Use of Tree/Grass Hedges for
Soil Erosion Control in the
Central Kenyan Highlands
S.D. Angima, M.K. O'Neill, A.K. Omwega, and D.E. Stott
A B S T M C T Three erosion control methods of using a tree hedge, a grass hedge, and a combination o f the two were used on an alfirol in central Kenya. Soil loss, biomass yield, andprofile survey of the runoffplots were measured during two cropping seasons. Average cumuLative soil loss
fiom plots with hedges of tree, combination, grass, and non hedged control were 5.6 7.4, 11.2,
and 10.9 Mg ha-', respective&. Dry matter yield were 2.98, 9.24, and 11.90 Mg ha-' yr-I for
tree, combination, and grass bedge, respectively. Topographic survey of the plots showed a near
un@rm terraceformation and decrease in slope of about 0.2 %for all bedges, but an increase in
slopefor the controlplots by the same magnitude. Small-scale farmers in the highlands of Central
Kenya who practice a mixed farming system could use this soil conservation technology as a step
towards sustainable farming practices.
S
everal methods exist for control of soil
erosion by water. In Kenya, most soil
conservation efforts have focused on
the use of contour plowing, residue lines
(windrows), rows of stones, terraces,
mulching and cutoff drains, all aimed at reducing the slope and increasing infdtration
of rain water. Farmers, however, may adopt
soil conservation measures that provide
benefits in addition to erosion control,
such as the use of tree hedges (hedgerows)
on contours for fodder production.
The use of hedgerows falls under the
designation of an agroforestry system.
Agroforestry usually refers to systems that
include trees in association with crops
and/or fodder (pasture) to maintain or improve soil fertility, crop production, and
erosion control through the use of multipurpose trees and contribute to farm income (Nair 1984; Leakey 1996; Ohlsson
and Shepherd 1992). Trees in hedgerows
act as vegetative barriers along the contour
of a slope and maintain soil organic matter
through leaf fall and root residues, while
the area between the hedges is used for
While this research was conducted, Samson Angima
was a graduate research assistant and M i d 0 'Neil1
was a senior agronomist with the International
Centre for Research in Agroforesty at Nairobi,
Kenya. Current&, Samson D. Angima is in the Department of Agronomy at Purdue University and
Mic& O'Neill is assistant professor and superintendent with the Agricultural Science Center at New
Mexico State University. Asenath Omwega is with
the Intermediate Technology Development Group
in Nairobi Kenya and Diane Stott is a soil microbiologist at the USDA ARS, National Soil Erosion
Research Laboratory at West Lafiyette, IN.
agricultural production (McDonald 1997).
Desirable characteristics of the trees in
hedgerows include an adequate supply of
viable seed, vigor, fast growth, nitrogen
fixation, copious biomass for production
of mulch, fodder, fuelwood, and other
useful by-products specific to the species.
Hedgerows lead to progressive development of terraces through accumulation of
soil upslope of the hedge and stabilization
of terrace banks by stems and roots
(Young and Sinclair 1997). Such systems
provide sustainable alternatives for areas
where the human population density is
increasing rapidly, reaching the point
where large scale land management is no
longer possible and marginal lands are
coming under cultivation (Fujisaka 1997).
One tree species that has had remarkable
success in soil conservation and nutrient
cycling and retention within agroforestry
systems is calliandra ( Callidndra calathyrsszw)
(Paterson 1994). Calliandra is a leguminous species indigenous to Central America that grows to about 10 m in humid
tropical regions up to an altitude of 1800
m. Through biological N fmation, erosion
control, and green manure/leaf litter, calliandra can improve soil quality and increase yields of associated crops and other
grass species such as Napier grass (Pennisetarn perpureurn) (National Research Council 1983; Nitrogen Fixing Trees Association 1988; Goudreddy 1992).
Napier grass is a tall perennial grass
reaching over 3 m high, resistant to
drought, and grows at altitudes of up to
2400 m with a minimum rainfall of 900
mm (Henderson and Preston 1959).
V O L U M E 5 5 N U M B E R 4 2000
478
Copyright © 2000 Soil and Water Conservation Society. All rights reserved.
Journal of Soil and Water Conservation 55(4):478-482 www.swcs.org
Keywords: Agroforestry, calliandra, erosion, fodder, Napier grass, runofi soil conservation, vegetative hedges
Propagation is facilitated through splits and highlands. It lies on latitude 000 30’ S,
cuttings and grows for three to four years longitude 370 27’ E with an altitude of
before yields start declining. Biomass yields 1480 m above sea level (O’Neill et al.
are limited by N and range between 12 and 1993). Dominant soils are typic Paleustdfs
150 Mg ha-’ yr-l, depending on soil fertili- (Soil Survey Staff‘ 1990) Humic Nitosols,
ty, management, and the variety of Napier (FA0 1990), with medium to high inhergrass used (Henderson and Preston 1959; ent fertility and relatively high base saturaPurseglove 1985; Orodho et al. 1992). To tion (O’Neill et al. 1993). The average
alleviate the N problem, fertilizer N is ap- bulk density of the Ap horizon is 1.10 g
~.
annual rainfall is 1250 mm,
plied or Napier grass is grown in association ~ m -Average
which
comes
in
two rainy seasons termed
with a leguminous species (Synders et al.
1992). Estimates indicate that legume/grass the long rains (March-September) and
associations may fur 100-200 kg N ha-’ yr-’ short rains (October-February).
Erosion plots. Runoff plots measuring 5
(Williams and Burt 1983). Erosion control
is achieved through the combination of cal- x 30 m were established on an18% slope.
liandra and Napier grass because of the There were eight plots, with four replicatcombined effect of stem strength from cal- ed treatments in a randomized complete
liandra and a massive near-surface lateral block design. The treatments were: 1.) a
control with no hedge, 2.) a grass hedge
root system from Napier.
consisting
of two rows of Napier grass
A study conducted in the central highlands of Kenya revealed a shortage of fod- (Pennisetum perpureum), 3.) a tree hedge
der for the growing dairy industry and a consisting of two rows of calliandra (Calprotein gap in the diet of the milking an- liandra calothyrsus),and 4.) a combination
imals (Minae and Nyamai 1988). Due to hedge consisting of one row of calliandra
the high population density in this region and one row of Napier grass. The hedges
(450-700 persons km-2) and the very were placed in the middle and bottom of
small farm sizes that range from 0.5-4 ha the plots at 15 m intervals. Spacing within
(O’Neill et al. 1993), integration of such the hedgerows was 50 cm for Napier grass
protein rich fodder species as calliandra and 25 cm for calliandra since Napier grass
into already existing Napier fodder banks develops suckers that quickly close up the
may increase production and provide the spacing gap. Spacing between the two
much needed protein requirement of rows of hedges in all treatments was 75
livestock. Contour planting of this cm. The calliandra row preceded the
species on slopes could play multiple Napier grass row upslope of the hedge in
roles by providing fodder, increasing order to ease competition by calliandra to
water infiltration, decreasing erosion, and adjacent crops. Calliandra was inoculated
enhancing soil fertility through the nitro- with an appropriate Rbizobium pp. before
planting to enhance the legume’s nitrogen
gen fixing ability of calliandra.
The objectives of this study were to de- fixing capability. Maize (Zed mays) was
termine to what degree various combina- grown on all plots during the two croptions of hedges would: a,) reduce runoff ping seasons at the recommended density
and soil loss, b.) assist in terrace forma- for the area of 5.3 plants m-2(0.25x 0.75
tion, and c.) produce biomass for use as m). Harvesting of the hedges was carried
mulch or fodder for animals. We hypoth- out twice during each growing season.
esized that a hedge consisting of both the Napier grass was cut at 5 cm above the
tree (Calliandra calothyrsus) and the grass ground while calliandra stems and leaves
(Pennisetum purpureum) components were cut at 50 cm above the ground.
Plot boundaries were constructed using
would reduce runoff and erosion, enhance
natural terrace formation, and produce galvanized iron sheets 50 cm high and inhigher quality biomass better than when serted to 20 cm below the soil surface.
Runoff was measured by a tipping bucket
either species is used separately.
system (Khan and Ong 1997). The tipping
Materials and Methods
bucket system consisted of a bucket separatStudy site. This study was carried out ed into two parts of three liters capacity
in Embu, Kenya as part of the National each, and a tray that collected the runoff
Agroforestry Research Project, which is a and poured it into one section of the bucket
collaborative venture between the Kenya at a time. A metallic drain pipe 5 cm in diAgricultural Research Institute (KARI), ameter attached to either side of the tipping
the Kenya Forestry Research Institute bucket collected a sub sample of sediment
KEFRI), and the International Center for each time the bucket tiped. The sub sample
Research in Agroforestry (ICRAF). The contained 1% of the runoff sample from
research site is representative of the high each tip of the bucket. This sample was
productive areas in the central Kenyan measured and dried and represented a per479
J O U R N A L OF S O I L A N D W A T E R C O N S E R V A T I O N
centage of the total runoff and soil loss for
that particular event (Khan and Ong 1997).
A data logger (Campbell CRD-10)’provided a minute-by-minute record of the runoff
rate paralleled with rainfall data fiom a tipping rain gauge.
Topographicsurvey. Two types of topographic surveys consisting of 39 profile
points per plot per survey were carried out
at the end of each cropping season to determine the general relief of the runoff
plots using a level and rod survey equipment. A general topographic survey was
carried out on all runoff plots to determine
the average slope. The distance between
successive readings down the slope was 2.5
m at three profile points across the plot. A
more detailed specific topographic survey
was carried out on the six plots with
hedges and was measured at 0.5 m intervals at 1.5 m above and below the middle
hedge, and 1.5 m above the lower hedge.
The purpose of this survey was to find the
extent to which each set of hedges would
prevent runoff through terrace formation
due to soil deposition upslope of the
hedge. Below the hedge, scouring of soil
was expected, and therefore, the survey
gave the magnitude of such scouring.
Statistics. Analysis of variance
(ANOVA) was determined for the runoff
and soil loss data using an SAS program
(SAS Institute 1998). This information
was used to determine if differences observed between the treatments and control
were significant using the F-ratio. Duncan’s multiple range test was used to determine if the differences between treatments
were significant at the 5% level.
Results and Discussion
Rainfall, runofl and soil loss. Erosion
data were collected from October 1993 to
September 1994. Rainfall during the study
period totaled 837 mm, considerably less
than the annual average for this area of
1250 mm per annum (Table 1). During
erosive storms, intensities ranging between
25 and 150 mm hr-’ lasting up to 20 minutes were recorded. About 40% of the precipitation fell in the short rainy season (October-February) while 60% fell in the long
rainy season (March-September)but only 7
of the 62 rainfall events (11%) were dassified as erosive because they produced runoff
and soil loss (Table 1). Of these seven
storms, an average of 5.5% of rainfall went
into runoff and the rest infiltrated, evaporated, or was used by the crops. The largest
numbers of erosive storms (five) were in
November and April when crops have just
germinated and ground surface cover is
minimal. In view of this and of the high
amount of rainfall totals in these two
months, emphasis should be placed on conservation measures that focus on reducing
sediment transport in runoff, as these periods contribute significantly to accelerated
soil erosion.
Measured runoff and soil loss were less
in the tree hedge and the combination
hedge as compared to the control but not
significantly different using Duncan's
multiple range test at the 5% level (Table
2). The grass hedge showed higher sediment losses than the control by 0.3 Mg
ha-' due to initial poor establishment
from drought that required replanting of
the hedge. However, after the end of the
second cropping season, the grass hedge
showed a reduction in overall slope
(Table 3). These results were not surprising, as the hedges were in the establishment phase. Complete establishment of
hedges takes up to four years depending
on the species used, prevailing climatic
conditions, and management. This study
covered only the initial two years.
Despite there being no significant differences between the control and treatments
for runoff and soil loss (Table 2), deposition upslope of the hedges and reduction in
surface rilling were visible on plots with
hedges using marked plastic stakes posi-
tioned at different points above and below
the hedges. This phenomenon was instrumental in educating farmers who visited
the experimental site on the effects of sheet
and rill erosion and how it can be controlled before the effects of soil degradation
on crop yields could become grossly apparent and soil fertility restoration becomes
too costly for the small-scale farmers.
Topographic samey. A topographic survey showed terrace formation of varying
degrees in all the hedge plots. Terrace formation reduces slopes, with lower slope
values indicating the potential for less
runoff and soil loss. There was an overall
increase in slope for the control plots according to the general topographic survey
(Table 3). In the specific topographic survey, the plots with hedges show an overall
slope reduction above the contour hedges.
The combination hedge had the lowest
overall decrease in slope of 0.1 Yo, while
the grass and tree hedge showed a 0.2 Yo
decrease in slope above the lower hedge.
All the hedges showed a 0.1% decrease in
slope above the middle hedge. The control plots had a 0.2% increase in slope at
the bottom of the plot at a position in
line with the plots with hedges. There was
negligible scouring below the middle
hedge in the hedge treatments.
In south central Honduras, live barriers of vetiver and Napier grasses for soil
erosion control showed soil accumulation
by barriers of 2.6-1 1.2 cm upslope of the
hedge and natural terrace formation of
5.2-13.8 cm after three years (Walle and
Sims 1998). This shows the potential of
using live barriers to help in soil erosion
reduction and gradual reduction in slope.
Biomassproduction. Biomass production
from the hedges was highest with the use of
Napier grass hedges, which produced 11.9
Mg ha-' yr-' because Napier produces high
tonnage of biomass. The combination
hedge yielded a combined biomass of 9.24
Mg ha-' yr-' and the tree hedge a biomass
of 2.98 Mg ha-' yr-' (Table 4).
Napier grass is used by farmers as
roughage for animals and has very low
crude protein but high fiber content. It is
also an effective soil mulch material
(Henderson and Preston 1959). Calliandra leaves are very high in crude protein
of about 24%, which improves animals'
diet and is a good nitrogen source if incorporated into the soil (Paterson 1994).
Calliandra stems could be used as a
source of fuel wood or stakes in vegetable
gardening. From these results there is an
indication that hedges of fodder species
can be used to control erosion while at
Table 1. Average monthly rainfall, monthly rainfall and storm characteristics during the
1993 short and 1994 long rainy seasons, Embu Kenya.
~
Month
Season
October
November
December
January
February
March
April
May
June
July
August
SeDtember
1993 SR
1993 SR
1993 SR
1993 SR
1994 SR
1994 LR
1994 LR
1994 LR
1994 LR
1994 LR
1994 LR
1994 LR
Total
Average
rainfall P+
(mm>
194.0
194.2
61.3
26.0
28.6
123.O
3 19.9
169.7
34.7
25.4
31.8
43 .O
1251.6
Monthly
rainfall
(mm)
51.2
241.8
35.8
1.4
0.2
70.0
363.0
74.4
0.0
0.0
0.0
0.0
837.8
~~
Number of
storms
Erosive
storms
3
17
3
1
1
7
23
7
0
0
0
0
0
2
0
0
0
0
3
2
0
0
0
0
62
7
I-
SR = Short rainy season, LR = Long rainy season
t'
Source: Regional Research Centre - Embu Meteorological Station No. 09037202
VOLUME 5 5 NUMBER 4 2000
480
Table 2. Total runoff and soil loss amounts using various combinations of hedges at the
end of the two-season study period in Embu, Kenya.
Runoff (mm)
Treatment
14.3
14.3
7.5
10.1
Control
Grass hedge
Tree hedge
Combination hedge
Soil loss (Mg ha-')
at
a
a
a
~~~
10.9
11.2
5.6
7.4
at
a
a
a
~~
' If same within-column letter appears, differences are not significant at the 5 % level by
Duncans multiple range test.
the same time provide useful farm products that increase gross productivity of
the farm. Production is likely to increase
as the hedges continue to develop.
Conclusion
O u r research shows that different
arrangements of hedges can be used to
control runoff and soil erosion and can be
utilized on the farm depending on the
choice of the vegetative species that meets
individual needs of the farmers. The calliandra hedge could be used by farmers
who want to control erosion and also utilize the plant protein source for the diet
of the animals or for use as mulch to enrich the soil of N. Napier grass could be
used by farmers who would like to control erosion and can afford commercial
protein supplements for the animals
while using Napier grass as a source of
roughage. The combination hedge could
be used as a balance of the two systems
both for erosion control and fertility enhancement or as a source of high quality
fodder for their livestock. All three methods will conserve soil for both crop and
animal production systems within an environmentally sustainable framework.
ENDNOTE
Trademark names are provided for the convenience of the reader and imply no endorsement by
the authors or their respective institutions.
ACKNOWLEDGEMENT
This work was a collaborative effort between the
Kenya Agricultural Research Institute (KARI), the
Kenya Forestry Research Institute (KEFRI), and
International Center for Research in Agroforestry
(ICRAF).The research was made possible through
funds provided to the National Agroforestry Re-
search Project by the Swedish International Development and Cooperation Agency (Sida).
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Revised legend. Rome: Food and Agriculture
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Forum 8:8-11.
Goudreddy, B.S. 1992. Fodder trees. Pp 4 6 4 8 . In:
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Henderson, G.R. and P.T. Preston. 1959. Fodder
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Khan, A.H.A. and C.K. Ong. 1997. Design and
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Table 3. Final component slopes for the different treatments from topographic survey at
the end of the two-season study period in Embu, Kenya.
Slope measurement
Control
Grass hedge
.............................
Original slope above
middle hedge
New slope above middle
hedge
Mean decrease in slope
above middle hedge
Original slope above lower
hedge
New slope above lower
hedge
Mean decrease in slope
above lower hedge
Combination
hedge
(%) .............................
--
14.7
14.7
15.1
--
14.6
14.6
15.0
--
0.1
0.1
0.1
15.2
19.8
19.2
17.6
15.4
19.6
19.0
17.5
0.2
0.2
0.1
-0.2
t The control treatment had an average slope increase of
481
Tree hedge
0.2%.
J O U R N A L O F S O I L A N D WATER CONSERVATION
Table 4. Total biomass production from the hedges expressed as 14 contour rows of
double hedge per hectare at the end of the two-season study period in Embu,
Kenya.
Biomass type
Calliandra leaves
Calliandra stems
Grass
Total biomass
0.00
0.00
0.00
0.00
McDonald, M.A., P.A. Stevens, J.R. Healey, and
P.V.D. Prasad. 1997. Maintaining soil fertility
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O'Neill, M.K., F.M. Murithi, O.Z. Nyaata, S.P.
Gachanja, D. Mugendi, P. Tuwei, and H.C.J.
Thijssen. 1993. National Agroforestry Research
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Synders, P.J.M., A. Nahuis, F. Wekesa, and A. P.
Wouters. 1992. Yield and quality of a mixture
of Napier grass and green leaf Desmodium at
2.14
0.84
0.00
2.98
0.00
0.00
11.90
11.90
two cutting regimes. Naivasha: National Animal
Health Research Center.
Walle, R.J. and B.G. Sims. 1998. Natural terrace
formation through vegetative barriers on hillside
farms in Honduras. American Journal of Alternative Agriculture 13:79-82.
Williams, W.T. and R.L. Burt. 1983. A multidisciplinary approach to tropical pasture improvement (Stylosanthes hamata, legume cultivars).
In: R.L. Burt et al. (eds). The role of Centrosema, Desmodium, and Stylosanthes in improving tropical pastures. Boulder: Westview Press.
Westview Tropical Agriculture Series
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0.77
0.28
8.19
9.24
Young, A. and F.L. Sinclair. 1997. The effectiveness
of contour hedgerows for soil and water conservation. Agoforestry Systems 82-4.
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