Long-term agronomic and environmental effects of rock wall terraces in southern
Honduras
T.L. Thurow and H. E. Sierra
University of Wyoming, Renewable Resources Department, Laramie, WY, USA;
PanAmerican University, Tegucigalpa, Honduras
ABSTRACT
The steeplands of Honduras are home to more than 40% of the country’s
population and yield 73% of the country’s basic grains. As the steeplands are
cleared of the native forest cover, their susceptibility to landslides increases 20 to
31 fold, leading to degradation of the production potential of the field and
resulting in sedimentation problems downstream. Soil characteristics and crop
production on 30% to 60% slope steeplands in southern Honduras was evaluated
for the period 10 to 13 years after rock wall terraces had been installed. The soil
characteristics on the terraced sites were significantly better (e.g., deeper A
horizon, more organic carbon) than on adjacent non-terraced sites. This was
associated with sorghum and maize yield being significantly better on the terraced
sites than on adjacent non-terraced sites. The difference in sorghum yield
between the local variety and region’s best-producing hybrid (DVM-198) was
greater on terraced sites than the difference between the local variety and the
hybrid on adjacent non-terraced sites, indicating that application of hybrid
technology paid greater dividends on the terraced sites. Terraces moderated the
impacts of drought in the context of maize production.
INTRODUCTION
Honduras is the most mountainous and agrarian nation in Central America; 80%
of the country is steepland (slopes greater than 20%), and 60% of the households
rely on agriculture as their main source of income. Historically, the steeplands of
Honduras were covered with forests that had root systems that tied the soil to the
hillside. Due to an increasing population (approximately three percent growth per
year, 1950-1990), and the associated demand for food, approximately a million
hectares of Honduras’ forested steeplands were cleared for cultivation of crops
between 1974 to 1990 (SECPLAN, 1994). Indeed, 73% of the staple crops (e.g.,
maize, sorghum and beans) are produced on deforested steeplands (IICA, 1995).
Haplustalfs, a common soil in the region, are typically 0.2 to 1 m deep on 30 to
70% slopes. These soils have rapid infiltration rates, but are underlain by argillic
subsoils that slow water percolation. This results in the A and Bt horizons
becoming saturated during prolonged rainfall events, setting the stage for mass
slippage along the B/C horizon interface. Comparative analysis of aerial photos
taken before and after Hurricane Mitch showed landslides occurring 31 times
more often on recently-harvested steep cropland than on steep forests, i.e.,
landslides occurred on 0.4% of forested steeplands, compared with 8.2% of
planted cropland and on 12.4% of recently harvested cropland (PerottoBaldiviesto, 2000). Sloughing is also prevalent during normal rainfall periods:
according to a five year study of field-size steepland catchments with 60% slopes
in southern Honduras, 1993-1997, landslides triggered by several days of
consecutive rain were responsible for an estimated 94% of the soil loss (Thurow
and Smith, 1998). Soil loss averaged 92 tons ha-1 yr-1 on slash-and-burn fields
(traditional method) and 17 tons ha-1 yr-1 on mulched fields. In contrast, soil loss
was documented at less than one ton ha-1 yr-1 on sites that were forested or had
rock walls or vetiver grass barriers that helped to tie the soil to the hillside.
Rock wall terraces in Central America and the Caribbean have a mixed track
record of success in tropical steeplands in terms of return on investment and
voluntary maintenance (Lutz et al., 1994). A joint project sponsored by the U. S.
Agency for International Development and the Honduran Ministry of Natural
Resources, which facilitated farmers’ voluntary establishment of rock wall
terraces on the steeplands of southern Honduras, is viewed as a success by the
farmers of the region. In a survey of 75 farmers who had adopted soil and water
conservation treatments, every one that had adopted rock walls (38) continued to
voluntarily maintain them 15 years after they had been installed; each stated that
the rock wall terraces were a valuable component of their farm (Santos et al.,
2000). The objective of this paper is to quantify the terrace versus adjacent nonterrace crop yield and soil characteristics.
MATERIALS AND METHODS
Site description
The study was conducted in the Namasique watershed (13o14'N; 87o05'W),
located about 15 km southeast of Choluteca, southern Honduras. The average
temperature is 26 C; the monthly average varies only +1 C between months.
Rainfall is distributed in a bimodal pattern. The first half of the rainy season
(primera) begins in early May and ends in early July. The second half of the rainy
season (postrera) begins in late July or early August and ends in early November.
The dry period (canicula) between the two rainy seasons lasts from two to six
weeks. Average annual rainfall is 1983 mm.
Planting begins with the onset of the rainy season. Maize is intercropped with
land-race sorghum varieties. These tropical sorghums are three to four meters tall,
drought tolerant and sensitive to photoperiod. Maize is planted at the onset of the
rainy season and sorghum is planted two weeks in alternating 40 cm mounds
spaced 40 cm apart.
In 1985, a food-for-work program rewarded work crews building rock walls at the
study area. The rock walls investigated in this study were built on steep slopes
(30% to 60% slopes) with Haplustalf soils. Part of the extension effort associated
with building the rock walls was to convince the farmers not to burn their fields in
preparation of planting. Therefore mulch was present on both the non-terraced
and terraced sites since the time rock walls were established. The rock walls were
built on the contour of the hillsides, spaced 5 m to 6 m apart. They were
constructed by excavating a foundation trench about 50 cm deep and then building
a rock wall to a height of 1 m above the surface; no cementing material was used.
This wall height/spacing configuration was not designed for maximum soil
retention, rather it was the best design that could achieve general consensus for
farmer participation. This meant that when the walls filled in with sediment the
slope of the terraces was about half of the original slope, i.e., maximum protection
that would be associated with bring the bench grade to 0% was not attainable.
Sampling methods
The 1985 program participants stopped building the terraces when the food-forwork subsidy ended, resulting in rock terraces sometimes stopping mid-field. We
selected six sites where the owner of a site farmed the same field the same way,
the only difference being that part of the field had a rock terrace and the other part
of the field did not. These sites were split to test performance of the local
sorghum variety with the best hybrid for the area (DVM-198; 50% local land-race
genotype, 50% exotic genotyes). The most common maize variety used in the
region (HR-15) was used for the maize trials. The local maize-sorghum intercrop
practice common for the area (described above) was used, with maize planted at a
rate of 20 kg ha-1 and sorghum planted at a rate of 18 kg ha-1. Maize was
fertilized with urea at the rate of 37 kg N ha-1 and 41 kg P2O5 ha-1. Sorghum was
fertilized with urea at a rate of 30 kg N ha-1.
Harvest data from six fields (i.e., six replications) were collected from a 2 m wide
by 5 m long (i.e., length between rock walls) plot within each of three terraces;
corresponding samples were also collected on the adjacent non-terraced areas on
each of the fields. The stover and grain from these plots were oven dried at 60oC
for 48 hr. Analysis of variance was conducted using significance levels of p<0.1,
with treatment means separated by Duncan’s multiple range test (p < 0.1).
On one of the study fields, soils were analyzed from 11 pedons within trenches on
four rock wall terraces and 10 pedons were analyzed in the adjacent non-terraced
sites. For this analysis of variance the terraces constituted the replications and the
pedons as samples within the replications.
RESULTS AND DISCUSSION
Farmers stated that they perceived their rock wall terraced land as being more
productive than adjacent non-terraced parcels seven to ten years after they had
constructed the rock walls. A testament to the perceived value of rock walls was
that farmers voluntarily maintain them without further subsidies.
This study quantifies and validates the perception of the farmers. The rock walls
were built in 1985, therefore they were ten to 13 years old during our study period
of 1995 to 1998. Table 1 illustrates that sorghum and maize grain yield and
biomass production was significantly better on the terraced sites than the adjacent
non-terraced sites.
The sorghum hybrid averaged 1.1 ton ha-1 greater grain yield than the local
sorghum variety on the terraced sites. On the adjacent non-terraced sites, the
sorghum hybrid averaged 0.4 ton ha-1 greater yield than the local. This indicates
that the value of hybrid seed development is enhanced when planted on terraced
sites. Conversely, this implies that if soil is not conserved, the benefits of future
hybrid seed technologies have substantially less potential than if the soil had been
retained on the site. This point is further reenforced by noting that the local
sorghum variety grain yield averaged 0.7 ton ha-1 more when planted on the
terraced sites compared with the adjacent non-terraced sites, but the sorghum
hybrid grain yield averaged 1.4 ton ha-1 more when planted on the terraced sites
compared with the adjacent non-terraced sites. Also, grain yield of the local
sorghum variety planted on the terraced sites produced significantly more grain
(0.3 ton ha-1) than was produced by the hybrid on the adjacent non-terraced sites.
Farmers in the region repeated stated that they especially valued the terraced sites
because they helped to reduce the risk and consequences of drought. The nature
of the bimodal precipitation pattern in the region and the variation in the start and
length of the intervening dry period (canicula) is particularly important with
regard to maize production, since the canicula coincides with the period when
maize grain develops. For example, in 1995 the canicula lasted about six weeks
Table 1. Grain yield and standing crop at harvest on terraced (T) and non-terraced
(No T) sites in Namasique, Honduras. Means for sorghum or maize followed by
the same number in the same year are not significantly different (p<0.1).
__________________________________________________________________
GRAIN YIELD (ton ha-1)
1995
1997
1998
3-yr Average
T
No T
T
No T
T
No T
T
No T
Sorghum
Local Variety 1.8b 0.8c
1.5c 1.2d
1.9b 1.1d
1.7b 1.0d
DVM-198
2.7a 0.9c
2.8a 2.0b
2.9a 1.4c
2.8a 1.4c
Maize
Local Variety
2.3a
1.3b
0.8a
0.7a
3.0a
2.2b
STANDING CROP (STOVER) AT HARVEST (ton ha-1)
1997
1998
T
No T
T
No T
Sorghum
Local Variety
10.5b 10.7b
8.7b 7.2d
DVM-198
11.7a 9.5c
10.4a 8.1c
Maize
Local Variety
8.1a
6.3b
10.3a 8.9b
2.0a
1.4b
2-yr Average
T
No T
9.6b 8.9c
11.1a 8.8c
9.2a 7.6b
resulting in a 77% greater maize grain yield on the terraced sites than the adjacent
non-terraced sites. In 1998, a more normal canicula length of about three weeks
resulted in a 36% greater maize grain yield on the terraced sites than the adjacent
non-terraced sites. In 1997 the first rainy season (primara) was dry much drier
than normal and the canicula started early and lasted longer than usual (about
eight weeks), resulting in a poor maize grain yield on both sites. However, even
during this severe drought year there was a significant difference in stover
production, with 29% more stover produced on the terraced sites than the adjacent
non-terraced sites (i.e., the maize grew better on the terraced sites, but apparently
ran out of water before the grain could develop). The stover provided some
benefit to the farmer in terms of forage for their livestock, thereby softening the
impact of drought even though grain yield was very poor.
Comparison between locations within terraces illustrates that grain yield is
significantly greater immediately behind the rock wall and then declines as the
Table 2. Comparison of grain yield between locations within terraced fields
(location 1 is the first meter of field behind the rock wall, location two is the
second meter of field behind the rock wall, etc.), Namasique, Honduras. Means
within each column followed by the same letter are not significantly different
(p<0.1).
__________________________________________________________________
Location
Yield of Sorghum varieties (ton ha-1)
Yield of Maize (ton ha-1)
Within
1997
1998
1997
1998
Terrace DVM-198 Local
DVM-198 Local
Local
Local
1
3.3a
2.3a
3.3a
1.7a
0.9a
3.3a
2
3.0b
2.2a
3.1b
1.7a
0.9a
3.1b
3
2.7c
1.9b
2.8c
1.4b
0.8b
3.0b
4
2.4d
1.7c
2.7c
1.2b
0.7b
2.8c
5
2.3d
1.7c
2.4d
1.4b
0.6c
2.6d
__________________________________________________________________
distance behind the rock wall increases (Table 2). This yield pattern within
terraces, and the yield differences between terraced and non-terraced sites (Table
1), corresponds with a detailed soil analysis on one of the terraced/non-terraced
field sites. The mean thickness of A horizons of four rock wall terraces was 24
cm compared with 7 cm on the adjacent non-terraced portion of the field. In the
meter behind the rock walls the A horizons averaged 27 cm and decreased to 20
cm at 5 m behind the rock wall. The average amount of total organic carbon
stored in the A and Bt horizons was 11.8 kg m-2 of terrace compared with 6.5 kg
m-2 of adjacent non-terraced field. Each of these soil characteristic comparisons
are significantly different (p<0.1). Differences in depth of A horizon and amount
of organic carbon stored in the soil are typically associated with soil fertility and
soil moisture retention capability. This, combined with the physical impoundment
of water in the area immediately behind the rock wall, helps to explain why the
terraced sites yield more and why terraced sites provide a buffer to the impacts of
the dry period associated with the canicula.
The greater A horizon soil depth on the terraced sites compared to adjacent nonterraced sites was likely a due to a combination of some soil washing into the
terraces from above (construction of terraces usually originated at a road or path at
a bottom of the field and progressed up the hillside, rarely resulting in the upper
portions of the hillside fields being terraced) and from a secession of accelerated
erosion on the terrace. Most of the non-terraced sites had experienced several
landslides since 1985, but there had been no landslides on the terraced sites.
Many of the farmers in the Namasigue region who adopted rock walls in the mid1980s reported having made incremental changes in the farming systems on their
terraced fields. They observed better grain production on the terraces and began
experimenting with planting a mixed assemblage of fruit trees on those sites. As a
result, many sites where rock walls were installed in the mid-1980s are now
undergoing transformation into orchards, resulting in as much as ten-fold increase
in annual income from the terraced sites (Santos et al., 2000). Thus, about 40
years after the native forests were cleared, many of these terraced sites are in the
process of returning to forest cover (albeit as fruit trees instead of native forest).
CONCLUSIONS
Small farmers faced with a choice between maximizing present income or making
an investment in long-term benefits from investments in soil conservation
techniques will usually opt for maximizing present income, even though this
increases the likelihood of deterioration of their natural resource base over the
long-term. Subsistence farmers knowingly make this tradeoff due to investment
poverty (Reardon and Vosti, 1995). Therefore, even though farmers in the region
readily recognize the advantage of having rock wall terraces in their fields, the
arduous labor demands associated with building rock walls, and the possibility of
earning cash if willing to work as a seasonal migrant, result in very few people
building rock walls without subsidization despite the long-run agronomic benefits
of terracing in southern Honduras detailed here. The farmers demonstrated
willingness to maintain the walls once established and serial benefits to
downstream investments of reduced vulnerability to floods and sedimentation
associated with stabilization of steepland portions of the watershed (Samayoa et
al., 2000) make a compelling case for future soil and water conservation
investments in the region.
ACKNOWLEDGMENTS
This work was supported by the Global Bureau, United States Agency for
International Development, under the terms of the Soil Management Collaborative
Research Support Program Grant No. LAG-G-00-97-00002-00. Dr. Larry
Wilding and Dr. Richard Drees of Texas A&M University helped with some of
the soil analysis and interpretation.
REFERENCES
IICA (Instituto Interamericano de Cooperacion Agricola) (1995). Honduras:
Diagnostico del sector agropecuario. San Jose, Costa Rica.
Lutz E., Pagiola S. and Reiche C. (1994). Economic and institutional analysis of
soil conservation projects in Central America and the Caribbean. World Bank
Environment Paper Number 8. Washington D.C.
Perotto-Baldivieso H.L. (2000). Landslide hazard zonation in Namasigue and El
Triunfo, southern Honduras. M.S. Thesis, Texas A&M University, College
Station, Texas.
Reardon T. and Vosti S.A. (1995). Links between rural poverty and the
environment in developing countries: Asset categories and investment poverty.
World Development 23,9: 1495-1506.
Samayoa A.M., Thurow A.P. and Thurow T.L. (2000). A watershed-level
economic assessment of the downstream effects of steepland erosion on shrimp
production, Honduras. Technical Bulletin No. 00-1, USAID-Soil Management
CRSP/Texas A&M University, College Station, Texas.
Santos H., Thurow A.P. and Thurow T.L. (2000) Linkages between investment in
extension services and farmers’ adoption of soil conservation practices in southern
Honduras. Technical Bulletin No. 00-2. USAID-Soil Management CRSP/Texas
A&M University, College Station, Texas.
SECPLAN (Secretaria de Planificacion) (1994). IV Censo nacional agropecuario.
Tegucigalpa, Honduras.
Thurow T.L. and Smith J.E. (1998). Assessment of soil and water conservation
methods applied to cultivated steeplands of southern Honduras. Technical
Bulletin No. 98-2. USAID-Soil Management CRSP/Texas A&M University,
College Station, Texas.
Corresponding Author Contact Information:
T.L. Thurow, University of Wyoming, Renewable Resources Department,
Laramie, WY 92071, Phone: 307-766-5130, Fax: 307-766-6403,
tthurow@uwyo.edu, POSTER, Engaging diverse stakeholders in support of small
farms