principles of soil conservation for cultivated land

SOIL T E C H N O L O G Y vol. 1, p. 101-116 Cremlingen 1988 ]

PRINCIPLES OF SOIL C O N S E R V A T I O N FOR CULTIVATED L A N D

H. Hurni, Berne

Summary

Soil erosion by water from cultivated land is the most threatening degradation process for sustainable soil productivity. Both in mechanized as well as in subsistence agricultural systems soil erosion is a long-term threat, although industrialized agriculture has hidden somewhat the damage of this degradation through increased fertilizer and other inputs. Soil conservation is a problem of implementation in both systems. One has to dif- ferentiate between on-farm and off-farm influenced measures. On-farm measures are again subdivided into biological and mechanical measures.

For the detection of soil erosion as a problem in the field, seven rules of thumb are given to detect and validate past degradation as well as actual processes. Potential soil erosion can also be estimated with the help of some formula and experience rules.

In mechanized cultivation systems, soil conservation measures must be off-farm influenced measures and subject to and beneficeries of policies, tax regulations or subsidies. On-farm biological conservation is implemented through minimum tillage, cropping patterns and landuse changes. Mechanical structures such as

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bunds or terraces may be developed, but need spacing to fit the slope steepness and a good design. An upper limit for cropping is given at about 15% slope gradient for mechanized cultivation systems.

In subsistence cultivation systems, on the other hand, off-farm influenced measures are mainly social or service incentives, where motivation and agitation play a more important role. On the farms, mechanical measures become more important, since biologically, subsistence farmers have cultivation practices which are highly optimized for their survival strategy, well adapted to the natural, social, political and economic pre- requisites. Bunds, i.e. earth walls across the slope, or terraces, i .e. systems of bunds with a levelling of the surface area between them, or other earthmoving activities, however, must be well planned according to agro-ecotogy, soils, and the populations concerned.

Zusammenfassung

Bodenerosion auf Kulturlandflgchen, verursacht durch Wasser, ist weltweit der bedrohlichste Degradierungsprozel3, der langfristig die nachhaltige Nutzung der Bodenressource beeintr~chtigen wird. Bodenerosion ist sowohl in einer mechanisierten wie auch in einer selbstver- sorgenden Landwirtschaft eine grol3e Gefahr, obschon sic in der ersteren durch

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die enorme Zufuhr yon Kunstdiinger und anderen Mitteln in den letzten Jahrzehn- ten verborgen blieb. In beiden Systemen ist die Durchf~ihrung yon Bodenkon- servierung problematisch und eine Un- terscheidung zwischen betrieblichen und augerbetrieblichen Magnahmen ist des- halb wichtig. Betriebliche Mal3nahmen werden wiederum in biologische und mechanische unterteilt.

Um Bodenerosion als Problem zu er- fassen, werden im Artikel sieben Faust- regeln beschrieben, mit denen frtihere Degradierung wie auch aktuelle Prozesse im Feld auf ihre Gef'~ihrlichkeit hin abgeschS.tzt werden k/Snnen.

In der mechanisierten Landwirtschaft sind aul3erbetriebliche Bodenkonservie- rungsmal3nahmen wichtig, die ~iber Poli- tik, Steuern oder Subventionen laufen. Betriebliche Mal3nahmen sind haupt- siichlich biologisch, z.B. Minimalbearbei- tung des Bodens, Nutzungsmuster oder -5.nderungen. Mechanische Strukturen wie Erdw~ille oder Terrassen auf Fel- dern k/Snnen ebenfalls entwickelt werden, brauchen abet grol3e Zwischen- r~iume und eine genaue Planung. Eine Obergrenze von 15% Hangneigung ist f'tir mechanisierte Kulturen und biologische Konservierungsmal3nahmen limitie- rend.

In der Subsistenz-Landwirtschaft bestehen aul3erbetriebliche MaBnahmen vor allem aus Anreizen oder Dienstlei- stungen, so dab hier die Motivation und Agitation eine besondere Rolle spielt. Auf den Feldern sind mechanische Mag- nahmen wichtig, da Subsistenzbauern traditionell auf biologischer Basis arbei- ten, die sic nicht leicht ver~ndern k/Sn- nen, da sie natkirlichen, sozialen, politi- schen und 6konomischen Engp~issen un- terliegen. Erdw~ille, Terrassen und ande- re arbeitsintensive Maf3nahmen miissen

allerdings unter Einbezug der Agro/Sko- logie, des Bodens und der lokalen BevN- kerung und ihrer Landnutzung durchge- ftihr t werden.

Resum~

U&rosion par l'eau des sols cultiv~s est le processus de d&gradation le plus mena~ant pour une productivit~ soutenue des sols. L'&rosion des sols est un grand danger aussi bien dans un syst~me d'agriculture intensive que dans un syst~me d'agriculture de subsis- tance, bien que l 'apporte de fertilisants et d'autres intrants, l'agriculture des pays industrialists ait pu minimaliser les el- lets de la d~gradation des sols. Dans les deux syst6mes, la r~alisation de la conservation des sols est probl~matique. Pour cette raison, une distinction doit ~tre fare entre les mesures prises au niveau de l'entreprise agricole et celles prises en dehors de l'entreprise. Au niveau de l'entreprise on peut distinguer les mesures biologiques et les mesures m~caniques.

Pour d~tecter fi partir de quel moment l'~rosion pose un probl~me sur le terrain, sept r~gles de base sont donn~es pour estimer la d~gradation pass~e ainsi que les processus actuels de d~gradation.

Dans un syst~me d'agriculture motoris~e, les mesures de conservation des sols doivent ~tre prises en dehors de l'entreprise sous forme de d~cision politiques, fiscales ou sous forme de subsides. Les mesures au niveau de l'entreprise agricole sont principalement d'ordre biologique, par exemple le semis direct, la configuration des cultures ou l'alternance des cultures. Des structures m~caniques telles que des diguettes en terre ou des terrasses peuvent ~galement ~tre dfiveloppfies fi condition d'etre bien

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con~ues. Les cultures motoris6es ainsi que les techniques de conservation de sols sont limit6es aux pentes de moins de 15%.

Dans un syst6me d'agriculture de sub- sistance, les mesures prises en dehors de l'entreprise sont plut6t d'ordre social ou se pr6sentent sous forme des services. Dans ce cas, la motivation et la propa- ganda jouent un grand r61e. Au niveau de l'entreprise, les mesures m6caniques deviennent tr4s importantes puisque les paysans cultivent d6jA, par tradition, d'une fa~on biologique, v u l e cadre na- turel, social, politique et 6conomique. Des diguettes en terre, des terrasses ou d'autres travaux intensifs doivent ~tre ex6cuths en tenant compte de l'agro- 6cologie, des sols et de la population locale.

1 Introduction

1.1 Soil Erosion - - a World Problem

In a definition adapted from BENNET (1939), "soil erosion" is the vastly accel- erated process of soil removal and rede- position brought about by human inter- ference with the normal dynamic equi- librium between soil formation and soil removal. The most important agent of soil erosion is water, while wind may be important in a minority of cropland situations only. Cultivated land is the landuse type where soil erosion is highest. Other landuse types normally produce 10-100 times less erosion.

Through measurements, assessments and modellings of soil erosion by water we know that from today's World croplands 16 tons of soil are lost per hectare per year (BROWN & WOLF 1984). Are these 1-2 mm of eroded soil depth a problem for the future of agricul-

tural production? Most research results support the hypothesis that present day industrialized agriculture hides the problem of soil erosion, because vast increases in productivity have been obtained by fertilising and improved crop varieties in the past 30 years. On the other hand, clearly negative effects of soil erosion on soil productivity are obvious in many subsistence agricultural systems of developing countries, where yields per capita are decreasing in many countries for a variety of reasons including soil erosion.

With these introductory thoughts, it becomes clear that "mechanized cultivation systems", or modern agriculture of most industrialized and many developing countries, should be separated from "subsistence cultivation systems" of the poorest developing countries. Both terms are used here in their broadest sense. Photo 1 and 2 demonstrate that soil erosion occurs in both mechanized and subsistence systems, although soil conservation might be considerably different (see also Section 5.).

1.2 Soil Conservation - - a Problem of Implementation

Opposed to the productive process of soil cultivation, "soil conservation" is a re-productive process. It is an activity applied by the individual or the community in order to maintain the soil fertility of a given area for sustainable use of the soil resource for future generations. "Reproductive" in economic terms un- fortunately means short-term costs, with benefits from soil conservation only in the long-term. Because it is economically negative, soil conservation has rarely been in the short-term interest of land users.

When listing principles of soil conser-

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Photo 1: Rill erosion and downslope accumulation on a potatoe field in a mechanized agricultural system near Berne, Switzerland. (H. H U R N I I979).

vation for cultivated land, a second sep- aration into two categories will be made, namely "on-farm" and "off-farm influenced" soil conservation measures. While on-farm measures deal with conventional methods to actually reduce overland flow and sediment transport on a given land area, off-farm influenced measures are defined here as any influence fi'om outside, e.g. through legislation, policy,

subsidy, motivation, taxation or enforce- ment. Off-farm influenced measures are very important for publicly owned lands like forests and rangelands, or for bad- lands and unproductive lands. However, cultivated land conservation can also be heavily influenced by off-farm measures. Sections 3 and 4 shall make this dif- ferentiation, and also separate between "mechanical", i.e. earth-moving, and "bi-

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Photo 2: Rill erosion during a 40 mm storm on a wheat field in a subsistence agricultural system near Abbo Ager, Wello, Ethiopia where cultivation is by ox-plough. (B. M E S S E R L I 1983).

ological", i.e. plant-based on-farm conservation measures.

As a first step, it could be helpful to de- fine some rules of thumb for field surveys of soil erosion to be used as guidelines for decision-making. Normally, conservation needs are defined using the relationship between soil loss and soil loss tolerance ( W I S C H M E I E R & SMITH 1978). In Section 2 of this paper, however, such

relationship is not applied, because soil loss tolerance rates, so-called T-values, are very difficult to quantify, even more difficult than the prediction of soil erosion rates. Alternatively, various simple field observations are described which help to find out if soil conservation is necessary or not. They can be made during dry periods or during storms, or even as potential risks if no field evidence of

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Photo 3: Tremendous gully developed fi'om rills, degrading the hind and disturbing tillage operations on a cultivated barley field near Gich, Simen, Ethiopia. (H. H U R N I 1974).

soil erosion is visible at the time of ob- servation.

2 Rules of T h u m b for Erosion Risk Assessment

2.1 Signs of Soil Degradation

An assessment of past and present soil erosion can be made if the soil colours, depths, stoniness and erosion features are observed on a cultivated field and the following rules of thumb are considered:

Rule of thumb 1: Obvious signs of soil erosion are gullies and rills which might hinder or aggravate land management operations for farming (photo 3). Care should be taken because rills of a previous year might

be covered through tillage operations thereafter, especially if they were small.

Rule of thumb 2: Clear indicators of past or current soil erosion are shal- low soils, highly variable in depth and colour, especially if there are spots with faint soil colouring which differ from their surroundings, but also if soils have a high concen- tration of stones at the soil surface (photo 4).

Rule of thumb 3: Soil accumulations on the more gentle Footslopes of hills are usually the result of former soil erosion (see photo 1).

Rule of thumb 4: Diminishing yields of agricultural crops, observed over longer periods, if no change in the

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Photo 4: Dense stone cover on a degraded field neat" Afdeyu, Eritrea, Ethiopia, with minimum yield of wheat additionally diminished by drought. (H. HURNI 1984).

cultivation system and technology took place, are most certainly the result of fertility decline very likely aggravated by soil loss (see photo 4).

2.2 Observations of Soil Erosion Processes

An even better assessment than through signs of degradation is possible if the process of soil erosion occurring during

a storm can be visualized on a given cultivated field. Three indicators clearly show excessive soil erosion beyond tolerable levels:

Rule of thumb 5: If surface sealing by clay and silt particles occurs during a storm and persists after the rains have stopped, the soil surface is insufficiently protected fi'om rain- drop impact. Although, perhaps,

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high runoff and soil erosion did not happen during that event, the next storm could result in extreme damages due to the reduced infiltration capacity of the soil.

Rule of thumb 6: If rills and small gullies are formed through runoff and overland flow, tolerable amounts are exceeded (see photo 1 and 2), even if the rills are small.

Rule of thumb 7: If runoff observed at the outlet of a field is coloured brown to dark brown, erosion is excessive and has to be reduced.

In all cases described above, soil conservation measures are indispensible if short or long-term degradation is to be avoided. However, even if no signs exist and no storm observations were made, measures might be needed, if the Univer- sal (WISCHMEIER & SMITH 1978) or other soil loss prediction equations show that soil, slope, and rainfall conditions are such that there is a risk of erosion.

2.3 Potential Soil Erosion

Potential soil erosion, or soil erosion risk, can be checked with the simple slope length - slope gradient relationship given in tab.1. Very low erosion risk values (line 1) might be defined for areas with very low total annual rainfall (below 500 mm), or areas with crops allow- ing a ground cover all year long, or zero tillage cultivation practices. Medium erosion risk values (line 2) might be defined for areas with about 750 mm rainfall, a normal cereal crop planted in the main growing season, and contour ploughing. Very high erosion risk values (line 3) might be defined for areas with high annual rainfall (1500 mm), a normal cereal

crop planted late in the rainy season, and ploughing. The values were calculated with the USLE empirical soil loss model ( W I S C H M E I E R & S M I T H 1978) and a slope gradient extension between 20 and 60% developed by H U R N I (1982).

In tab.l, 3 erosion risk classes are given. Maximum tolerable LS values of the USLE are defined as LS = T/RKCP, where higher figures indicate lower risks. Medium erosion risk could exist in an area with about 750 mm annual rainfall (R = 400), a soil erodibility of K = 0.15, a cereal crop of C = 0.15, contour ploughing of P = 0.75, and a maximum tolerable soil loss of T = 10 t/ha.

As is seen, serious erosion risks for cultivated land start at about 5-15% slope gradients, where slope lengths should be limited below 50 metres. This can be achieved eg. by regular drainage ditches or bunds across the slope at these horizontal spacings. According to erosion risk, crop cultivation on slopes steeper than 10-40% (average: 15%) is not recommended, even if slope lengths are reduced to about 10 metres. Here, also slope gradients must be reduced, eg. by terracing. Sections 3 and 4 de- scribe methods to achieve such reduc- tions. However, considerable variations between different agroclimatic zones and management practices exist. Conserva- tion measures must be adapted to such additional parameters.

Other potential soil erosion damages must be expected if cultivated areas are kept fallow without ground cover over longer periods of the year, especially at the onset of the rainy season, when the soil has been ploughed or tilled, and when crops are seeded, but have not yet germinated and are not yet covering the ground. Soil erosion risk is highly variable according to rainfall variability and

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Slope gradient (%) 1 3 5 10 20 30 40 50

l) Max. slope length: any any 665 102 29 15 10 7 2) Max. slope length: any 732 239 37 10 6 4 3 3) Max. slope length: 401 81 26 4 1 none none none

I Key to erosion risk: 1) Very low erosion risk (maximum tolerable LS value = 2.5) 2) Medium erosion risk (maximum tolerable LS value = 1.5) 3) Very high erosion risk (maximum tolerable LS value = 0.5)

Tab. 1: Relationship between maximum slope lengths in metres for given slope gradients and variable erosion risks using the USLE adapted for steep slopes.

season, soil texture, organic matter, infiltration capacity, structure, the type of crop and cultivation calendar, and the multiple variations of land management and treatments.

3 Principles of Conservation for Mechanized Cultivation Systems

Mechanized cultivation is the dorninant form of agriculture in all industrialized countries, producing food for almost 1 billion people, while probably less than 100 million farmers and technicians are involved at the production level. Due to the close integration of mechanized farming into the economic system of a country, conservation can be implemented using instruments such as pric- ing, marketing, taxation, insurance and other " o f f farm" activities.

3.1 Principles for Off-farm Measures

Off-farm influenced measures are mostly intended to promote on-farm conservation through changes of cropping patterns, crop types or landuse in general. Such changes are generally induced by means of indirect incentives. Incentives

can be either fiscal, social (FAO 1987), or in the form of services. Fiscal incentives include tax exemptions, tax al- lowances, tariff policies, insurance or subsidies. Since mechanized agricultural systems are generally well embedded in a national economy, they may directly react to such legislative measures (cf LOVE JOY & NAPIER 1987). Incentives in the form of services and social support are used more in non-mechanized systems (see 4.1.).

While tax regulations and subsidies are adapted more to industrialized countries where Government influence and control is generally higher, Third World countries more often implement off-farm influenced measures in the form of procla- mations and administrative procedures, eg. on state farms and large estate enter- prises.

3.2 Principles for On-farm Measures

On-farm measures can be biological. These include changes in landuse, im- provements in crop densities through cropping pattern, crop rotation, strip cropping, or a reduction of tillage operations. While biological measures of- fer excellent opportunities to reduce soil loss to tolerable levels, there are some

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Photo 5: Graded Fanya-juu (throw uphill) bund, with a drainage ditch below the earth wall, at work during a 30 mm storm on cultivated land near Anjeni, Gojam, Ethiopia. Such structures are suitable both for mechatiized and subsistence agricultural systems if properly designed. (H. H U R N I 1985).

severe constraints hindering easy implementation. Firstly, ecological conditions should allow a higher density of vegetation throughout the rainfall period. Sec- ondly, vegetation ground cover should not be increased at the cost of reduced production. Thirdly, mechanized land and crop management should not be complicated by these measures. Fourthly,

reduced tillage may require larger inputs of herbicides, the long-term ecological effects of which are not known. De- spite these constraints, however, biological measures can more easily be implemented in mechanized systems due to the higher flexibility of the economy- oriented farmer for landuse alternatives and changes, and due to the possibil-

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Photo 6: Peasant mobilisation through food-for-work campatgns intended to organize the construction of graded Fanya-juu bunds on privately cultivated land near Andit Tid, Shewa, Ethiopia. (H. HURNI 1984).

ity to better integrate scientific develop- ments such as improved seeds, fertilizers or other farm implements, which are not easily accessible or affordable for subsistence farmers. Biological measures can be effective on their own on slope gradients up to about 15%.

On-farm measures on the other hand can also be mechanical. These include contour tillage and soil property im- provements, or earth-moving activities such as terraces or bunds of all forms (photo 5). The main intention is always a reduction of either the slope length alone, or of both slope length and slope gradient if terraces are formed. Me- chanical masures can be effective especially in the long term, since they persist

over more than one cropping season, if well designed and maintained (see Sec- tion 4.2.). However, mechanical measures should be spaced sufficiently to allow mechanized farming between. As a con- sequence, upper slope limits for mechanical on-farm measures are reached quite quickly, at around 15% slope gradients, the same as for biological measures (cf. tab.l).

4 Principles of Conservation for Subsistence Cultivation Systems

Subsistence agriculture on steep slopes concerns about 500 million people in the Third World (HURNI 1987). Conserva-


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tion in the context of subsistence farming may be defined as activities involv- ing only limited external inputs. Many bottlenecks and constraints to soil conservation exist in such systems, and the peasant family is barely surviving with no possibilities to take any risks.

4.1 Principles for Off-farm Measures

Due to the low economic integration of subsistence farmers into national systems of politics and economy, off farm influenced measures such as taxes, subsidies, or insurance may hardly serve as indirect incentives to practice soil conservation on the farm, or to change landuse patterns. Here, services to promote soil conservation can be offered from outside, including technical assistance, incentives (photo 6), motivation, education, train- ing or farm equipment. Social incentives may be offered in exchange for soil conservation through the provision of social services, community buildings, or through community organization of conservation campaigns where all mem- bers of a community must participate. Another way of promoting off-farm influenced conservation is watershed development projects as a best means to reduce land degradation in the medium term.

4.2 Principles for On-farm Measures

Since subsistence agricultural systems have common economic and political independence, conservation options for cultivated land are rather similar for all peasants. Biological measures, changes in landuse or in land cover are very much limited because the peasant can take no risks in the production of his basic food crop, and because he has no

landuse and income alternatives. Fur- thermore, his agricultural calendar is al- ready optimized at the input level avail- able to him. Biological measures, there- fore, are acceptable especially for steep slopes (fig.l), where agroforestry systems such as alley cropping are recommended, or on slopes up to about 15% gradients, where small grass strips can be alter- nated with cropped strips (fig.2). Inter- cropping, mixed cropping, specific seed- ing and weeding are activities usually al- ready applied traditionally, so that no change in soil erosion can be expected from these cultivation practices.

A major realistic option is mechanical conservation measures such as bunds (graded or level, fig.3), and terraces (developed or constructed, fig.4). Bunds are walls of earth or stones aligned across the slope, which do not alter the slope gradient between two bunds. Terraces, on the other hand, are intended to level the area between two riser slopes, Ter- races can be developed from bunds with maintenance and upgrading. Concern- ing the design, it will be necessary to follow strict rules according to agroeco- logical zonation, traditional knowledge, landuse patterns, and local parameters of slope and soil. An example developed for Ethiopia is presented in H U RN I (1986). For example, bunds and terraces graded across the slope are recommended in areas of higher rainfall or if soils with a high clay content are abundant. Gradi- ents are needed to allow surplus runoff to be drained sideways into safe waterways or natural drainage systems. Level bunds are important in areas where water is a serious constraint, where more infiltration is needed and where soils are silty or sandy. The structure and composi- tion of earth material on bunds depends on the availability of material, but also

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Fig. 1: Alley cropping shown Jbr biological steepland conservation in a situation where some strips have to be cropped. They are placed inbetween rows of leguminous trees to minimize soil loss. (Drawing by J. WETZEL I986).

'":"---:~

Fig. 2: Grass strips are suitable also for mechanized cultivation systems (/" they are sufficiently spaced. This biological conservation measure is recommended in Ethiopia for cultivated slopes up to about 15%. (Drawing by J. WETZEL 1986).


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. - - - - ~ - ~ - k ' ~ ~ ~ I ' - ~ ,.,~ , , . . r ~

Fig. 3: Level bund as mechanical water retention strttctttre.)tot' cultivated land. In the long term, a terrace will develop between two bunds, additionally reducing slope and soil loss. (Drawing by J. WETZEL 1986).

Fig. 4: Slightly outward graded bench terraces can be the ultimate goal oJ'steepland conservation for cropped land, !; t ̀maintenance is guaranteed continuously. (Drawing by J. WETZEL 1986).

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on the ecological zone. In dry areas with low regeneration potential for vegetation stone bunds are more suitable, while in wet areas soil bunds can be stabilized with grasses. For the formation or development of bench terraces, soils should be deep. Here, the vertical interval between two bunds intitially spaced on the field must be two and half times the depth of the soil. I f it is more, terraces will not completely fill the space between two lines because of lack of soil material to form them large enough. If terraces are well developed, regularly maintained and quite level, they provide ecologically sta- ble soils tbr cultivation on slopes up to about 50% slope gradients. On naturally steep slopes, terraces will be very small, but this should be no serious problem for labour-intensive agricultural activities.

5 General Conclusions

As a major principle, mechanized agricultural systems are systems where soil conservation measures are introduced and implemented primarily through off- farm policy measures, such as tax regulations, subsidies, tariffs and insurances. Mechanized cultivation should not be practiced on slopes above 15% gradients. Changes of landuse should be pro- moted and supported for such slopes. Biological measures should be incorpo- rated in the cropping activities as much as possible. Mechanical measures such as terraces should be introduced only if no other alternatives exist.

Subsistence agricultural systems differ considerably. Here the role of the state is primarily to motivate and organize farmers to carry out conservation, and to provide social incentives and services for peasants willing to adopt on-farm measures (BLAIKIE 1985). On cultivated

land, biological measures will be acceptable only to a very limited extent. At the low innovation level due to high risks in subsistence situations, mechanical measures seem to be more easily acceptable in most cases. Bunds and terraces, however, must be well designed, graded where needed, and regularly maintained as long as they are needed to stabilize the soil in cultivation.

Acknowledgement

The author would like to acknowledge the permanent assistance given by Mr. Kebede Taro, Head of CFSCDD of the Ethiopia:: Ministry of Agriculture and by Prof. Dr. Bruno Messerli of the Geography Institute of the University of Berne. Both provided their help with per- sonal enthusiasm, support and guidance since the inception of the Soil Conserva- tion Research Project in 1981. The Swiss Directorate for Development Coopera- tion is gratefully acknowledged for its continuous financial and technical support of the project, which the author directed until January 1987.

References

BENNET, H.H. (1939): Soil Conservation. New York, London,

BLAIKIE, P. (1985): The political economy of soiI erosion in developing countries. Longman, London/New York, 188 pp.

BROWN, LR. & WOLF, E.C. (1984): Soil erosion: Quiet crisis in the world economy. World- watch Paper 60, 49 pp.

FAO (1987): Incentives for community partici- pation in soil conservation programmes. FAO Conservation Guide 12, Rome

HURNI, H. (1982): Soil erosion in Huai Thung Choa - Northern Thailand: Concerns and constraints. Mountain Research and Development, Vol. 2, No. 2, 141-156.

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HURNI, H. (t986): Guidelines for development agents on soil conservation in Ethiopia. Com- munity Forests and Soil Conservation Devel- opment Department, Addis Abeba, Ethiopia, 100 pp.

HURNI, H. (1987): Options for steepland conservation in subsistence agricultural systems. Workshop on soil and water conservation on steep lands. San Juan, Puerto Rico. 15 pp.

LOVE,JOY, S.B. & NAPIER, T.L. (1987): Insti- tutional constraints to soil conservation on steep lands. Workshop on soil and water conservation on steep lands. San Juan, Puerto Rico.

WISCHMEIER, W.H. & SMITH, D.D. (1978): Predicting rainfall erosion losses - - a guide to conservation planning. U.S. Dept. of Agricul- ture, Washington. Agriculture Handbook Nr. 537, 58 pp.

Address of author: Hans Hurni Soil Conservation Group Geography institute, University of Berne Hallerstraf3e 12 3012 Berne, Switzerland

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