TECHNICAL PAPER #58
UNDERSTANDING SOIL
CONSERVATION TECHNIQUES
By
Fred Weber
Carol Stoney
Dr. Edward Pytlik
Illustrated By
Frederick J. Holman
Published By
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax:
703/243-1865
Internet: pr-infor@vita.org
Understanding Soul Conservation Techniques
ISBN: 0-86619-277-8
[C]
1989, Volunteers in Technical Assistance
PREFACE
This paper is one of a series published by Volunteers in
Technical Assistance to provide an introduction
to specific state-of-the-art technologies of interest to
people in developing countries. The
papers are
intended to be used as guidelines to help people choose
thechnologies that are suitable to their situations.
They are not intended to provide construction or
implementation details. People are
urged to contact
VITA or similar organizations for further information and
technical assistance if they find that a
particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
illustrated almost entirely by VITA volunteer
technical experts on a purely voluntary basis.
Some 500 volunteers were involved in the
production of
the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff
included
Suzanne Brooks handling typesetting and layout and Margaret
Crouch as editor and project manager.
Co-author Fred Weber, a pioneer in the community forestry
concepts presented here, has advised
projects for over 20 years.
He wrote the original edition of the VITA publication Reforestation in
Arid
Lands, from which much of this paper is drawn, based on a
training manual he prepared for Peace Corps
volunteers in Niger.
Carol Stoney collaborated with Mr. Weber on the revisions for the
new edition of
Reforestation.. Dr.
Edward Pytlik teaches appropriate technology at West Virginia University.
Frederick
J. Holman, a landscape architect, illustrated both the book
and the additional material in this paper.
VITA is a private, nonprofit organization that supports
people working on technical problems in
developing countries.
VITA offers information and assistance aimed at helping individuals and
groups
to select and implement technologies appropriate to their
situations. VITA maintains an
international
Inquiry Service, a specialized documentation center, and a
computerized roster of volunteer technical
consultants; manages long-term field projects; and publishes
a variety of technical manuals and papers.
UNDERSTANDING SOIL CONSERVATION TECHNIQUES
by
Fred Weber, Carol Stoney, and Dr. Edward Pytlik
I. INTRODUCTION
Soil conservation efforts protect the soil from the two
primary forces of erosion, wind and water.
A
wide assortment of different soil conservation techniques
are being used today. Windbreaks and
dune
stabilization, for example, are effective methods of halting
wind erosion. Terracing, planting trees
and
other vegetation in contour strips or along contour ridges,
and gully control plantings are techniques
used in combination with physical control measures to reduce
soil erosion from water. Conservation
tilling refers to a variety of methods used to control both
wind erosion.
Some of these methods are based on traditional practices
that have been carried out by local people
for generations.
Others are relatively new, "invented" by technicians working
with local farmers or
pastoralists and still being adapted to varying site
conditions. The methods described here
provide a
practical guide for use in the field, rather than extensive
coverage of background information, theory,
and reference sources.
The Reference List and Information Source List should be consulted for
further
documentation.
The techniques described here can contribute to the
increased productivity and sustainability of land
use systems. Most
can be implemented by rural households or communities using locally available
materials. Nearly
all of the techniques involve the establishment of vegetation cover, primarily
trees
and shrubs. Some
also involve physical soil conservation methods as well, such as contour
ridges,
terraces, or walls.
This approach is intended to increase awareness of ways in which vegetative
methods can be used in combination with physical methods.
Trees and shrubs play a critically important conservation
role. They can reduce soil surface
temperatures, increase infiltration and retention of soil
moisture, provide organic matter, pump
nutrients, fix nitrogen, reduce erosion from water and wind,
form live fences, and provide shade, all
of which create better growing conditions for crops and
grasses. In addition, certain tree
species may
provide food (fruit, leaves, edible seeds, etc.) not only
for people but also for livestock, fuel, building
materials, and other important products.
Soil conservation project planning should not take a
cookbook approach. Rather, the project
design
should be adapted to specific site conditions and current
land use patterns. Species trials are
required
to meet site requirements.
Demonstration sites using more varied species, including more indigenous
species, are needed so that future selection can be made on
the basis of what has worked.
The material in this technical paper is drawn largely from
Reforestation in Arid Lands (Weber and
Stoney, 1986), which provides a comprehensive review of
reforestation methods including project
design, site and species selection, soil preparation,
nursery management, and many of the conservation
techniques presented here.
Additional material, on physical methods to control erosion, were
provided
by Dr. Pytlik.
II. CONTROLLING WIND
EROSION
Windbreaks
Windbreaks are strips of trees and other vegetation that slow
the flow of the wind, reducing wind erosion,
evaporation, and wind damage to crops.
They are sometimes referred to as
shelterbelts, although this
term usually implies a wider strip of vegetation, which
incorporates more rows of trees and shrubs than
are usually found in a windbreak.
Windbreaks have an especially high potential in farming
areas where cereal crops such as millet and
sorghum are grown.
The most successful windbreak projects to date are those found on
enclosed farm
lands and in some demonstration or pilot projects under
government or private control. The
major
obstacle to windbreak establishment in other areas has been
the difficulty and high cost of protecting the
trees against animal grazing.
Some large-scale successes have been achieved in areas where
donors,
government agencies, and local people have worked closely
together.
Highly impressive results have been observed at a CARE
project in the Majjia Valley in Niger, where
crop yields from fields protected by windbreaks are consistently
higher than those from unprotected
fields. Studies
indicate that total yields are approximately 20 percent higher, even after
accounting for
losses from land that has been taken out of crop production
to provide space for the windbreaks
(Dennison, 1986).
The effectiveness of a windbreak depends on how efficiently
the wall of vegetation blocks the wind and
confines the wind's turbulence to the zones close to the
windbreak. A vegetation density of, 60
to 80
percent seems to work best in arid zones.
A barrier dense enough to block wind passage
completely will
cause turbulence close to the ground, loosening soil
particles that can then be picked up by the wind.
As
well as removing needed topsoil, wind that is carrying soil
particles causes damage to crops through the
abrasive effect of the sediment on plant tissues.
A row of trees that provides less complete wind reduction
will also ensure that the effects of the wind
are felt farther away.
Gaps or openings in the windbreak should be avoided as much as
possible. Wind
is funneled through gaps in the tree rows, concentrating its
force and speed, so that the final effect can
be very damaging.
<WINDBREAK>
22p02.gif (300x600)
Windbreaks can furnish protection for downwind areas up to
10 times the height of the trees, provided
the windbreak consists of at least two rows of plants of
different heights. Large trees should
be chosen
for one row (see A, below).
Fast-growing species can be mixed with slower growing, longer-lived
trees,
depending on local preference.
Row B should be composed of shorter species, chosen if possible
for their
by-products, and rows C and D are auxiliary rows.
These are planted with lower, bushier trees,
shrubs,
and grasses. A well
chosen vegetation mix for windbreak composition will not only provide
protection
from the wind, but will yield secondary products as
well. The windbreak trees themselves,
if properly
harvested, can also provide significant quantities of
fuelwood and poles without jeopardizing their
primary function.
<WINDBREAK>
22p03a.gif (600x600)
Windbreaks and shelterbelts can be laid out to include
roads, trails, or driveways for livestock.
In this
way, animals and people can benefit from a shaded passageway
that otherwise would be very hot. Any
path through the windbreak should be at an oblique angle
rather than perpendicular to the tree rows.
This
will allow people and livestock to move through the
windbreak without opening a gap for the wind to
roar through.
<WINDBREAK>
22p03b.gif (486x486)
Some other points to consider about windbreaks:
o Species chosen
should obviously be suited to the soil and climate where they will be grown.
Local species
are preferred Good selections can be made from species protected by law.
Use species
local residents themselves have chosen and value.
o The most
efficient windbreaks are those with one or two rows of low-growing
shrubs or
trees on the outside and two or three rows of taller trees on the inside.
o The utility of
wider shelterbelts can be enhanced by the selection of multiple use
species for
the middle rows. Species that provide
locally consumed fruits and
medicines
should definitely be considered.
o Frequently a
combination of planting methods is highly practical when establishing
windbreaks. In other words, a
combination of nursery transplants, live fencing,
cuttings, and
stumps can be used depending on planting times in the area.
o Preparation
and protection of the site involved are possibly more important for
windbreaks
than for regular plantations. During
the rainy season when crops are
being
cultivated, the fields are effectively protected from livestock, however, after
the
harvest the
animals are often allowed to browse the crop residues left in the
fields.
Keeping livestock away from the windbreaks
during this time is difficult, and
fencing a long
narrow strip of land is costly.
o Where complex
land ownership patterns exist, it may not be possible to establish
continuous
straight tree rows across individual fields and parcels.
In this case
windbreaks may
be staggered so that they conform with established boundaries such as
borders of
fields, roads, trails, stream, and other natural or man-made features.
Staggered
windbreaks also provide the most effective
protection around towns and villages, where
they are laid
out in a pattern of overlapping blocks.
<SHELTERBELTS>
22p04.gif (486x486)
o Another possible
planting pattern is to line farm fields with wide windbreaks and to
Plant dispersed
trees inside the field.
o Many nurseries
in arid zones could benefit from the establishment of a windbreak to
protect the
seedlings from drying winds. The
nursery windbreak also serves as a
demonstration to
visitors to the nursery. If the nursery
is very small, however, a tall
windbreak might
cast too much shade on the seedlings.
<SAND STABILIZATION>
22p05.gif (270x540)
Sand Stablization
Sand stabilization is an important aspect of revegetation
and conservation activities in many arid and
coastal areas.
Shifting and blowing sand causes great damage to farmland, buildings,
installations, and
roads. Entire
settlements can be threatened by the movement of shifting dunes.
The best protection against drifting or blowing sand is to
prevent the sand from being picked up by the
wind and becoming airborne.
Conservation of existing grass and other vegetation cover is necessary
to
hold the sand in place.
Even a small disturbance such as a footpath can start the process of
erosion on
fragile dunes. Once
airborne, drifting sand can be made to settle, nevertheless, and can be kept
from
further shifting.
The first step is to determine why the natural vegetation
has not recolonized the area that is being eroded.
Various options that will remove any constraints to natural
vegetation should then be considered.
Often
the problem is being caused by animals.
Under these circumstances, little if
anything will be gained by
planting trees, unless access is first controlled.
There are basically two approaches to dune fixation:
biological and physical. The best ultimate
results
are obtained when the open area where sand is picked up can
be permanently covered by vegetation.
Biological methods include:
o Fencing off the
area to protect it from animals, so that the vegetation can regenerate
naturally.
o Establishing
hedge rows of species that can be successfully regenerated from
cuttings even in
areas where annual rainfall does not exceed 300-400mm.
Freshly cut
branches can be
partially buried in rows of shallow trenches.
o Direct seeding,
particularly of grasses, but also of woody plants such as vines,
shrubs, and
trees.
Often before grasses and other ground cover can be
reestablished, however, the movement of the sand
must be halted.
Physical dune stabilization measures include:
o Wind-baffles
(palisades), which are constructed of a variety of materials, generally
whatever is
locally available.
o
"Fore-dunes," which consist of sand or soil ridges set at
right angles to the major
winds.
They can be 1 to 5 meters high and stretch
over hundreds of meters in
length.
Heavy construction equipment is required for
large-scale efforts.
o Mechanical
surface stabilization, which is accomplished by covering exposed areas
to reduce
further erosion. Plastic sheeting,
nets, cloth, or some other fiber are used.
o Chemical surface
stabilization, which involves spraying a binder (rubber, oil, or
plastic base) on
the surface to bind particles together.
Grass seeds and mulch can
also be mixed
with the binder and sprayed on the area to be protected.
Preference should be given to biological control measures
whenever possible. However, some
physical
construction is often needed for initial plant
establishment. Usually some type of
low-cost materials are
available locally.
This barrier can take many forms and be made of a variety of materials.
<WINDBREAK FENCES USED FOR SAND STABILIZATION>
22p06.gif (486x486)
Stems and poles (3-8cm in diameter and up to 2m long) can be
used to construct a diamond pattern of
criss-cross rows across areas of open sand.
Branches can be staked out in dense rows, or
fences can be
woven from branches to construct the palisade.
By breaking the force of the wind, the palisades
keep
the exposed sand from being picked up, and the sediment load
already carried by the wind is deposited
in or behind the barrier.
Sand will become entrapped in such rows, and ridges will gradually
form. Plant
growth then becomes possible in the protected areas behind
the ridges.
Fenced in squares and other sand traps can also be
constructed of materials as basic as bundles of grain
stalks or other crop residues.
Additional possibilities include palm fronds, sticks, branches,
cardboard,
or any material that is reasonably sturdy, easily available,
and low cost. Some of the problems that
may
be encountered in maintaining the barriers include damage
from animals and termites that are attracted
to them for food.
Where sand accumulations are heavy, the barriers may have to be raised
or added to
periodically.
<TYPICAL WIND BARRIER PATTERN>
22p07a.gif (486x486)
Before beginning a sand or dune stabilization project,
planners should consider the following:
o Dune fixation
is not an appropriate conservation investment if the area that is being threat
ened by
shifting sands has no inherent value.
Unless some benefit will accrue in terms of
protection of
farmland, homes, or other property, the cost is prohibitive.
Furthermore, those
who will gain
the most from the project should also be willing to exert the most effort,
particularly
in terms of
sustaining and protecting the vegetation cover.
<DETAILS OF PALISADE NETWORK>
22p07b.gif (486x486)
o Dune fixation
projects should not be undertaken without first carefully evaluating
traditional
and current land
use attitudes, especially those governing grazing and wood cutting.
If these
are incompatible
with the restrictions needed to protect the vegetation, then changes in land
use policies and
practices must take place before dune fixation activities are initiated.
o The shifting of
live dunes is influenced by a complex set of variables, and may
change with the
seasons. It is worthwhile to observe
and measure dune movements for a
period of 12 months before starting
stabilization activities.
<DUNE STABILIZATION>
22p08.gif (600x600)
o Except under
extreme desert conditions, it is more effective to stabilize the zone of
origin of the
shifting sand, rather than concentrating efforts on the areas where the sand is
being
deposited. It is important, therefore,
to determine the location from which the sand is
being removed by
the wind.
o Project sites
that are close to or within actual desert zones will require more
intensive
efforts to stabilize shifting dunes.
Maintenance inputs will also be higher.
o The more exposed
a specific location is to the wind (near the crest of large dunes, or in
saddles between
ridges), the more difficult it is to establish vegetation.
Physical protection is
often
needed. If it is not possible to use
physical control measures, however, the area can still
sometimes be
stabilized after the top has been lost to wind erosion.
o Locally
occurring trees and shrubs have great resiliency.
In species selection, the indigenous
vegetation
should receive priority over exotics, particularly for large-scale projects.
III. CONTROLLING
WATER EROSION
Contour Strips
The most likely, logical place to use trees and shrubs to halt
erosion caused by water is across slopes,
particularly where hillside cultivation is practiced.
Properly maintained trees and shrubs,
planted in
combination with grasses and other vegetation, can
effectively control surface runoff, thereby reducing
soil losses. One
successful technique involves establishing parallel vegetation bands along
contour lines.
These contour strips will reduce runoff from the slopes
above if they are designed and maintained to
ensure a dense, multi-layered permanent ground cover.
The ground surface is protected by
successive
layers of litter, grasses, other ground plants, bushes, and
trees. A dense vegetation belt will not
only stop
or slow down runoff, but will also trap soil particles
suspended in the water that have been removed from
the more exposed areas between the strips.
<CONTOUR STRIPS>
22p09.gif (540x540)
Correct dimensioning of the D and W variables indicated in
the illustration above is important.
Many
factors affect the spacing of the strips, but the degree of
slope is the most important. If
previous efforts
to establish contour strips in the area are available for
study, these sites should be observed for evidence
of erosion to determine if the dimensions are in
proportion. Conservation services may
also have tables
or formulas appropriate for local site conditions.
If no information of this kind is available,
dimensions
can be calculated using the following table as a rough
indication of spacing in arid and semi-arid areas:
Slope
(degrees) W (meters)
D (meters)
0 2
50
5 4
47
10 5
43
20
8
38
30 10
33
40 13
28
50 17
24
60
20
20
Basis: 0-600mm mean
annual precipitation
In areas with rainfall between 600-1,000mm:
increase W by 20%
decrease D by 10%
In areas with rainfall more than 1,000mm:
increase W by 50%
decrease D by 20%
Revegetation efforts on these strips can be approached in
many ways. To simply establish some
groundcover, scarification of the ground along the contour
may be sufficient site preparation.
Furrows
can be dug by hand or by using a harrow or disc blade.
More intensive effort may consist of
additional
seedbed preparation, for instance, loosening up the soil
surface and raking along the contour.
Direct
seeding of desirable trees and shrubs may be feasible for
some species. Other trees can be
established
by cuttings. The
most direct, but also most costly, method of establishing contour strips is by
planting
nursery raised seedlings.
The primary consideration for species selection should be
local preference, because the contour strips
take a certain percentage of the land out of cultivation,
even though they are intended to increase
productivity of the total area.
Many different species can be used, often in combination. Fruit
trees are
frequently a high priority on farmland.
In other areas, trees that produce poles for
construction, rafters,
and fences may be preferred.
Particular attention should be given to vegetation layers
nearer the ground surface. Fodder plants,
such
as Guinea, napier, clover, or elephant grasses, may be of
interest for feeding to penned livestock.
Perennial bean species, produced on small woody shrubs for
human consumption, may appeal to the local
inhabitants. Contour
strips can be a good location for introducing new species on a small-scale,
experimental basis as well.
Terracing
For centuries, farmers living in hilly and mountainous
regions of the world have been terracing their
hillsides as a means to prevent soil erosion.
Terraces are simply channels cut into
hillsides, embankments
built onto hillsides, or a combination of the two
constructed across the slope of the land. They have proven
to be the most effective mechanical means of erosion control
on slopes planted in continuous row crops.
As much as 85 percent of the sediment eroded from a field
can be trapped by terracing.
There are four basic terracing designs.
In the level bench design the terraces are
parallel with the horizon,
whereas in the sloping bench design the terraces are leveled
so that their planting surfaces have a slight
downward angle. The
reverse slope or step terracing design has terraces that have planting surfaces
that
angle upward slightly.
The fourth terracing design, used primarily in conjunction with flood
irrigation,
has terraces that are parallel with the horizon and have a
built up outer edge to prevent water runoff down
the hill.
<FOUR TERRACING METHODS>
22p10.gif (540x540)
Trees Along Contour Ridges
For information on the various applicable soil conservation
measures that involve construction of
contour ridges or terraces, or excavation of infiltration
ditches, a number of texts are available.
The
Centre Technique Forestier Tropical (CTFT), the Centro
Agronomico Tropical de Investigacion y
Ensenanza (CATIE), the International Council for Research in
Agroforestry (ICRAF), and the United
Nations Food and Agriculture Organization (FAO) have all
published handbooks and technical materials
on the subject. In
addition, many of the bilateral donor organization have developed standard
texts on
the subject during the past decade.
Construction designs and extension materials
have been developed
specifically for certain countries, among them Honduras,
Kenya, Burkina Faso, and the Philippines.
There is still relatively little information available,
however, that deals with the effective combination
of biological and physical erosion control measures.
Vegetation, especially trees and shurbs, can
play
a vital role in increasing the effectiveness of soil and
water conservation efforts. Properly
established and
managed woody plants can reduce maintenance and costs on
hillside erosion control projects as well.
The following sketch shows where trees and shrubs can make
an important contribution to physical ridge
or ditch formations along the contour lines of sloping
surfaces.
<GRADUALLY DEVELOPING BENCH TERRACES>
22p11.gif (600x600)
Gully Reclamation
Permanent vegetation, especially shrubs and trees, can
reduce bank or channel bottom erosion as long
as the flow of water is not too powerful.
Vegetation can also help stabilize
mechanical protection
materials, such as large rocks positioned along banks or
bottom (rip-rap), wire mesh baskets filled with
rocks (gabions), or bales of straw or branches staked in
place to reduce water velocities.
Gullies present special problems, because they often occur
on steep slopes, and even brief peak flows
can cause serious damage.
Gully erosion is difficult to reverse once it has gotten started, and it
can quickly
destroy valuable agricultural land.
To prevent the formation of gullies along waterways, the
banks should be lined with trees and shrubs.
Trees, shrubs, and other vegetation can be established
within the gullies to control further erosion and
to help rebuild the soil layers that have been removed.
Improperly placed trees can, however, have
the
undesired effect of narrowing the channel and increasing the
speed of stream flow. The following
sketch
shows how to combine vegetation with mechanical gully
erosion control methods for optimal results.
<GULLY CONTROL:
COMBINING PHYSICAL AND VEGETATIVE METHODS>
22p12.gif (600x600)
IV. CONSERVATION
TILLING
Conservation tilling is a general term that includes a
number of tilling methods, used alone or in
combination, to conrol erosion caused by both wind and
water. The methods have in common the
goal
of disturbing the surface of the soil--as by plowing--as
little as possible.
In general, the dominant factor in determining the
effectiveness of conservation tillage practices is the
amount and distribution of crop residue left on the soil
surface. However, the amount of crop
residue
mixed into the soil during tilling, the type of soil, size
and location of untilled residue strips, contour
ridging, and surface roughness are all important factors
contributing to soil loss prevention.
Wind erosion control can be established by developing
vegetative and non-vegetative land cover,
reducing field lengths along the prevailing wind direciton,
roughening or clodding the land, and terracing
slopes and hilltops where converging winds increase velocity
and shear stress.
Crop residue and mulches help to reduce both water runoff
and the amount of sediment contained in the
runoff. Ground
roughness and clods created through tilling increase water absorption and
reduce water
runoff velocity.
Ridging on the contour also substantially reduces runoff velocity and
soil loss.
Low-till and no-till farming practices combined with residue
mulch cover and contour planting can
reduce the soil loss ratio of a field from .63 in
conventional across-slope plowing and planting to .12 in
the first year of implementation and to .04 by the end of
the fourth year. Examples of no-till
farming
systems include the following:
1. Sod-planting--in
which maize, for example, is grown in combination with cool-season
perennial
grasses.
2. Sod-strip
planting--in which six rows of maize are alternated with 8m parallel
strips of
established grasses across the general slope of the land.
Each year 1 1/4
rows of maize is
advanced down the slope and the upper border is reseeded to a
mixture of grass
and legumes.
3. Complete pasture
renovation--sod-planted maize method extended into entire
fields where
erosion is too severe to permit conventional tillage.
4. Interseeding
legumes and/or grasses into established grass.
5. Planting in
winter cover crops.
6. Planting in crop
residues.
7. Multi-cropping
systems--maximizes production by providing three crops in two
years or five
crops in four years.
<CONTOUR PLOWING AND PLANTING>
22p13.gif (600x600)
Contour plowing and planting are more popular than terracing
because of their lower production and
maintenance costs (both real money and time).
In the contour system, both the plowing and
planting are
done across the slope and follow the natural contour of the
land. Contour strip cropping is an even
ore
efficient means of soil erosion, but this efficiency is
offset by a loss of the major crop yield.
However,
the alternating forage crop compensates somewhat for this
loss.
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Buck, L.E. (ed.). 1983.
Proceedings of the Kenya National Seminar on Agroforestry,
Nov. 1980.
Nairobi:
International Council for Research in Agroforestry and the University of
Nairobi.
Delehanty, J., J. Thomson, and M. Hoskins.
1985 Majjia Valley Evaluation Study:
Sociology
Report. Niamey:
CARE International Report.
FAO. 1977.
Guidelines for Watershed Management.
Rome: Food and Agriculture
Organization
Conservation
Guide Series No. 1., 298 pp.
FAO. 1977.
Conservation in Arid and Semi-Arid Zones.
Rome: FAO Conservation
Guide
Series No. 3.
FAO. 1977. Special
Readings in Conservation Techniques.
Rome: FAO Conservation Guide
Series No. 4.
FAO. 1983.
Management of Upland Watersheds; Participation of the Mountain
Communities. Rome:
FAO Conservation Guide Series No. 8.
FAO. 1985. Sand Dune
Stabilization: Shelterbelts and
Afforestation in Dry Zones. Rome:
FAO
Conservation Guide Series No. 10.
FAO. 1985. FAO Watershed
Management Field Manual: Vegetative and
Soil Treatment
Methods.
Rome:
FAO Conservation Guide Series No. 13.
Flannery, R.D. 1981.
Gully Control and Reclamation.
Arlington, Virginia; Volunteers in
Technical
Assistance (VITA), 26 pp.
Gulick, F.A. 1984.
Increasing Agricultural Food Production through Selected Tree
Planting
Techniques: A summary memorandum with
selected references.
Washington,
D.C.: USAID/Bureau for Africa, 149 pp.
Hagedorn, H. et al. 1977.
Dune Stabilisation: a survey of
literature on dune formation and
dune
stabilization. Eschborn, W.
Germany: GTZ 193 pp.
Ilaco, B.V. (ed.) 1981.
Agricultural compendium for Rural Development in the Tropics and
Subtropics.
New York: Elsevier Scientific Publishing
Co., 239 pp.
Kunkle, S.H. 1978.
Forestry Support for Agriculture Through Watershed Management,
Windbreaks and
Other Conservation Actions. Position
Paper, Eighth World Forestry
Congress,
Jakarta, Indonesia, 28 pp.
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