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Effect of soil salinity on the productivity of arid lands, with special reference to the Sudan

Mohamed O.H. el-Karouri National Council for Research, Khartoum

The increasing demand for food and other agricultural products makes the conservation and optimal utilization of soil resources increasingly imperative. This is more so in arid regions, which have narrow resource margins.

Saline soils are a widespread element of the landscapes of arid zones and soil salinization is one of the main processes responsible for the degradation and reduced productivity of agricultural lands in these regions.

Soil salinity may affect the productivity of agricultural land directly as a consequence of the presence of critical amounts of soluble salts in the root zone, which limit water and nutrient uptake by plants and cause physiological and metabolic disorders. Indirectly, salts may affect plants through adverse soil properties caused by sodicity and alkalinity imposed on the soil by soluble salts which may or may not be present in amounts detrimental to plants. The adverse effects of sodicity and alkalinity may be of both a chemical and a physical nature.

Many factors influence the effect of salinity on plant growth. Some of these are the composition of the salts, the moisture regime of the soil, its nutrient status, and the physical composition of the soil profile, as well as the plant species, including its rooting habit, age, and tolerance both to total salt content and to specific salts in the soil solution. Soil and cropmanagement practices also modify the effects of soil salinity.

It is estimated that more than 50 per cent of the irrigated lands in arid and semi-arid regions of the world are affected to some degree by salinization and that millions of hectares of agricultural land have been abandoned because of salinity hazards.

Besides soils already affected by salinity there are large areas which are potentially vulnerable and which could easily be damaged by salinization through irrigation. With few exceptions, wherever and whenever crops are irrigated in arid regions, salinity is a potential, if not an actual, problem.

A classic example where salinization has caused severe damage to agricultural land is that of Mesopotamia, where a once-fertile land has been transformed into a salt-caked waste. The soil that supported "the gardens of Babylon has come grudgingly to grant no more than an irregular crop of salttolerant barley." A recent example of impending disaster resulting from introducing irrigated agriculture to an arid region, if remedial measures are not taken, is that of the West Nubarya area in Egypt. According to Schultz and de Ridder (1974), this region is experiencing a rising watertable which has resulted in waterlogging, secondary salinization and influent seepage of saline groundwater into irrigation canals. The percolation of irrigation water through soils in fields with high salinity characteristics has raised the salt content as well as the water level of the groundwater.

Similarly, it was reported that in the Jhelum River plains in Pakistan the productivity of the agricultural lands of that region have declined because of salinity and waterlogging caused by the rising water table as a result of improper planning of the irrigation system (UNCOD, 1977).

Salinity Problems in the Sudan

In the Sudan, with the rapid expansion of irrigated agriculture, the question of soil salinity is becoming more and more urgent, not only for putting naturally salt-affected lands under cultivation, but also for maintaining the productivity of existing irrigated areas.

The Sudan lies wholly within the tropics, between latitudes 3° N and 22°N. Ecological conditions vary considerably from the desert in the north to the arid and semi-arid savanna areas of central Sudan and to the sub-humid areas of the southern provinces.

At present, the total area of irrigated land in the Sudan comprises about 1.6 million ha but there are several new irrigation projects under construction. These could increase the total irrigated area to perhaps 4 million ha. Thus in the near future problems of soil salinity are likely to be encountered on a larger scale.

Gezira

Work on soil salinity in the Sudan may be traced back to the turn of the century,when it was established by the early work of Beam (1911) that there were potential salinity and alkalinity hazards associated with the Gezira Scheme. Many workers expressed concern about the possible deterioration of the soils of the Gezira, the first and largest irrigation enterprise in the Sudan, when it was put under irrigation. Vageler and Alten (1932) and Balls (1935) advocated the installation of a sub-soil drainage system if the rapid deterioration of the Gezira soils was to be avoided. Greene and Snow (1939), on the other hand, opposed this view because of the poor permeability of the soil and the restricted lateral movement of water, which are prerequisites for an efficient drainage system. However, because of the highquality irrigation water with its low salt content (less than 200 ppm) and high Ca:Na ratio, as well as inclusion of long fallows in the rotation during which salts moving to the surface can be washed down by rains, in addition to the growing of salt-tolerant crops such as cotton, no visible signs of soil deterioration have been observed in the Gezira Scheme.

Although no signs of soil deterioration were detected, efforts were directed towards improving the inherent properties of the Gezira soils, particularly soil permeability. Greene and Snow (1939) pioneered the use of gypsum in reclaiming Gezira sodic soils, but found that teaching following gypsum application was insufficient to effect any profound change in soil productivity. Their attempts at using sodium accumulators (A triplex spp.) also failed significantly to lower sodium levels in the soil, quite apart from its depletion of plant nutrients, particularly nitrogen.

The Gezira vertisols are similar to the regurs of India, the black cotton soils of USA, and the grumusols or self-mulching soils elsewhere. However, the problems relating to poor infiltration and internal drainage appear to be more severe in Gezira soils, which may account for their resistance to improvement.

Northern Sudan

Unlike central Sudan and the Gezira area, the soils of northern Sudan are affected to a greater degree by soil salinity, particularly from Khartoum northwards along both banks of the Nile. The area affected by salinity in the Sudan can only be estimated, since few systematic surveys have been made. However, the areas which are potentially irrigable but where salinity is the main limiting factor for its development cover more than 200,000 ha.

In view of the large population and the relatively limited amount of good arable land in northern Sudan, there is an urgent need, particularly in the two northern provinces, to develop marginal lands which are often salt-affected.

The soils of the two northern provinces are derived largely from the alluvial deposits of the Nile mixed with some nonalluvial sands from the Nubian Sandstone. The soils are divided into two main groups, namely soils of the Recent Flood Plain and soils of the High Terraces. Solls of the Recent Flood Plain, which are a mixture of entisols and vertisols, include the Gerif and Gureir (local names) and the basin soils.

The Gerif soils are distributed along both banks of the Nile and contain high amounts of silt of fairly recent origin. Crops are grown without resort to irrigation, after subsidence of the annual flood. The moisture-release curve of these soils indicates that plants could utilize most of the moisture in the profile (Drover,1966).

The Gureir soils, which occur near the river bank and adjacent to the Gerif soils, are subject to flooding by moderately high flows. They are similar to Gerif soils in many aspects, but have a finer texture. They are rich free-working alluvial loams and, when irrigated, are one of the most productive soils in the Sudan.

The soils of the basins are alluvial deposits which vary greatly in age and nature. It is believed that the basins are the remains of former channels of the Nile. They are flooded only at very high flood stages. Basin soils are characterized by a high clay content, low salinity, and occasionally by high sodicity.

Most of the soils of the Recent Flood Plain, particularly the Gerif and Gureir soils, are fully exploited for agricultural production, and any envisaged agricultural development in the two northern provinces will depend mainly on the utilization of High Terrace soils. Besides some of the topographic and physical drawbacks, the main factors limiting the development of High Terrace soils are salinity and sodicity.

High Terrace soils occur on the landward side of the Recent Flood Plain. They could be classed as aridisols which have a weak structure and a texture which varies from sandy loams to sandy clays. A survey report (Huntings, 1964) and inspection of the data presented in Table 1, which was extracted from the work of Karouri (1967), demonstrates that considerable variation in salt content and composition exists, not only between different sites but also with depth, reflecting the diversity of parent materials within any one profile. Electric conductivity (EC) and exchangeable sodium percentage (ESP) values range between 0.2 and 49.5 mmhos/cm and between 1 and 100 respectively. Sodium is by far the dominant cation, and the dominant anions are chloride and sulphate. Therefore, the main salts are generally sodium sulphate or sodium chloride.
Little information is available on the reclamation of High Terrace soils. However, experience at the Gezira and the work of Karouri and Kaufmann (1966) have revealed that fine-textured soils are difficult to reclaim. On the other hand, it is expected that light-textured High Terrace soils can be reclaimed fairly easily. Preliminary investigations pertaining to the use of gypsum as an ameliorant on lighttextured High Terrace soils have indicated its effectiveness in improving the permeability of these soils (Karouri, 1968). Tahir and Fadl (1976) compared EC and ESP from two sites of High Terrace soils, one from virgin land and the other from a plot cultivated and irrigated from a surface well. Table 2 shows the effect of irrigation on leaching the salts and reducing the level of exchangeable sodium.

One cannot, however, overlook the importance of carrying out more intensified studies and pilot projects before embarking on large-scale project developments.

The area south of Khartoum

Similarly the area south of Khartoum between the Blue and the White Nile and extending as far south as the northwest boundary of the Gezira Scheme is predominantly saline and/or sodic. The total area is estimated at 81,000 ha. Due to salinity problems this area has never been utilized agriculturally in any form except for a short-lived animal fattening scheme which was established in an area of 4,000 ha, but low productivity of the land was one of the factors that contributed to its failure.

However, because of high-quality irrigation water from the two Niles, good roads which connect the area with Khartoum throughout the year, and availability of health and electric services, the area has recently gained importance for growing high-value crops for local consumption and for export.

Two semi-detailed soil and land classification studies were undertaken on the area by Huntings (1964) and the Soil Survey Division (Tom,1973). The Soil Survey Division has upgraded Huntings'Class Vl soils to Class IV following a reassessment of land capability classification in the Sudan. Nonetheless, the two studies were in agreement that most of the area is unsuitable for development and that the two limitations of these soils are salinity and sodicity.

The area south of Khartoum is generally flat. The soils are of alluvial origin laid down by the Blue Nile and are derived from the basaltic rocks of the Ethiopian Highlands. According to the Soil Survey report, three soil orders were identified: namely, vertisols, aridisols, and entisols; but the bulk of the soils are entisols. Associated with the three orders are four main soil types. These are:

Esailat Sodic Series. These constitute about 46 per cent of the surveyed area. The clay content is variable, ranging from 13 to 51 per cent. The soil matrix is calcareous throughout. In the top layer salinity is light to moderate and electric conductivity ranges from 0.5 to 6 mmhos/cm. The subsoil is more affected by salinity, and EC ranges from 4 to 30 mmhos/cm. Similarly, ESP varies with depth and ranges from 8 to 71, with the lower values being associated with the surface horizons.

TABLE 1. Chemical Analyses of Saturation Extract

TABLE 2. EC and ESP of Virgin and Cultivated Plots

Eilafun Series. This is a brown clayey soil that forms deep cracks when dry, and surface cracks are common but often covered with a thick surface mulch. Clay content varies from 25 to 61 per cent. The occurrence of soluble salts and exchangeable sodium is rather variable. EC ranges from 0.5 to 17 mmhos/cm, and the ESP from 4 to 42. Both EC and ESP are generally lower in the upper layers, but are present in moderate to high amounts lower down.

Gureir Series. This has a loamy texture, with the clay content ranging from 15 to 39 per cent. Excess amounts of exchangeable sodium are contained in the lower horizons. Soluble salts content is low at the top of the profile but is moderate to high at greater depths. EC ranges from 1 to 16 mmhos/cm, and ESP from 15 to 43.

Bageir Series.This has a light texture, with a clay content that ranges from 9 to 36 per cent. Generally it is non-saline but sodic. EC ranges from 0.5 to 6 mmhos/cm and ESP from 11 to 68. The lower values are those of the surface horizons.

The chemical analyses of four profiles representative of the four soil types (Tables 3 - 6) show the magnitude of salinity and sodicity in the area.

Recently the Ministry of Agriculture, Food, and Natural Resources in the Sudan has recognized the need for applied research in the field of soil salinity in order to permit the development of the area south of Khartoum. Thus the Soba Research Station for soil salinity studies was established.

The programme of work at this station covers a wide range of research pertaining to the field of saline and sodic soils reclamation, such as the assessment of leaching and drainage requirements, water needs of crops, use of organic and chemical ameliorants, establishment of a salt-tolerance index, economics of reclamation and management, etc.

Although only a short time has elapsed since the establishment of Soba Research Station in 1974, some useful findings have been made. In a four-year experiment chemical and organic ameliorants were compared using wheat, maize, fodder sorghum, beans, and broad-beans as indicator crops. Resulting crop Yield indicated the superiority of farmyard manure to gypsum and green manure (Karouri, 1977). Although gypsum improved the soil physical properties (increased hydraulic conductivity and reduced mechanical resistance and crust strength) and increased seedling emergence, it did not result in any substantial increase in crop yield.

In a series of experiments more than 25 crops were screened for salinity tolerance under field conditions. The yields of these crops were found to range from 20 to 70 per cent of the normal productivity of nomsaline soils in the region (Karouri, 1977). The magnitude of the reduction in yield depended on the type of crop and its level of tolerance to salts. The crops that gave fairly moderate yields were beetroot, radishes, cucumbers, onions, fodder sorghum, alfalfa, sunflowers, safflower, soybeans, etc. On the other hand, peas, beans, broad beans, lentils, lupine, watermelons and jojoba performed rather poorly.

Cultural practices and management could modify to a great extent the performance of crops grown on saline soils. It has been found that planting on ridges or beds increases seedling emergence and plant populations significantly, particularly in areas where crust formation is a him drance to seedling emergence.

Conclusion and Recommendations

Although salinity problems have long been identified in the Sudan, research in this field was neglected, sporadic and unpromising and consequently many practical problems still remain unanswered. This was attributed to the abundance of good arable land and the lack of severe salt problems in irrigated schemes. However, since the trend is towards intensively irrigated agriculture and utilization of marginal lands which are often salt-affected, there is a pressing need for a more concerted effort in the field of reclamation of saline and sodic soils, particularly in northern Sudan.

In order to maintain successful agriculture in the present irrigation schemes such as the Gezira, which contain some salts though not enough to take them out of production, and in order to prevent waste and failure in the development of newly reclaimed projects, more exact and detailed information is required on the physical and chemical properties of the soils, the effect of salts on plant growth, and on the quality of irrigation water particularly if underground water is to be used, as expected in the Wadi el-Khawi Scheme in the Dongola area.

The Institute of Environmental Studies could play a leading role in this field through co-operation with existing national institutes such as the Agricultural Research Corporation and the National Council for Research, which are also engaged in salinity studies. The role of the institute should not be to create a new infrastructure but to strengthen and mobilize existing efforts by co-operating and supplementing on-going work. Thus the available national expertise and facilities, which are rather limited, could be fully utilized and duplication of effort avoided.

The research programme to be pursued should be development-oriented and closely associated with national development strategies and social goals.

TABLE 3. Chemical Analyses of Esailat Series

TABLE 4. Chemical Analyses of Eilafun Series

TABLE 5. Chemical Analyses of Gureir Series

TABLE 6. Chemical Analyses of Bageir Series

The following objectives could serve as a guide in developing the research programme at the institute:

(a) study of the effect of salts on soil physical and chemical properties;
(b) study of the response of crops to the naturally occurring salts and the formulation of a salt-tolerance index for most of the economically important crops;
(c) determination of the leaching requirements in order to avoid harmful salt concentrations;
(d) exploration of the possibilities and limitations of the installation of an efficient drainage system;
(e) determination of the efficiency of various ameliorants for the improvement of sodic soil; and
(f) study of the economics of reclamation and management of saline and sodic soils.

Training is as important as research. The Sudan has an inadequate number of scientists and middle-level technicians in the various disciplines concerned with reclamation, improvement, and management of salt-affected soils. Since research is problem-oriented, students studying for the institute's diploma could do part of their training at the experimental stations, particularly at Soba and Hudeiba-the two main national institutes engaged in soil salinity work. Equally important is the training of middle-level staff who operate at the interface between research and the realities of field practice. Through instructional courses, in-service training, field demonstrations, and study tours, the institute could help create and strengthen such staff.

The training programme could serve as a meeting place of technology and field practices, where concepts, ideas, innovations, and research findings could be exchanged and harmonized with practical realities.

References

Balls, W.L. 1935. "Drainage in the Sudan Gezira." Emp. Cott:. Gr. Rev, 12, pp. 32-37.

Beam, W. 1911. "Soils of the Gezira: A Preliminary Note." Cairo Scient. J., 5, pp. 181-89.

Drover, D.D. 1966. "Moisture Retention in Some Soils of the Sudan." African Soils, 11, pp. 483-87.

Greene, H., and O.W. Snow, 1939. "Soil Improvement in the Sudan Gezira." J. Agric Sci., 29, pp. 1-34.

Huntings Technical Services, 1964. "Roseres Soil Survey," Report No. 5, Min. of Agric., Sudan.

Karouri, M.O.H. el-. 1967. "Annual Report of the Research Division," Min. of Agric., Sudan.
-. 1968. "Annual Report of the Research Division," Min. of Agric., Sudan.
-. 1977. "Annual Report: Soba Research Station," Agric. Research Corp., Sudan.
-, and H.J. Kaufmann, 1966. "Salinity and Alkalinity Problems in Soils of Northern Province." Proc. 10th Agric Res. Colloquim. Res. Div., Min. of Agric., Sudan.

Schultz, F.E., and N.A. de Ridder. 1974. "The Rising of Water Table of the West Nubarya Area." Nature and Resources, 10, No. 1, pp. 12 - 17.

Tahir, A., and H. Fadl, 1978. "Sahara Agricultural Company Soil Report: Land Use, Soil Conservation and Water Programme." Min. of Agric., Sudan.

Tom, O.A.1973. "Semi-Detailed Survey of South Khartoum Area and Blue Nile Provinces." Soil Survey Dept., Wad Medani, Sudan.

United Nations Conference on Desertification (UNCOD).1977. "Case Study on Desertification: Mona Reclamation Experimental Project, Pakistan." (A/CONF. 74113)

Vageler, P.,and F. Alten, 1932. Cited by F Crowther in J.D. Tothill, ea., Agriculture in Sudan, 1952.

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