soil nutrient harvesting in indigenous teras water harvesting in eastern sudan

SOIL NUTRIENT HARVESTING IN INDIGENOUSWATER HARVESTING IN EASTERN SUDAN

D. NIEMEIJER*

Department of Environmental Sciences; Group of Erosion and Soil and Water Conservation, Wageningen Agricultural University,Nieuwe Kanaal 11, 6709 PA Wageningen, The Netherlands

ABSTRACT

The indigenous water harvesting system o�ers greater production security, but its major bene®t is that it raises thenutrient-limited yield from some 150±250 kg haÿ1 to some 650 kg haÿ1 through its nutrient harvesting e�ects. Soilfertility was found to be two to four times higher inside the ®eld. Calculated nutrient-limited yields are considerablyhigher than real yields, which points at other yield-limiting factors such as labour and water availability. The example ofthe system shows that nutrient harvesting may in some cases be a viable alternative to application of mineralfertilizers. This implies that indigenous technologies can be sustainable and should be thoroughly examined before theyare complemented by external inputs. # 1998 John Wiley & Sons, Ltd.

KEY WORDS: nutrient harvesting; systems; soil fertility; Sudan (eastern); soil and water conservation

INTRODUCTION

In instances where the farming systems are faltering because they do not (or no longer) support a sustainableenvironment and livelihood, external inputs in the form of fertilizer and other technologies are oftenconsidered the only solution. However, care must be taken to ensure that su�cient attention is given to theindigenous knowledge of the local environment and agriculture, otherwise relevant local experience may getlost in a ¯ood of external recommendations and technical advice. Indigenous techniques can sometimes besurprisingly e�ective, even when at ®rst sight they contradict the most basic assumptions of modern farming.A good example is the water harvesting system as it is practised in Kassala Province in eastern Sudan(Van Dijk, 1995; Van Dijk, 1997; Niemeijer, 1998). In conventional agricultural thought it is assumed thatwhen a natural ecosystem is taken into production and becomes an agroecosystem, nutrient losses due tocrop harvest must be compensated for by fallow, manuring and/or mineral fertilizer application. The Bejaagropastoralists of Kassala's Border Area (Figure 1) do not engage in fallowing, manuring or mineralfertilizer application and still manage to cultivate individual ®elds for many decades in this arid area(long-term average yearly rainfall of 299 mm (1901±91), but only 225 mm over the last 15 years). The secretof their success lies in the nutrient harvesting e�ect of their indigenous water harvesting system.

This study presents a quantitative analysis of nutrient harvesting on a ®eld and examines the variousproduction-limiting factors.1 The system maintains soil fertility so e�ectively that other constraintsbecome more limiting to production.

THE SYSTEM IN KASSALA'S BORDER AREA

The system as it is practised in large parts of northern Sudan basically consists of a ®eld surrounded onthree sides by low earth bunds, while the upstream side functions as an inlet for surface runo� (Figure 2).

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LAND DEGRADATION & DEVELOPMENT

Land Degrad. Develop. 9, 323±330 (1998)

*Correspondence to: D. Niemeijer, Department of Environmental Sciences, Wageningen Agricultural University, Nieuwe Kanaal 11,6709 PA Wageningen, The Netherlands.

CCC 1085±3278/98/040323±08$17.50# 1998 John Wiley & Sons, Ltd.

According to Craig (1991) elementary forms of this technique date back to the Funj Kingdom (1504±1820)and later spread over large parts of (semi)arid Sudan. Early in the present century, the technique was pickedup by the agropastoral Beja that live on the piedmont plains of the Border Area administrative unit ofKassala Province (Figure 1). In this 8600 km2 area to the east of the seasonal Gash river, the systemforms the most important arable technique (40 per cent of cultivated area) of a farming system based on an

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Figure 1. The Border Area is located in eastern Sudan on the border with Eritrea.

Figure 2. Sketch of a with bunds and weeding ridges (not to scale). Note that not all weeding ridges have the same orientation.Farmers adjust the orientation to achieve an optimal distribution of water within the .teraseeee teraseeee

324 D. NIEMEIJER

# 1998 John Wiley & Sons, Ltd. LAND DEGRADATION & DEVELOPMENT, 9, 323±330 (1998)

integration of animal husbandry and various forms of runo� farming. At present, an estimated 10 000 to15 000 hectares are under , and this cultivation delivers an important contribution to the householdsubsistence of the more than 200 000 inhabitants populating the area (Van Dijk and Amhed, 1993; Kuhlman,et al., 1987).

Over decades individual Border Area farmers have adapted the system to the local environment and theirneeds, and shape, size and orientation is, especially in the northern parts of the Border Area, highlyvariable. The farmers adapt to the local hydrology through a system of bunds and weeding ridges (Figure 3)to harvest an adequate amount of water and nutrient-rich silt and detritus from overland ¯ow and from thenumerous ephemeral streams that ¯ow in an east±west direction over the piedmont plains (Niemeijer, 1998).

From measurements on several ®elds it was found that near the base contour bund an average 2±3 cm ofsediment is deposited annually (Figure 4). Near the inlet sedimentation was found to be less, but was in onecase still some 1.5 cm per year (Niemeijer, 1993).

QUANTITATIVE ANALYSIS OF NUTRIENT HARVESTING

To quantify both the annual input of nutrients and the nutrient balance in the soil, samples were taken fromone of the older ®elds. This was based on the assumption that after long-term cultivation of at least twodecades comparison of soil samples inside (cultivated) and outside the ®eld (natural conditions) woulddecisively show whether sorghum cultivation had led to soil nutrient depletion inside the ®eld or not. Thecrop is normally sown with a planting stick in the untilled soil, but in the season studied no cropping hadtaken place due to a serious drought. Stalks are usually removed from the ®eld to be used as animal fodder,or, in some cases left on the ®eld to be grazed in situ, stubbles remain on the ®eld.

A total of four pro®les were examined (Figure 5), two inside the ®eld (sorghum monocrop, locallandrace), one upstream just outside the inlet (sparse grasses) and one some 100 m downstream of the

(bare soil). In late December, samples were taken of the surface sediment (approximately 0±1 cm) andthe topsoil (10±15 cm).2 The soils are formed in slightly calcareous Quaternary colluvio-alluvial sedimentsderived from Pre-Cambrian basement complex rock-outcrops (inselbergs) as wells as the foothills of the

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Figure 3. A ®eld on the piedmont plains of eastern Sudan's Border Area.teraseeee

INDIGENOUS WATER HARVESTING SYSTEM 325TERASeeeeeee


Eritrean Highlands. The basement complex consists mainly of igneous and metamorphic rocks such asgranitic and hornblende gneisses, calc-silicates, marbles, granites, pegmatites and secondary quartz veins(Kraayenhagen, 1987; Whiteman, 1971; Van der Kevie and Buraymah, 1976; TNO, 1982). Texture rangesfrom silt loam to loam for the surface sediment and from sandy loam to sandy clay loam and silty clay loamfor the topsoil samples. Table I gives the values for pH, organic carbon, total nitrogen, total phosphorus,P-Olsen, exchangeable potassium and magnesium and the calcium carbonate equivalent for the surfacesediment.3 Both inside ®eld samples (IN1 and IN2) only di�er signi®cantly in organic carbon and totalnitrogen. This may be explained by the fact that IN2 no longer receives detritus because a new bund hasrecently been constructed upstream from this pro®le (Figure 5). Because for IN2 mineralization continueswithout new organic matter input (no detritus and no crop residues) the amount of organic carbon and totalnitrogen slowly diminishes. Even with this di�erence between IN1 and IN2, the di�erence between the inside®eld samples and the upstream sample (UP) is very marked.

With the exception of the pH all properties of the surface sediment have higher values inside the ®eld thanupstream of it. The organic matter related properties (organic carbon, total nitrogen and P-Olsen) are as

Figure 4. Detritus (in the form of excrement, grasses, and crop residues) harvested near the base contour bund of a ®eld.teraseeee

Table I. Some chemical properties of the surface sediment (0±1 cm)

Property Inside ®eld Outside Inside divided®eld by outside

IN1 IN2 UP

pH (H2O) 7.98 7.84 8.31 0.95Organic carbon (%) 1.94 1.23 0.60 2.6Total nitrogen (ppm) 1551 757 507 2.3Total phosphorus (ppm) 847 852 708 1.2P-Olsen (ppm) 11.3 11.1 3.1 3.6K-exchangeable (ppm) 407 411 325 1.3Mg-exchangeable (ppm) 484 396 352 1.3CaCO3 equivalent (%) 0 0.3 0 ±

326 D. NIEMEIJER


much as 2 to 3.5 times higher. Thus while the soil surface of the upstream pro®le (where no sedimentationoccurs) functions as a transport zone of organic detritus and sediment, the soil inside the ®eld is clearlyenriched by a yearly input of organic matter and to a lesser extent of sediment derived minerals.Manure deposited by animals grazing on the sorghum stubbles also adds to the organic matter input, but isunlikely to explain the large di�erence that was observed between the inside ®eld and the outside ®eldsamples. The pH is lower (i.e. more suitable to plant growth) inside the ®eld probably because of the higherorganic matter content and the increased water percolation caused by the water retaining bunds. Leastdi�erent is the total phosphorus content, which may be explained by the fact that the soils themselves areformed in phosphorus-rich sediments (probably in the form of apatite). Even at a depth of 1 m totalphosphorus was found to be still above 500 ppm.

While Table I illustrates the annual input of nutrients through sediment and detritus harvesting, it doesnot show the balance between nutrient input and consumption through long-term cultivation. To get thislong-term picture it is necessary to look at the topsoil samples presented in Table II. As a whole both inside®eld samples are comparable, and so are both outside ®eld samples. The di�erence between both groups ofsamples is again very prominent. Soil reaction is again lower inside the ®eld, whereas all other properties aresigni®cantly (1.5 to 4.5 times) higher inside the ®eld. This indicates that even after two decades of cultivationthe nutrient status inside the ®eld is better than outside the ®eld. The higher nutrient status within the ®eldmust thus primarily be explained by the input of organic matter and (partially) weathered minerals throughruno� harvesting.

However, yields are not impressive. For the studied ®eld, grain yields ranged from zero during the 1990drought to almost 500 kg haÿ1 in the wet year 1988, while yields around 300 kg haÿ1 are apparently thenorm for mediocre years. Considering the fact that both water and nutrients are harvested one may wonder

Figure 5. Map of the studied with bunds, ephemeral streams, and sampling locations. (Note the bund to the east (upstream) ofIN2 that was constructed just before 1986 and cuts it o� from sediment deposition).

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what is more limiting to plant growth, insu�cient water or nutrients. The following calculations give anindication of the nutrient-limited yields.

NUTRIENT-LIMITED YIELDS

For simplicity the nutrient-limited yields will only be calculated for phosphorus and nitrogen as theseelements are in low supply and show the greatest di�erence between the inside ®eld and the outside ®eldsituation. Exchangeable potassium and magnesium values are relatively high (Landon, 1991) and veryunlikely to be limiting. The same is true for calcium as the soils are slightly calcareous.

Pro®le IN1 receives on the average 1.5 cm of sediment containing 1551 ppm of total nitrogen and 11.3 ppmP-Olsen annually (Table I). Therefore, with a measured topsoil bulk density of 1�23 g cmÿ3, this 1.5 cm layercontains 286 kg of nitrogen and 2�1 kg haÿ1 of P-Olsen. If we estimate that 2 per cent nitrogen becomesavailable annually through mineralization and that 50 per cent of P-Olsen may be regarded as available toplant growth, this gives approximately 5.7 kg available nitrogen and 1.0 kg available phosphorus per hectare.With estimated crop uptake ratios of 0.026 for nitrogen and 0.005 for phosphorus, this gives an approximatesorghum grain yield of some 200 kg haÿ1 for both nitrogen and phosphorus limited yields.4 This isconsiderable for a 1.5 cm thick layer and clearly indicates how e�ective nutrient harvesting is. On the topsoilas a whole a much larger yield is attainable. If the conservative assumption is made that the 1.5±15 cm deeplayer is best described by the sample taken at a depth of 10±15 cm (see Table II), the approximate amounts ofavailable nitrogen and phosphorus are respectively 18�8 kg haÿ1 and 4�7 kg haÿ1 for this layer. For thetopsoil as a whole this gives 24�5 kg haÿ1 of nitrogen and 5�7 kg haÿ1 of phosphorus. Taking into accountestimates for leaching (1�5 kg haÿ1 of nitrogen; 0�05 kg haÿ1 of phosphorus) and gaseous losses (6�1 kg haÿ1

of nitrogen) this leaves 16�9 kg haÿ1 of nitrogen and 5�7 kg haÿ1 of phosphorus available to crop growth.5

With the mentioned uptake ratios this gives potential yields for the whole topsoil of 650 kg haÿ1 and1150 kg haÿ1, for respectively nitrogen and phosphorus limited yields.6 Since real yields are usually below500 kg haÿ1 for the studied ®eld, these rudimentary calculations indicate that it is likely that only in very wetyears a nutrient-limited yield is attained, while in other years limiting factors must probably be sought in theplant density, pests, diseases, weeds, labour or water availability.

OTHER YIELD-LIMITING FACTORS

The few measurements available on plant density and seed weight (Niemeijer, 1993; El Mosbah, et al., 1988;El Hassan and El Seed, 1989) suggest that with average plant densities around 58 000 to 62 000 plants perhectare the total number of plants is by itself not likely to be a yield-limiting factor. So, while there areindications that some farmers adapt plant densities relative to the expected soil moisture gradient within their

Table II. Some chemical properties of the topsoil (10±15 cm)

Property Inside ®eld Outside ®eld Inside dividedby outside

IN1 IN2 UP DOWN

pH (H2O) 8.06 8.04 8.41 8.64 0.94Organic carbon (%) 0.97 0.95 0.39 0.41 2.4Total nitrogen (ppm) 567 679 172 92 4.7Total phosphorus (ppm) 830 871 527 621 1.5P-Olsen (ppm) 5.7 6.5 1.4 2.3 3.3K-exchangeable (ppm) 270 434 196 102 2.4Mg-exchangeable (ppm) 384 480 360 168 1.6CaCO3 equivalent (%) 0.3 0.3 0 0 ±

328 D. NIEMEIJER


®eld (Niemeijer, 1993), it seems unlikely that, for the purpose of risk aversion, they reduce the total plantdensity to such an extent that this limits yields in wet years.

As far as pests and diseases are concerned quantitative data is completely lacking, but there can be nodoubt that pests like locusts, grasshoppers, stalkborers, and birds, and diseases like loose smut a�ect yieldsnegatively. Weeds are another problem, and farmers spend considerable time on weeding up to three times aseason. The total time spent on weeding can amount up to 17 to 28 person days per hectare depending on theenvironmental conditions, water availability and yield expectancy (Van Dam and Houtkamp, 1992).

In addition farmers have several plots usually in di�erent farming areas and travel times are on average asmuch as four hours a day. Labour availability is clearly a production-limiting factor, even more so because(for religious reasons) women do not participate in arable farming and because many people also engage inlivestock keeping and o�-farm activities (Van Dam and Houtkamp, 1992; Niemeijer, 1993; Van Dijk, 1995).Finally, water availability is likely to be a yield-limiting factor in most years (recently 8 out of 10 years werebelow 250 mm). In very wet years, yields are appreciably higher than during normal or dry years when a

may receive only a few showers and ephemeral stream or overland ¯ows during a whole rainy season. Inexceptional cases, a single shower and runo� event can be su�cient for a small grain yield. Apparently, evenwith the water harvesting system, yields are still limited by the amount of rainfall in most years. Thesystem does, however, o�er a much greater security since pure rainfed farming would not give a yield at all in8 out of 10 years. Its real bene®t, however, is that it not only harvests water, but also nutrients, because,without nutrient harvesting, yields would be limited to some 150 to 250 kg haÿ1.7

CONCLUSION

The example of the system as practised on the piedmont plains of Kassala's Border Area, clearly showsthat (ecologically) sustainable farming systems exist in Africa at present. Especially in (semi)arid zonesfarmers tend to cultivate their ®elds in areas that in one way or another receive some additional overland¯ow. This ¯ow not only helps their crops through dry spells, but also contributes signi®cantly to soil fertilitythrough the deposition of sediment and organic matter. Unfortunately, indigenous nutrient harvesting is alittle-studied subject, and there is a lack of quantitative data to assess its importance for rainfed farming. Acomparable indigenous nutrient and water harvesting system by the Papago Indians was studied by Nabhan(1983; 1984). In the North American Sonora, Nabhan compared soil samples in long-term cultivated siteswith samples from uncultivated sites in similar environmental positions and found that there was nosigni®cant di�erence. He concludes that evidence is lacking that long periods of (Papago-style) cultivationhave depleted soil nutrients or altered the organic matter status.

The examples of the Beja and Papago systems show that, even in marginal environments, application ofmineral fertilizer is not always necessary to maintain soil fertility. Not only would disregard of indigenousforms of nutrient management and straightforward application of recommended standard farm manage-ment practices be a waste, but it might also disturb the already fragile balance between inputs and outputsand may seriously a�ect the household livelihood. For households balancing on the verge of existence (as isthe case for many African farmers) adjustment to `modern' management practices easily lead to a downwardspiral of indebtedness.

External recommendations should not replace indigenous practices, but be complementary to thosepractices, and should be based on a thorough understanding of the local environment and farming systems.

ACKNOWLEDGEMENTS

The author wishes to express gratitude to the following organizations and individuals: Kassala Departmentof Soil Conservation, Land Use and Water Programming; Kassala Area Development Activities; JohanBerkhout; Anita van Dam; Ton Dietz; Johan van Dijk; Hashim Mohamed El Hassan; John Houtkamp;Valentina Mazzucato; Rudo Niemeijer; Jan Sevink; and Leo Stroosnijder.

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REFERENCES

Craig, G. M. (ed.) 1991. The Agriculture of the Sudan, Oxford University Press, Oxford.El Hassan, A. M. and El Seed, W. A. 1989. Crop Assessment in Hedadeib Area, Ministry of Agriculture and Natural Resources,

Department of Soil Conservation, Land Use and Water Programming, Kassala.El Mosbah, A. M., El Mubarak, O. and El Hassan, A. M. 1988. Crop Assessment in Hedadeib Area. Ministry of Agriculture and Natural

Resources, Department of Soil Conservation, Land Use and Water Programming, Kassala.Janssen, B. H., Guiking, F. C. T., Van der Eijk, D., Smaling, E. M. A., Wolf, J. and Van Reuler, H. 1990. À system for quantitative

evaluation of the fertility of tropical soils (QUEFTS)', Geoderma, 46, 299±318.Kraayenhagen, J. 1987. Kassala Area Development Activities. Foothill Rehabilitation through Waterspreading, KADA/DHV, Kassala.Kuhlman, T., Ibrahim, S. and Kok, W. 1987. Refugees and Regional Development. Final Report of the Research Project `Èritreans in

Kassala'', Free University, Centre for Development Cooperation Services, Amsterdam, and University of Khartoum, DevelopmentStudies and Research Centre.

Landon, J. R. (ed.) 1991. Booker Tropical Soil Manual: A Handbook for Soil Survey and Agricultural Land Evaluation in the Tropics andSubtropics, Longman, Harlow.

Nabhan, G. P. 1983. Papago ®elds: Arid lands ethnobotany and agricultural ecology, PhD thesis, University of Arizona.Nabhan, G. P. 1984. `Soil fertility renewal and water harvesting in Sonoran desert agriculture: the Papago example', Arid Lands

Newsletter, 20, 21±28.Niemeijer, D. 1993. Indigenous Runo� Farming in a Changing Environment: The Case of Kassala's Border Area, Sudan Landscape and

Environmental Research Group, University of Amsterdam and Department of Irrigation and Soil and Water Conservation,Wageningen Agricultural University.

Niemeijer, D. 1998. Environmental Dynamics, Adaptation, and Experimentation in Indigenous Sudanese Water Harvesting, in D. M.Warren, S. Fujisaka and G. Prain (eds.) Biological and Cultural Diversity: The Role of Indigenous Agricultural Experimentation inDevelopment, Intermediate Technology Publications, London.

TNO 1982. Water Resources Assessment Kassala Gash-Basin, Final Technical Report, TNO, Delft.Van Dam, A. J. and Houtkamp, J. A. 1992. Livelihood and runo� farming; the case of the Border Area village of Hafarat, Eastern

Region, Sudan, MA thesis, University of Amsterdam.Van der Kevie, W. and Buraymah, I. M. 1976. Exploratory Soil Survey of Kassala Province; a Study of Physiography, Soils and

Agricultural Potential, Soil Survey Report (FAO) No. 73, unpublished, Soil Survey Department, Wad Medani.Van der Pol, F. 1992. Soil mining: An Unseen Contributor to Farm Income in Southern Mali, Royal Tropical Institute, Amsterdam.Van Dijk, J. A. 1995. Taking the Waters: Soil and Water Conservation among Settling Beja Nomads in Eastern Sudan, Avebury,

Aldershot.Van Dijk, J. A. 1997. Ìndigenous soil and water conservation by teras in eastern Sudan', Land Degradation & Development, 8, 17±26.Van Dijk, J. A. and Amhed, M. H. 1993. Opportunities for Expanding Water Harvesting in Sub-Saharan Africa: The Case of the Teras of

Kassala, International Institute for Environment and Development, London.Whiteman, A. J. 1971. The Geology of the Sudan Republic, Oxford University Press, Oxford.

ENDNOTES

1. Fieldwork took place in 1990 within the framework of the Water Spreading Research Kassala (WARK) programme of the SudaneseNational Council for Research.

2. For the downstream pro®le surface sediment was not sampled, while the topsoil was sampled at 12±17 cm.3. Mycorrhizal and rhizobial status of the soils was not examined. Micronutrients were not determined, but (most) are likely to be

available because the soils contain an abundance of (partially) weathered minerals.4. Parameter estimates are based on Van der Pol (1992) and Janssen, et al. (1990).5. Leaching and gaseous losses: For nitrogen respectively 1�5 kg haÿ1 and 25 per cent of total available nitrogen; for phosphorus

0�05 kg haÿ1 and nil; based on Van der Pol (1992). Losses through erosion are not considered because it is an accumulatory area.6. Even if we consider the problem of pH-related phosphorus ®xation, nitrogen is still likely to be the most limiting factor, as the

nitrogen-limited yield was estimated at practically half of the phosphorus-limited estimate.7. Calculations as before, but based on outside samples.teraseeee

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soil nutrient harvesting in indigenous teras water harvesting in eastern sudan

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