the impact of indigenous soil and water conservation practices on soil productivity: examples from...

THE IMPACT OF INDIGENOUS SOIL AND WATERCONSERVATION PRACTICES ON SOIL PRODUCTIVITY:EXAMPLES FROM KENYA, TANZANIA AND UGANDA

J. ELLIS-JONES1* AND A. TENGBERG2

1Silsoe Research Institute, Wrest Park, Silsoe, Bedfordshire, MK45 4HS, UK2GoÈteborg University, Department of Earth Sciences, Physical Geography, Box 460, SE-405 30 GoÈteborg, Sweden

Received 15 October 1998; Accepted 15 January 1999

ABSTRACT

Farmers in many parts of Africa use indigenous soil and water conserving (ISWC) practices as an integral part of theirfarming systems. Farmers have developed such methods which have maintained productivity and contributed to long-term sustainability, while introduced measures have often been rejected or simply failed to achieve their technicalobjectives.This paper examines the strengths and weaknesses of some ISWC practices in Kenya, Uganda and Tanzania. An

evaluation has been carried out based on farmers' evaluation criteria as well as criteria identi®ed by researchers. Farmers'criteria are often based on the quality of their natural resources ( farmers are more likely to conserve those soils that willgive the highest return on their investment), the resource level of the household (particularly income levels and labouravailability), cropping intensity as well as cultural traditions related to age, education and gender. Of great importance,however, is the need to maintain or increase soil productivity. Methods that conserve moisture, reduce soil erosion,maintain soil fertility and increase productivity, which are socially acceptable and economically viable, are those whichfarmers favour. ISWC practices often have these qualities but farmers have not always been able to adjust the techniquesto rapid changes in farming systems and increasing intensity of land use.Soil productivity and economic modelling indicate that although yields declines are lower when traditional

technologies are used, long-term productivity remains a problem. There is an urgent need to work closely with farmers toimprove and develop traditional SWC techniques. Copyright # 2000 John Wiley & Sons, Ltd.

KEY WORDS: indigenous soil and water conservation; soil productivity; participatory research and development; subSaharan Africa

INTRODUCTION

To sustain food production in smallholder farming in developing countries, soil and water conservation(SWC) has been strongly promoted in almost every developing country over the last 50 years. As aconsequence, a large number of conservation technologies have been developed and many soil conservationprojects and programmes have been implemented. Despite this, land degradation remains a major threat toagricultural production in many developing countries. Although there is an awareness that the underlyingcauses of land degradation are often social and economic in nature (Blaikie, 1985; Blaikie and Brook®eld,1987; Biot, et al., 1995), soil erosion has conventionally been perceived as the chief biophysical cause ofdeclining productivity. Yet, the limited e�ectiveness and low adoption of widely promoted antierosionmeasures make it necessary to reconsider the causes of productivity decline as well as to consider social andeconomic constraints of improving SWC practices.

Under increasing population pressure, rotational fallow periods which promote soil recuperation areincreasingly being shortened. Measures such as conservation banks, terraces and ditches, widely introduced

LAND DEGRADATION & DEVELOPMENT

Land Degrad. Develop. 11: 19±36 (2000)

Copyright # 2000 John Wiley & Sons, Ltd.

*Correspondence to: J. Ellis-Jones, Silsoe Research Institute, Wrest Park, Silsoe, Bedfordshire, MK45 4HS, UK; e-mail: [email protected]

Contract/grant sponsor: Department for International Development, UK.Contract/grant sponsor: Wenner-Gren Foundation.

to farmers using top-down paternalistic approaches, often under coercion or with subsidized programmes,often had little lasting e�ect and land productivity has continued to decline. Reasons for this identi®ed byShaxson, et al. (1997) include:

. The often gradual nature of productivity decline, making it di�cult for farmers to identify the cause.

. Pressure on farmers to secure food production in the short term.

. Physical measures may not stop soil erosion because of poor construction or maintenance.

. Physical structures do not necessarily, on their own, raise yields.

. A proportion of the cropping area may be taken out of production. When land is scarce, this may beunacceptable or have a high cost.

When farmers are faced with declining productivity, they are able to select from a range of technologies toreverse this decline. This includes fertilizers, pesticides and improved seeds as well as SWC technologies,which are inherently di�erent from crop improvement technologies. Farmers, particularly those with leastresources, would expect to see bene®ts within a cropping season from such investments. However, SWCmeasures usually involve signi®cant initial and ongoing investment in both cash and labour with bene®tsbeing realized in the longer term.

This paper examines a number of indigenous soil and water conservation (ISWC) technologies that haveevolved as an integral part of farmers' land management practices in three areas of subSaharan Africa:semiarid Kenya and hillside areas of Tanzania and Uganda. We are adopting the dynamic interpretationof ISWC given in a recent inventory of ISWC (Reij, et al., 1996) where ISWC is seen as the result ofaccumulative responses to a range of in¯uences over time. ISWC can thus encompass traditional SWCtechniques that have developed without external interventions as for example the system in Tanzania,as well as locally adapted/modi®ed introduced technologies such as the in Kenya (see page 24 forexplanation of these terms). The impact of ISWC on soil productivity is investigated, returns on investmentare calculated and socioeconomic characteristics/constraints of the di�erent techniques are discussed.

METHODOLOGY

The research has endeavoured to build on a new approach to land management, the principal of which is thatdevelopment should be initiated from building on what farmers are already doing. As such it has been basedon three basic premises (Critchley and Ellis-Jones, 1998):

(1) To build on farmers' traditional practices and local knowledge.(2) To focus on moisture conservation and fertility enhancement for crop production, with conservation of

soil being achieved simultaneously.(3) To involve farmers throughout the process of identifying, planning, implementation, monitoring,

evaluation and dissemination of results.

Local knowledge is an important resource in development and according to a recent typology (Blaikie,et al., 1997). Local knowledge in the context of the present study can be described as knowledge mutuallyconstructed between local people and external agents. It is perceived as having intrinsic value, thoughsometimes it may be less suitable than scienti®c knowledge in new contexts. Local knowledge is developed tocreate a product which is greater than the sum of the contributions from either system. The main mechanismpromoting this synergy is negotiation between all parties throughout the entire research process.

Productivity of Soil and Water Conservation Systems

In order to assist in promoting viable and socially acceptable SWC systems it is important to understand howdi�erent systems a�ect productivity. There are many ways of estimating erosion/conservation±productivityrelations, which can range from estimates based on farmers' own perception of productivity changes tosophisticated biophysical models of crop, soil and water interactions, such as USLE or EUROSEM. Soil

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20 J. ELLIS-JONES AND A. TENGBERG

Copyright # 2000 John Wiley & Sons, Ltd. LAND DEGRADATION & DEVELOPMENT, 11: 19±36 (2000)

erosion±productivity relationships for tropical soils indicate a strongly curvilinear yield decline with erosionhaving large impacts for initial soil losses. By using the relationships for yield decline with cumulative soilloss for di�erent levels of management, it is possible to predict yield changes over time (Stocking and Peake,1996; Tengberg, et al., 1998a). However, economists have recently questioned the usefulness of complexmodels because of data availability limitations in many developing countries (De Graa�, 1995; Bojo, 1991).They have suggested a more straightforward cost±bene®t approach for evaluating costs and bene®ts,utilising input from complex models, where available, to validate local experimentation, experience andperceptions of farmers and extension workers.

The impacts of erosion on productivity have been researched for decades, although not necessarily in a formsuitable for economic analysis. Erosion yield e�ects can be very location speci®c and, if applied to other areas,resultsmay be distorted as we see from these three case studies. Themost critical question is not necessarily thecost of soil conservation, but rather, whether the long-term bene®ts of reduced soil erosion or land degrada-tion make the costs of conservation worth bearing (Pagiola, 1992). A number of techniques can be used,ranging from simple partial budgets (PBs) and break-even analysis to a more comprehensive cost±bene®tanalysis, using discounted cash ¯ow investment appraisal techniques. PBs are best utilized for conservationmeasures that are repeated on an annual basis and a constant crop productivity response is likely. They cannotbe used where SWC requires an initial investment and annual maintenance and where productivity is expectedto increase over a number of years. In this case cost-bene®t analysis is more suitable, and is best used tocompare di�erent measures. It requires quanti®cation of costs and bene®ts over a period of time, which canthen be discounted to determine net present values, internal rates of return and/or bene®t: cost ratios (B :Cratios).

Cost±bene®t analysis provides a logical framework for collecting, presenting and analysing informationon costs and potential bene®ts. It is a common tool for project appraisals and is designed to assist decisionmakers in choosing alternative courses for action and allocating scarce resources (Gittinger, 1984). The greatadvantage with cost±bene®t analysis is that implicit judgements are made explicit and subject to analysis,and as such is a decision tool that lends itself to developing into a participatory process, where stakeholderscan assist in the de®nition of options and their likely impacts. Cost±bene®t analysis gives no pretence atoptimization. It indicates whether it would be in farmers' interests to adopt a particular measure in theirspeci®ed circumstances. Those elements having greatest impact on the costs and bene®ts can be subjected toa sensitivity analysis.

The costs and bene®ts that need to be considered in an evaluation of conservation technologies include thefollowing factors.

Establishment and maintenance costsAll costs in the ®rst 12 months have been regarded as establishment costs. Costs thereafter have beenregarded as annual or maintenance costs. Purchased materials and hired labour have been valued at cost,with farm and household supplied inputs being valued at their opportunity cost.

Productivity lost on land used by SWCAn estimate of land lost to conservation based on ®eld estimates (Altshul and Okoba, 1997; Miiro, et al.,unpublished) shows considerable variation between farmers, re¯ecting the ¯exibility and dynamic nature ofISWCs. Based on this we have assumed a distance between barriers of 15 m, often being the closest thatfarmers are willing to accept where the width is over 0.5 m. In practice land lost is likely to vary with slope.Productivity lost is the loss in gross margin on that area, when no SWC is practised.

Soil productivity gainsGains in productivity have been based on the di�erence in productivity with conservation and withoutconservation, as determined by either productivity models, farmers' estimates, extension workers' estimatesor yield data from research trials or a combination of all four. Regional statistics, although often regarded asunreliable were consulted where available to provide some con®rmation of the base level of yields.

SOIL AND WATER CONSERVATION PRACTICES IN SUBSAHARAN AFRICA 21


In semiarid Mbeere District, Kenya, with highly variable rainfall, long periods of monitoring of factorsa�ecting soil productivity is required before any reliable scenarios using soil productivity models can beconstructed. In the Kenyan case we therefore focus on farmers' perception of changes in soil productivity aswell as yields achieved over four seasons on-station trials (Okoba et al., 1998a).

For Mbinga District, Tanzania, we compare results from scenarios of soil productivity changes developedfrom a simple soil loss±productivity model with the perception of di�erent stakeholders in the area. Evidencefrom soil erosion±productivity experiments in southern Brazil (Stocking, 1996), indicates that soil erosionhas a considerable impact on the productivity of Ferralsols, the same soil type as that found in MbingaDistrict. Decreasing yields, reduction of soil organic matter, increasing soil acidity and free aluminium, andP-®xation are the most common problems following erosion (Tengberg, et al., 1998a). However, total soilloss is the single most important factor in explaining productivity changes for these soils. An equation formaize yield decline with cumulative soil loss was established for Ferralsols and Cambisols in southern Brazil.The general form of the relationship between soil erosion and yield decline has proved to be similar indi�erent farming systems (Tengberg and Stocking, 1997), although the empirically derived constants need tobe calibrated to a particular situation. The equation for maize yield decline with cumulative soil loss forFerralsols and Cambisols in southern Brazil is:

y � 5805 ÿ 802 Ln *�x� �1�

where y is yield in kg haÿ1 and x is cumulative soil loss in tonne haÿ1 (Tenngberg, et al., 1998a). However,although the above relationship pertains to no fertilized conditions, the potential productivity of the farmingsystems in Brazil is much higher than in the Tanzania case ± around 5800 kg haÿ1 of maize compared to1500 kg haÿ1 for low input systems in Tanzania (Ellis-Jones, et al., 1996). Equation (1) was thereforecalibrated to Tanzanian levels of crop productivity to:

y � 2202 ÿ 802* Ln �x� �2�

To model erosion induced productivity changes over time, information on erosion rates for the siteconditions investigated is also needed. In the present study we used seasonal soil loss for maize cropping,respectively, for slopes of 20 per cent and 45 per cent measured in the higher rainfall part of the area (Thadei,et al., 1997). These slopes are typical of those found in the area. Measured annual soil losses on di�erentslopes and with di�erent conservation structures are given later in this paper in Figure 1. We also used yieldresults over a two-year period from on-farm trials (Dihenga, et al., 1998).

For beans also, we assume a logarithmic yield decline with erosion. Potential bean yield for low inputfarming systems for the SWC system with least erosion, , is 300 kg haÿ1 and soil loss in the ®rst year on20 per cent slopes is equal to 2.4 t haÿ1. For ridges, similarly, the yield is 200 kg haÿ1 and soil loss 7.3 t haÿ1

(Thadei, et al., 1997). We thus obtain the following relationship:

Y � 402 ÿ 109* Ln �x� �3�

where Y is yield and x cumulative soil loss. Although the productivity levels in terms of kilograms per hectareare lower for beans than for maize, the relative drop in productivity is less sharp than for maize. This is notsurprising as it is well known that leguminous crops are less sensitive to the impact of erosion than cereals(Stocking, 1996).

In Kabale District, Uganda, we used the same modelling approach. The relationship between soil loss andyield decline for Acrisols has a negative exponential form (Tengberg and Stocking, 1997). The generalrelationship was calibrated for sorghum yields (Y) in Kabale District thus giving the following equation:

Y � 4060* eÿ0�003x �4�

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Annual soil loss rates (x) for di�erent slopes and degrees of cover were obtained from Biteete-Tukahirwa(1995), and are given later in this paper in Figure 3. Rainfall variability in this area is not large (Biteete-Tikahirwa, 1995). However, we also use farmers' perceptions of productivity decline and yield results over atwo year period from on-farm trials (Briggs, et al., 1998).

CHARACTERISTICS OF THE STUDY AREAS

Di�erent ISWC techniques have been studied in the three areas, with its own contrasting biophysical andsocioeconomic environments (Table I). In semiarid Mbeere District, Kenya, the most striking characteristicis the environmental and socioeconomic diversity, one of the underlying factors being the high rainfallvariability, leading to high variability in crop production. In both Mbinga, Tanzania and Kamwezi,Uganda, the climate is subhumid and, although rainfall variability can be a problem, the priority problem inboth areas is a long-term decline in soil productivity. These varying characteristics also result in di�erences inlivelihoods and farming strategies between the areas. In Mbeere, risk management is the main priority witho�-farm income opportunities increasingly seen as the best way to o�set food and income shortages fromfarming. In contrast, cultivation of cash crops has become increasingly important in Mbinga and Kamwezi,where the maintenance of long-term soil productivity is the key natural resource problem.

This paper concentrates primarily on those measures that farmers are keen to have investigated.

Kenya

Research has centred on Mbeere ( formerly Lower Embu) District, and the following ISWC and introducedtechnologies, being primarily mechanical measures, were investigated.

Trash lines, the use of which dates back to the early 20th century (Altshul and Okoba, 1997), are usuallymade from millet or sorghum residues that are laid across the slope and are left to decompose and new trashlines are laid annually between the originals. Their e�ectiveness is dependant on their composition and care

Table I. Characteristics of the study areas

Characteristic Kenya Mbeere Tanzania Mbinga Uganda Kamwezi

EcosystemRainfall

Semiarid730 mm (bimodal)

Subtropical hillsides1000±1600 mm(unimodal)

Subhumid hillsides750 mm (bimodal)

Altitude 700±1000 m 1200±1900 m 1700 mSoils Ferrasols, Luvisols

and CambisolsFerralsols Ferrasols/Acrisols

Livelihoods Cultivation of foodcrops and o�-farmactivities

Cultivation of cashcrops (co�ee) and foodcrops

Cultivation of foodcrops with surplusesmarketed

Main soil and waterconservationpractices

Trash lines, stonebunds, log lines,

andcombinations ofthese techniques

pits and(ridge) systems with orwithout incorporationof organic matter onannual crops. Terracesand mulching on co�ee

Trash lines on annualcrops, mulching andrainwater harvestingon bananas

Priority naturalresource problem

High rainfallvariability andunpredictable yields

Long-term decline insoil productivity

Long-term decline insoil productivity

Farming strategy Risk management Sustaining soilproductivity

Sustaining soilproductivity

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with which they are established (Critchley, et al., 1994). Provided they are well established, they are capableof withstanding deluges of rain, slowing run o� and letting excess water percolate through the trash andtrapping soil. Trash lines can be ®xed or mobile, large or small, depending on material availability. About45 per cent of farmers in the study area are using trash lines.

Stone bunds, which are used by approximately 30 per cent of farmers, are con®ned to areas where stonesare in abundant supply, often where stone has to be cleared to permit cultivation. Construction quality anddimensions vary considerably from simple stone lines to low walls. Bunds are semipermeable allowingwater to pass through and trapping soil, which can lead to the formation of substantial terraces over time.Although labour intensive in construction, labour can be spread through and over a number of years as andwhen it is available.

Log lines, used by 10 per cent of farmers, are largely limited to newly opened lands where the timber isunsuitable for charcoal production. As with trash lines and stone bunds, log lines result from makingrational conservation-e�ective use of a resource that is available in the ®eld and are often followed by othermeasures such as grass strips, trash lines and even . Their use, however, is declining as new landbecomes less available, we have therefore excluded them from the analysis.

terraces, initially enforced on a wide scale during the colonial era and more recently encouragedthrough food-for-work programmes, are constructed by digging ditches and throwing soil upslope. Relativelyfew have been constructed without a carrot-or-stick approach, but these terraces are nevertheless found in 75per cent of farms in the area. They are labour intensive to construct and require tools, often unavailable inmany households, and can, if not adequately maintained or correctly constructed, result in worse erosionthan having no protection measures. Many farmers in fact use combinations of these techniques.

Tanzania

Attention was focused on the ISWC systems of the Matengo Highlands, Mbinga District in southwestTanzania, in particular the pit and ridge systems used in annual food crops.

or pit systems are the most prevalent land preparation system in the area on slopes from 10±60 percent. They have been in use for some 200 years (Stenhouse, 1944), but largely restricted to the MatengoHighlands and neighbouring areas. E�orts to extend the system elsewhere in Tanzania during the 1950s werenot successful (DALDO, personal communication). are formed towards the end of the rainy seasonin March and April when grass is slashed after a fallow period, laid as a matrix, some two metres square andthen covered with soil dug from the centre of each square. There is a high labour requirement with genderdi�erentiation by task with much of the burden falling on women. Annual crops are planted on the bundsand weeds are composted in the pit. The pits help to maintain soil fertility, reducing soil erosion andincreasing soil moisture availability that is especially important at the end of the rainy season when a beancrop is planted. New bunds are formed in the pit area, without the need to be aligned on the contour.can be reformed every two years after two crops with a six-month fallow over the second rainy season.Traditionally up to six years fallow was the norm, as they do require a lengthy fallow to maintain soilfertility, but this has reduced to one year and less as land pressures increase.

or ridge systems are of two distinct kinds, those that incorporate organic matter and are thereforesimilar to the bunds surrounding pits of and those which do not, where organic matter is burnt beforeconstruction. Ridges are constructed along the contour with reconstruction occurring every year by splittingthe ridges after ®lling the furrow with crop residue or cut grass and weeds, where such organic matter has notbeen burnt. They are less labour intensive than with men and women working together in theirconstruction. As a result women, especially non- and those in non-polygamous relationships,prefer ridge systems. They can be as e�ective as on gentler slopes, especially where the organic matteris incorporated, and they can be more easily mechanized. They do, however, need to be constructed annuallyand are less e�ective on steep slopes, especially when they are not aligned exactly on the contour. Due to theincreasing costs and availability of labour, burning of organic matter has become increasingly commona�ecting longer term productivity.

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As the productivity of co�ee has improved through better management practices, better marketing andsupply of inputs, supported by steadily increasing prices, the area under co�ee has expanded. This hasincreased the labour demand, as well as increasing incomes. Fertilizer sales are rising with some now evenbeing applies to . Application of fertilizer after burning is seen as labour-saving strategy (Ellis-Jones,et al., 1996). As fertilizer use has increased, the incidence of burning has also increased (DALDO, personalcommunication).

Uganda

The Kabale District in southwest Uganda has a wealth of ISWC (Miiro, et al., unpublished) of which two,trash lines and mulching of bananas, were investigated in the drier parts in Kamwezi subcounty. Othersincluded mixed cereal, potato, legume rotations, rainwater harvesting and ditches. Only trash lines areinvestigated at this stage as the complexities of the mulching systems are still being investigated.

Trash lines are used in ®elds of annual crops on hillsides typically from 20±30 per cent. They areconstructed from weeds and crop residues (mostly the roots of maize, millet and sorghum) and serve to retainsoil and enhance fertility and soil moisture. They generally last for two seasons before decomposing, beingspread and incorporated into the soil. They e�ectively constitute mobile compost strips. However, there aremixed views among farmers as to the size of trash lines and the best times to break or move them. Trash linesat farm boundaries are often left as permanent structures with resulting high earth embankments whichultimately collapse, sometimes this is done on purpose as a soil fertility strategy.

Mulching has been used since pre-colonial times for bananas (Simmonds, 1970) serving to controlmoisture and reduce weeding requirements. Materials used comprise not only banana leaves and haulms, butalso material transported from the hillside cropping areas, notably sorghum and bean stover. The practiceallows banana production with considerably lower rainfall than normally required for bananas.

Other forms of rainwater harvesting using run o� from roads and paths is also widely practised in bananasproduction. We have excluded SWC in bananas from this analysis because of the lack of accurate long-termproductivity data.

Understanding the advantages and disadvantages of ISWC can assist in identifying the criteriadetermining farmers' use of di�erent soil and water conservation practices as well as identifying scienti®cproperties of techniques that are economically viable and socially acceptable. Most ISWC practices arecharacterized by their multiple functions, ¯exibility, spreading of labour requirements for construction andmaintenance, and by gender di�erentiation of labour input (Reij, et al., 1996), as well as being an integralcomponent of land management and cropping systems that minimize loss of land. Introduced measures areoften speci®c to a single requirement requiring new management practices. The awareness of farmers aboutconservation of soil moisture, soil fertility and soil itself is high in all three areas with ISWC measures beingmore widely used than introduced technologies. in Kenyua and ditches and rainwater harvestingtechniques in Uganda are the main introduced technologies which have been adopted.

Problems associated with ISWC include decreasing availability of organic material especially in periods ofdrought or through competing demands for livestock fodder or reduced biomass due to declining soilfertility. In addition pest and disease problems may be encouraged and ISWCmay, as in the case of inTanzania, rely on fallow periods.

RESULTS AND DISCUSSION

Costs of Conservation

Detailed costing of each SWC were obtained in each country (Briggs, et al., 1998; Dihenga, et al., 1998;Okoba, et al., 1998b). These are summarized in Table II.

Permanent structures such as stone bunds and , usually have higher establishment but lowermaintenance costs than other non-permanent structures. This high cost in the initial stage and the uncertainbene®ts discourage farmers from adoption. If maintenance is inadequate, soil erosion can occur at anaccelerated rate with loss of the initial investment. Often ISWC technologies have higher annual costs, but

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lower than the initial establishment costs than permanent SWC. In addition, they do not have the problem ofincreasing soil erosion if inadequately constructed.

Productivity Changes

Mbeere District, KenyaHunt (unpublished paper, 1996) compared total grain yield from millet dominated plots without SWC atvarious locations in Mbeere for the short rains season 1972±93 and 1992±93. The results show a mean yielddecline of about 30 per cent for this 20-year period. We will assume that, without any SWC, yields willdecline approximately 1.5 per cent per year, being equivalent to 30 per cent decline over 20 years. This ismore or less in line with the impression farmers give in interviews (Okoba, et al., 1998a). For the subsequenteconomic analysis, we use yields achieved in on-station trials over four seasons, acknowledging that thesetend to be indicative of better resourced farmers on higher fertility soils (Table III). On-station trials werecarried out on land cleared, after a fallow period on one of the main soil groups with low inherent soilfertility. Farmers from all resource categories were involved in trial design, which was based on a maize±cowpea intercropped system commonly used in the area.

These trials show the extreme variation that is characteristic of semiarid areas with highly variable grossmargins (Table IV). However, over the four seasons we have established a consistent base-line. Inputs arebased on those used on-station, which again are indicative of those used by better resourced farmers (Okoba,et al., 1998a).

There was little di�erence in gross margins for the alternative SWC in the ®rst season, but di�erencesincreased substantially in the second and subsequent seasons. Over the four seasons (two years) the highestgross margins were achieved using trash lines (except small trash lines), followed by large stone bunds and

. These gross margins have not taken into account the establishment or maintenance costs ofSWC. These are accounted for in the cost±bene®t analysis investment appraisal determining net presentvalues (Table V).

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Table II. Resource requirements for conservation technologies

Country Conservation technology Lifespan(years)

Landlost(%)

Labour requirements(days per ha)

Cost(US$ per ha)

Construction Annual orMaintenance

Construction Annual orMaintenance

Kenya Large ®xed trash lines 1±2 7 ± 10±20 ± 10±20Large movable trash lines 1±2 7 ± 10±20 ± 10±20Small trash lines 1±2 4 ± 10±20 ± 10±20Small double-spaced trashlines

1±2 8 ± 10±20 ± 10±20

Large stone bunds 10 7 62 12 62 12Small stone bunds 10 5 36 12 36 12

10 13 54 18 54 18

Tanzania 2 0 45±55 15±20 45±55 15±20� O 1 0 ± 25±35 ± 25±30ÿO 1 0 ± 8±10 ± 8±10

Uganda Trash lines-annual crops 1±2 10 ± 25±35 ± 25±35

Notes: 1. Labour costs have been based on US$1.00 per day, although these vary from US$0.75ÿ $1.25 per day in thedi�erent case studies.2. Crop residues and/or weeds used in trash lines comprise up to 3000 kg haÿ1, depending on availability.3. For , use of additional tools is required, particularly picks and shovels.4. � 0 � with organic matter incorporated; ÿ 0 � without organic matterincorporated.

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Table III. Yields achieved on station over four cropping seasons (kg haÿ1), Mbeere District, Kenya.*

Conservation technology Nov. 1995 April 1996 Nov. 1996 April 1997

Sorghum Maize Cow peas Maize Cow peas Maize Cow peas

Grain Stover Grain Stover Grain Stover Grain Stover Grain Stover Grain Stover Grain Stover

Large ®xed trash lines 186 120 404 2031 1823 1479 0 875 360 324 522 1940 705 851Large movable trash lines 179 148 641 1528 1740 1435 0 690 334 345 561 2151 474 546Small trash lines 262 371 425 2291 1448 1228 0 858 373 388 414 1584 488 594Small double-spaced trash lines 225 101 679 1729 1818 1397 0 739 349 335 571 1876 524 640Large stone bunds 330 155 395 1290 1646 1070 0 705 392 384 570 2037 522 616Small stone bunds 167 98 221 1284 1486 1261 0 640 312 307 414 1942 537 569

238 151 433 1051 1670 1550 0 734 382 365 518 2164 537 577Nothing 253 93 295 1051 1319 1065 0 503 306 249 367 2146 526 482

*Average yields obtained from three a randomized block design, each plot being 45 m long and 6 m wide.Source: Okoba, et al., 1997.

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Net present values have been calculated using a 20 per cent discount rate, re¯ecting farmers' timepreferences, over a ten-year period, at four scenarios of labour and trash values. A productivity declinewithout conservation of 1.5 per cent per year and no increase (or decrease) with conservation, based on theaverage gross-margin over the four seasons has been assumed. In the scenarios examined, the use of largestone bunds (assuming no transport costs to ®eld) o�ers best returns, except where crop residues and labourare low cost. Where stones are unavailable, either trash lines (particularly large ®xed trash lines and smalldouble-spaced trash lines) or have the highest net present values, dependant on the cost of trashand labour. When crop residues have low opportunity cost, trash lines are the best SWC option. However, asthe opportunity cost of crop residues increase, as is likely in times of drought or increasing demand bylivestock, become increasingly attractive. If the opportunity cost of labour can be reduced byundertaking work out of peak periods, become more attractive. If, however, the cost of labourincreases, as is likely as o�-farm employment increases, the use of trash lines becomes more attractive. Whenboth crop residues and labour are low cost, trash lines are the most attractive option. Where both are highcost, are most attractive.

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Table IV. Gross margins for alternative SWC over four seasons (US$ per ha), Mbeere District, Kenya

Conservation technology Nov. 95 April 96 Nov. 96 April 97 Average peryear

Rank

Large ®xed trash lines ÿ191 639 ÿ104 178 261 1Large movable trash lines ÿ193 644 ÿ118 86 210 3Small trash lines ÿ166 485 ÿ97 53 137 6Small doublespaced trash lines ÿ180 688 ÿ111 106 252 2Large stone bunds ÿ150 543 ÿ92 108 204 4Small stone bunds ÿ197 442 ÿ130 78 97 7

ÿ176 565 ÿ96 105 199 5No conservation ÿ173 380 ÿ134 67 70 8Fanya juueeeeeeeeeeTable V. Net present values of alternative SWC with di�erent assumptions regarding cost of trash and labour, MbeereDistrict, Kenya (US$ per ha).

Conservation technology Scenario

High labourLow trash

High labourHigh trash

Low labourLow trash

Low labourHigh trash

NPV Rank NPV Rank NPV Rank NPV Rank

Large ®xed trash lines 635 2 297 3 665 1 327 3Large movable trash lines 456 4 63 5 486 4 93 5Small trash lines 128 6 0 7 161 6 33 7Small double-spaced trash lines 631 3 232 4 655 2 256 4Large stone bunds 640 1 517 1 651 3 528 1Small stone bunds 11 7 38 6 41 7 68 6

381 5 410 2 392 5 421 2AssumptionsCost of trash (Ksh per kg) 0 2 0 2Cost of trash (Ksh per tonne) 0 2000 0 2000Cost of trash (US$ per tonne) 0 29 0 57Cost of labour (Ksh per day) 70 70 35 35

Exchange rate is Ksh 60 to US$1.00

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Mginga District, TanzaniaAccording to the model predictions, the superior e�ectiveness of the system in reducing soil erosioncompared to is clearly demonstrated. On the steeper (45%) slopes is the only feasible system asthe high erosion rates for lead to very low yields in the ®rst year of cultivation. However, evenhave a relatively short life span with the short fallow period used (Figure 1).

These model predictions were validated with the perception of productivity declines of di�erentstakeholders in the area investigated (Figure 2). This depicts the perception of yield decline over time byextension workers, groups of farmers as well as individual men and women. The productivity levels for

and , respectively, were better di�erentiated by women than by men, probably because womenare responsible for the growing of food crops (Ellis-Jones, et al., 1996). Interestingly, extension workersindicated a higher productivity level of new land than did most of the farmers. These ®ndings also agree withperceptions of productivity decline in Mbinga District presented in Mattee, et al. (1997), where it is shownthat most farmers are of the opinion that the productivity of food crops is decreasing. Although farmers'perceptions can be coloured by the development discourse, their actual practices of land fallowing andabandonment provide further con®rmation of model predictions.

It is, however, necessary to take into account the di�erent rotations used with and . These,together with the yields used in the economic analysis, are shown in Table VI. These yields are based onmodel predictions ( for maize with no fertilizer), stakeholder perceptions (maize and beans with nil and highfertilizer) and on-farm trials (maize and beans with nil and low fertilizer). The di�erent levels of fertilizerinput re¯ect the di�erent resource categories of farmers in the area.

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Figure 1. Model predictions of maize yield decline over time for and systems. Mbinga District, Tanzania.ngoroeeeeee matutaeeeeeee



Although there are many variations to the crop rotation shown, including intercropping of maize, beansand sometimes cassava, mainly on , the ones shown are widely used. Gross margins over a ten-yearperiod and NPVs achieved with this cropping pattern are shown in Table VII. Costs of production have beenbased on data obtained from farmers' estimates as well as records maintained on on-farm trials (Martin,et al., 1998).

Over the period gross margins decline in line with yield levels at all levels of fertilizer application. At nil andlow application rates the land would probably have been abandoned after ®ve or six years, though withincreasing land shortages this is often not the case and farmers continue to crop achieving negative returns. Ifthe costs of family labour are excluded, it is likely that positive gross margins are achieved even at very lowproductivity levels. At nil and low fertilizer rates give the highest productivity, largely due to the forcedfallow period that the system demands every third year. At high fertilization application rates, increasedproductivity allows the land to be cropped for marginally longer, with � 0 giving highest produc-tivity. In all three scenarios and � 0 net present values are higher than ÿ 0. However itis signi®cant that ÿ 0 at high fertilizer application rates gives a higher net present value thanand � 0 at nil rates and higher NPV than � 0 at low rates. Clearly, as the opportunity cost oflabour increases it becomes rational to burn and not incorporate organic matter, an increasingly commonpractice. Such a system is unlikely to be sustainable in the long term without signi®cant fallow periods.

provide the best opportunity for sustained productivity at low input levels, but they are dependanton women's labour and threatened with increased labour costs as well as declining land availability. As labour

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Figure 2. Di�erent stakeholders' perception of maize yield decline over time in Mbinga District, Tanzania. 10 fm � group of10 farmers; Li M � male farmer in Lipumba; Li F � female farmer in Lipumbas; ext � the extension; Mh M � male farmer in

Mhekela, Mh F � female farmer in Mhekela.



Table VI. Di�erent rotations and yields achieved for and (kg haÿ1), Mbinga District, Tanzania

Year No fertilizer With fertilizer (low) With fertilizer (high)

� 0 ÿ 0 � 0 ÿ 0 � 0 ÿ 0

1 Beans Beans 300 300 250 500 400 300 600 600 6002 Maize Maize 1500 1000 800 2000 500 1000 3000 3000 28003 Fallow Maize 0 800 600 0 1250 800 0 2500 23004 Beans Fallow 250 0 0 400 0 0 500 0 05 Maize Beans 1000 250 150 1500 300 200 2500 400 5006 Fallow Maize 0 600 500 0 1000 600 0 2000 18007 Beans Maize 200 500 400 300 800 500 400 1800 16008 Maize Fallow 500 0 0 1000 0 0 2000 0 09 Fallow Beans 0 150 100 0 250 150 0 300 400

10 Beans Maize 150 400 300 250 600 400 300 1500 1300

Notes: : planting pits with incorporation of organic matter; � 0: ridges with incorporation of organic matter; ÿ 0: ridges withoutincorporation of organic matter; no fertilizer: common farmers' practice; low fertilizer: increasing farmers' practice; high fertilizer: extensionrecommendations.

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becomes scarce with increased demand for co�ee production and as new lands are opened, it is increasinglylikely that burning of organic matter will increase. As fertilizer becomes more readily available, throughincreased co�ee productivity it is also likely that fertilizer will increasingly become a substitute for labour.

Kamwezi sub-county, Kabale District, UgandaHere we used the same modelling approach as in the Tanzania case study as described previously. Estimatedproductivity decline for di�erent degrees of ground cover is presented in Figure 3.

It can be seen that yields can be sustained with a good soil cover, whereas poor cover results in an immediatesharp drop in yields. The relatively gentle declines on gentler slopes and with moderate cover helps to explainresearch undertaken by Linblade, et al. (1996) who found that the area under cultivation had decreased since1945, while the area under fallow had increased. Although this may be due to the survey being carried outclose to Kabale, where farmers are relatively well o�, relyingmore on livestock and o�-farm incomes, in otherareas less fallow and decreasing fertility are seen to be priority issues (Extension workers and farmers,Kamwezi, personal communication). In Kamwezi, farmers indicated that although yields rapidly declinedeach year after fallow, the use of trash lines extended the cropping period from ®ve to six years before itbecame necessary to leave the land to fallow again (Figure 4). Farmers indicated that after two years of fallow,the cycle could then be repeated but over the longer term productivity had continually declined.

Actual yield levels that were recorded in on-farm trials (Briggs, et al., 1998) con®rm that these farmerperceptions are indicative of productivity levels. The number of years to reach critical production levels forhousehold food security can di�er greatly between soils and management levels. Ferralsols and Acrisolsfound in the study areas in Tanzania and Uganda erode easily but the impact in terms of yield loss per unit ofsoil loss is moderate. Cultivation without SWC can lead to a critical level of production after only one to fouryears on moderate slopes (Tengberg and Stocking, 1997). Gross margins based on model predictions andfarmer perceptions, together with input levels recorded by farmers and net present values over a ten-yearperiod are shown in Table VIII.

This clearly demonstrates the bene®ts of trash lines, but also indicates that when the cost of family labouris included in the costs of production, as is the case in Table VIII, it becomes uneconomic to continuecropping beyond year 4±5 with trash lines and years 3±4 without trash lines. As in Tanzania, increasing landshortages mean that households often crop land for longer periods before abandonment or fallow. This is

Table VII. Gross margins per ha and net present values for and (US$ per ha), Mbinga District, Tanzania

Application Year NPVs

1 2 3 4 5 6 7 8 9 10 Discount rate20% 5%

No fertilizer85 76 0 45 20 0 5 ÿ36 0 ÿ35 141 161

� 0 97 30 7 0 57 ÿ15 ÿ26 0 ÿ23 ÿ38 105 102ÿ 0 96 20 ÿ2 0 16 ÿ14 ÿ25 0 ÿ24 ÿ36 78 56

Low fertilizer245 58 0 165 2 0 85 ÿ54 0 45 343 475

� 0 177 14 ÿ20 0 97 ÿ42 ÿ65 0 57 ÿ87 149 145ÿ 0 97 ÿ27 ÿ50 0 17 ÿ72 ÿ83 0 ÿ23 ÿ94 ÿ27 ÿ140

High fertilizer241 55 0 161 ÿ1 ÿ281 81 ÿ57 0 1 325 430

� 0 236 162 106 0 76 50 28 0 ÿ4 ÿ6 424 573ÿ 0 236 60 4 0 156 ÿ52 ÿ75 0 66 ÿ108 260 288

Notes: : planting pits with incorporation of organic matter; � 0: ridges with incorporation of organicmatter; ÿ 0: ridges without incorporation of organic matter; no fertilizer: common farmers' practice; lowfertilizer: increasing farmers' practice; high fertilizer: extension recommendations.


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Figure 4. Farmers' perceptions of productivity changes, Kamwezi, Uganda.

Figure 3. Productivity decline for maize without fertilizer application or fallow periods, Kabale District, Uganda.



especially so for poorer resourced households who have limited areas of land. They often seek to sell cropresidues from their hillside crops to those who use the material for mulching bananas (Briggs, et al., 1998),thus losing the bene®ts of crop residues for SWC.

CONCLUSIONS

Although land with di�erent ISWC measures maintains a higher soil productivity than unconserved land,long-term productivity decline and reducing returns to soil conservation are still considered major problemsespecially for resource-poor farmers, as has been illustrated in the three case studies. One of the main factorsbehind the decline in soil productivity seems to be decreasing fallow periods (or land abandonment) incombination with nil or low external inputs.

New measures are required to support ISWC. These are likely to include the use of in®eld measures thatcan improve soil moisture and nutrient availability and include combinations of reduced tillage systems,incorporation of organic matter from cover crops and green manures, the use of crop residues as protectivemulches, use of composts and other soil improving technologies such as appropriate live barrier species(both grasses and trees/shrubs), which have had considerable success in higher rainfall environments butless so in lower rainfall ones (Briggs, et al., 1998; Dihenga, et al., 1998; Okoba, et al., 1998). Research andcapacity building needs to be oriented towards the development and extension of technologies adapted toland-user conditions, which create incentives in the short run.

An important way forward is to identify farmer innovators at all resource levels, who experiment withinthe framework of their existing farming systems using locally available materials. Such an approach to soilproductivity enhancement is likely to build on the strengths of ISWC and, at the same time, recognize thethreats from their inherent weaknesses. It is essential to build on the existing diversity and ¯exibility of ISWCand to develop and understanding of the complex interactions between environmental and socioeconomicfactors that give rise to di�erent ISWC practices. Modern techniques need to encompass the ¯exibility ofISWC, providing options that can be modi®ed and adopted to ®t local biophysical and socioeconomiccircumstances. (Tengberg, et al., 1998b). They will need to:

. Enhance soil fertility and conserve soil and moisture.

. Be an integral component of the farming system.

. Use existing materials.

. Not increase labour requirements at peak periods.

. Address gender di�erences in labour utilization.

. Be low cost, not requiring a high initial investment and have an immediate impact on productivity.

. Reduce risk.

Participatory methodologies will be key in insuring farmers remain central to the research and develop-ment process. However, the conditions for success are multiple and include a conducive policy environment,an e�ective institutional setting as well as personal changes among researchers and development workers(Pretty, 1995).

Table VIII. Gross margins per ha and net present values of trash lines under a sorghum and bean rotation,Kamwezi, Uganda (US$)

ConservationTechnology

Year NPVs

1 2 3 4 5 6 7 8 9 10 Discount rate20% 5%

With trash lines 1201 948 572 335 ÿ154 ÿ220 0 0 1201 948 2401 3843Without trash lines 1056 598 185 ÿ79 ÿ250 0 0 0 1056 598 1564 2493



ACKNOWLEDGEMENTS

We would like to acknowledge the input of the research teams fromKenya, Tanzania and Uganda in supportof this paper. The project has been supported by the UK Government's Department for InternationalDevelopment and Anna Tengberg had a post-doctoral grant from the Wenner-Gren Foundation during thecourse of the study.

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