a study on cracking in some vertisols of the sudan

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Geoderma - Elsevier Publishing Company, AmsterdamPrinted in The Netherlands

A STUDY ON CRACKING IN SOME VERTISOLS OF THE SUDAN

A. ZEIN EL ABEDINEand GLENNH. ROBINSON

F.A.O. Regional Office, Bangkok Thailand)

Received May 23, 1969)Resubmitted June 26, 1970)

ABSTRACT

Z e in e l A b e d i n e , A . a n d R o b i n s o n , G . H ., 1 9 7 1 . A s t ud y o n c r a c k i n g i n s o m e Ve r t i s o l so f t h e S u da n . G e o d e r m a , 5: 2 2 9 - 2 4 1 .

A p r o c e d u r e f o r m e a s u r i n g s o il c r a c k s i n c e r ta i n v e r ti s ol s o f t h e S u d a n w a sdeve loped . Th i s inc luded the use o f p l a t e cu t l i ke a b i l at e ra l i t h a base 12 c m lo nga n d a h e a d a n g le o f 2 2 ° 3 0 . T h e t w o s i d e s o f t h e b i la t er a l w e r e g r a d u a t e d t o r e a d t h ew id t h o f a cr ac k . T h e d e p t h w a s m e a s u r e d b y f l ex ib le g r a d u at e d m e t a l p r ob es . M e a s u r e -m e n t s w e r e m a d e a l o n g a s tr ai gh t l in e a n d t h e f r e q ue n c y a n d v o l u m e o f c r a c k s d e t e r m i n e d .

It wa s found tha t t he wid th s o f t he c rac ks a re a ff ec ted by the l eng th o f d r y i ngpe r iod, t he k ind o f so i l an d by the c l ay con ten t , i r r iga tion r ed uce s the vo l um e o fc r a c k s i n t h e G e z i r a c o t t o n - g r o w i n g ar e a. T h e v o l ~ m e o f c r a c k s u n d e r n at u r alvegetation was not alt ere d by kind of vegetation. However, grass incr ease s theirvolume 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 bysoil history and vegetation.

I N T R O D U C T I O N

The soils of the ea st er n part of the central clay plain of the Sudanare mos t ly Ver t i so l s . These Ver t i so l s were s tud ied in represen ta t ive p ar t sof the clay plain as follows: the C, ezira area which l ies between the WhiteNile and Blue Nile, the Link Canal area which extends in a broad stretcheast of the Blue Nile, and the Gash delta which lies some 300 km east ofthe Gez i ra ( see F ig . l ) .

Of the dis t ingui shing c hara cter is t ics of the Vert isol or der of soi ls thepres ence of crac ks, c la y content of 30 or more, s l ickensides andparal le lepiped s t ru ct ur al aggreg ates are def ini t ive (Soi l Survey Staff, 1967) .

The cracks develop when an expanding type of clay dries. The widthand depth of cracking are associated with the degree of desiccat ion, theamount and type of cl ay , and the amount and frequ ency of rainfall . Thedesiccation is also often related to the type and density of vegetation.

Crack ing in f luences so i l deve lopment and so i l charac te r i s t i cs to sucha degree that i t i s considered at the highest level of soi l c lass i f icat ion.Some of the associated effects of soil . cracking include the formation ofg i lga i mic ro- r e l i e f , s l i ckens ides , a churn ing or mix ing o f t h uppe r pa r t

of the soi l prof i le , and in some instances increasing the amount and depthof water penetrat ion.

Geoderma, 5 (1971) 229



3 2o 4 ° 36°

) ' m #I I

A S S A L A ~ f

e n t r a t tO M E D N I I

| 5 0 u t h • ~ t ~ ' ~ I

~ l ,o s ., ,E s ~ r : . . - ~ ~ ' :

. t2 3 4 e 3 6

Fig.1. E as te rn p ar t of the centra l Clay Plain, Sudan.

I n a r e a s w i t h a t hi c k w e l l - d e v e l o p ed s u r f a c e m u l c h a n d w i d e c r a c k s ,t h e m u l c h w i l l f a l l i n t o t h e c r a c ks . D u r i n g t h e n e x t r ai n t he e x p a n s i o n o f t h es o il p l u s t h e m u l c h d u e t o w e t t in g w i l l b e s u c h t h a t it c a u s e s t h e s o f t t oh e av e , t h us c a u s i n g g i lg a i s u rf a ce r el ie f. T h i s p r o c e s s c h u r n s a n d m i x e st h e so i l m a t e r i a l t o a p p r o x i m a t e l y t h e d e p t h o f c r a c ki n g , t h u s o f t e n p r e -v e nt i ng t h e d e v e l o p m e n t o f a te x tu r al B h o ri z on . T h e c h u r n i n g a n d m i x i n gt e nd to i n c o r p o r a t e o r g a n i c m a t t e r a n d m a y h e lp in m a in t ai n in g a b e t t e rs t r u ct u r e . I n a d d i ti o n , t he e x p a n s i o n w i t h w et t i n g c a u s e s s l i p p l a n e s , a n d ,w i t h t h e m o v e m e n t o f s oi l, f o r m s s l ic k en s id e s a n d p a ra l le l ep i pe d s t r u c t u r a laggrega te s .

T h e p e n e t r a t i o n of w a t e r t h r o u g h t he c r a c k s i n t h e G e z i r a a r e a w a se s t i m a t e d b y J. C o u l t e r p e rs o na l c o m m u n i c a t i o n , 1 9 67 ) t o b e 2 .5 i n c h e s

6 3 r a m ) o f w a t er , w h e n t he c r a c k s d e ve lo p t o t h e i r m a x i m u m a t t he e n d o f

the d r y season . In a s tudy o n fo l low i r r iga t ion in Egyp t , E l Sha l 1954 )f o u n d t h a t f il li ng t h e c r a c k s d e v e l o p e d i n a c l a y s o i l i n m i d d l e E g y p tr e q u i r e d 75 m m o f w a t e r m o r e t ha n t he a m o u n t n e e d e d t o r ai se t hem o i s t u r e t o t h e f i e l d c a p ac i ty.

Within the Gez ir a area the thickness of the surf ace mulch is re la t i ve lysmal l, 1-5 ram, and gilgai relief is not well developed. However, evi de nc eof cracking can be observed in pits to depths of more than 1 m by theprese nce of wedge shaped in t rus ions of b rown mater ia l f rom the su r f ac einto a mor e gra y ma ter ia l of subsoil and substratum. I t has been obs er ve dby many scie nt ist s Greene, 1928; Finck and Ochtman, 1981; Soil Su rv eyStaff , 1968) that the re of ten is a dark gray horizon underlying the br ow ne rsurf ace soi ls , the de pth of which ranging between 40 cm and mor e t han100 cm. In a reas whe re th is dark gray hor izon occur s near i the su r f ac e ,the cracking and churning often break and mix the layer to a point that i tis near ly des tro yed with only fragm ents remaining. Where the dark gr ayhorizon is deeper i t be comes more prominent and consequent ly more



continuous. In pro fil es where this horizon is below 90 cm, ther e is l i t t le o rno mixing with the browner soil material other than that which has falleninto the deeper cracks. This phenomenon has been used by Zein el Abedineet al . 1971) to es ti ma te the age of the central Gez ira Vertisols.

A considerable pa r t of the Gezira area is , a t present , i r r i gated w hichchanges the natural moi st ure balance in the soil , and consequently thepat tern of cracking. Fur the rmo re, in the common pract i ce of mana geme ntof thes e soils, a yea r of fallowing is inse rte d in the cr op rotation as abeneficial measure for fer t i l i ty conservat ion and a helpful one for waterpenetration in the soil through the developed cracks.

The Gash delta Vertisols are brown clays of alluvial fan formationand ar e bel ieved to be younger than the Gezira and Link Canal soils. Theyrec eiv e les s than 300 mm of precipitation and ar e cultivated by a rotatio nin which one i r r ig at io n by f looding takes place e very three years . Theusual procedure is to f lood one third of the area one year and fal low i t fortwo year s . The flood wat er is dis t r ibuted by canals and control led by dykeswhich enable the water to stand on the land to a period of about three weeks.

When the water is lost by evaporation and penetration to the deep layers,the soi l becomes ready for cul t ivat ion of cer ta in crops with no fur therirrigation. Such periodical flooding may affect the cracking pattern of thesoils.

The Link Canal Vert isols are dark grayish brown clays al l throughthe prof i les . The cl i mat e is mor e humid than in the Gezira with rain-fal l of 500-600 mm p er year. The are a is one of the proposed are as foragricul tural development , but is s t i l l under natural condi t ions.

Neither the changes in the cracking of the Gezira soils due to thechange of moistur e b alance, nor the amounts and depths of water pene-trat ion through the cracks have been ser iously invest igated.

In this study, the depth, volume, moisture conditions and certainother fac tor s re la t ive to crac ks in the aforementioned soi ls of the centralClay Plain of the Sudan are evaluated.

MATERIALS AND METHODS

The invest igat ions were carr ied out using 9 soi l prof i les : 2 prof i les ,GTO3 and GTO4, in central Gezira, 2 profiles, GTO5 and GTO6 in southGezira , 1 prof i le , GRF in central Gezira f rom a plot in the Gezira Re se ar chFarm that is non-irr igated and lef t under natural condi t ions, 2 prof i les~GHATO5 and GHBTO7, in the Gash delta area, and 2 profiles, O4H, andO4MH, in the Link Canal area. The f i rs t 4 prof i les represent i r r igatedsoi ls in Gezira , and the rest are under natural condi t ions, bear ing in mindthat the Gash del ta prof i les are ar t i f ic ia l ly f looded once every three years .Mois ture de te rmina t ions were car r ied ou t a t the t ime of c rack measur ingand are repo rted fo r thre e of the nine prof i les Table HI). The per iods th ate lapsed be tween the las t i r r iga t ion or ra in and the measurements o f c racksare given in Table I for a l l prof i les .

Measuring the depths of cracks was carr ied out using thin f lexiblegraduated metal probes with pointed smooth noses . Deep cracks wereme as ur ed with a 150 cm long probe of a diam ete r of 3 ram, and shallowcracks were measured by a 50 cm long one with a 1.5 mm diameter. The

probe was gently pushed down vertically into the crack until i t stops,then i t was moved few cent imeters back and for th a long the crack and



TABLE I

C rops, last i rri ga ti on dates, time of sampling, and period of fallowing of pr of il esunder investigation

Area Pro fil e Crop and Date of Date of P er io d ofno. conditions last sampling dr yi ng

before last irr igation mon thsirrigation or rainor rain

(GT O3 cotton Jan . 1968 May 1968 4Ce nt ra l ~GTO4 fallow ra in Oct. 1967 May 1968 7Gez ira ~GRF fallow ra in Oct. 19 67 June 1968 8

South /GTO5 wheat Fe b. 1968 May 1968 3Gezi ra (GTO6 wheat Feb. 1968 May 1968 3

Link (O4H na tu ral fo re st ra in Oct. 1967 Apr. 1968 6Canal ~O4MH ra in Oct. 19 67 Apr. 1968 6

Gash fGHATO5 cas tor flood Aug. 196 7 Feb. 1968 6delta ~ . G H B T O 7 fallow rai n Aug. 1967 Feb. 1968 6

pushed aga in , so tha t t he nose changed i t s r e s t ing po in t in the bo t tom of thec rack . By do ing th i s , t he p robe , in mos t c ases , en te r ed a fu r t he r dep t h . Byr e p e a t i n g th i s m o v e m e n t t h e fi n a l m e a s u r a b l e d e pt h of t he c r a c k w a sr e a c h e d a n d r e c o r d e d . I n c e n t r a l G e z i r a p r o f i l e s th e r e s u l t s w e r e c h e c k e da n d f ou nd i n a g r e e m e n t w i th 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 d ug p i t s .

The wid ths o f c rac ks a t t he so i l su r face l eve l were mea sur ed by aspec ia l ha rd ma te r i a l p l a t e cu t l i ke a b i l a t e ra l w i th a base 12 cm long and ahead ang le o f 22030 . The two s ides o f the b i l a t e ra l a rea g radua ted to r eadthe wid th of a c rac k where the two s ides of the b i l a t e ra l wedge re s t on thes i d e s o f t he c r a c k s . T h is m o de l w a s u s e d b e c a u s e t he u p p e r m o s t e d g e s o ft h e 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 ne d d ue to th e f a l l i n g o fs u r f a c e m u l c h i n t o t h e c r a c k s .

To mea sur e the c rack s in the f i e ld a s t a r t ing po in t A was ch ose n f ro mwhich a t ape 20 m long was s t r e t ched in a conven t iona l d i r ec t ion AB. Thes t r a i g h t l i n e A B c r o s s e d a n u m b e r of c r a c k s t h at w e r e m e a s u r e d f o r d e p t h sand wid ths , and fo r d i s t ances f rom the po in t A . The wid ths r ecor ded wer ethose f a l l ing on the l ine A B . A n o t h e r l i n e A C l a t e r a l t o A B w a s m a r k e dby the t ape , fo r th e sam e d i s t ance o f 20 m, s t a r t in g f rom po in t A , and thes a m e m e a s u r e s w e r e m a d e o f t he c r a c k s c r o s s e d b y A C . Al though ther e c o r d e d w i d t hs i n t h i s m e th o d a r e n ot t h e 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 dupon the ang les be tween the c racks and the l ines A B and A C y et t h e r e s u l t sr e p r e s e n t t he w i d t h s i n 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 t he l i n e s A B a n d A Cw h i ch a r e u s e d a s b a s e s f o r c a l c u l a t i n g t h e i r v o l u m e .

T h e a v e r a g e s p a c i n g o f c r a c k s a l o n g a s t r a i g h t l i n e c an b e o b t a i n e db y d iv i d i n g t h e t o t a l l e ng t h of m e a s u r e m e n t s b y t h e n u m b e r of c r a c k s .

T h e s t e ps t o c a l c u l a t e t h e l i n e a r s h r i n k a g e of t he s o i l f r o m t h e s u r f a c edown to the bo t tom of the deepes t c rack , and the f ina l computa t ion o f thev o l u m e of c r a c k s , a r e b a s e d on t h e a s s u m p t i o n t h at th e c r o s s - s e c t i o n o f ac rack i s a b i l a t e r a l t he base of wh ich i s the wid th o f the c rack a t t h esu r face , and i t s he igh t i s t he dep th .

232 Geo der ma, 5 (1971)



These steps are as follows:(a) The crack s me asu res are arran ged in a descending 0rder of

magnitude according to their depths.(b) Using a suitable scale, they are drawn in additive order on an

XYgraph , base s on X, the deepest next to Y, an dth e heights are related toY a x i s .

c ) A f in a l a c c u m u l a t i v e c u r v e i s o b t a i n e d r e p r e s e n t i n g t h e l in e ars h r i n k a g e o f t h e s o i l i n re l at i o n t o d e pt h . T h e s c a l e o n X i s c o r r e c t e d t or e a d p e r c e n t a g e o f l i n e a r s h r i n k a g e F ig . 2A ).

L i n e a r s h r i n k a g e

~ 30 2 a ~ - -

Fig.2. A. Dia gra m of producing linear shrinkag e curve. B. Diag ramof volume calculation of soil bulk and cracks.

(d) The total volume of soil bulk without cracks can be calculated forany unit square area by using the frustum of pyramid equation of volume:

v = 1/3 h (A 1 + A 2 + VA 1 • A 2)

wher e u = volume of fru stu m of pyramid ; A1 = ar ea of upper base; A2 =area of bottom base; h = height.

(e) The total volume, ABCDEFG (Fig.2B), is calculated by dividing itinto seg me nt al la ye rs of known height: SO, S1, 2...

(f) Each layer has an upper base and a bottom one, the side lengthsof which can be defined from the curve of linear shrinkage, and by usingthe frust um of py ram id equation, its volume is calculated.

(g) The total bulk volu me of the lay er s is the total shrunken volumeof the soil without cracks.

(h) By subtracting this volume from the initial geometric volume,abcdE~G of the unit area, from the surface to the deepest crack, the total

volume of cracks is obtained.Three comments must be made here c oncerning the resul ts obtained

by this method.(1) It is realized that the linear shrinkage is related, in this method,

to the dimensions of the soil at the time of measurement and is expressedas percentage of them.

(2) The ultimate lower end of any crack will be so narrow that it willnot allow the probe to touch its final end, thus, the figures obtained are ofles s magnitude than the a ctual depths. The e rr or will be within 5%, which isthe ratio of the probe diameter to the average width of the crack at thesurface, i .e. , 1.5:30 and 3:60 ram.

Geoderma, 5 (1971) 23J



(3) In ex t rem e cas es o f des icca t ion of so i l , the c ro ss - sec t io n o f acrac k, esp ecia l ly a deep one, wi l l not be a bi l a te ra l . The s ides of th e cr ac ka t the top l ayer s , in ex t rem e dryness , a re nea re r to be ing pa ra l l e l to eachother than being the s id es of a bi l a te ra l .

Thus th is metho d would be a lways l iable to give res ul t s of to ta lmea su re s l e ss than the ac tua l va lues of the c rack dep ths, and, co ns eq uen t l ys l igh t ly sma l le r vo lume. But we be l i eve that such resu l t s a re sa t i s fa c t o r i lycomparab le to each o the r.

RESULTS AND DISC USSION

The clay contents of the soi ls of the four ar ea s are not very wi de lydifferent . Clay content s ar e 51-58 in cen tra l Gez ira , 60-64 in so ut hGezi ra , 71-76 in Link Canal and 55--67 in Gash de l t a ' . Exc ha ng ea bl esod ium perce n tag es in a l l l aye rs o f L ink Canal and Gash de l t a p r o f i l e sa re 3 -6 . In the Gez i r a p rof i l e s ESP i s 4 -8 in the su r face l ay ers in -

c re as i ng to 20-30 in the deeper l aye rs .T h e d i s c u s s i o n o f r e s u l t s i s p r e s e n t e d i n f o u r p a r t s : c r a c k d e p t h s ,widths , spacing, and tota l volume.

rack depths

T h e d a ta s u m m a r i z e d i n Ta b l e II p r e s e n t a n o ve r al l p i ct u re of t h ed i ff e re n ce s i n c r a c k d e p t h s a m o n g t h e n i ne s i te s, a s w e ll a s t h e f r e q u e n c i e so f o c c u r r e n c e p e r c e n t o f t h e de p th s .

T h e f i rs t p o i n t n o t e d i n t he r e s u l t s , i n s pi t e o f t he d i f f e r en t d r y i n gp e r i o d s w h i c h m a y c h a n g e t h e o r d e r if e q ua l iz e d, i s t h e g e ne r al a d v e r s ec o rr e la t io n b e t w e e n t h e a v e r a g e c r a c k d e p th s a n d t h e a m o u n t o f p r e c i p i -t at io n. T h e d e s c e n d i n g o r d e r o f m a g n i t u d e i n c r a c k d e pt h s r u n s a s: C a s hd e l t a ( n o n -f l o o de d ) > c e n t r a l G e z i r a ( n o n i r ri g a t ed ) > L i n k C a n a l ~ t h ei r r i ga t e d s o i ls o f G e zi r a . T h e d e s c e n d i n g o r d e r o f r ai n fa l l o r i r r i g a t i o n i s:a b o u t 1 , 50 0 m m i n i r r i ga t e d G e z i ra , 6 0 0 m m i n L i n k C a n a l, 3 0 0 - 4 0 0 i nG ez ir a, a n d le ss t h a n 3 0 0 m m in G a s h d el ta . T h e a m o u n t o f w a t e r u s e d f o rf lo od in g p ro fi le G H A T O 5 G a s h d el ta i s n ot k n o w n , b ut f or r e a s o n s d i s c u s s e di n a l a t e r p a r t t h e a v e r a g e d e p t h o f t h i s p r o f il e i s e q u a l t o G I - I B TO 7 w h i c hrec e iv ed na tu ra l p rec ip i t a t ion on ly.

T h e m a x i m u m d e p th r e a c h e d in t he i rr ig at ed G e z i r a a r e a d o e s n o te x c e e d 6 5 c m , w h i l e i n t h e n o n- i rr i ga t ed o n e t h e c r a c k s r e a c h e d 1 3 5 c m .

It i s a l so n o t i c e d th a t a l th o ug h t he m a x i m u m d e p t h s i n t h e G e z i r ai r ri g at e d s o il s r a n g e b e t w e e n 4 5 a n d 6 5 c m , y e t, t h er e a r e d i f f e r e n c e sa m o n g s t t h e p e a k s o f o c c u r r e n c e . P r o f i l e G T O 3 h a s 7 3 o f t h e t ot aln u m b e r of c r a c k s b e t w e e n 1 6 a n d 3 5 c m w it h a n a v e r a g e of 2 6 c m . T h i ss h a l l o w c r a c k i n g i s d u e t o t h e f a ct t h at t h e s it e w a s o c c u p i e d b y c o t t o np l a n ts a n d t h e s t a lk s w e r e s ti ll s t a n di n g fu r n i sh i n g a c o v e r t h a t r e d u c e se v ap o ra t io n . I n t h e s a m e t i m e t h e a b so r pt i on o f m o i s t u r e b y t h e m a t u r e dp l an t s w a s n a tu r al l y l o w. B o t h c o nd i ti o ns c h e c k l o ss e s b y e v a p o r a t i o n a n dconsequen t ly r e du c i ng the to t a l sh r ink ing and c rack ing .

1 Data obtained by Soil Survey Laboratory Staff, Wad Medani, Sudan.

234 Geoderma, 5 1971)



TABLE lI

Fre quen cies of occu rren ce perc ent, and average depths of cracks in the Gezira,Link Canal and Gash delta soil s

Rangecm)

Gezi ra Link Cannal Gash deltanatu ral flooded every

irrigated natural condition 3 yearscond.

GTO3 GTO4 GTO5 GTO6 GRF O4H O4MH GHAT5 GHBT7

4 1 6 12 5 4 2 1 10 3 11 416--25 46 17 9 8 24 30 14 16 826-35 27 38 23 41 21 18 24 11 836-45 15 29 31 44 11 14 17 8 1646-55 - i0 27 5 8 13 25 5 28

56-65 - 1 6 - 6 10 9 4 6

66-75 . . . . 9 5 5 15 3

76-85 . . . . 6 - 3 15 9

86--95 . . . . 7 -- -- 7 8

96--105 . . . . 1 - -- 3 4106-115 . . . . 4 - -- 4 3

~>115 . . . . 2 -- -- 1 3

Profile26 34 39 35 51 42 36 60 60aver. cm

Prof i l e GTO4, o r ig i na l ly f a l lowed a f t e r the r a in fa l l o f 1967, has 84%of the cracks between 16 and 45 cm with an average of 34 cm. Prof i lesGTO5 and GTO6 have more than 80% of the i r cracks between 26 and 55 crn ,wi th ave rag es of 39 and 35 cm, r e spec t ive l y. I t i s no t i ced tha t a l though the

la s t two p ro f i l e s were under whea t , wh ich had been i r r iga ted on ly 3 monthspr i o r to sampl in g , ye t t hey have abou t the same pa t t e rn o f c rack in g asGTO4 which was fa l lowed for 7 months . This i s due to the fac t tha t wheatc rop , wh ich may be cons ide red a s a dense g rowing g ra ss , would r ap id l yc o n s u m e t h e m o i s t u r e f r o m t h e t op l a y e r s . A f t e r h a r v e s t i n g wh e at t he s o i lw a s e x p o s ed t o t he a t m o s p h e r i c e l e m e n t s t h at e n h a n c e m o i s t u r e l o s s e s b ye 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 th e u p p e r p a r t of t he p r o f il e n e a r e r t othe cond i t ions o f the long fa l lowed p ro f i l e GTO4 and the non- i r r iga tedc o n d i t i o ns of pr o f i l e G R F. T h e m o i s t u r e c o n t e nt s d e t e r m i n e d i n p r o f i l e sGTO4, GTO5 and GRF conf i rm th i s s t a t emen t (Tab le I I I ) .

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 w h ic h r e p r e s e n t s t h e s t a t e of c r a c k i n gunder na tu ra l cond i t ions , has a d i ff e ren t pa t t e rn . The peak o f c rack dep thsap pe ar s a t the shal low ra ng e of 16 to 25 cm, fo l lowed by a gradua ldec rea se towards d eep e r l eve l s un t i l 135 cm. As the da rk g ray hor i zona p p e a r s i n t h i s p r o fi l e a t 6 5 - 1 2 0 c m thus , a t leas t 40% of the cracks tapi t and ve ry f ew of the m cu t th rough i t s en t i r e th i ckness . These f ind ingsa r e i n a g r e e m e n t w i t h 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 b y t h e S oi lSurvey S taff, where t he top b rown and the da rk g ray ma te r i a l s mix o r fo rmw e dg e s h a p e d i n t r u s i o n s o f b r o w n i nt o t h e d a r k g r a y m a t e r i a l of t h e s u b s o i l .

The two p ro f i l e s i t e s o f the L ink Cana l a rea a re s l igh t ly d i ff e ren t intha t s i t e O4H i s under g ra s s and scan ty t r ees , and s i t e O4MH i s undert r e e s a n d s c a n t y g r a s s .

Geo derm a, 5 (1971) 23 5



TABLE III

Moisture contents at ti me of crack s me asuri ng in cer tain profiles (GTO4,GTO5 and GRF ) in Gezira

GTO4 GTO5 GRF 1

depth (cm) perc ent, depth (cm) perce nt, depth (cm) percent .

0-1 6.0 0-2 7.5 0--20 6.31-30 7.9 2-10 8.0 20-40 10.5

30--40 11.0 10--35 13.0 40-60 16.940-70 15.5 35-67 18.0 60-80 20.370-105 19.2 67-102 24.5 80-100 21.6

105-140 24.0 100-120 22.1

1 With kind permission of H. Farbrother, Cotton Research Corporation,Wad Medani, Sudan.

The e ffect of na tu ra l vege ta t ion exp re sse s i t se l f in the dep ths o fc rac ks . P rof i l e O4H, under g ra ss , has more than 60 of i t s c r ac ks be t wee n16 and 45 cm depth, wh er ea s prof i le O4MH, unde r t ree s , has 75 of thec ra cks be tween 26 an d 55 cm. I t i s ev iden t tha t g r as s deve lops sha l lo wc r a c k i n g an d 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 a nd f r o md e e p e r l a y e r s .

The two s i t e s o f the Gash de l t a d i ffe r in the i r h i s to ry. S i t e GHATO5w a s f lo o de d 6 m o n t h s p r i o r t o c r a c k m e a s u r i n g a n d s ta l k s of c a s t o r p l a n t swere s t i l l s t and ing . S i t e GHBTO7 has no t been f looded and rece iv ed on lythe ra ins o f 1967. Al though bo th p rof i l e s have the same aver age de p th , ye t ,the peak of oc cur re nc e app ear s a t the sha l low dep th o f 26 cm in GHAT OS,

and appears be tween the dep ths 46-55 in GHATO7 tha t r ece ived ra in fa l lonly. The su rf ace la ye r of GHATO5 must have swe l led to a ma xi mu m und erf looding and when i t dr ie d i t cr ack ed mo re than GHBTOT.

I n c o n c l u s i o n i t i s s t a t e d th a t t he a v e r a g e d e p th of c r a c k s i s i n v e r s e l yr e l a t e d t o t he a m o u n t of p r e c i p i t a t i o n o r i r r i g a t i o n . T he f r e q u e n c i e s o focc ur r enc e of c r ac k dep ths a re a ffec ted to a cons ide r ab le ex ten t by thekind and densi ty of vegeta t ion and his tory of the soi l .

r a c k w i d t h s

The da ta o f c r ac k wid ths do not show gre a t d i f fe renc es among the

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 t ho s e da t a of c r a c k d ep t hs . T h e r e a s o n , o fc o u r s e , i s th at t h e p l a i n o r s u r f a c e s h r i n k i ng a l m o s t c o m p l e t e l y s t o p s , a ti t s m a x i mu m , w h e n t h e m o i s t u r e r e a c h e s t he h y g r o s c o p i c r a n g e. T h i s i sc o n t r a r y t o t he d e p t h s o f c r a c k s w h ic h w i ll i n c r e a s e a s l on g a s d e p l e t i o nof m o i s t u r e c o n t i n u e s a d v an c i ng f r o m t h e s u r f a c e d o w n wa r ds t o t h e d e e play er s with no l im i ta t i on excep t the t ime fac to r and the dep tn o f we t l a ye rs .

The da ta o f c ra ck wid ths i s p res en ted in Tab le IV fo r 9 p r of i l e s , a sf r e q u e n c i e s o f o c c u r r e n c e p e r c e n t an d p r o f i l e a v e r a g e s .

I t i s no ted f r om the da ta o f the Gez i ra p rof i l e s , tha t the pe ak s o fo c c u r r e n c e of c r a c k w i dt h s a r e m o r e o r l e s s r e l a t e d t o t h e p e r i o d o ffa l lowing . The pea k o f 28 appe ars in the non - i r r i ga t ed p rof i t e s i t e GRF

236 Ge od er ma , 5 (1971}



TABLE IV

Frequencies of occ urr enc e percent and average crack widths of the soils of Ge zi raLink Canal and Gash de lt a a re as

Crack w i d t h Gez ira Link Canal Gash delta(cm)

GTO3 GTO4 GTO 5 GTO6 GRF O4H O4 MH GHATO 5 GHBTO7( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )

<2.6 8 9 18 14 9 6 2 30 222.6--3.5 27 23 42 51 25 22 3 30 313. 6-4. 5 38 32 27 24 22 31 37 19 194. 6-5. 5 19 24 11 11 28 16 27 4 195.6--6.5 4 7 2 - 13 6 15 17 66.6--7.5 4 4 -- -- 2 13 7 -- -

7.5 -- 1 - -- 1 6 9 - 3

A v e r a g ew i d t h o fp r o f i l e ( c m )

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 hi s s i te has the longest per iod under d ryingconditio ns. The pea ks of 38 and 32 in si tes GTO3 and GTO4, that we refa l lowed fo r 4 and 7 months respe c t ive ly, appear a t c rack wid th 3 .6 -4 .5 cm .Si tes GTO5 and GTO6, that wer e fa l lowed for 3-4 months have th ei rpeaks of 42 and 51 re sp ec t i ve ly a t 2 .6-3.5 cm width.

T he a v e r a g e s o f c r a c k w i d t hs i n G e z i r a a r e a l s o r e l a t e d to th e p e r i o d sof fallowing. The d es ce nd in g or de r of length of fallowin g is 8, 7, 4 and 3months and co r r es po nds wi th the descend ing o rd er o f c rack wid ths o f 4 . 2 ,4 .2 , 3 .9 and 3 .4 cm respect ively.

The effect of veg eta t io n in the Link Canal ar ea is dominant . In s i t eO4H, under gr as s , the av er ag e cra ck width is 5 .1 cm, whi le in s i te O4MI-I ,under t r ee s , the aver age i s 4.8 cm. The da ta ind ica te a g r ea te r su r fac edryness in the s i t e wi th dominan t g rass vege ta t ion .

The meas ur es o f the Gash de l ta indica te that the s i t e tha t was f lo od ed7 months p r io r to c rack measur ing has an average wid th o f 3 .4 cm, whi lethe s i t e tha t was und er ra in on ly has an average o f 3 .7 cm. I t i s a l so no t i c edt h at t he f r e q u e n c i e s o f o c c u r r e n c e i n t he f o r m e r s i t e a r e h i g h e r th an t h el a t t e r a t t h e r a n g e s o f n a r r o w c r a c k s . I n c o m p a r i s o n w it h o t h e r s i t e s u n d e rna tu ra l cond i t ions , the Gash de l t a p ro f i l e has the na r rowes t average wid thof cr ac ks owing to i t s low er c lay content .

The effect of c lay content on the width of cracks can be t raced onlyby compar ing Gash de l t a with Link Cana l so i l s . The Gash de l t a so i l s co n t a i n55-67 c lay and th ei r av er ag e width of cr ack s is 3 .4-3.7 cm. The LinkCana l so i l s have 71-76 c lay and the i r averag e width of c ra cks i s4.8-5.1 cm.

In conc lus ion i t i s c l e a r th a t c rack wid ths a r e a ffec ted by the l e ng t hof dryi ng per iod, by th e his tor y of the soi l , and by the c la y content . Thed i ffe r ences in the c ra ck wid ths a re ra the r l imi te d to a na r r ow range o fv a r i a b i l i t y.

Geoder ma, 5 (1971) 237



r a ck spac i~zg

Al though the gene ra l shape of so i l b locks enc i rc l ed by c rac ks i s apo lygon , and a measu rem en t of spac ing a long a s t r a igh t l i ne wi l l no tn e c e s s a r i l y m e a s u r e t h e a r b i t r a r y c e n t r e l i n e of t he p o l yg o n s , y e t , t h e d a t ah e lp i n p r e s e n t i n g a n a p p r o x i m a t e s u r v e y o f t h e a v e r a g e s p a c i n g b e t w e e nc r a c k s o n a l i n e a r b a s i s .

T he r e s u l t s a r e g i ve n i n Ta b l e V f o r a l l s i t e s . T h e da t a a r e p r e s e n t e da s a v e r a g e s p a c i n g i n c e n t i m e t e r s a n d a s f r e q u e n c i e s of o c c u r r e n c e ofs p a c i ng i n t e r v a l s .

TABLE V

Average spacings and frequencies of occurrence percent of spaces between cracksin Gezira Link Canal and Gash delta soils

Spaces Gezira Link Canal Gash deltacm)

GTO3 GTO4 GTO5 GTO6 GRF O4H O4MH GHATO5 GHBTO7

<21 20 14 22 5 39 19 31 17 621-40 40 41 29 30 53 26 35 49 1941--60 32 19 20 35 7 21 18 22 416 1 8 0 4 8 2 0 1 9 1 3 1 8 8 1 9

8 1 1 0 0 4 6 7 i i 3 6 4 9

> I 0 0 1 2 2 2 6

Averagespacing ofprofile cm)

39 52 45 54 28 51 39 39 62

T h e c o m p a r i s o n a m o n g s t a r e a s a n d 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 ep o i n t s o f i n t e r e s t .

The i r r iga te d so i l s o f Gez i ra and Gash de l t a f looded) c rack a tave rage spac ing be tween 39 and 54 cm, wi th peaks o f occur rence a t21-60 cm spac ing .

T h e G e z i r a s o i l 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 io n s s h o w s am u c h n a r r o w e r s p a c i n g wi t h a n a v e r a g e o f 28 c m . 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 , i n t he G e z i r a , i n c r e a s e s b r e a k - d o w n an d f i s s u r i n g o ft h e b i g g e r s u r f a c e b l o c k s .

T h e L i nk C a n a l s o i l s s h o w d i f f e re n t a v e r a g e s . T h e s o il u n d e r d o m i n a n tg ra ss , O4H, c ra cks a t an ave ra ge spac ing of 51 cm, whi l e tha t und er t r e es ,O4MH, has an ave ra ge o f 39 cm. Th i s i s r en der ed to the mec han ic a l b ind i nge ffec t o f the sha l low roo t sy s t ems o f g ra s s wh ich ac t a s in fo r cem ent fo r thes u r f a c e b l oc k t h u s r e d u c i n g t he f i s s u r i n g of t h e i r s t r u c t u r e .

I t can be conc luded tha t the spac ing of c rac ks i s i n f luenced by theh i s t o ry o f the s o i l and by the na tu re o f the vege ta t ive cover.

L i n e a r s h r i n k a g e a nd t o ta l v o l u m e o f 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 o f t he 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 ep r e s e n t e d i n F i g . 3 , c a l c u l a t e d as p e r c e n t o f 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

o f m e a s u r i n g .238 Geode rma , 5 1971)



i n e a r s h r in k a g e Y )0 0 5 I 0 15

4 • ,,

~+o1[I~ s o ~ GT 0 3 0 4 H

. G T 0 4 . . . . . . . 0 4 M H

I t z ~ . . . . G T 0 5...............G T 0 6 . - , . - , _ ~ G H B T 0 7

~ G R F . . . . G H AT O 5

Fig.3. Linear shrinkage of soils: GTO3 and GTO4 in central Gezira,GTO5 and GTO6 in south Gezira, GRF permanent fallow in central Gezira,O4H and O4MH in Link Canal, and GHATO5 and GHBTO7 in Gash delta.

It is noticed that although the surface shrinkage in the Gezirairr iga ted soi ls is between 7 and 10 , yet the shr inkag e decr eas es to le ssthan 0.7 cm at a depth of 50 cm from the surface. The Gezira soil undernatur al conditions, GRF, has the gre ate st value of shrin kage of 15 in thesurfac e, d ecr eas ing to about 3 a t 50 cm depth, and cont inues to de cr ea semo re slowly to 0 at the depth 130 cm. Thus irr iga tio n has reduc ed theshrinkage to about half that under natural conditions.

The Link Canal soi ls have smaller shr inkage than Gezira soi ls undernatural conditions in both surface and deep layers. The shrinking in theLink Canal soils ends at about 70 cm depth. This difference is mainly dueto the difference in ra infal l and par t ly to that of the drying per iod, as LinkCanal area receives about 600 mm against about 300 m m in Gezira, whilethe drying per iods are 6 and 8 months respect ively.

The Gash del ta soi ls have a different pat tern due to their lower c laycontents and higher permeabi l i ty. The surface shr inkage is ra ther small ,between 5 and 9 , but the shrinking de cre as es at a much slowe r rat e thanall other profiles to reach the deepest level of all , between 130 and 150 cm.The deep cracking of the soi ls of Gash del ta is rendered to their compara-

t ively higher permeabi l i ty which helps to drain them more eff ic ient ly thanthe other area s , thus reducin g the moist ure contents in the deep layer s a ndshortening the t ime of drying.

The differences observed in l inear shr inkage are emphasized in thetotal volume of cracks.

The data of tot al crac k volumes are pre sen ted in Table VI. Theres ul t s are pre sen ted so that crack volumes are given for each 10 cmlayer of the prof i le down to the deepest crack. The resul ts are calculatedas cubic met re s pe r he ctare , mil l imetr es of water, and cubic met ers ofwater per feddan*.

The to tal vo l ume of c racks in the i r r iga ted a re a of the Gez i ra ra nge sbetween 222 m 3 h a and 313 m3/ha, equivalent to 22.2 mm and 31.3 mm ofwate r respect ively. In the si te GRF of non- irr igat ed soi l , the cra cksvolume r eac he s 867 m3/ ha, equivalent to 86.7 mm of water, i .e. , about 3--4t imes the volume of the i r r igated soi ls .

*Feddan = 4,200 m 2.



T A B L E V I

Vo l u m e s o f c r a c k s i n G e z i r a , L i n k C a n a l , a n d G a s h d e l t a s o i l s i n m e t r e c u b e p e rh e c t a r e p e r 1 0 c m l a y e r

L a y e r s G e z i r a L i n k C a n a l G a s h D e l t a(cm)

G T O 3 G T O 4 G T O 5 G T O 6 G R F O 4 H O 4 M H G H A T O 5 G H B T O 7

0 - 1 0 1 5 7 1 4 0 1 3 0 1 0 2 2 4 7 1 8 4 1 7 2 1 5 91 0 - 2 0 8 5 9 0 9 4 7 0 1 8 7 1 2 2 1 2 5 1 2 52 0 - 3 0 3 3 4 8 5 4 3 6 1 3 1 7 4 8 2 9 23 0 - 4 0 1 0 1 9 2 5 1 2 9 2 4 0 4 6 7 14 0 - - 5 0 2 6 8 2 6 9 2 0 2 3 5 65 0 - - 6 0 - 2 2 - 5 2 8 9 4 26 0 - 7 0 . . . . 3 8 2 4 2 97 0 - 8 0 . . . . 2 4 - 1 1 88 0 - 9 0 . . . . 1 4 - - 79 0 - 1 0 0 . . . . 8 - - - 4

1 0 0 - 1 1 0 . . . . 5 - - 31 1 0 - 1 2 0 . . . . . . . 21 2 0 - 1 3 0 . . . . . . . 1

T o t a lm 3 / h a 2 8 7 3 0 5 3 1 3 2 2 2 8 6 7 4 5 0 4 6 2 6 0 9

m 3 / f e d d a n 1 2 1 1 2 8 1 3 1 9 3 3 6 4 1 8 9 1 9 4 2 5 6

2 8 . 7 3 0 . 5 3 1 . 3 2 2 . 2 8 6 . 7 4 5 , 0 4 6 . 2 6 0 . 9

1 0 58 36 44 73 22 21 51 0

7421

3 9 2

1 6 5

3 9 . 2

C o n c e r n i n g t h e q u e s t i o n o f w a t e r p e n e t ra t i o n i n t h e s o i l p r o f i le , itw il l b e n o ti c ed t h at o n ly b e t w e e n 1 . 2 m m a n d 3 . 5 m m o f w a t e r d o p e n e t r a t et h e s o il r o fi l e t o t h e de p t h s b e l o w 3 0 c m f r o m t h e s u rf a ce i n t h e i r r i g a t e ds o i l s o f G e z i r a. T h e s i t e u n d e r n a t u r al c o n di t i o ns i s m u c h b e t t er t h i s w a y ,a s 3 0 m m o f w a t e r p e n e tr a te b e l o w 3 0 c m . T h u s , i n t h is r e sp e ct , t h eb e n e fi c i a l e f f ec t o f f a l l ow i n g t h e la n d f o r o n e y e a r i n t h e c o m m o n r o t a t io na p pl i ed i n G e z i r a , b e c o m e s q u es t io n ab l e f r o m t h e p o i n t o f v i e w o f w a t e rpene t r a t i on i n t h e p ro fi l e .

T h e t w o L i n k C a n a l p ro fi le s, O 4 H a n d O 4 M H , h a v e al m o s t t he s a m ev o l u m e o f c r a c k s , 4 5 0 a n d 4 6 2 m 3 / h a , e q ui v al e nt t o 4 5 .0 a n d 4 6. 2 r n m o fw a t e r r e sp e ct i ve l y. T h o u g h t h e t ot al v o l u m e i s a b ou t t h e s a m e i n b o t hs it es y e t , it s h o u l d b e n o t i c e d t h a t i n O 4 H , u n d e r g r a s s , t h e v o l u m e o fc r a c k s b e l o w 3 0 c m d e p t h i s o nl y 7 0 m 3 / h a , w h i le i n O 4 M H , u n d e r t re es ,t he v o l u m e b e l o w t h e s a m e d ep th is 8 3 m 3 /h a. T h i s i nd ic at es m o r e

s h r i nk i n g i n t h e d e e p l a y e r u n d e r t r e e s t h a n u n d e r g r a s s .T h e t w o G a s h d e l t a s it es h a v e a p p r ec i a b ly di ff er en t v o l u m e a o f c r a c k s ,

t h e o n e p r e v i o u s l y f l oo d ed a n d cu l ti v at e d w i t h c a st o r c r o p, G H A T O S , h a s6 0 9 m 3 / h a , a n d t h e o th e r o n e th a t r e c e i v e d ra i n o n ly, I I B T O 7 , h a s 3 9

r n3 /ha . .Th i s d i f f e r ence i s du e t o tw o f ac to r s : ( a) t he g r ea t e r dep l e t i on o fm o i s t u r e f r o m t h e s u r f a c e b y t h e c R s t o r pl a nt s ; a n d ( b) t h e f l oo d in g f o r 3w e e k s o f G H A T O 5 a l l o w e d t he s oi l t o s we ll m o r e f re el y t ha n t he s c a n t yr ai n r e c ei v e d b y G I I B T O 7 , a n d c o n s e q u e nt l y t he s hr in ki ng o f t h e f o r m e rw i ll b e g r e a t e r t h a n t h e l at te r. T h e v o l u m e o f c r a c k s i n t h e t o p 3 0 c m i nt h e s i te u n d e r c a s t o r p l a n t s i s 3 7 6 m 3 / h a g a in s t 2 5 5 m 3 / h a i n t h e s i tew i t h r a in on ly.

2 4 0 G e o d e r m a , 5 1 9 7 1 }



I n c o n c l u s i o n i t i s c o n s i d e r e d t ha t i r r i g a t i o n r e d u c e s t h e v o l u m e ofc r a c k s i n t h e G e z i r a t o o ne t h i r d o r o n e f o r t h o f t h e v o l u m e u n d e r n a t u r a lcond i t ions . The vo l ume o f c ra cks in L ink Cana l a rea i s no t a ffec ted by thek i n d of v e ge t a t i o n a l t h o u gh 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 t h e s u r f a c el a y e r s , a n d t r e e s i n c r e a s e s i t a t t h e d e ep l a y e r s . T h e s o i l s of G a s h d e l t ac r a c k t o g r e a t e r d e p t h s t h an i n a l l o t h e r a r e a s , b u t t h e c r a c k s a r e n a r r o w .The to t a l vo lume of c ra cks in the upper l aye r s i s mu ch a ffec ted by soi lh i s to ry and vege ta t ion .

REFERENCES

El Shal, M.J., 1954. Wat er Requ irem ents of Main Crops in Menoufia Dist rict . T he s i s,Fac ult y of Engi neer ing, Cairo Unive rsity , Cairo, U.A.R., 62 pp.

Finck, A. and Ochtman, L.H.J. , 1961. P rob lem s of soil evaluation in the Sudan. J. SoilSci., 12, 87-95.

Gre ene , H., 1928. Soil prof il e in the ea st er n Gezir a. J. A gr. Sci., 18: 518-530.Soil Survey Staff, 1968. Re port s on the Gezira soils. Divisio n of Soil Survey, Mi ni st ry

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a study on cracking in some vertisols of the sudan

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