s b e t w e e n 26 and 55 c r n , with a v e r a g e s of 39 a n d 35 c m , r e s p e c t i v e l y . It is n o t i c e d that although t h e l a s t two p r o f i l e s w e r e u n d e r wheat, which had b e e n i r r i g a t e d only 3 m o n t h s p r i o r to s a m p l i n g , y e t t h e y have about the s a m e p a t t e r n of c r a c k i n g a s GTO4 which w a s f a l l o w e d f o r 7 months. T h i s i s due to the fact that w h e a t c r o p , w h i c h m a y be c o n s i d e r e d as a d e n s e g r o w i n g g r a s s , would r a p i d l y c o n s u m e the m o i s t u r e f r o m the top l a y e r s . A f t e r h a r v e s t i n g wheat the s o i l was e x p o s e d to the a t m o s p h e r i c e l e m e n t s that e n h a n c e m o i s t u r e l o s s e s b y e v a p o r a t i o n . T h e s e c o n d i t i o n s b r i n g the u p p e r p a r t of the p r o f i l e n e a r e r t o the c o n d i t i o n s of the l o n g f a l l o w e d p r o f i l e GTO4 and the n o n - i r r i g a t e d c o n d i t i o n s of p r o f i l e G R F . The m o i s t u r e c o n t e n t s d e t e r m i n e d in p r o f i l e s GTO4, GTO5 and G R F c o n f i r m t h i s s t a t e m e n t ( T a b l e III). T h e n o n - i r r i g a t e d p r o f i l e G R F which r e p r e s e n t s the s t a t e of c r a c k i n g u n d e r n a t u r a l c o n d i t i o n s , h a s a d i f f e r e n t p a t t e r n . The peak of c r a c k d e p t h s a p p e a r s at the s h a l l o w r a n g e of 16 to 25 c m , followed by a g r a d u a l d e c r e a s e t o w a r d s d e e p e r l e v e l s u n t i l 135 cm. As the "dark g r a y h o r i z o n a p p e a r s i n t h i s p r o f i l e a t 65-120 c m , t h u s , at l e a s t 40% of the c r a c k s t a p it and v e r y few of t h e m cut t h r o u g h i t s e n t i r e t h i c k n e s s . T h e s e f i n d i n g s a r e in a g r e e m e n t with t h e m o r p h o l o g i c a l f e a t u r e s o b s e r v e d by the Soil S u r v e y Staff, w h e r e t h e top b r o w n and the d a r k g r a y m a t e r i a l s m i x o r f o r m wedge s h a p e d i n t r u s i o n s of b r o w n into the d a r k g r a y m a t e r i a l of the s u b s o i l . T h e two p r o f i l e s i t e s of the Link C a n a l a r e a a r e s l i g h t l y d i f f e r e n t i n that s i t e O4H i s u n d e r g r a s s and s c a n t y t r e e s , and s i t e O4MH i s u n d e r trees and scanty grass.
Geoderma, 5 (1971)
235
TABLE III Moisture contents at time of cracks measuring in certain profiles (GTO4, GTO5 and GRF ) in Gezira GTO4
GTO5
GRF 1
depth (cm)
percent,
depth (cm)
percent,
depth (cm)
percent.
0-1 1-30 30--40 40-70 70-105 105-140
6.0 7.9 11.0 15.5 19.2 24.0
0-2 2-10 10--35 35-67 67-102
7.5 8.0 13.0 18.0 24.5
0--20 20-40 40-60 60-80 80-100 100-120
6.3 10.5 16.9 20.3 21.6 22.1
1 With kind permission of H. Farbrother, Cotton Research Corporation, Wad Medani, Sudan. The effect of n a t u r a l v e g e t a t i o n e x p r e s s e s i t s e l f in the d e p t h s o f c r a c k s . P r o f i l e O4H, u n d e r g r a s s , has m o r e than 60% of i t s c r a c k s b e t w e e n 16 and 45 cm d e p t h , w h e r e a s p r o f i l e O4MH, u n d e r t r e e s , has 75% o f t h e c r a c k s between 26 a n d 55 cm. It is evident that g r a s s d e v e l o p s s h a l l o w c r a c k i n g and t r e e s d e p l e t e s o i l m o i s t u r e to a g r e a t e r d e g r e e and f r o m deeper layers. The two s i t e s of the Gash d e l t a d i f f e r in t h e i r h i s t o r y . Site G H A T O 5 was flooded 6 m o n t h s p r i o r to c r a c k m e a s u r i n g and s t a l k s of c a s t o r p l a n t s w e r e s t i l l s t a n d i n g . S i t e GHBTO7 has not b e e n flooded and r e c e i v e d o n l y the r a i n s of 1967. A l t h o u g h both p r o f i l e s have t h e s a m e a v e r a g e d e p t h , yet, the peak of o c c u r r e n c e a p p e a r s at the shallow depth of 26 cm in G H A T O S , and a p p e a r s b e t w e e n the depths 46-55 in GHATO7 that r e c e i v e d r a i n f a l l only. The s u r f a c e l a y e r of GHATO5 must have s w e l l e d to a m a x i m u m u n d e r flooding and when i t d r i e d it c r a c k e d m o r e than GHBTOT. In c o n c l u s i o n i t i s s t a t e d that the a v e r a g e depth of c r a c k s i s i n v e r s e l y r e l a t e d to the a m o u n t of p r e c i p i t a t i o n or i r r i g a t i o n . The f r e q u e n c i e s of o c c u r r e n c e of c r a c k depths a r e affected to a c o n s i d e r a b l e extent b y t h e kind and d e n s i t y o f v e g e t a t i o n and h i s t o r y of the soil.
Crack widths The data of c r a c k widths do not show g r e a t d i f f e r e n c e s a m o n g t h e d i f f e r e n t p r o f i l e s a n d a r e a s a s those data of c r a c k depths. The r e a s o n , of c o u r s e , is that t h e p l a i n o r s u r f a c e s h r i n k i n g a l m o s t c o m p l e t e l y s t o p s , at i t s m a x i m u m , w h e n t h e m o i s t u r e r e a c h e s the h y g r o s c o p i c r a n g e . T h i s i s c o n t r a r y to the d e p t h s of c r a c k s which will i n c r e a s e a s long a s d e p l e t i o n of m o i s t u r e c o n t i n u e s a d v a n c i n g f r o m the s u r f a c e downwards to t h e d e e p l a y e r s with no l i m i t a t i o n e x c e p t the t i m e f a c t o r and t h e deptn of w e t l a y e r s . The d a t a of c r a c k widths i s p r e s e n t e d in T a b l e IV f o r 9 p r o f i l e s , a s f r e q u e n c i e s of o c c u r r e n c e p e r c e n t and p r o f i l e a v e r a g e s . It is noted f r o m the d a t a of the G e z i r a p r o f i l e s , that the p e a k s of o c c u r r e n c e of c r a c k widths a r e m o r e or l e s s r e l a t e d to the p e r i o d o f fallowing. The p e a k of 28% a p p e a r s in the n o n - i r r i g a t e d p r o f i t e s i t e G R F 236
Geoderma, 5 (1971}
TABLE IV F r e q u e n c i e s of o c c u r r e n c e p e r c e n t and a v e r a g e c r a c k widths of the s o i l s of G e z i r a , Link C a n a l and Gash d e l t a a r e a s
Crack width (cm)
<2.6 2.6--3.5 3.6-4.5 4.6-5.5 5.6--6.5 6.6--7.5 7.5
Average width of profile (cm)
Gezira
Link Canal
Gash delta
GTO3 GTO4 GTO5 GTO6 GRF
(%)
(%)
(%)
(%)
(%)
O4H O4MH
(%)
(%)
GHATO5 GHBTO7
(%)
(%)
8 27 38 19 4 4 --
9 23 32 24 7 4 1
18 42 27 11 2 --
14 51 24 11 ---
9 25 22 28 13 2 1
6 22 31 16 6 13 6
2 3 37 27 15 7 9
30 30 19 4 17 --
22 31 19 19 6 3
3.9
4.2
3.4
3.4
4.2
5.1
4.8
3.4
3.7
at 4.5-5.5 cm width. T h i s s i t e has the longest p e r i o d u n d e r d r y i n g conditions. The p e a k s of 38 and 32% in s i t e s GTO3 and GTO4, that w e r e fallowed f o r 4 and 7 m o n t h s r e s p e c t i v e l y , a p p e a r at c r a c k width 3.6-4.5 c m . Sites GTO5 and G T O 6 , that w e r e fallowed f o r 3-4 months have t h e i r p e a k s of 42 and 51% r e s p e c t i v e l y at 2.6-3.5 cm width. The a v e r a g e s of c r a c k widths in G e z i r a a r e a l s o r e l a t e d to the p e r i o d s of fallowing. The d e s c e n d i n g o r d e r of length of fallowing is 8, 7, 4 and 3 months and c o r r e s p o n d s with the d e s c e n d i n g o r d e r of c r a c k widths of 4 . 2 , 4.2, 3.9 and 3.4 cm r e s p e c t i v e l y . The effect of v e g e t a t i o n in the Link Canal a r e a is dominant. In s i t e O4H, u n d e r g r a s s , t h e a v e r a g e c r a c k width i s 5.1 cm, while in s i t e O4MI-I, u n d e r t r e e s , the a v e r a g e is 4.8 cm. The data i n d i c a t e a g r e a t e r s u r f a c e d r y n e s s in the s i t e w i t h dominant g r a s s vegetation. The m e a s u r e s of the Gash d e l t a indicate that the s i t e that was f l o o d e d 7 months p r i o r to c r a c k m e a s u r i n g has an a v e r a g e width of 3.4 cm, w h i l e the s i t e that was u n d e r r a i n only has an a v e r a g e of 3.7 cm. It is a l s o n o t i c e d that the f r e q u e n c i e s of o c c u r r e n c e in the f o r m e r s i t e a r e h i g h e r than t h e l a t t e r at the r a n g e s of n a r r o w c r a c k s . In c o m p a r i s o n with o t h e r s i t e s u n d e r n a t u r a l conditions, t h e Gash d e l t a p r o f i l e has the n a r r o w e s t a v e r a g e w i d t h of c r a c k s owing to i t s l o w e r c l a y content. The effect of c l a y content on the width of c r a c k s can be t r a c e d o n l y by c o m p a r i n g G a s h d e l t a with Link Canal s o i l s . The Gash d e l t a s o i l s c o n t a i n 55-67% c l a y and t h e i r a v e r a g e width of c r a c k s i s 3.4-3.7 c m . The Link Canal s o i l s have 7 1 - 7 6 % c l a y and t h e i r a v e r a g e width of c r a c k s is 4.8-5.1 cm. In c o n c l u s i o n it i s c l e a r t h a t c r a c k widths a r e affected by the l e n g t h of d r y i n g p e r i o d , by t h e h i s t o r y of the soil, and by the c l a y content. The d i f f e r e n c e s in the c r a c k widths a r e r a t h e r l i m i t e d to a n a r r o w r a n g e of variability.
Geoderma, 5 (1971)
237
Crack spaci~zg Although t h e g e n e r a l shape of soil b l o c k s e n c i r c l e d by c r a c k s i s a polygon, and a m e a s u r e m e n t of s p a c i n g a l o n g a s t r a i g h t l i n e will n o t n e c e s s a r i l y m e a s u r e the a r b i t r a r y c e n t r e l i n e of the p o l y g o n s , y e t , t h e d a t a help in p r e s e n t i n g an a p p r o x i m a t e s u r v e y of the a v e r a g e s p a c i n g b e t w e e n c r a c k s on a l i n e a r b a s i s . The r e s u l t s a r e g i v e n in T a b l e V f o r all s i t e s . The d a t a a r e p r e s e n t e d a s a v e r a g e s p a c i n g in c e n t i m e t e r s and a s f r e q u e n c i e s of o c c u r r e n c e of spacing intervals. TABLE V Average spacings and frequencies of occurrence percent of spaces between cracks in Gezira, Link Canal and Gash delta soils Spaces
Gezira
Link Canal
Gash delta
GTO3 GTO4 GTO5 GTO6 GRF
O4H O 4 M H
GHATO5
GHBTO7
<21 21-40 41--60
20 40 32
14 41 19
22 29 20
5 30 35
39 53 7
19 26 21
31 35 18
17 49 22
6 19 41
61-80 81-100
4 4
8 6
20 7
19 ii
1 -
31 3
8 6
8 4
19 9
2
-
-
-
2
-
6
45
54
28
51
39
39
62
(cm)
> I 0 0
Average spacing of profile (cm)
-
39
12
52
The c o m p a r i s o n a m o n g s t a r e a s and i n d i v i d u a l p r o f i l e s p r e s e n t s o m e p o i n t s of i n t e r e s t . The i r r i g a t e d s o i l s of G e z i r a and G a s h d e l t a (flooded) c r a c k a t a v e r a g e s p a c i n g b e t w e e n 39 and 54 cm, with p e a k s of o c c u r r e n c e a t 21-60 cm spacing. T h e G e z i r a soil u n d e r n a t u r a l ( n o n - i r r i g a t e d ) c o n d i t i o n s s h o w s a m u c h n a r r o w e r s p a c i n g with an a v e r a g e of 28 cm. T h i s i n d i c a t e s t h a t e x t r e m e d e s i c c a t i o n , in the G e z i r a , i n c r e a s e s b r e a k - d o w n and f i s s u r i n g of the b i g g e r s u r f a c e b l o c k s . The L i n k C a n a l s o i l s show d i f f e r e n t a v e r a g e s . The s o i l u n d e r d o m i n a n t g r a s s , O4H, c r a c k s at an a v e r a g e s p a c i n g of 51 c m , w h i l e that u n d e r t r e e s , O4MH, h a s an a v e r a g e of 39 cm. T h i s i s r e n d e r e d to the m e c h a n i c a l b i n d i n g effect of the s h a l l o w root s y s t e m s of g r a s s w h i c h act a s i n f o r c e m e n t f o r t h e s u r f a c e b l o c k t h u s r e d u c i n g the f i s s u r i n g of t h e i r s t r u c t u r e . It c a n b e c o n c l u d e d that the s p a c i n g of c r a c k s i s i n f l u e n c e d b y t h e h i s t o r y of the s o i l a n d by the n a t u r e of the v e g e t a t i v e c o v e r .
L i n e a r shrinkage and total volume of c r a c k s T h e l i n e a r s h r i n k a g e c u r v e s of the 9 s i t e s u n d e r i n v e s t i g a t i o n a r e p r e s e n t e d in F i g . 3 , c a l c u l a t e d a s p e r c e n t of e x i s t i n g d i m e n s i o n s a t t h e t i m e of m e a s u r i n g . 238 Geoderma, 5 (1971)
Linear shrinkageY%) 00
40
.
•
5
I0
15
,,
~+o1[I! ,
~ so~ Itz~
GT03 04 H . GT04 ....... 04MH . . . . GT05 ...............GT06 .-,.-,_~GHBT07 ~GRF . . . . GHATO5
Fig.3. L i n e a r shrinkage of soils: GTO3 and GTO4 in central Gezira, GTO5 and GTO6 in south Gezira, GRF permanent fallow in central G e z i r a , O4H and O4MH in Link Canal, and GHATO5 and GHBTO7 in Gash delta. It is noticed that although the surface shrinkage in the G e z i r a i r r i g a t e d soils is between 7 and 10% , yet the shrinkage d e c r e a s e s to l e s s than 0.7 cm at a depth of 50 cm f r o m the surface. The G e z i r a soil under natural conditions, GRF, has the g r e a t e s t value of shrinkage of 15% in t h e s u r f a c e , d e c r e a s i n g to about 3% at 50 cm depth, and continues to d e c r e a s e m o r e slowly to 0% at the depth 130 cm. Thus i r r i g a t i o n has reduced the shrinkage to about half that under natural conditions. The Link Canal soils have s m a l l e r shrinkage than Gezira soils u n d e r natural conditions in both surface and deep l a y e r s . The shrinking in the Link Canal soils ends at about 70 cm depth. This difference is mainly due to the difference in rainfall and partly to that of the drying period, as L i n k Canal a r e a r e c e i v e s about 600 mm against about 300 m m in Gezira, while the drying periods a r e 6 and 8 months respectively. The Gash delta soils have a different pattern due to their lower c l a y contents and higher permeability. The surface shrinkage is r a t h e r s m a l l , between 5 and 9%, but the shrinking d e c r e a s e s at a much slower rate than all o t h e r profiles to r e a c h the deepest level of all, between 130 and 150 c m . The deep c r a c k i n g of the soils of Gash delta is r e n d e r e d to their c o m p a r a tively higher p e r m e a b i l i t y which helps to drain them m o r e efficiently than the other a r e a s , thus reducing the moisture contents in the deep l a y e r s a n d shortening the t i m e of drying. The d i f f e r e n c e s observed in linear shrinkage a r e emphasized in the total volume of c r a c k s . The data of t o t a l crack volumes a r e p r e s e n t e d in Table VI. The r e s u l t s a r e p r e s e n t e d so that c r a c k volumes a r e given for each 10 cm l a y e r of the profile down to the deepest crack. The r e s u l t s a r e c a l c u l a t e d as cubic m e t r e s p e r h e c t a r e , m i l l i m e t r e s of water, and cubic m e t e r s of w a t e r p e r feddan*. The total v o l u m e of c r a c k s in the i r r i g a t e d a r e a of the Gezira r a n g e s between 222 m 3 / h a and 313 m3/ha, equivalent to 22.2 mm and 31.3 mm o f w a t e r respectively. In the site GRF of n o n - i r r i g a t e d soil, the c r a c k s volume r e a c h e s 867 m 3 / h a , equivalent to 86.7 m m of water, i.e., about 3--4 t i m e s the volume of the i r r i g a t e d soils. *Feddan = 4,200 m 2.
TABLE
VI
Volumes of cracks in Gezira, hectare per 10 cm layer Layers
Link Canal,
and Gash delta soils in metre
Gezira
cube
Link Canal
Gash Delta
per
(cm) GTO3
GTO4
GTO5
GTO6
GRF
O4H
O4MH
GHATO5
GHBTO7
0-10 10-20 20-30 30-40 40--50 50--60 60-70 70-80 80-90 90-100 100-110 110-120 120-130
157 85 33 10 2 . . . . . . .
140 90 48 19 6 2
130 94 54 25 8 2 . . . . . . .
102 70 36 12 2 -
247 187 131 92 69 52 38 24 14 8 5
184 122 74 40 20 8 2 -
172 125 82 46 23 9 4 1 --
159 125 92 71 56 42 29 18 7 4 3 2 1
105 83 64 47 32 22 15 10 7 4 2 1
Total m3/ha
287
305
313
222
867
450
462
609
392
m3/feddan
121
128
131
93
364
189
194
256
165
28.7
. . . . .
. . . . . . .
. .
30.5
31.3
. .
. .
22.2
. .
86.7
45,0
46.2
60.9
39.2
Concerning the question of water penetration in the soil profile, it will be noticed that only between 1.2 m m and 3.5 m m of water do penetrate the soil profile to the depths below 30 c m from the surface in the irrigated soils of Gezira. The site under natural conditions is much better this way, as 30 m m of water penetrate below 30 cm. Thus, in this respect, the beneficial effect of fallowing the land for one year in the c o m m o n rotation applied in Gezira, becomes questionable from the point of view of water penetration in the profile. The two Link Canal profiles, O 4 H and O 4 M H , have almost the s a m e volume of cracks, 450 and 462 m3/ha, equivalent to 45.0 and 46.2 r n m of water respectively. Though the total volume is about the same in both sites, yet, it should be noticed that in O4H, under grass, the volume of cracks below 30 c m depth is only 70 m3/ha, while in O 4 M H , under trees, the volume below the same depth is 83 m 3/ha. This indicates m o r e shrinking in the deep layer under trees than under grass. The two Gash delta sites have appreciably different volumea of cracks, the one previously flooded and cultivated with castor crop, G H A T O S , has 609 m3/ha, and the other one that received rain only, GIIBTO7, has 392 rn3/ha..This difference is due to two factors: (a) the greater depletion of moisture from the surface by the cRstor plants; and (b) the flooding for 3 weeks of G H A T O 5 allowed the soil to swell more freely than the scanty rain received by GIIBTO7, and consequently the shrinking of the former will be greater than the latter. The volume of cracks in the top 30 c m in the site under castor plants is 376 m3/ha 'against 255 m3/ha in the site with rain only.
240
Geoderma,
5 (1971}
In c o n c l u s i o n it i s c o n s i d e r e d that i r r i g a t i o n r e d u c e s the v o l u m e of c r a c k s in the G e z i r a to one t h i r d o r one f o r t h of the v o l u m e u n d e r n a t u r a l c o n d i t i o n s . The v o l u m e of c r a c k s in Link C a n a l a r e a i s not affected by t h e k i n d of v e g e t a t i o n a l t h o u g h g r a s s i n c r e a s e s t h e i r v o l u m e i n the s u r f a c e l a y e r s , a n d t r e e s i n c r e a s e s it at the deep l a y e r s . T h e s o i l s of Gash d e l t a c r a c k to g r e a t e r d e p t h s than i n a l l o t h e r a r e a s , b u t the c r a c k s a r e n a r r o w . T h e t o t a l v o l u m e of c r a c k s in the u p p e r l a y e r s i s m u c h affected by soil history and vegetation.
REFERENCES El Shal, M.J., 1954. Water Requirements of Main Crops in Menoufia District. T h e s i s , Faculty of Engineering, Cairo University, Cairo, U.A.R., 62 pp. Finck, A. and Ochtman, L.H.J., 1961. Problems of soil evaluation in the Sudan. J. Soil Sci., 12, 87-95. Greene, H., 1928. Soil profile in the eastern Gezira. J. Agr. Sci., 18: 518-530. Soil Survey Staff, 1968. Reports on the Gezira soils. Division of Soil Survey, M i n i s t r y of Agriculture, Wad Medani, Sudan. Soil Survey Staff, 1967. Soil Classification, a Comprehensive System, Supplement t o 7th Approximation. 1967. U.S. Dept. of Agriculture, 207 pp. Zein el Abedine, A., Robinson,. G.H. and Commissaris, A., 1971. Approximate age of the Vertisols of Gezira, central Clay Plain, Sudan. Soil Sci., in p r e s s .
Geoderma, 5 (1971)
241