slope position effects on soil fertility and crop productivity and implications for soil...

Agriculture, Ecosystems and Environment 91 (2002) 113–126

Slope position effects on soil fertility and crop productivityand implications for soil conservation in upland northwest Vietnam

A. Wezela,∗, N. Steinmüllerb,1, J.R. Friederichsenc,2a Institute for Botany and Landscape Ecology, Ernst-Moritz-Arndt-University of Greifswald, Grimmer Str. 88, 17487 Greifswald, Germanyb Institute for Plant Production and Agroecology in the Tropics and Subtropics, University of Hohenheim 380, 70593 Stuttgart, Germany

c University of Hohenheim, Mergenthalerstr. 15, 73760 Kemnat, Germany

Received 15 August 2000; received in revised form 7 June 2001; accepted 13 June 2001

Abstract

Agriculture is increasingly practised on the very steep slopes of mountainous Vietnam with serious problems of soil erosionand degradation. In five Black Thai villages of Yen Chau and Mai Son district, northwest Vietnam, soil parameters and cropyields of 19 maize (Zea maysL.) and 25 cassava (Manihot esculentaCrantz) fields with 33–91% inclination at upper and lowermid slope positions were studied. Farmers’ preferences for different farming systems components was assessed by interviewsin Black Thai, Hmong, Xinh Mun and Khmu villages and soil conservation strategies evaluated. The 10–22% lower organicmatter, nitrogen and phosphorus content at the lower mid slope was associated with decreased maize yields by 27% andcassava yields by 31%, compared to upper mid slope positions. This accelerated soil degradation at lower slope positions mayprimarily be attributed to an enhanced mineralisation and crop export rather than to soil erosion due to more frequent historiccropping activities towards lower slope positions. The general implication to toposequence studies for assessing erosion effectsis that they are likely to be confounded with historic farmers’ preferences to cultivate easier accessible, lower slope positions.Irrespective of the causes of degradation, soil fertility was not a priority for farmers whose cropping management is currentlyfocusing on the introduction of improved maize varieties. Consequently, only soil conservation and fertilisation strategies thatinclude long- and short-term interests of farmers should be promoted in the future: vegetative barrier and cover crops withfood and fodder species, fruit and timber trees and minimum tillage.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Degradation; Erosion; Ethnic minorities; Toposequence; Steep slope; Upland agriculture

1. Introduction

Three-quarters of the land area of Vietnam is dom-inated by mountainous topography. The proportion

∗ Corresponding author. Tel.:+49-3834-864185;fax: +49-3834-864187.E-mail addresses:[email protected] (A. Wezel),[email protected] (N. Steinmüller),[email protected] (J.R. Friederichsen).

1 Tel.: +49-711-4593738.2 Tel.: +49-711-4569346.

of agricultural land (24%) is low (SPH, 1996). Thenorthwestern part of Vietnam is almost exclusivelyhighland with elevations between 300 to over 1000 m(Le Ba, 1997). Agriculture is more and more practisedon steep hillsides as the population of Vietnam’s north-ern mountainous regions increased between 1960 and1984 by 300%, and is assumed to double again withinthe next 20 years (Jamieson et al., 1998). As a conse-quence, in the study region Yen Chau, agricultural landper person decreased from 0.5 ha per person in 1980 to0.2 ha per person in 1998 (Yen Chau, 1999). The forest

0167-8809/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0167-8809(01)00242-0

114 A. Wezel et al. / Agriculture, Ecosystems and Environment 91 (2002) 113–126

areas in the northern mountains of Vietnam decreasedfrom 95% in 1943 to 17% in 1991 (Sikor, 1995).

People in this mountainous area belong to differ-ent ethnic minorities. In Son La province, where thestudy region is located, the Black Thai comprise themajority of the population with 54%, followed by theKinh (the Vietnamese) (18%) and the Hmong (12%)(Eeuwes, 1995). Ethnic groups with fewer people in-clude amongst others the Khmu (1.3%) and the XinhMun (0.2%).

On upland areas, swidden (slash and burn) agricul-ture is normally practised by the Black Thai, Hmong,Khmu and Xinh Mun. This system is characterised bythe clearing of primary or secondary forest to culti-vate upland rice (Oryza sativaL.), maize (Zea maysL.) and cassava (Manihot esculentaCrantz) for afew years before fields are abandoned and new forestor fallow areas have to be cleared. At present, landscarcity induces changes in the cultivation practicefrom short cultivation–long fallow periods to longcultivation–short fallow periods.

At the valley bottoms, the Black Thai practice“composite swiddening”, a system of agriculture inwhich households combine cultivation of wet ricefields in the valleys with rotational swiddening onthe hillsides (Jamieson et al., 1998). Today, rotationalswiddening in the uplands of the study region is moreand more replaced by permanent cultivation (Bui HuyHien et al., 1994). Besides the main crops rice, maizeand cassava, others such as fruit trees, cotton (Gossyp-ium hirsutumL.), sugarcane (Saccharum officinarumL.), different bean species, sesame (Sesamum indicumL.), indigo (Indigofera tinctoriaL.) and vegetables inhome-gardens play important roles. Fruit trees, sugar-cane and to a certain extent maize are planted as cashcrops, the other crops are used for self-consumption.Fertilisers are generally applied to paddy rice and sug-arcane and rarely to maize fields. Pesticides are usedfor paddy rice and sugarcane cultivation. Animals arekept at the compound for meat and egg production(cows, pigs, chicken and ducks), whereas water buf-faloes are mainly used for field work and transport.

Agriculture in mountainous areas of southeastAsia is often blamed for being unsustainable by en-hancing deforestation and soil erosion (Sam, 1994;Souvanthong et al., 1994 in: Rerkasem and Rerkasem,1995). The existence of soil degradation and ero-sion on farmers’ fields was thus hypothesised at the

beginning of the study. To analyse this, the objectivesof this study in northwest Vietnam were the assess-ment (1) of soil fertility and crop yield changes alongsteep slopes, (2) of farmers’ preferences for differentfarming system components, and (3) of the extentfarmers use soil conservation measures to combat soildegradation.

2. Materials and methods

2.1. Study region

The study region is located in the mountainous areaof Son La province in northwest Vietnam (Fig. 1). SonLa province includes amongst others Yen Chau andMai Son district. The climate in this region is char-acterised by tropical monsoons with a rainy season insummer and relatively dry cold winters (Le Ba Thao,1997). Average annual precipitation is 1114 mm (BuiHuy Hien et al., 1995). A maximum of 1810 mm was

Fig. 1. Location of the study region in northwest Vietnam.

A. Wezel et al. / Agriculture, Ecosystems and Environment 91 (2002) 113–126 115

measured in 1956. The natural vegetation of tropicalsemi-deciduous monsoon forests and bamboo forestsis still found on areas where human impact on vegeta-tion is low (Nguyen, 1995), elsewhere being replacedby secondary forests and agricultural land. Uplandsoils in the study area mostly belong to the group ofred and yellow soils in highland regions between 100and 1000 m altitude (Ferrasols and Leptosols accord-ing to FAO classification). Limestone and schist arethe dominating bedrocks (Bui Huy Hien et al., 1995),but also conglomerates have been observed.

The former cultivation cycles in the villages con-sisted of cultivation periods of 1–2 years of uplandrice, followed by 2–3 years of maize and/or cassava,ending with a fallow period of around 5–7 years.Today, cultivation cycles have changed to 0–1 yearsof rice cultivation, followed by 2–5 years of maize,followed by cassava (2–3 years) and finally a fallowperiod of 0–3 years. Often maize and cassava are in-tercropped following directly upon a rice cultivationperiod.

2.2. Soil investigations

Soil parameters were surveyed on 19 maize fieldsof four villages (Houi Thon, Nam Un, Chai, Cang)and on 25 cassava fields of five villages (Houi Thon,Nam Un, Chai, Na Dong, Hiem) in Yen Chau District(Fig. 1). Three study villages of the Black Thai arelocated on middle elevations in the mountains (HouiThon, Cang, Hiem), the other two villages in a rivervalley (Nam Un, Chai), with all of their maize andcassava fields on hillsides. The survey period extendedfrom the end of August to the beginning of October in1998. Soils were sampled at different slope positions:two basal positions (bottom, concave), two mid slopepositions (lower, upper) and two top slope positions(convex, ridge). Since most fields were located withinthe mid slope area, statistical evaluations were impos-sible in lower and top slope areas due to insufficientobservations. The criterion for the separation betweenupper and lower mid slope positions was the changefrom concave slope (upper mid slope) to the convexslope (lower mid slope).

At each slope position, two soil samples with adistance of 3 m along the contour line of 0–10 cmdepth were collected in September 1998. Prior tothe laboratory analysis, the two samples were mixed,

air-dried and sieved (2 mm screen). The pH of KClwas measured in a 1:2.5, soil:liquid mixture, or-ganic matter (OM) content was analysed using theWalkley–Black procedure, total nitrogen (N) by theKjeldahl method, available phosphorus by the Bray-1procedure, exchangeable potassium by flame emis-sion spectrophotometry, cation exchange capacity(CEC) by the ammonium acetate method and soiltexture by a combined sieve-pipette analysis. Depthof A-horizon was measured by digging small pits,then measuring the depth where soil colour changedusing the Munsell soil colour charts (Munsell (1954)cited in Schlichting et al. (1995)). Topsoil erodibilitywas estimated with the help of a nomogram with theparameters texture, organic matter content, and bya field assessment of the upper topsoil material ofaggregate stability and bulk density (Wishmeier et al.(1971) cited in Schlichting et al. (1995)).

At Nam Un, one complete toposequence could bestudied across a 125 m long slope comprising fiveslope positions: two basal positions (bottom, con-cave), two mid slope positions (lower, upper) andone top slope position (forest; Fig. 2). At the topslope position a secondary forest was present, on themid slope, concave and bottom positions this wasmaize. Soil erosion was not directly measured in thefields, but inferred using the universal soil loss equa-tion (USLE). The expected relative erosion losses ofeach slope segment between two adjacent measuringpoints was determined by the topographicLS factor(L = length;S = steepness of slope) of the USLE:LS = (L/22.13)0.3(16.8 sinθ − 0.5), where slope is≥9% andθ is the angle of slope. According to fieldtests this equation is valid for slopes up to 60% butflume tests produced similar results up to 84% (Mc-Cool et al., 1982, 1987). TheLS factor for the ithsegment was calculated by an equation for irregularslopes that adds to the sediment of theith segmentthe sediment load flowing onto the segment from the(i−1)th segment above (Foster and Wishmeier, 1974):Lm+1

i − (Lm+1i−1 Si)/(Li − Li−1)(22.13)m, whereLi is

the length from the top of the slope to the lower endof the ith segment,m = 0.3 (see USLE above), whereSi is the average slope of theith segment calculated asthe mean of the slope at the upper and lower measur-ing point of theith slope (Fig. 2). Assuming negligibleerosion in the forest zone at the ridge of the slope, theupper mid slope was considered as the top (i = 1).


Fig. 2. Slope positions of soil sampling on a toposequence with forest and maize in Nam Un, northwest Vietnam. Numbers in bracketsare lengths and average slopes of segments between successive measuring points.

2.3. Yield measurements

At upper and lower mid slope positions, maizegrain yields were determined in six fields of the vil-lages Houi Thon and Cang. Cassava root yields wererecorded in three fields of the villages Houi Thonand Chai. In three fields, maize was also harvestedon concave top slope positions. Two plots of maize,3 m × 3 m in size, were harvested at each slope po-sition in August 1998. Maize cobs were weigheddirectly in the field. Maize grains were separatedfrom spindles to determine the grain–spindle ratio.Grain dry matter (DM) percentage was obtained fromoven-dried samples at 65◦C of each harvested plot.No other biomass was harvested.

Cassava roots of three plants, each, were harvestedfrom two plots per slope position, 2 m× 2 m in size.Tubers were weighed in the field, thus only fresh mat-ter was determined. In all harvested maize and cassavaplots soil samples were collected as described before.

2.4. Statistics

All data were analysed by SAS (Version 6.12).PROC GML was used for comparing means of upperand lower mid slope positions using fields as blocks.Cassava yields where analysed without blocks, be-cause fields had at-value of only 0.5 which was not

significant (P = 0.66). In the correlation analysis withPROC CORR, maize and cassava fields were pooledafter adjusting cassava yields to the maize yield level.For this adjustment all cassava yields were multipliedby 3984/17,250 which is the average maize grainyield divided by the average cassava root yield of all12 and 6 fields, respectively. The normal distributionof residuals was checked by PROC UNIVARIATEand the homogeneity of variances by plotting resid-uals versus predicted values. These conditions weremet except for the normal distribution of N, P andK in soils of maize fields (P < 0.01). Means werecalculated as LSMEANS for correcting unbalancedobservation numbers. Frequency data were analysedby a chi-square test using PROC FREQ.

2.5. Interviews with farmers

Farmers were interviewed in 11 villages in YenChau and Mai Son districts of Son La province. Six ofthe 11 villages belonged to the ethnic group of BlackThai, three to the Hmong and one each to the XinhMun and the Khmu group. Assisted by a Vietnameseinterpreter, interviews were held between March andMay 1999 at farm compounds using different partici-patory tools such as ranking techniques, standardisedquestionnaires and seasonal calendars. In this studyonly results of the priority ranking is presented.


In groups of four to seven persons, repeated fourtimes, farmers were asked to order seeds representingfarming system components on cards according toimportance. Many seeds on a card thus representedhigh importance, few seeds have low importance.The farmers where free to choose the most importantitems (see Fig. 5) from a list which was developedduring the various interviews with the farmers, wherethey were also asked about farming components.

Because major socio-economic differentiations co-incide with ethnic affiliation, the villages were dividedinto two groups of 39 interviews, each. The first vil-lage group (VG1) comprised the Black Thai villages,which have better access to markets and governmentservices such as schools, health care, state extensionservice, credit and in some cases electricity. Theircropping pattern mostly included paddy terraces andwas relatively dynamic. Innovative crop managementmethods and crops such as sugarcane, longan (Dimo-carpus longanLour.) and high yielding maize hybridswere more frequent than in VG2. Village group 2,which included all other investigated ethnic groups,had less access to the above named government ser-vices and to markets. The cropping pattern in thisgroup was mostly limited to rainfed upland crops.Innovations seemed to reach farmers later and to alesser extent. The government strongly influenced thetechnological progress by extension and opium sub-stitution programs, because long distances to marketsare limiting market integration processes. Develop-ment agencies have been promoting erosion controlmeasures in 9 out of the 11 villages.

3. Results

3.1. Soil investigations

Differences of soil parameters in upper and lowermid slope positions of maize and cassava fields areshown in Table 1. In average, the upper positionsexceeded the lower positions by 22% for P, 13% forOM, 10% for N and 4% for CEC. Among the othersoil parameters only pH approached a significancelevel of 10% in cassava fields, whereas all other errorlevels exceeded 40%.

For visualising differences in potential erosionamong slope positions caused by topography, i.e.

slope length and steepness, theLS factors of theUSLE of all slope segments below the forest zone areshown in Fig. 3. TheLS factors have been presentedas differences to the maximumLS factor occurring atthe lower mid slope position to generate trends in thesame direction as the soil parameter curves. Accord-ing to thisLS factor curve, erosion would be high atthe upper mid slope and highest at the lower mid slopebut should decrease considerably at the concave andbottom positions. In contrast, all curves of soil fertil-ity parameters except K and P, reached a minimum atthe concave position. Although between the concaveand the bottom position all these parameters increasedexcept for P, the magnitude of this increase was muchless than expected by theLSfactor, except for pH. Thecurves of CEC, available P and clay percentage canbe distinguished from all other curves by the absenceof the decline between forest and the upper mid slopeposition. The only major change in soil texture was theincrease in silt content at the expense of clay betweenthe upper and the lower mid slope position (Fig. 4).

3.2. Yield of maize and cassava

Maize grain and cassava root yields were signifi-cantly higher by 1 Mg ha−1 (dry matter maize yield)and 4.7 Mg ha−1 (fresh matter cassava yield) at theupper than at the lower mid slope position (Table 2).In relative terms, these yield increases amountedto 27 and 32%, respectively. The decline in maizegrain yield by 0.4 Mg ha−1 between the upper midslope and the highest, convex slope position was notsignificant.

3.3. Correlation between yield and soil parameters

The correlation between yield and soil parametersand among soil parameters was calculated by pool-ing all 44 fields (Table 3). Crop yields were onlysignificantly affected by topsoil depth. Among thesoil parameters, high and positive correlations (r =0.7–0.9) occurred only between OM and N or K. Othersignificant correlations showed that a deeper topsoilwas associated with higher pH, OM and K values.Steeper fields had more clay and N. The pH was pos-itively related to OM, N, K and CEC. A higher CECwas associated with a higher pH, clay, OM, N, P andK values.

118A

.W

eze

le

ta

l./Ag

ricultu

re,E

cosyste

ms

an

dE

nviron

me

nt

91

(20

02

)1

13

–1

26


Fig. 3. TopographicLS factor, depth of A-horizon and soil parameters along a toposequence of a maize field in Nam Un, northwestVietnam. Equations are linear regressions significant at+P ≤ 0.1 or ∗P ≤ 0.05.


Fig. 4. Soil texture along a toposequence of a maize field in Nam Un, northwest Vietnam. Equations are linear regressions significant at+P ≤ 0.1 or ∗P ≤ 0.05.

Table 2Dry matter maize grain and fresh matter cassava tuber yield ontoposequences in Vietnam

Position Maize grains Cassava tubers

Mg ha−1 na Mg ha−1 na

Convex top slope 4.1 3Upper mid slope 4.5 6 19.6 3Lower mid slope 3.5 6 14.9 3Probabilityb 0.03 0.02CV (%) 14 9

a Number of plots measured.b Probability of significant difference ofF-test; upper vs. lower

mid slope.

3.4. Future importance of farmingsystem components

Generally across both village groups, mechanisationfor tillage and improved upland crops ranked higherin the farmers’ priorities than animals (Fig. 5). Soilconservation measures were only considered relevantin VG1, credit only in VG2. Credit and horticultureranked lowest in VG1, animals and soil conservation

measures in VG2. However, it has to be noted that thedata of the Black Thai ethnic group (VG1) are proba-bly more precise due to an easier communication withthe better educated Thai villagers. Language barriers,shyness and discomfort were recognised during thecomplicated interviews with farmers belonging to theHmong, Xinh Mun and Khmu ethnic groups (VG2).

4. Discussion

4.1. Decline of soil fertility and crop yields

The higher soil fertility and crop yields at theupper than at the lower mid slope position was inline with soil losses and yields on upper and lowerlinear slope positions in a case study from Nebraska(Jones et al., 1990). Both results may be attributedto the constant slope angle at the upper and lowerposition, because erosion increases with slope lengthif the slope angle is constant. However, these resultsare in contradiction to almost all published topose-quence studies which indicate a down-slope gradient


Table 3Pearson correlation coefficients between yield and soil parameters in lower and upper mid slope positions of maize and cassava fields asshown in Tables 1 and 2a

Depth ofA-horizon(cm)

Slope(%)

Clay(g kg−1)

pH OMb

(g kg−1)N (g kg−1) P (mg kg−1) K (cmol kg−1) CECc

(cmol kg−1)

Yield 0.48∗ −0.11 −0.21 −0.36 −0.09 −0.14 0.29 −0.37 0.24Depth of A-horizon 0.10 0.17 0.28∗∗ 0.33∗∗ 0.14 −0.11 0.19+ 0.13Slope 0.43∗∗∗ 0.02 0.15 0.20+ −0.06 0.04 0.13Clay 0.15 0.06 0.16 −0.03 0.11 0.41∗∗∗pH 0.40∗∗∗ 0.37∗∗∗ 0.06 0.42∗∗∗ 0.25∗OM 0.90∗∗∗ −0.16 0.70∗∗∗ 0.26∗N −0.10 0.66∗∗∗ 0.34∗∗∗P −0.07 0.26∗K 0.33∗∗

Average 13 62 310 5.9 20 1.3 10 0.39 11Minimum 4 33 100 4.1 6 0.4 3 0.12 7Maximum 30 91 500 7.5 68 4 42 1.20 16

a n = 18 for yield andn = 88 for soil parameters.b Organic matter.c Cation exchange capacity.+ P = 0.1.∗ P = 0.05.∗∗ P = 0.01.∗∗∗ P = 0.001.

of increasing soil fertility and crop yield. In all thesestudies, the less pronounced physical and chemicalerosion symptoms at lower positions were attributedto sedimentation processes induced by decreasingrunoff velocity (Mahler et al., 1979; Pan and Hop-kins, 1991; Fiez et al., 1995; Mohammad et al., 1995;McConkey et al., 1997; Fenton et al., 1999). Thus,other factors than soil erosion must play an importantrole for decreasing soil fertility and yields downslope.

4.2. Soil erosion versus cropping frequency

Another main factor for decreased yields and soilfertility at lower slope positions is the cropping fre-quency. Because these areas are more convenient forcropping they were historically more frequently used(own interviews; Sikor, 1999). Thus, an enhancedmineralisation (Tiessen et al., 1994) and nutrientexport via crops can be expected. In the following,arguments pro and contra for erosion versus prolongedcultivation will be discussed.

The depletion of OM, N and available P is con-sidered a typical erosion symptom at more erodedpositions caused by the selective transport of fine

aggregates which are chemically richer than thecoarser ones (Wan and El-Swaify, 1997), but differ-ences of soil texture between upper and lower midslope positions in this study were very small. Thepreferential depletion of P in the lower slope posi-tions as compared to OM and N (Table 1) may beexplained by a stronger association of available P tofine clay particles, while total N and OM are prob-ably more uniformly distributed in the various soilsize categories (Sharpley, 1985; McIsaac et al., 1991).However, the selectivity of erosion processes shouldalso have affected K (Sharpley, 1985; McIsaac et al.,1991), which was even higher at the lower mid slopeposition. Thus, the loss of OM, N and P must alsobe attributed to an enhanced mineralisation and cropexport due to historically more frequent croppingactivities. The cropping frequency explains also whyin the toposequence study erosion symptoms did notdiminish with decreasing steepness from upper andlower mid slope to concave and bottom positions asit was expected by the topographicLS factor (Fig. 3).In addition, at a constant cropping frequency the claycontent should decrease with steepness due to selectiv-ity of the erosion process. In this study, steeper fields


Fig. 5. Preference ranking of farming system components in two village groups of northwest Vietnam.

contained more clay (Table 3), indicating differentland use frequencies at the upper and lower slopepositions.

Although cropping frequency seems to have the ma-jor impact on soil fertility decline, erosion is existent inthe fields of the study area. Field observations detectedheavy soil erosion symptoms in 2 of the 44 fields.In one field a dark, former topsoil horizon was over-laid by two more reddish horizons (17 cm), indicatingdeposition of eroded material from higher positions. Inanother field, depth of topsoil horizon was only 3 cmat the lower field position compared to 11 cm at thetop. At the lower position of this field, 64% less maizeyield was measured than at the top. Minor soil ero-sion features like exposition of stones or rocks werefound in several other fields. The reason for the rar-ity of severe erosion symptoms is probably the geo-logical endowment. Most fields are situated in areaswith limestone, some in areas with conglomerates thathave topsoils of low to medium erodibility; no topsoils

with high erodibility were found (Table 4). In neigh-bouring districts of Yen Chau, where soils derivedfrom sandstone are more frequent, much stronger soilerosion symptoms can be observed. A lower erodi-bility of soils derived from limestone as comparedto sandstone or other parent materials has also beobserved in Spain and Zaire (Sanroque et al., 1990;Miti, 1991).

In conclusion, decline of soil fertility is mainlycaused by more frequent cropping at bottom, concaveand lower mid slope positions. At the lower mid slopeposition erosion also has an impact on soil fertilityas indicated by field observation of the soil erosionsymptoms and theLS factor which has its maximumat this position. In addition, today almost all fields ex-tend from bottom to upper mid slope positions, thusthe lower parts are not exclusively cropped anymore.Already, due to land scarcity, permanent cropping isfound in some fields. Only the top slope positions areleft uncropped with forests or shrubs.


Table 4Topsoil erodibility (low, medium) of upper and lower mid slope positions of maize and cassava fields in northwest Vietnam

Maize Cassava Total

Low (%) Medium (%) n Low (%) Medium (%) n Low (%) Medium (%) n

Upper 53 47 19 32 68 25 41 59 44Lower 68 32 19 44 56 25 55 45 44Chi-square P = 0.32 P = 0.38 P = 0.20

4.3. Farmers preferences for farmingsystem components

In the study region, farmers are aware of decline ofsoil fertility, but their solutions are more short-termoriented with improved upland crops or credits forfertiliser and to a lesser extent in a long-term strat-egy such as investing in soil conservation measures.Although farmers stated soil degradation and ero-sion were responsible for declining upland cropyields, they also mentioned increasing yields ofmaize after the introduction of improved varietiesand increased input of mineral fertiliser, and wereable with these short-term solutions to compensateyield losses (Wezel et al., 2001). This, probably, con-cealed that soil degradation at the lower position hasalready depressed maize yields by 21% and cassavayields by 24% compared to upper, more fertile po-sitions (Tables 1 and 2). Here, better informationand extension is necessary before signs like rill orgully erosion will bring back farmers’ attention tothis topic. Anyhow, problem awareness for the needof soil conservation measures, which is a commonproblem throughout Asia (Napier et al., 1991), alonewill not improve adoption behaviour, unless the pro-moted measures are more attractive to farmers, i.e.short-term returns to labour and land should notdecrease.

4.4. Soil conservation measures

In the past, many proposed soil conservation tech-nologies were not adopted by farmers in upland areas(Ongprasert and Turkelboom, 1996; Cramb et al.,1999) because of technical, social, economic andinstitutional constraints (Napier et al., 1991; Napier,1996; Lapar and Pandey, 1999; Cramb et al., 1999).

Nevertheless, farmers might become more interestedif conservation measures would provide direct benefitssuch as animal fodder or weed suppression, while soilconservation is a secondary, long-term benefit. For ex-ample the promotion of erosion control measures withTephrosia candidaDC. hedgerows failed in the studyregion (Friederichsen, 1999), becauseTephrosiacanonly be used as mulch and has no direct benefit such asfodder, so that farmers were unwilling to provide thehigh labour input of this system. However, also foddertree species were rarely adopted by farmers, becauseexotic species such asLeucaena leucocephala(Lam.)De Wit, Cajanus cajan(L.) Millsp. or Indigoferasp.,the latter is adapted to higher altitudes due to its frostresistance, were promoted without informing farmersabout their feed value. Even the potential mulchingbenefit was not fully used, because cuttings of thehedgerows were mainly applied besides the tree rowsand not spread across the food crop area betweenthe hedgerows.

Weed suppression is a very attractive benefit ofsoil conservation measures by cover crop systems,because weeds were mentioned as primary limitationin a ranking of current crop production constraintsin the study region (Friederichsen, 1999). Secondlyranked were pests (diseases, insects), followed by ratsand soil fertility, and lastly by erosion. Therefore,some farmers were undersowing cover crops such asLablab purpureus(L.) Sweet andVigna umbellata(Thunb.) Ohwi and H. Ohashi for weed control andN enrichment in maize. Some other farmers plantedPachyrhizus erosus(L.) Urb. (yam bean) betweenmaize for bean and tuber production. This creepingplant species is still persistent after maize harvestremaining as mulch until the beginning of the fol-lowing cropping season. Intercropping of maize withcassava may have some positive side-effects for soil


conservation. Farmers in the study region normallygrow cassava over a 2-year period using a wide plantspacing of(1–1.5) m × (1–1.5) m. In the first year,they often plant maize between the young cassavaplants providing some soil cover in the initial fewmonths when the canopy cover of cassava plants isextremely low compared to other crops, thus cre-ating the most severe soil losses of different crops(Putthacharoen et al., 1998). At the beginning ofthe second year, erosion is no longer problematic,because a nearly closed cassava canopy and estab-lished weeds provide sufficient soil cover. This sec-ond year cassava could theoretically be promising asa vegetative barrier with a food crop species whichmight be more attractive to farmers than barriers withfodder species. A broad spectrum of indigenousstrategies for improved fallow land is applied insoutheast Asia (Cairns and Garrity, 1999), some ofwhich were also found in the study region. In thelast years, the process of market infrastructure de-velopment provided favourable economic conditionsfor the establishment of fruit tree orchards withpermanent vegetation cover (see Bui Huy Hien et al.,1995). Also perennial timber systems withTectonagrandisL.f. were planted.

Except cropping systems, another promising soilconservation method is minimum tillage which re-stricts clearing or weeding to smaller spots aroundplanted crops, as it was observed on a few maize fields.Occasionally, declining ditches have been applied inthe study region. Mulch lines, stone barriers and grassor weed strips have successfully been used on hill-sides in northern Thailand (Turkelboom et al., 1996).In the study region, weed strips were observed on onefield only. Microterraces have been promoted by a soilconservation project since 1998 and were adopted bysome farmers, although its long-term effectiveness hasstill to be proven.

Burning of fields and fallows for land preparationis still a common practice with short-term advan-tages in labour economy, weed and pest control, butwith disadvantages of increased nutrient leaching byinducing nutrient flushes. Farmers still have few al-ternatives to burning and need assistance to replace orat least improve this practice. Potential improvementsare the timing of fires, top–down burning, firebreaksand different aspects of ash fertilisation and tillage(van Keer et al., 1998).

5. Conclusions

Although visible soil erosion symptoms were rarein maize and cassava fields of Yen Chau District,water erosion may have contributed to the decliningsoil fertility and yield at lower mid slope positionsdue to an accelerated runoff velocity with slope lengthas indicated by the topographicLS factor. In contrast,the LS factor cannot explain the continued decreaseof topsoil depth and nutrient content from lower midslope to concave positions in the complete topose-quence study. Thus, although historic records areunavailable, it can be assumed that more frequent his-toric cropping activities towards the hill bottoms haveplayed the major role in the soil fertility and yield de-cline at lower slope positions. The increased croppingfrequency may also have enhanced soil organic mat-ter decomposition which is supported by the selectivedepletion of OM, N and available P which are closelyrelated to organic matter. The general implication totoposequence studies for assessing erosion effects isthat they are likely to be confounded with historicfarmers’ preferences to cultivate easier accessible,lower slope positions. Irrespective of the causes ofdegradation, soil fertility ranks low in farmers priori-ties which are currently focusing on stabilising yieldsthrough the introduction of new maize varieties. Thus,soil conservation measures are rarely applied byfarmers. Consequently, soil conservation and fertili-sation strategies such as vegetative barrier and covercrops with food and fodder species, fruit and timbertrees and minimum tillage should be promoted thatconsider farmers’ long- and short-term interests.

Acknowledgements

Many thanks to Dr. Dao Chau Thu (SustainableAgriculture Research and Development Center, HanoiAgricultural University) for preparing the field trip andcooperation. Highly acknowledged is the collectionof the cassava field data by N. Hirth. Further thanksto B. Heider and A. Luibrand who went through themanuscript with valuable remarks, and especially tothe reviewers who improved the paper substantially.Financial support was provided by the Eiselen Foun-dation Ulm, Germany and the Schimper FoundationStuttgart-Hohenheim, Germany.


References

Bui Huy Hien, Dao The Anh, Le Van Tiem, Mai van Thiet, NguyenManh Trung, 1995. Etude sur la durabilité de l’agriculture d’unecommune de montagne du Nord-Ouest Vietnam. In: INSA,CIRAD-URPA (Eds.), Durabilité du développement agricole auNord-Vietnam. Etudes de cas. Maison d’édition de l’agriculture,Hanoi, pp. 114–172.

Cairns, M., Garrity, D.P., 1999. Improving shifting cultivation insoutheast Asia by building on indigenous fallow managementstrategies. Agrofor. Syst. 47, 37–48.

Cramb, R.A., Garcia, J.N.M., Gerrits, R.V., Saguiguit, G.C.,1999. Smallholder adoption of soil conservation technologies:evidence from upland projects in the Philippines. LandDegradation Dev. 10, 405–423.

Eeuwes J.G.C.M., 1995. Upland use in Vietnam. Institutionsand regulations of forest management systems: Black Thaicooperatives in Son La province. Wageningen AgriculturalUniversity, Wageningen, 146 pp.

Fenton, T.E., Brown, J.R., Mausbach, M.J., 1999. Effects oflong-term cropping on organic matter content of soils:implications for soil quality. In: Lal, R. (Ed.), Soil Quality andSoil Erosion. CRC Press, Boca Raton, FL.

Fiez, T.E., Pan, W.L., Miller, B.C., 1995. Nitrogen use efficiencyof winter wheat among landscape positions. Soil Sci. Soc. Am.J. 59, 1666–1671.

Foster, G.R., Wishmeier, W.H., 1974. Evaluating irregular slopesfor soil loss prediction. Trans. ASAE 17, 305–309.

Friederichsen, J.R., 1999. Assessment of erosion control infarming systems in northwestern Vietnam. InterdisciplinaryStudy Project 1999, Hohenheim University, Germany; HanoiAgricultural University, Vietnam; Chiang Mai University,Thailand, 10 pp.

Jamieson, N.L., Le Trong Cuc, Rambo, A.T., 1998. Thedevelopment crisis in Vietnam’s mountains. East–West CenterSpecial Report No. 6, Honolulu, Hawaii, 32 pp.

Jones, A.J., Selley, R.A., Mielke, L.N., 1990. Cropping and tillageoptions to achieve erosion control goals and maximum profiton irregular slopes. J. Soil Water Conserv. 45, 648–653.

Lapar, M.L.A., Pandey, S., 1999. Adoption of soil conservation:the case of the Philippine uplands. Agric. Econ. 21, 241–256.

Le Ba, T., 1997. Viet Nam: the country and its geographicalregions. The Gioi Publishers, Hanoi, 617 pp.

Mahler, R.L., Bezdicek, D.F., Witters, R.E., 1979. Influence ofslope position on nitrogen fixation and yield of dry peas. Agron.J. 71, 348–351.

McConkey, B.G., Ulrich, D.L., Dyck, F.B., 1997. Slope positionand subsoiling effects on soil water and spring wheat yield.Can. J. Soil Sci. 77, 83–90.

McCool, D.K., Wishmeier, W.H., Johnson, L.C., 1982. Adaptingthe universal soil loss equation to the Pacific Northwest. Trans.ASAE 26, 928–934.

McCool, D.K., Brown, L.C., Foster, G.R., Mutchler, C.K., Meyer,L.D., 1987. Revised slope steepness factor for the universalsoil loss equation. Trans. ASAE 30, 1387–1396.

McIsaac, G.F., Hirschi, M.C., Mitchell, J.K., 1991. Nitrogen andphosphorus in eroded sediment from corn and soybean tillagesystems. J. Environ. Quality 20, 663–670.

Miti, T., 1991. The relationship between channel erosion in maizeunder mechanized monoculture and the erodibility of soilssurrounding Lubumbashi (Shaba, Zaire). Geogr. Econ. Trop.15, 105–123.

Mohammad, M.J., Pan, W.L., Kennedy, A.C., 1995.Wheat responses to vesicular–arbuscular mycorrhizal fungalinoculation of soils from eroded toposequence. Soil Sci. Soc.Am. J. 59, 1086–1090.

Napier, T.L., 1996. Socio-economic factors affecting adoption ofsoil and water conservation practices in lesser-scale societies. In:Buerkert, B., Allison, B.E., von Oppen, M. (Eds.), Proceedingsof the International Symposium on Wind Erosion in WestAfrica: The Problem and its Control, University of Hohenheim,Germany, 5–7 December 1994, Margraf Verlag, Weikersheim,Germany, pp. 297–306.

Napier, T.L., Napier, A.S., Tucker, A., 1991. The social, economicand institutional factors affecting adoption of soil conservationpractices: the Asian experience. Soil Tillage Res. 20 (2/4), 365–382.

Nguyen, T.D., 1995. Geography of Vietnam. The Gioi Publishers,Hanoi, 191 pp.

Ongprasert, S., Turkelboom, F., 1996. 20 years of alley coppingresearch and extension in the slope of northern Thailand.In: Turkelboom, F., van Look-Rothschild, K., van Keer, K.(Eds.), Proceedings of the Conference on Highland Farming:Soil and the Future? Chiang Mai, Thailand; Soil FertilityConservation Project, Maejo University, Thailand; KULeuven,Belgium, 21–22 December 1995, pp. 55–71.

Pan, W.L., Hopkins, A.G., 1991. Plant development, and N andP use of winter barley. I. Evidence of water stress-induced Pdeficiency in an eroded toposequence. Plant and Soil 135, 9–19.

Putthacharoen, S., Howeler, R.H., Jantawat, S., Vichukit, V., 1998.Nutrient uptake and soil erosion losses in cassava and six othercrops in a Psamment in eastern Thailand. Field Crop Res. 57,113–126.

Rerkasem, K., Rerkasem, B., 1995. Montane mainland southeastAsia: agroecosystems in transition. Global Environ. Change5 (4), 313–322.

Sam, D.D., 1994. Shifting cultivation in Vietnam. Country Report,IIED Forestry and Land Use Series No. 3.

Sanroque, P., Rubio, J.L., Izuierdo, L., 1990. Relationships amongerodibility, parent materials and soil type in areas of Valenciaprovince (Spain). Soil Technol. 3, 373–384.

Schlichting, E., Blume, H.P., Stahr, K., 1995. BodenkundlichesPraktikum: Eine Einführung in pedologisches Arbeiten fürÖkologen, insbesondere Land- und Forstwirte und fürGeowissenschaftler. Blackwell-Wissenschafts Verlag, Berlin,295 pp.

Sharpley, A.N., 1985. The selective erosion of plant nutrients inrunoff. J. Soil Sci. Soc. Am. 49, 1527–1534.

Sikor, T., 1995. Decree 327 and the restoration of barren land in theVietnamese Highlands. In: Rambo, A.T., Reed, R.R., Le TrongCuc, DiGregorio, M.R. (Eds.), The Challenges of HighlandDevelopment in Vietnam, East–West Center, Honolulu, Hawaii;Center for Natural Resources and Environmental Studies, HanoiUniversity; Center for Southeast Asian Studies, University ofCalifornia, Berkley, CA, pp. 143–156.


Sikor, T., 1999. The political economy of decollectivization: astudy of differentiation in and among Black Thai villages ofnorthern Vietnam. Ph.D. University of California, Berkeley, CA.

Souvanthong, P., Symmavong, N., Keosacksit, K., 1994. Shiftingagriculture in Laos. In: Proceedings of the Workshop on ShiftingCultivation, Chiang Mai University, Chiang Mai.

Statistical Publishing House (SPH), 1996. Statistical Data ofAgriculture, Forestry and Fishery 1985–1995. Hanoi, Vietnam.

Tiessen, H., Cuevas, E., Chacon, P., 1994. The role ofsoil organic matter in sustaining soil fertility. Nature 371,783–785.

Turkelboom, F., Anderson, M., Ongprasert, S., 1996. Indigenoussoil conservation strategies in the highlands of northernThailand. In: Turkelboom, F., van Look-Rothschild, K., vanKeer, K. (Eds.), Highland Farming: Soil and the future?Proceedings of the Conference on Highland Farming: Soil andthe Future? Chiang Mai, Thailand; Soil Fertility Conservation

Project, Maejo University, Thailand; KULeuven, Belgium,21–22 December 1995, pp. 72–90.

van Keer, K., Comtois, J.D., Turkelboom, F., Ongrasert, S., 1998.Options for soil and farmer friendly agriculture in the highlandsof northern Thailand. Tropical Ecology Support Programme,GTZ, Eschborn, Germany, 184 pp.

Wan, Y., El-Swaify, S.A., 1997. Flow-induced transport andenrichment of erosional sediment from a well-aggregated anduniformly-textured Oxisol. Geoderma 75, 251–265.

Wezel, A., Luibrand, A., Le Quoc Thanh, 2001. Temporal changesof resource use, soil fertility and economic situation in uplandnorthwest Vietnam, in press.

Wishmeier, W.H., Johnson, C.B., Cross, B.V., 1971. A soilerodibility nomograph for farmland and construction sites. J.Soil Water Conserv. 26, 189–193.

Yen Chau, 1999. Statistical Data of Yen Chau District. StatisticalDepartment, unpublished.

slope position effects on soil fertility and crop productivity and implications for soil...

Documents