GeoJournal 23.3 2 3 3 - 2 3 7

© 1991 (Mar) by Kluwer Academic Publishers 233

Soil Properties and Moisture Characteristics and their Relationship with Crop Mid-day Stress in the Sudan Gezira

Ismail, A. M.A., Prof. Dr., University of Qatar, Department of Botany, POB 2713, Doha, Qatar

ABSTRACT: This study is mainly about the relationship of the moisture holding and release capacity of soils with their texture, particularly the clay fraction. Soil samples collected from various sites in the Sudan Gezira were analysed for physical and chemical properties and moisture characteristics. Generally the soils were clayey except the recent alluvium which was loamy. The pH was alkaline and the electric conductivity < 1.0 dS m -i. The studied soils were calcareous and very poor in organic matter, their intermediate to high water holding capacity being due mainly to the presence of large quantities of clay. Matric potentials of -0.4 to -1.5 MPa reduced soil moisture for all types of soils but the effects were not necessarily quantitatively equal. Permanent wilting point - moisture content at -1.5 MPa - varied widely (10.5-22.0 %) amongst the different soils and this was mainly attributed to texture and mineralogy of the soil clays. Bulk density decreased with increase in clay content. Calcium carbonate content was found to have no apparent relationship with the moisture content of these soils. The amount of silt fraction seems to have an overriding effect on the relationship between soil moisture availability in the recent alluvium soil and its subsequent release to growing plants. The results are dicussed in relation to the phenomenon of mid-day stress observed in all grown crops of the Sudan Gezira.

Introduction

The ever increasing popula t ion of the Sudan requires more supply of food. Efforts of increasing food crops product ion is encouraged through agricultural intensification practices by ameliorat ing and expanding arable land. The two main staple food crops of the Gezira are sorghum and wheat. Sorghum, because of the drought in some parts of the Sudan, is becoming increasingly impor tant as a cash crop in the Gezira , while yields of wheat are still insufficient for the coun'try's needs (Ageeb 1988).

Three agricultural systems are general ly adopted in the Sudan: (a) Gerif cultivation, where, after inundat ion by f lood water , the alluvial silts of the river terraces are cult ivated, use being made of the moisture thus s tored in the soil; (b) i rr igated schemes, using canals and pumps; and (c) rain-fed farming (E1-Arifi 1982). Beside the

presence of aquatic weeds the flow of water in the Gezira canals was also impeded by silt accumulated over the past 1 2 - 1 5 years. In addit ion, poor drainage coupled with low infil tration rate and waterlogging due to poor management of the Gezira soils causes patchiness and low crop yields (Ageeb 1988). In the Gezira drought is not so severe as to cause crop failure but water stress due to inadequate irrigation, poor and errat ic rainfall and high evapotranspira t ion presents the major physical constraints on crop yields. Rehabi l i ta t ion of the Gezira irr igation facilities may res tore more acreage of land for growing crops. The World Bank is investing 308 million dollars during the per iod 1987-1992 as a fund to improve irr igation methods and management (World Bank 1990).

Childs (1940) observed that the physical proper t ies of soils are the factors governing soil-moisture characteristics while Ghosh (1980) argued that the soil- moisture characteristics for any soil can be established

234 GeoJournal 23.3/1991

directly from the mechanical properties of the soil and by a single measurement of ~ (water potential) for 0 (moisture content). The techniques used to determine these constants are time-consuming, tedious and laborious (Salter 1967). Dahiya et al. (1988) obtained estimation of the moisture constants for several hundreds of soil samples from their easily determinable parameter saturation percentage and regression equations. However, soil-moisture retention curves provide a convenient method describing the moisture release properties of different soils (Winter 1974). To design efficient irrigation systems a thorough knowledge of the balance between crop demands and the capacity of soil to hold and release water is essential. An estimation of the available water capacity requires information about the field capacity and wilting point, the generally acceptable upper and lower limits of water availability to the plants (Abrol et al. 1969), who also found a positive correlation between percentage CaCO3 and available water.

This present study was to obtain information on moisture retention and release curves using both physical and chemical properties of the soils. The second objective was to advance our understanding of the physico/chemical behaviour of the soils in relation to (an agronomic problem) mid-day water stress in the Sudan Gezira.

Study Area

The climate of the Gezira is classified as semi-arid. Mean annual rainfall is about 342-424 mm (Ishag et al. 1985) mean temperature during the main crop sowing period is 25-39 °C. For about 7-8 months of the year there is no rain. The soil profile rarely becomes sufficiently wetted to allow adequate infiltration below the root zone and essentially all soil water is abstracted each year with no carry over for next year (Greene 1928; Ishag et al. 1985; Omer et al. 1988).

Materials and Methods

Eight sites were chosen to represent the range of soils found in the Gezira and in order to examine whether differences in soil structure which may influence moisture availability and subsequent crop productivity. For each site one sample of soil (2.5 kg, divided into three sub- samples) from the top 30 cm was collected. Soils No. 1 and 2 (group A-aridisols), soils Nos. 1-3 (group B- vertisols) were kindly supplied by Soil Survey Administration, Ministry of Agriculture, Wad Medani, Sudan. Soils Nos. 4-5 (vertisols) were sampled from Central Gezira. Recent flood-plain alluvium with no B horizon (Group C) was collected from the west bank of the Blue Nile near Hassa-Heissa.

Physical, chemical analyses and water retention curves were carried and determined for each soil sample. All soil samples were air-dried ground to pass a 0.2 mm sieve and

analysed (three replicates from each sample). Soil texture was determined by the Bouyoucos hydrometer method (Day 1965). Saturation percentage was determined on a soil paste made with distilled water. The bulk density of the disturbed soil samples was determined by the clod method (Blake 1965, p. 381). Moisture retention and release curves were determined using pressure plate and pressure membrane apparatus (Richards 1954 Soil Moisture Equipment Corporation, Santa Barbara, California). The available water was determined from the difference between -0.033 Mpa matric potential (field capacity) and the -1 .5 Mpa matric potential (wilting point). The water content of soil, as measured by this method, is a satisfactory approximation of the available water (Kramer 1983, p. 98). Electrical conductivity and pH were determined from the saturation extract. Soluble cations and anions were determined from the saturation extract as follows: calcium and magnesium by titration with versenate, sodium and potassium were determined photometrically, carbonate, bicarbonate by titration with H2SO4, chloride by titration with silver nitrate and sulphate by precipitation as barium sulphate. Calcium carbonate was determined by gravimetric loss of carbon dioxide (Richards 1954), organic matter by the Walkley- Black method, total nitrogen by Kjeltec Auto 1030 Analyser (Tecator AB-Sweden).

Results

A full list of the site locations and major physical and chemical soil properties is given in Tab 1. The textural classes represented are: six clayey, one sandy clay and one loamy soils. In the aridisols, vertisols and recent alluvium, the respective clay fraction was 59.0-62.0 %, 44-64 % and 27 %. Silt fraction 4 -10 %, 2.5-10 % and 37.5% and the sand fraction 2 8 - 3 6 % , 3 0 - 5 3 % and 37 %. Generally soils have intermediate to high moisture holding capacity (saturation percentage > 45%). The water holding capacity increased (P < 0.05) with increase of clay content for the aridisols and vertisols and with increased silt content for the recent alluvium. Bulk density increased with increase in sand and silt fractions while particle density was almost unchanged for all groups of soils. As expected and well-known the pH was similar, with no significant difference, and alkaline in all soil samples. The electric conductivity of the saturation extract was generally very low, Central Gezira I soil and recent alluvium had lower EC (P < 0.05) compared to the other six sites. The bicarbonate was generally high in all samples but significantly lower (P < 0.05) in Central Gezira and recent alluvium soils. Soluble carbonates were invariably low, chlorides and sulphates were low but their content was similar in all locations. Generally, Ca ions and Mg ions were low in aridisols but gradually increased for vertisols and the recent alluvium samples, Na ÷ was low in all sites with the lowest Na- content (P < 0.05) in the recent alluvium while K ÷ was extremely low in all

Geodournal 23 .3 /1991 235

Se- Class and Clay Silt Sand rial name of soil % % % Den- No. 2 2-50 50-2000 sit)

Mi- Mi- Mi- g ml -a cron cron cron

(A) Aridisols 1 Wad-AI-Hadad 59.3 4.0 35.7 1.30

i822)

2 Wad-At-Hadad 61.8 10.0 28.2 1.29 1760"1

(B) Vertisols ] Fahal (492) 44.3 2.5 53.2 /.48

2 Faha11493/ 54,3 2.5 43.2

3 CentraIGezira 1 54.3 5,0 40.7

a Central Gezira 2 64.3 5.0 30.7

5 CentralGezira3 49.3 10.0 40.7

(C~ Recent alluvium 26.8 37.5 35.7

Bulk Patti- Tex- Saturation pH ECe mg 1 i mg 1-1 Or- Total cle ture percentage (dSm -~ CO3 HCO:, CI SO4 Ca Mg Na K CaCO3% ganic Nitro-

Den- class 25 °C/ mauer gen sity (%) (%)

g ml -~

2.67 Clay 55.6+1.40" 8.41 0.52 9 217 45 36 16 5 i04 5 7.98-+0.19 0.58 0.21

2.67 Clay 59.4-+1.48 8.40 0.62 9 240 34 89 26 5 120 5 5.91-+0.15 0.75 0.57

2.67 Sandy 47.0_+1.20 8.25 0.53 11 222 50 37 26 6 97 5 11.43+_0.28 0.40 0.27 Clay

1.32 2.67 Clay 47.7+1.19 8.40 0.66 9 219 61 79 24 6 124 5 7.68_+0.19 0.76 0.27

1.31 2 .68 Clay 58.9_+1.47 8.19 0.48 13 194 34 76 31 12 78 5 5.04+-0.15 0.49 0.25

1.28 2.68 Clay 65.1_+1.62 8.15 0.63 16 231 53 109 36 13 113 5 5.90_+0.14 0.54 0.22

1.28 2.68 Clay 49.9+_1.24 8.18 0.76 I61 63 156 42 6 120 5 5.90+0.17 0.46 0.21

1.54 2.65 Loam 51.8_+1.29 7.95 0.46 9 112 66 49 65 11 16 5 7.82_+0.19 0.77 0.47

" S . E .

s tud ied sites. Ca lc ium c a r b o n a t e occu r r ed t h r o u g h o u t the

s a m p l e d si tes in an er ra t ic pa t t e rn . F r o m the resul ts the

o r d e r of C a C O s c o n t e n t was Faha l 492 > Faha l 493 =

W a d - A 1 - H a d a d 822 -- r ecen t a l luvium > W a d - A 1 - H a d a d

760 -- CG1, 2 and 3 (P < 0.05). Organ ic m a t t e r was

e x t r e m e l y low, rang ing b e t w e e n 0.40 % (aridisols) and

0.77 % in the r ecen t a l luvium. L ikewise , to ta l n i t r o g e n e

was 0.2 % in Faha l (493) bu t was b e t w e e n 0 . 4 7 - 0 . 5 6 %

in W a d - A 1 - H a d a d (822) and the r ecen t a l luvium soils.

T h e r e was no c lear-cut r e l a t ionsh ip b e t w e e n CaCO3

c o n t e n t and avai lable w a t e r fo r all s a m p l e d soils (Tab 1).

M o i s t u r e r e t e n t i o n curves (Fig 1) s h o w e d tha t the

m o i s t u r e c o n t e n t o f any mat r i c po t en t i a l t h r o u g h o u t t he

range --0.033 to - 1 . 5 M p a are h ighes t in c layey soils and

lowes t in l oamy and sandy clay soils. F ie ld capaci t ies

fo l lowed the o rde r : Cen t r a l Gez i ra 1 = Cen t ra l Gez i r a 2

= W a d - A 1 - H a d a d 760 > W a d - A 1 - H a d a d 822 > Cen t ra l

Gez i r a 3 = Faha l 493 > r e c e n t a l luvium > Faha l 492.

P e r m a n e n t wil t ing po in t fo l lowed the r eve r se order :

r e cen t a l luvium < Faha l 492 = Cen t ra l Gez i ra 3 Faha l

493 < W a d - A 1 - H a d a d 760 = W a d - A 1 - H a d a d

822 < Cen t ra l Gez i r a 2 < Cen t r a l Gez i r a 1.

Fig 2 shows mo i s t u r e re lease charac te r i s t ics curves for

the l oamy soil (Ger i f cul t ivat ion) c o m p a r e d to tha t o f a

Gez i r a clay soil. T h e loamy soil has a la rger m a x i m u m

w a t e r c o n t e n t ( A W l he ld at re la t ively low mat r ic

Fig 1 Soil-moisture retention curves of some Gezira soils

o o Wad-EI-Hadad (760)

= = Fahal (492)

n Wad-E1-Hadad (822)

A A Fahal (493)

o-.---~ Recent alluvium

o----4 CentralGezira (1)

A----a Central Gezira (2)

A----, Central Gezira (3)

FC = Field capacity, WP = Wilting Point. The magnitudes of measurements are smaller than the symbols SE are not represented.

o • SO.O

,.~ 40.0

_¢._

30.0 .4- - -

20.0 ..&...

10.0 --.

I I I I I m, I -0.033 -0.3 -0.6 -0.9 -1.2 -1.5.J

236 GeoJournal 23.3/1991

SO.O

~,0.0 a

30.0 c i:!~si:-.):~

20'Oh

d ~;~':'~':;'";':'(:~

0.0 -0.033 -0.2 -0.3 -0.4 -0,6 -0.9 -1.2 -1.5

Fig 2 The characteristic moisture release curves for a typical Gezira clayey soil and a loamy soil (Recent alluvium). Half of water in the loamy soil is held at matric potential below - 0.4 Mpa - but in the clayey soil most of the water is held above -0.4 Mpa (hatched = water held at high suction stippled = water held at low suction), a-b and c-d represent AW for the clayey soil and loamy soil respectively.

potential ( - 0 .2 Mpa); the Gezira clayey soil has almost most of its water content (AW = available water) held at heigh matric potential ( - 0 .4 Mpa). The total water content of the Gezira soil (area under Curve A) is almost twice that of the recent alluvium (area under Curve B).

Discussion

Low soil water contents are the major constraints on crop production in semi-arid areas and their effect is sometimes accentuated by salinity, particularly where osmotic and matric stress often occur simultaneously (Hao and de Jong 1988). Klute (1965) emphasized the importance of available water capacity as the prime measure to assess the moisture characteristics of a soil. The wider the range between field capacity and permanent wilting percentage, the longer plants can resist moisture stress. However, this statement does not always hold for some clays, particularly the Gezira clays which have high wilting percentage. Lund (1959) explained this effect by stating "Clay particles by themselves form pores so small, suctions holding much of the water are greater than a plant can overcome in obtaining moisture".

Most crop plants have osmotic potentials in the range of -1 .5 Mpa to -2 .0 Mpa, i.e. the -1 .5 Mpa is within the range at which soil moisture becomes severely limiting. The results suggest that the total water content at - 0 .4 Mpa coincides with Gezira farmers observation of mid-day stress. Cotton and groundnuts exhibit loss of turgidity (mainly by influencing leaf morphology) especially during periods of high water demands, in the middle of the day. At sunset these crops recover completely from temporary wilting. This phenomenon occurs despite adequate available water in the soil (Fig 2). However, it is the temporary lag between the rate of

transpiration and of absorption that leads to mid-day stress. Jarvis and Jarvis (1963) indicated that the actual water potential at the root surface depends on the rate of transpiration and the conductivity of the soil to water. Peter (1965, p. 282) stated that "There is no unique soil water retentivity value at which the uptake of water by plants suddenly ceases. The wilting points refer to the soil water content at which soil cannot supply water at a sufficient rate to maintain turgor". Mild to moderate water stress has a serious effect upon canopy photosynthesis and other major metabolic processes. A key point seems to be the high wilting point of Gezira clays. This is very important because it is difficult to correct even with good water management. However, it is not clear how far mid-day stress specifically affects metabolism (and yields) independent of general moisture stress due to inefficient water management. This temporary perturbation of metabolism due to the observed mid-day stress - in the Sudan Gezira - and its subsequent recovery awaits further investigation.

The Gerif cultivation system on the recent alluvium, which dates back to prehistoric times, is successful in the Gezira and even in the whole Nile valley because the loamy soil which supports the growth of plants contains a smaller volume of water readily available and easily donated to the plants which is frequently replenished from the high but continually falling water-table (Chadwick and Obeid 1963; Winter 1974). Lund (1959) pointed out that silt particles are the most important factor in forming a large available water-holding capacity in a soil and that pores between silt particles are ideal to hold water and easily release it to plants.

The above results show that particle size fractions of the Gezira soils influence the release pattern more significantly than the retention pattern. It is possible that the distribution of calcium carbonate between the

GeoJournal 23.3/1991 237

mechanical fractions (and particularly in the silt fraction) may be worthy of further examination as it has a general bearing on soil/moisture relationships in the whole area. For instance, different soil types may be expected to have different calcium carbonate distributions in their mechanical fractions. The soils of the Gezira have a high fertility potential if irrigation is made available (E1-Arifi 1982) and the proper balance of nutrient is maintained. However, before the relationships (conclusions) given.in the present study can be used with confidence for other Gezira soils, regression analyses of the moisture characteristics on various combinations of particle-size

fract ions mus t be carr ied out. In the p resen t inves t igat ion the l imited n u m b e r of soils was the ma in reason that such analyses could no t be unde r t aken .

Acknowledgements

I am greatly i ndeb t ed to Professor O s m a n A h m e d Al i Fadl (Univers i ty of Gezira) for c o m m e n t i n g on the manuscr ip t and to Dr . Shama D a w - A l b e i t (Soil Survey Admin i s t r a t i on , Minis t ry of Agr icu l ture , W a d Medan i ) for her k ind help.

References

Abrol, I.P.; Khosla, B.K.; Bhumla, D.R.: Relationship of testure to some important soil moisture constants. Geoderma 2, 33-39 (1969)

Ageeb, O. A. A.: Wheat production in the Sudan. Proceeding of the First Regional Coordination Meeting of Barley and Wheat Scientists in the Arabian Peninsula, 18-20 October, 1988, Sana'a, Yemen Republic, pp. 28-32 ICARDA, Aleppa, Syria 1988.

Blake, G.R.: Bulk density. In: Methods of Soil Analysis, Part I (Ed. C.A. Black), pp. 374-390. Agronomy Series, American Society of Agronomy, Madison, Wisconsin 1965.

Chadwick, M.J.; Obeid, M.: The response of variations in nitrogen level of some weed species of the Sudan. Weed Research, 3, 230-241 (1963)

Childs, E. C.: The use of soil moisture characteristics in soil studies. Soil Science 50, 239-252 (1940)

Dahiya, I.S.; Dahiya, D.J.; Kuhad, M.S.; Karwasta, S.P.S.: Statistical equations for estimating field capacity, wilting point and available water capacity of soils from their saturation percentage. Journal of Agricultural Science, Cambridge, 110, 515-520 (1988)

Day, P.R.: Particle fractionation and particle size analysis. In: Methods of Soil Analysis. Part I (Ed. C.A, Black), 545-556. American Society of Agronomy, Madison, Wisconsin 1965.

E1-Arifi, S. A.: Some irrigation problems in the Sudan. In: Problems of the management of irrigation lands in areas and modern cultivation (Ed. H.G. Mensching). Published by International Oeographical Union Working Group, pp. 71-84, Hamburg 1982.

Ghosh, R.K.: Estimation of soil-moisture characteristics from mechanical properties of soils. Soil Science 130, 60-63 (1980)

Greene, H.: Soil profile in the eastern Gezira. Journal of Agricultural Science 18, 518-530 (1928)

Hao, X.; De Jang, E.: Growth of wheat and barley seedlings at different matric and osmotic potentials. Agronomy Journal 80, 807-811 (1988)

Ishag, H. M.; Fadl, O. A.; Adam, H. S.; Osman, A. K.: Orowth and water relations of groundnuts (Arachis hypogaea) in two contrasting years in the irrigated Gezira. Experimental Agriculture 21, 403-408 (1985)

Jarvis, P. G.; Jarvis, M. S.: Effects of several osmotic substrates on the growth of Lupinus albus seedlings. Physiologia Plantarum 16, 485-500 (1963)

Klute, A.: Water capacity. In: Methods of Soil Analysis. Part I. (Ed. C.A. Black) pp. 273-278. Agronomy Series. American Society of Agronomy, Madison, Wisconsin 1965.

Kramer, P.J.: Water Relations in Plants, p. 489, Academic Press, Inc., 1983.

Lund, Z.F.: Available water-holding capacity of alluvial soils in Louisiana. Soil Science Society of America, Proceedings 23, 1-3 (1959)

Omer, M.A.; Saxton, K.E.; Bassett, D.L.: Optimum sorghum planting dates in western Sudan by simulated water budgets. Agricultural Water Management 13, 33-48 (1988)

Peter, D.B.: Water availability. In: Methods of Soil Analysis, Part I. (Ed. C.A. Black), pp. 279-285. Agronomy Series, American Society of Agronomy, Madison, Wisconsin 1965.

Richards, L.A.: (Ed.) Diagnosis and improvement of saline and alkali soils. USDA, Agricultural Handbook, No 60. US Government Printing Office, p. 158, Washington DC 1954.

Salter, P.J.: Methods of determining the moisture characteristics of soils. Experimental Agriculture 3, 163-173 (1967)

Winter, E.J.: Water, Soil and the Plant, p. 141. The Macmillan Press Ltd. London 1974.

World Bank: Report of Sudan Gezira Rehabilitation Project. Progress Report for period January-March (1990)