Soil Erosion Control 1: December 2017
Soil Erosion Control 1: December 2017
Soil Erosion Control 1: December 2017
net/publication/322069790
CITATIONS READS
0 1,675
1 author:
Reinhardt Howeler
Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia
196 PUBLICATIONS 2,120 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
All content following this page was uploaded by Reinhardt Howeler on 26 December 2017.
CHAPTER 20
Reinhardt Howeler2
INTRODUCTION
With populations increasing at 2-3% per year in most developing countries, there is
an ever more pressing need to increase food production. In the past, the increase in food
production was mostly achieved through increases in area cultivated. However, since the
best arable land is already under cultivation, the further expansion of agricultural land will
be more expensive and the areas brought under cultivation will be ever more marginal in
terms of climate, soil fertility and slope. Most of the extension of the agricultural frontier
occurs by felling and burning trees in natural forests or by cutting and burning brush and
grasses in degraded forests or natural savannas. In forested areas, the ash produced from
burning the biomass normally adds sufficient nutrients to the soil to allow 2-3 cycles of
food crops to be grown before the land is abandoned again and returned to fallow to restore
its fertility. This system of “slash and burn” or “shifting cultivation” agriculture is still
practiced mainly in Sub-Saharan Africa, but is also common in parts of South America and
Asia. In tropical Asia the system is most prevalent in the outer islands of Indonesia, in
Vietnam and India. In Indonesia forests were disappearing at a rate of 400,000 ha per year
and in Thailand the forested area was decreasing at a rate of 1.6% per year, according to
1980 data.
Estimates of erosion rates from suspended sediments in rivers indicate that soil losses due
to water erosion is much more serious in Asia than in South America or Africa (Table 1).
Milliman and Meade (1983) calculated that the annual discharge of sediments from the
major river systems in continental SE Asia amounts to about 3.2 billion tons, while that of
insular SE Asia is almost equally high at 3.0 billion tons. In fact, the rivers of tropical Asia
discharge about four times more sediments than those of tropical America, and more than
ten times as much as those of Africa. Some of this erosion is due to natural processes,
especially in the rather unstable and geologically young Himalayan mountain ranges, but
much of it is directly due to, or accelerated by, human activity through deforestation, the
intense cultivation of hillsides and the opening of roads in unstable mountain areas.
according to rainfall and soil fertility. In the Philippines and southern India, cassava is
often grown in either recently established or in older coconut plantations. In Vietnam about
34-40% of cassava farmers grow cassava in intercropping systems, mainly with maize
(Pham Van Bien et al., 1996). The regular and often intensive land preparation employed
for growing these annual food crops can lead to soil losses due to erosion of as much as 500
t/ha/year (Hardjono, 1987). In most cases, soil losses range from 10-100 t/ha/year.
Once the runoff water collects and concentrates into small rivulets, the force of the
running water can detach particles, and this may result in rill erosion, which may progress
into the formation of gullies. The objective of most soil conservation techniques are 1) to
protect the soil from direct rainfall impact by the establishment of either a live or dead
(crop residue or mulch) vegetative cover, which can absorb the energy of the impact of
raindrops, and 2) to reduce the quantity and slow the speed of the runoff water by
improving water infiltration into the soil and to reduce the length or steepness of the slope
by contour cultivation, contour ridging, contour grass barriers or hedgerows, and by
terracing or bunding.
The erosion process selectively removes mainly the organic matter and certain clay
fractions, which provide the soil with its water and nutrient holding capacity. Thus, surface
runoff results in a direct loss of potentially soil-stored water as well as that of washed-out
nutrients, especially from fertilizers, while soil loss due to erosion removes mainly the most
productive part of the soil containing a considerable amount of nutrients, especially organic
N, P and S, as well as very important micro-organisms, such as N-fixing bacteria and VA-
mycorrhiza. The loss of clay and OM also results in a lower cation-exchange capacity
(CEC) as well as a lower water holding capacity. Finally, the physical removal of part of
the topsoil reduces the effective rooting depth to underlying bedrock or subsoil layers. This
also reduces the water storage capacity of the soil and further exacerbates rainfall runoff
and erosion (Figure 1).
(Marsh, 1971). In Alfisols of India, with average annual soil losses of 40 t/ha (or 5 mm),
yields declined 1.25% per year for the first five years and 0.95% during the subsequent
years (Magrath, 1990). In cassava-based cropping systems in Java, annual soil losses of
76-144 t/ha resulted in estimated productivity losses of 3.8-4.7% per year (Magrath and
Arens, 1989). Cassava yields in severely eroded soils in Mondomo, Cauca, Colombia, were
about 50% of those in adjacent non-eroded soil (Howeler, 1986) (Figure 2), but this
depended also on the fertilizers used (Howeler, 1987) and the susceptibility of the variety
(Howeler, 1991).
Figure 1. Conceptual representation of the differential effect of erosion in various parts of the
landscape on soil depth, nutrient distribution and growth of crops.
20
10
0
I II III IV
Replication
Figure 2. The average yield of 18 cassava varieties planted in two replications on an eroded slope
and two replications on an adjacent non-eroded flat area in Mondomo, Cauca,
Colombia in 1983/84.
Source: Howeler, 1986.
Table 2. Average dry soil losses due to erosion measured in cassava trials in various countries
in Asia as well as in Colombia, S. America.
Table 3. C-values for various land uses and crops calculated by the Universal Soil Loss
Equation, as reported by four sources in the literature.
Forest
Primary forest (with dense undergrowth) 0.001 0.001
Second-growth forest with good undergrowth and high mulch cover 0.003
Industrial Tree Plantations
Benguet pine with high mulch cover 0.007
Mahogany, Narra, eight years or more with good undergrowth 0.01-0.05
Mixed stand of industrial tree plantation species, eight years or more 0.07
Agroforesty Tree Species
Coconuts, with annual crops as intercrop 0.1-0.3
Leucaena leucocephala, newly cut for leaf meal or charcoal 0.3
Cashew, mango and jackfruit, less than three years, without 0.25
intercrop and with ring weeding
Oil palm, coffee, cacao with cover crops 0.1-0.3
Grasslands
Imperata grassland, well established and undisturbed, with shrub 0.007
Shrubs with patches or open, disturbed grasslands 0.15
Savanah or pasture without grazing 0.01
Grassland, moderately grazed, burned occasionally 0.2-0.4
Overgrazed grasslands, burned regularly 0.4-0.9
Guinea grass (Panicum maximum) 0.01
Cover Crops/Green Manures
Rapidly growing cover crop 0.1
Velvet bean (Mucuna sp) 0.05
Annual Cash Crops
Maize, sorghum 0.3-0.6 0.3-0.9 0.05
Rice 0.1-0.2 0.1-0.2
Peanut, mungbean, soybean 0.3-0.5 0.4-0.8
Cotton, tobacco 0.4-0.6 0.5 0.14
Pineapple 0.2-0.5
Bananas 0.1-0.3
Diversified crops 0.2-0.4
New kaingin areas, diversified crops 0.3
Old kaingin areas, diversified crops 0.8
Cassava monoculture 0.2-0.8 0.18
Cassava with well-established leguminous ground cover 0.01-0.02
Crops with a thick layer of mulch 0.001
Other
Built-up rural areas, with home gardens 0.2
Bare soil 1.0 1.0 1.0 1.0
1)
Sources: Data from David, 1987, for watersheds in the Philippines.
2)
Data from Roose, 1977.
3)
Data from Margolis and Campos Filho, 1981, for Pernambuco, Brazil.
4)
Data from Leihner et al., 1996, for Cauca, Colombia.
531
The perennial plantation crops and fruit trees, like oil palm, cacao, coffee, cashew,
mango, jackfruit and bananas have C-values of 0.1-0.3, which is not too different from
some annual crops like upland rice or moderately grazed pastures. Other annual crops, like
maize, sorghum, peanut, soybean, cotton and tobacco seem to cause slightly more erosion
than pineapple, but less erosion than cassava. Cassava has a very wide range of C-values,
which indicates that erosion depends mainly on the way the crop is managed, such as plant
spacing, fertilizer application or ridging. Leihner et al. (1996) actually reported very low
C-values, comparable to those of well-managed range land, when forage legumes were
grown as a ground cover under cassava. While highly sustainable, this practice is seldom
economically viable as the cover crops compete strongly with cassava, resulting in very
low cassava yields (see below).
Engineering structures
This includes land leveling, the construction of contour banks or bunding and
various types of terracing. Although these structural solutions were emphasized in the past,
and still play an important role in some countries (especially Indonesia), their cost
effectiveness has generally been rather poor. This is due to their high cost of installation
($400-1,000/ha for terraces) as well as high cost of maintenance (Magrath and Doolette,
1990). If terraces or contour banks are not well designed or maintained, they can easily
collapse causing severe loss of land. Moreover, drainage ways need to be constructed and
maintained to safely conduct the water down slope. Besides the loss of land by terrace
risers, there is additional loss of land of 3-5% for drainage ways. Also, depending on slope
and soil depths, there may be considerable exposure of infertile subsoils, resulting in
reduced productivity or increased fertilizer requirements during the first years after
construction. If terraces are built with heavy machinery, this may also lead to soil
compaction and extremely high rates of erosion during and shortly after construction.
While farmers may construct terraces if given adequate incentives, they will never
spontaneously construct terraces because of their high cost, and dubious or only long-term
benefits
Vegetative techniques
These include various crop and soil management practices that will provide a
vegetative cover of the soil to reduce the impact of raindrops and increase infiltration, or
provide barriers to reduce the speed of runoff. Some examples of these techniques are:
Contour cultivation has been recognized as one of the most effective ways to reduce
runoff and erosion, capture soil moisture and increase yields. Compared with the
traditional system of up-and-down cultivation, runoff was reduced by 25%, while
yields of sorghum increased on average 35% during 30 years of experiments in India
(Dhruva Narayana, 1986). On moderate slopes (up to 15%) this can be done by tractor,
although it may take more time than up-and-down tillage. On steeper slopes (up to
50%) land can be prepared with oxen- or water buffalo-drawn equipment. A reversible
plow, utilized in the Andean zone of Colombia was very effective in contour plowing
of steep slopes (Howeler et al., 1993).
532
Minimum tillage and/or stubble mulching can be very effective in reducing runoff and
erosion. In loose and friable soil, seeds can be planted directly using a pointed stick to
make holes, while cassava can be planted by pushing the stakes directly into the soil.
In compacted soil or in weedy plots it may be necessary to prepare individual planting
spots with a hoe. Another form of minimum tillage is to reduce the intensity of tillage
(one plowing instead of various passes with plow or harrow) of the area to be tilled, or
alternating contour strips of tilled and untilled soil. While minimum tillage can
decrease erosion significantly, it often leads to a reduction in yield due to soil
compaction, weed competition and reduced efficiency of fertilizers when these are left
on the soil surface. When soils are compacted or the soil surface is sealed by heavy
rainstorms, runoff may actually increase and water infiltration decrease.
Contour ridging was found to be very effective in reducing runoff and erosion on gentle
slopes and in stable soil; it often also increases yields by concentrating topsoil in the
ridge, increasing rooting depth and conserving soil moisture. However, on steep slopes
or with unstable soils, too much water accumulating behind the ridges may cause them
to break resulting in concentrated water flow and gully erosion.
Mulching with crop residues or grass on the soil surface greatly improves water
infiltration, protects the soil from direct raindrop impact and reduces runoff and
erosion. Mulch application have been shown to increase yields of various crops up to
140% (Suwardjo and Abujamin, 1983) by supplying nutrients, increasing soil moisture
during dry spells and reducing soil temperature fluctuations. However, sufficient
mulching materials are often not available or their collection and transport is costly.
Thus, in situ production of mulch by rotating or intercropping food crops with
leguminous cover crops may be a more practical solution. Permanent cover crops or
“live mulches” of Calopagonium, Pueraria phaseoloides or Macroptillium
atropurpureum have been used successfully for erosion control under perennial trees
such as rubber or oilpalm. Attemps to use perennial legumes as cover crops in cassava
have been less successful due to severe competition of the cover crops with cassava.
Cassava yields were reduced on average 20-50% by nine cover crop species in
Thailand (Howeler, 1992; see Chapter 18).
Vegetative barriers may include:
1. Contour strips of cut-and-carry grasses such as elephant grass or napier grass
(Pennisetum purpureum), king grass (Saccarum sinense), Bermuda or Bahama
grass (Cynodon dactylon), Bahia grass (Paspalum notatum), etc. These have been
used successfully to reduce runoff and erosion and supply feed for cattle or water
buffaloes.
Contour strips of about 1 m width are usually planted at 1-2 m vertical intervals.
The drawback of this system is that 15-20% of the land must be taken out of crop
production, the grass trimming is labor-intensive , feed production is often more
than the family can use, and the grass stolons or feeder roots can seriously reduce
yields of adjacent rows of food crops.
2. Contour hedges of “inert” grasses such as vetiver grass (Vetiveria zizanioides) can
be very effective in reducing runoff and erosion and may increase crop yields by
improved water conservation and reduced nutrient loss. Single row hedges of
about 50 cm width are generally sufficient, thus taking less than 10% of land out of
production. Moreover, the deep vertical root system of this grass does not compete
seriously with adjacent crops. However, the low forage quality of the grass is a
serious drawback for those farmers who need to produce animal feed. Also, since
the seed of most vetiver grasses are infertile, the hedgerows have to be planted with
533
vegetative tillers, which are costly to produce, transport and plant, especially in
mountainous areas. On the other hand, once planted, the hedgerows can be very
effective for many years without the need of replanting.
3. Hedgerows of leguminous trees. The system is generally called
”alley cropping” and consists of planting fast-growing leguminous tree species
such as Leucaena leucocephala or Gliricidia sepium in contour lines about 4-5 m
apart. Crops are grown in the space between the hedgerows. To prevent light
competition the trees need to be pruned regularly to about 30-50 cm height and the
prunings can be used as animal feed or placed between the hedgerows as mulch
and are a good source of nutrients, mainly N fixed by the trees. While rather labor
intensive and slow to establish, this system can eventually be very effective in
forming terraces, reducing erosion and increasing yields (see Chapter 18). Basri et
al. (1990) reported an increase in rice yields of 25-30% by alley cropping with
Cassia spectabilis in northern Mindanao of the Philippines.
Many of these vegetative techniques can be applied solely or in combination, and in many
cases they act synergistically to increase productivity as well as reduce erosion. However,
each technique has its own benefits and its own limitations, which may require certain
trade-offs.
increases yields and maintains soil fertility, annual crop production can be limited to the
permanent cultivation of only the flattest and most fertile part of the farm, leaving the
steeper slopes for production of perennial trees, for grazing or forestry. Proper land use
planning, diversification and intensification of the farming enterprise will often be the most
effective way to control erosion, maintain soil fertility and sustain productivity.
4
Soil loss (t/ha)
Sweetpotato
Maize+bean
Sugarcane
Irish potato
Soybean
Cassava
Peanut
Maize
Cotton
Bean
Rice
14
12
Runoff (% of rainfall)
10
8
6
4
2
0
Sweetpotato
Maize+bean
Sugarcane
Irish potato
Cassava
Soybean
Peanut
Cotton
Maize
Bean
Rice
Figure 3. Effect of various crops on annual soil loss by erosion (top) and on runoff (bottom). Data
Figureare
13.average
Effect ofvalues
crops (corrected
on annual soil
for loss by erosion
a standard (top)rainfall
annual and onofrunoff
1.300(bottom).
mm) fromData are 48
about
average values (corrected for a standard annual rainfall of 1,300 mm) from
experiments conducted from 1943 to 1959 on sandy, clayey and Terra Roxa soils in Sao about 48
experiments conducted from 1943 to 1959 on sandy, clayey and
Paulo, Brazil with slopes of 8.5-12.8%. Source: Quintiliano et al., 1961Terra roxa soils
in Sao Paulo state of Brazil with slopes of 8.5-12.8%.
Source: Quintiliano et al., 1961.
535
Highest soil losses and runoff were observed in castor bean, common bean
(Phaseolus vulgaris) and cassava, followed by peanut, rice, cotton, soybean, potato,
sugarcane, maize and sweet potato. Using the relative soil loss as the criterion, with castor
bean considered 100, then cassava would have an index of 83, below that of beans (92), but
higher than peanut (64), rice (60), cotton (60), soybean (48), sugarcane (30), maize (29) and
sweet potato (16).
In other trials conducted for ten years on 12% slope on a red-yellow Podzolic soil
in Pernambuco, Brazil, Margolis and Campos Filho (1981) reported that cassava on
average produced an annual soil loss of 11.0 t/ha, compared with 8.3 t/ha for cotton, 3.0 for
maize, 2.8 for velvet bean (Mucuna sp.) and 0.4 t/ha for guinea grass (Panicum maximum),
while the soil loss on bare soil was 59.9 t/ha. Although annual soil losses were much
higher than those reported by Quintiliano et al. (1961), crops are listed in a similar order.
Table 4 shows similar data for soil losses in eight crops planted during four years
on 7% slope in Sri Racha, Thailand (Putthacharoen et al., 1998). By far, highest levels of
erosion were observed in cassava for root production (planted at 1.0 x 1.0 m), followed by
cassava for forage production (planted at 0.5 x 0.5 m), mungbean, sorghum, peanut, maize
and pineapple. Annual erosion losses for cassava averaged about 75 t/ha, while the average
yield was 16 t/ha of fresh roots. Thus, nearly 5 tons of soil were lost for every ton of roots
produced. These are extremely high rates of erosion on a slope of only 7%.
Table 4. Total dry soil loss by erosion (t/ha) due to the cultivation of eight crops during four
years on 7% slope with sandy loam soil in Sri Racha, Thailand from 1989 to 1993.
First Second
No. of crop period period Total Average
Crops cycles (22 months) (28 months) (50 months) t/ha/year
Cassava for root production 4 142.8 a 168.5 a 311.3 74.7
Cassava for forage production 2 68.8 b 138.5 ab 207.3 49.8
Maize 5 28.5 d 35.5 cd 64.0 15.4
Sorghum 5 42.9 c 46.1 cd 89.0 21.4
Peanut 5 37.6 cd 36.2 cd 73.8 17.7
Mungbean 6 70.9 b 55.3 cd 126.2 30.3
Pineapple1) 2 31.4 cd 21.3 d 52.7 12.6
Sugarcane1) 2 - 94.0 bc - -
F-test ** **
cv (%) 11.4 42.7
1)
second cycle is ratoon crop; sugarcane only during second 28-month period
Source: Putthacharoen et al., 1998.
Erosion losses for cassava in the Thai study were much higher than those of other
crops mainly because cassava was planted at a rather wide spacing while initial plant
growth was slow, leaving much soil exposed to the direct impact of rainfall during 3-4
months after planting and before the canopy closed. In contrast, the other annual food
crops were planted at much higher population densities (50,000-100,000 plants/ha) and had
a faster initial growth. Moreover, these row crops were planted along contour lines, which
helped considerably in reducing runoff and erosion. Except for mungbean, which was
planted six times in four years, all other food crops could be planted only once a year due to
536
the relatively short (6 month) rainy season in Thailand. Once harvested, the fields
remained in weeds with crop residues protecting the soil from further erosion
(Putthacharoen et al., 1998).
In regions with a longer wet season it is often possible to plant short-cycle food
crops, such as maize, rice, soybean, mungbean and peanut, twice a year. In that case,
because of more frequent land preparation and weeding, soil losses tend to increase.
Comparing one crop of cassava with two successive crops of maize, soybean, peanut and a
rice-soybean rotation, Wargiono et al. (1998) reported that annual soil losses for cassava
were similar to those obtained with two successive crops of soybean, slightly higher than
the rice-soybean rotation or two crops of maize, and about twice as high as that of two
crops of peanut.
Sheng (1982) reported that in Taiwan, with 2500 mm annual rainfall and on slopes
of 20-52%, erosion in cassava was 128 t/ha, compared with 62 for pineapple, 92 for
banana, 172 for sweetpotato and 208 t/ha for sorghum, peanut, sweetpotato, soybean and
maize grown in rotation. In that case, cassava cultivation resulted in less erosion than the
growing of several short-cycle crops in rotation during the same year.
Finally, when four successive crops of beans (Phaseolus vulgaris) were grown on
15% and 30% slope in Popayan, Cauca, Colombia, during the same 17 month period as one
crop of cassava1, soil losses for beans in both trials were about four times higher than for
cassava, due to the frequent land preparation and weeding required for the beans (Howeler,
1987; 1991). Once the cassava canopy was well established, runoff and erosion losses
were greatly diminished; this was also reported by Tongglum et al. (1992), Howeler
(1995), Tian Yinong et al. (1995) and Wargiono et al. (1995, 1998).
While slow initial growth and the need for wide plant spacing are intrinsic
characteristics of the crop, they can be mitigated against somewhat by planting at a closer
spacing, by selecting more vigorous varieties, and by enhancing early growth through
fertilizer application. All these have been shown to markedly reduce erosion (see below).
1
Due to the year-round low temperature at about 1800 masl, cassava grew slowly and required 17
months to produce a reasonable yield.
537
Table 5. Nutrients in sediments eroded from cassava plots with various treatments in Thailand
and Colombia.
Dry (kg/ha/year)
soil loss
Location and treatments (t/ha/year) N1) P2) K2) Mg2)
Phommasack et al. (1995, 1996) reported total nutrient losses in sediments and
runoff from maize fields with 25-35% slope in Luang Prabang, Laos: in the second year of
cropping, N, P and K losses in the eroded sediments (9.2 t/ha) were 53.9, 9.3 and 24.0
kg/ha, respectively, while those in the runoff (2,120 m3/ha) were 2.3, 0.9 and 26.1 kg/ha,
respectively (Howeler and Thai Phien, 2000). Although in this case soil loss and runoff
were not particularly high, nutrient losses in the sediments and runoff were substantial,
especially that of N and K in the sediments and K in the runoff.
To determine the effect of various agronomic practices on cassava yields and soil
erosion, many erosion control trials were conducted, both in Colombia and in various
countries in Asia. Most of these experiments used the simple methodology, described in
detail in Chapter 13, in which plots with different treatments are laid out side by side on a
uniform slope. Along the lower side of each plot a trench is dug and covered with a sheet
of plastic in such a way that the runoff water and sediments eroded from the plot are
captured in the trench. The runoff water is allowed to seep away through small holes made
in the plastic while the wet sediments remain on the plastic. This wet sediment is
periodically removed and weighed and a small sample is dried to determine the dry matter
content in order to calculate the dry soil loss per ha. Precautions must be taken that no
538
water enters the plots from the slope above the trial and no runoff water leaves the plots
through the side borders. Some experiments were conducted on experiment stations with
replications, but most were conducted on farmer’s fields or by farmers with help from
researchers or extensionists. The latter normally did not have replications. However, if
these farmer participatory research (FPR) trials were conducted with the same treatments
by several farmers in the same village, the average results were calculated and presented to
show farmers the amount of soil lost and the yields obtained in each treatment. In addition
the gross income, total production costs and net income from each treatment were
calculated and presented to the farmers, so they could discuss the pros and cons of each
treatment and select and adopt those most suitable for their own conditions.
Another trial on the effect of manual land preparation was conducted at the same
time on an adjacent site with 30% slope, again with the same cassava and bean varieties,
with very similar results. Again soil losses by erosion were about four times higher with
the four crops of beans as with one crop of cassava grown during the same 17 month
period. No preparation or hoe preparation of 1 m wide strips alternated with 1 m
unprepared strips were most effective in reducing erosion, but these treatments also resulted
in the lowest yields. Highest yields of both crops were obtained with complete land
preparation with hoe, while lowest yields were obtained with strip preparation. Soil losses
due to erosion were much lower in the plots prepared by hoe as compared with those
prepared by tractor shown in Table 6 (CIAT, 1988).
539
Cassava
30 1 pass plow
1 pass chissel
20 1 pass rotavator
strip preparation
10
110
Beans
100 2 passes plow
1 pass rotavator
90 2 passes rotavator
1 pass chissel
80 strip preparation
planting beans
70
Dry soil loss (t/ha)
60
50
40
30
20
10
0
0 2 4 6 8 10 12 14 16 18
Months after planting
Figure 4. Effect of various mechanical land preparation methods on soil losses due to erosion
on 15% slope in Popayan, Colombia, grown with cassava and Phaseolus beans.
Arrows indicate when beans were planted.
Source: CIAT, 1988.
540
Table 6. Effect of mechanical land preparation methods on yields of cassava and beans, as
well as on soil loss due to erosion on a 15% slope in Popayan, Cauca, Colombia.
Crop Method of land preparation Yield (t/ha) 1) Dry soil loss (t/ha) 2)
1)
Cassava 1 pass with plow 21.4 26.50
1 pass with rototiller 16.6 10.71
1 pass with chisels 17.5 16.21
1 m strips with rototiller alternated with 1 12.3 7.50
m strips without preparation
Table 7. Effect of method of land preparation on cassava yields and on dry soil loss due to
erosion when cassava was planted on 25% slope at CATAS in Hainan, China
in 1989.
In the same trial conducted in the same plots at CATAS in 1991, soil losses were as
high as 259 t/ha in treatment 4 due to exceptionally high rainfall in June, July and August,
while the lowest soil loss of 167 t/ha were recorded with only one time plowing without
541
ridging. Highest yields were obtained by planting in planting holes (Tian Yinong et al.,
1995)
Table 8. The effect of various soil and crop management treatments on cassava yields and soil
erosion in a farmer’s field with 40% slope in Mondomo, Cauca, Colombia.
D. Effect of fertilizers
1. Without fertilizers or lime 0.3 3.50
2. With fertilizers: 500 kg/ha lime and 750 kg/ha 10-30-10 9.3 2.96
542
Figure 5 shows the effect of cassava plant spacing, both in monoculture or when
intercropped with upland rice and maize, on the total crop value (gross income) and on soil
losses by erosion in Tamanbogo, Lampung Indonesia. At all plant spacings intercropping
resulted in a slightly higher gross income than planting in monoculture, but planting
cassava at 1x1 m resulted in a slightly higher income and lower erosion than planting at a
wider row spacing, especially in case of monoculture. In case of intercropped cassava there
was not much difference between the various spacing treatments, both in terms of gross
income or erosion (Wargiono et al., 1995).
A. Cassava spacing (m)
Total crop value (‘000 Rp/ha)
1.0x1.0 2.0x0.5
1600 2.73x0.6x0.6
1200
800
400
100 B.
Cassava monoculture
80 Cassava intercropped
Dry soil loss (t/ha)
60
40
20
Figure 5. Effect of cassava plant spacing on total crop value (A) and on soil loss by
erosion (B) when cassava was grown in monoculture or intercropped with
upland rice and maize in Tamanbogo, Indonesia.
Source: Wargiono et al, 1995.
543
Figure 6 shows the results of an erosion control trial conducted for five
consecutive years at Jatikerto Experiment Station in Malang district of East Java,
Indonesia. Cassava intercropped with maize was planted without hedgerows (check) or
with hedgerows of Pennisetum purpureum (elephant grass), Gliricidia sepium or Flemingia
macrophylla. The data on cassava yields and soil loss due to erosion in the treatments with
the various hedgerows are expressed as a percentage of those in the check plot without
hedgerows
Flemingia
160
Gliricidia
140
Relative root yield (%)
Elephant grass
120
80
60
1 2 3 4 5
120
Gliricidia
80
Elephant grass
60
Flemingia
40
1 2 3 4 5
Years after planting hedgerows
Figure 6. Trend in relative yield and relative soil loss due to erosion when cassava intercropped
with maize was planted with contour hedgerows of elephant grass, Gliricidia sepium
and Flemingia macrophylla during five consecutive years of cropping on 8% slope at
Jatikerto, Malang, Indonesia from 1991/92 to 1996/97.
544
It is clear that initially the hedgerows decreased cassava yields by occupying land,
but during the 4th and 5th year they caused a significant increase in yield, especially
Flemingia and Gliricidia by supplying N to cassava in this extremely N-deficient soil. In
the first year after establishment, the two leguminous tree species were also not effective in
reducing erosion, but in subsequent years all three hedgerows became increasingly more
effective, and during the 4th and 5th year had reduced soil losses to about 60% of those in
the check plots without hedgerows (Wani Hadi Utomo, personal communication). Thus,
planting cassava on slopes with contour hedgerows of leguminous tree species in an alley
cropping system (see Chapter 18) can both increase yields and reduce erosion.
Similar results were also observed during 11 years of continuous cropping in South
Vietnam. Table 9 shows the results of an erosion control trial conducted at Hung Loc
Agric. Research Center in Dongnai during the eleventh year of continuous cropping, using
various intercrops and contour hedgerow species to reduce erosion. Highest cassava root
yields were obtained by intercropping with peanut, but planting hedgerows of vetiver grass,
Leucaena leucocephala or Gliricidia sepium markedly reduced erosion as compared to the
check plot without hedgerows. Intercropping also reduced erosion but was not as effective
as the hedgerows, especially those of vetiver grass. Figure 7 shows that the effectiveness of
the hedgerows in reducing erosion increased over time and that vetiver grass was
consistently more effective than the other two leguminous tree species. The hedgerows
also increased cassava yields about 10-20%. Similar results were obtained with hedgerows
of vetiver grass or Tephrosia candida, which both reduced erosion to about 20% as
compared to the check without hedgerows in FPR erosion control trials (Howeler, 2008).
Figure 8 shows the effect of various soil and crop management treatments on the
accumulative soil losses due to erosion during a 10 month growth cycle of cassava in Sri
Racha, Thailand. As in most other erosion control trials, soil losses were most serious
during the first 4-5 months of growth, after which it decreased markedly because of
complete canopy cover and the onset of the dry season. This and many other trials showed
that soil losses were greatest in the absence of fertilizers, as this greatly delayed canopy
formation. Least amount of soil loss was observed in the treatments of no tillage and with
contour ridging. Intercropping with peanut also reduced erosion. However, this treatment
resulted in the lowest cassava yield of 16.1 t/ha, slightly lower than those obtained without
fertilizers (17.6) and no tillage (21.2). This compares with a yield of 27.1 t/ha for the
treatment with complete tillage (2 plowing, 2 disking) plus contour ridging and fertilizer
application.
545
Table 9. Effect of cropping systems and the planting of contour hedgerows on the yield of
cassava and intercrops, on dry soil loss by erosion, and on gross and net income
during the 11th consecutive year of cropping on 12% slope at Hung Loc Agric.
Research Center in Thong Nhat district, Dong Nai, Vietnam in 2007/08.
120 Gliricidia =
100
check without hedgerows
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10 11
no fertilizers
40
no tillage
10
0
0 2 4 6 8 10 12
Since most soil conservation practices have advantages and disadvantages, trade-
offs will need to be made. Those are best made by farmers themselves as they will greatly
depend on the specific bio-physical as well as the socio-economic situation at each site.
Thus, farmers were encouraged to conduct simple erosion control and various other types
of trials on their own fields with guidance from researchers and extension workers. These
were called Farmer Participatory Research (FPR) trials. From 1994 to 2004 farmers
conducted a total of 1,621 FPR trials in 99 villages of Thailand, Vietnam, China and
Indonesia, of which 378 erosion control trials. Some typical examples of these trials are
shown in Tables 11-13.
During farmer field days at time of harvest, farmers from the village (participating
and non-participating) and surrounding villages would visit each trial and evaluate and
score each treatment according to their own criteria. Later in the day the average results of
the each type of trial were presented for discussion with the farmers; this included estimates
of the gross income, total production cost and net income for each treatment. Farmers were
asked to raise hands to show how they had scored each treatment in order to calculate the
farmers’preferences, as shown in the last columns of Tables 11, 12 and 13.
547
Table 10. Effect of various soil/crop management practices on erosion and yield, as well as on labor and monetary requirements and
long-term benefits in cassava-based cropping systems.
Effect on
Erosion Terrace cassava Labor Monetary Long-term
Erosion control practices control formation yield requirement cost benefits Main limitations
Minimum or zero tillage ++ - - + -- + compaction, weeds
Mulching (carry-on) ++++ - ++ +++ + ++ mulch availability, transport
Mulching (in-situ production +++ - ++ ++ + ++ competition
Contour tillage +++ + + + + ++
Contour ridging +++ + ++ ++ ++ + not suitable on steep slopes
Leguminous tree hedgerows ++ ++ + +++ + +++ 1) delay in benefits
Cut-and-carry grass strips ++ ++ -- +++ + +++ 1) competition, maintenance
Vetiver grass hedgerows +++ +++ + + + +++
Natural grass strips ++ ++ - + - ++ high maintenance costs
Cover cropping (live mulch) ++ - --- +++ ++ + severe competition
Manure or fertilizer application ++++ - +++ + +++ +++ high cost
Intercropping ++ - - ++ ++ +++ labor intensive
Closer plant spacing +++ - + + + ++
+ = effective, positive or high
- = not effective, negative or low
1)
= value added in terms of animal feed, staking material or fuel wood.
548
Table 11. Effect of various crop management treatments on the yield of cassava and
intercropped peanut a well as the gross and net income and soil loss due to erosion
in a FPR erosion control trial conducted by six farmers in Kieu Tung village of
Thanh Ba district, Phu Tho province, Vietnam in 1997 (3rd year).
Dry .
soil Yield (t/ha) Gross Product Net Farmers
Slope loss income2) costs income ranking
1) 1)
Treatment (%) (t/ha) cassava peanut ----(mil. dong/ha)-----
1. C monocult., with fertilizer, no hedger. 40.5 106.1 19.17 - 9.58 3.72 5.86 6
2. C+P, no fertilizer, no hedgerows 45.0 103.9 13.08 0.70 10.04 5.13 4.91 5
3. C+P, with fertilizer, no hedgerows 42.7 64.8 19.23 0.97 14.47 5.95 8.52 -
4. C+P, with fertilizer, Tephrosia hedger. 39.7 40.1 14.67 0.85 11.58 5.95 5.63 3
5. C+P, with fertilizer, pineapple hedger. 32.2 32.2 19.39 0.97 14.55 5.95 8.60 2
6. C+P, with fertilizer, vetiver hedgerows 37.7 32.0 23.71 0.85 16.10 5.95 10.15 1
7. C monocult, with fert., Tephrosia hedger. 40.0 32.5 23.33 - 11.66 4.54 7.12 4
1)
Fertilizers = 60 kg N + 40 P2O5, + 120 K2O/ha; all plots received 10 t/ha pig manure
2)
Prices: cassava (C) dong 500/kg fresh roots
peanut (P) 5000/kg dry pods
Source: Howeler, 2001.
Table 12. Average results of two FPR erosion control trials conducted by farmers in Khook
Anu village, Thep Sathit district of Chayaphum province, Thailand, in 2001/02.
Table 13. Average results of five FPR erosion control trials conducted by farmers in Tien
Phong and Dac Son villages of Pho Yen district, Thai Nguyen province, Vietnam,
in 1997.
1)
Farmer’s practice: cassava monoculture, 11.4 t/ha of FYM+68 kg N+20 P2O5+50 K2O/ha;
all other plots received 10 t/ha of FYM+80 kg N + 40 P2O5 + 80 K2O/ha
2)
dry pods
3)
Prices: cassava: dong 600/kg fresh roots
peanut: 5,000/kg dry pods
4)
Costs FYM: dong 100/kg
urea (45%N): 2,500/kg
SSP (17% P2O5): 1,000/kg
KCl (60%K2O): 2,500/kg
peanut seed: 6,000/kg; use 50 kg/ha
labor: 7,500/manday
1 US $ = 11.000 dong
Source: Nguyen The Dang et al., 2001.
The average effect of the various soil and crop management practices on cassava
yields and on dry soil loss due to erosion were calculated as a percentage of a check
treatment without the practice for all erosion control experiments and FPR trials conducted
in Thailand and Vietnam. The results are shown in Tables 14 and 15. In both countries
contour hedgerows of vetiver or Paspalum atratum, were most effective in controlling
erosion, while in Vietnam hedgerows of Tephrosia candida, Flemingia macrophylla and
pineapple were also very effective. In Thailand these hedgerows slightly reduced yields
because they take up some space in the field, but in Vietnam they actually increased
cassava yields 10-15%. Planting cassava at a closer spacing was also quite effective in
reducing erosion in Thailand but not in Vietnam; in both countries closer spacing increased
cassava yields. Hedgerows of leguminous tree species like Leucaena or Gliricidia were
intermediately effective in controlling erosion and increased cassava yields only in long-
term trials in Vietnam. Application of fertilizers was one of the most effective ways to
increase cassava yields and markedly reduce soil losses by erosion, especially in Vietnam.
Intercropping with peanut, melon or sweet corn did not reduce erosion and decreased
cassava yields in Thailand (although they may have increased total income), while
intercropping with peanut was intermediately effective in reducing erosion and slightly
increased cassava yields in Vietnam.
550
Table 14. Effect of various soil conservation practices on the average1) relative cassava yield and
dry soil loss due to erosion as determined from soil erosion control experiments, FPR
demonstration plots and FPR trials conducted in Thailand from 1994 to 2003.
Relative Relative
cassava yield dry soil loss
Soil conservation practices2) (%) (%)
1. With fertilizers; no hedgerows, no ridging, no intercrop (check) 100 100
2. With fertilizers; vetiver grass hedgerows, no ridging, no intercrop** 90 (25) 58 (25)
3. With fertilizers; lemon grass hedgerows, no ridging, no intercrop** 110 (14) 67 (15)
4. With fertilizers; sugarcane for chewing hedgerows, no intercrop 99 (12) 111 (14)
5. With fertilizers; Paspalum atratum hedgerows, no intercrop** 88 (7) 53 (7)
6. With fertilizers; Panicum maximum hedgerows, no intercrop 73 (3) 107 (4)
7. With fertilizers; Brachiaria brizantha hedgerows, no intercrop* 68 (3) 78 (2)
8. With fertilizers; Brachiaria ruziziensis hedgerows, no intercrop* 80 (2) 56 (2)
9. With fertilizers; elephant grass hedgerows, no intercrop 36 (2) 81 (2)
10. With fertilizers; Leucaena leucocephala hedgerows, no intercrop* 66 (2) 56 (2)
11. With fertilizers; Gliricidia sepium hedgerows, no intercrop* 65 (2) 48 (2)
12. With fertilizers; Crotalaria juncea hedgerows, no intercrop 75 (2) 89 (2)
13. With fertilizers; pigeon pea hedgerows, no intercrop 75 (2) 90 (2)
14. With fertilizers; contour ridging, no hedgerows, no intercrop** 108 (17) 69 (17)
15. With fertilizers; up-and-down ridging, no hedgerows, no intercrop 104 (20) 124 (20)
16. With fertilizers; closer spacing, no hedgerows, no intercrop** 116 (10) 88 (11)
17. With fertilizers; C+peanut intercrop 72 (11) 102 (12)
18. With fertilizers; C+pumpkin or squash intercrop 90 (13) 109 (15)
19. With fertilizers; C+sweet corn intercrop 97 (11) 110 (14)
20. With fertilizers; C+mungbean intercrop* 74 (4) 41 (4)
21. No fertilizers; no hedgerows, no or up/down ridging 96 (9) 240 (10)
1)
number in parenthesis indicates the number of experiments/trials from which the average values were calculated.
2)
C = Cassava
** = most promising soil conservation practices; * = promising soil conservation practices
Source: Howeler, 2001.
Concerning the adoption of contour hedgerows, it is clear that these were adopted
mainly by those farmers that had actively participated in the project. Interestingly, the
great majority of farmers in Thailand preferred the planting of vetiver grass, while those in
North Vietnam preferred Tephrosia candida and in South Vietnam Paspalum atratum.
Other types of hedgerows, like lemon grass or pineapple, while being quite effective in
reducing erosion, were seldom adopted. This clearly indicates that farmers select those
practices that fit best into their existing farming practices and are most suitable for their
own particular conditions
551
Table 15. Effect of various soil conservation practices on the average1) relative cassava yield
and dry soil loss due to erosion as determined from soil erosion control experiments,
FPR demonstration plots and FPR trials conducted in Vietnam from 1993 to 2003.
Participants Non-participants
1)
Data are based on census forms filled by 417 households in Thailand and 350 in Vietnam, of
which 109 and 126 had been participants of the project, respectively.
Source: Dalton et al., 2007.
atratum because it provides feed for cattle and buffaloes. Thus, in order to achieve
adoption of soil conservation practices, researchers should not promote a single technology
because it happens to be effective in experiments, but they should let farmers conduct their
own soil erosion control trials, and let farmers select the practices that are most suitable for
their own conditions.
REFERENCES
Basri, I.H., A.R. Mercado Jr. and D.P. Garrity. 1990. Upland rice cultivation using leguminous tree
hedgerows on strongly acid soils. IRRI, Manila, Philippines. In: ILEILA Newsletter, May
1991. p. 32.
Centro Internacional de Agricultura Tropical (CIAT). 1985a. Cassava Program. Annual Report for
1982 and 1983. CIAT, Cali, Colombia. 521 p.
Centro Internacional de Agricultura Tropical (CIAT). 1985b. Cassava Program. Annual Report for
1984. Working Document No. 1. CIAT, Cali, Colombia. 249 p.
Centro Internacional de Agricultura Tropical (CIAT). 1988. Cassava Program Annual Report for
1985. Working Document No. 38, 1988. CIAT, Cali, Colombia.
Chan, S.K., S.L. Tan, H. Ghulam Mohammed and R.H. Howeler. 1994. Soil erosion control in
cassava cultivation using tillage and cropping techniques. MARDI Research J. 1: 55-66.
Chorley, R.J. 1969. Water, Earth and Man. Metheun Co. Ltd. Londen, UK.
Dalton, T.J., N.K. Lilja, N. Johnson and R. Howeler. 2007. Impact of participatory natural resource
management research in cassava-based cropping systems in Vietnam and Thailand. In: H.
Waibel and D. Zilberman (Eds.). International Research on Natural Resource Management.
Advances in Impact Assessment. CABI, Wallingford, Oxfordshire, UK. pp. 91-117
David, W.P. 1987. Soil erosion and land classification. Report for World Bank Farm Mission.
Dhruva Narayana, V.V. 1986. Soil and water conservation research in India. Indian J. Soil
Conservation 14: 22-31.
Hardjono, D. 1987. Demonstrasi UPSA dan permasalahannya dalam lokakarya pelaksanaan
rehabilitasi lahan dan konservasi tanah secara terpadu di Sub-DAS Konto. Malang/Batu.
March 11-12, 1987. Departemen Luar Negeri Kerajaan Belanda. pp. 2.1-2 - 2.1-7.
Howeler, R.H. 1986. El control de la erosión con prácticas agronómicas sencillas (Erosion control
through simple agronomic practices). Suelos Ecuatoriales 16: 70-84.
Howeler, R.H. 1987. Soil conservation practices in cassava-based cropping systems. In: T.H. Tay,
A.M Mokhtaruddin and A.B. Zahari. (Eds.). Proc. Intern. Conference on Steepland Agriculture
in the Humid Tropics, held in Kuala Lumpur, Malaysia. Aug. 17-21, 1987. pp. 490-517.
Howeler, R.H. 1991. Long-term effect of cassava cultivation on soil productivity. Field Crops
Research 26, 1-18.
Howeler, R.H. 1992. Agronomic research in the Asian Cassava Network – An overview. 1987-1990.
In: R.H. Howeler (Ed.). Cassava Breeding, Agronomy and Utilization Research in Asia. Proc.
3rd Regional Workshop, held in Malang, Indonesia. Oct 22-27, 1990. pp. 260-285.
Howeler, R.H. 1993. Integrated crop and soil management to prevent environmental degradation in
cassava systems in Asia. In: Cassava Starch Special Edition, 4th Issue, May 1993. Guangxi Starch
Association, Nanning, Guangxi, China. pp. 39-44. (in Chinese)
Howeler, R.H. 1994. Integrated soil and crop management to prevent environmental degradation in
cassava-based cropping systems in Asia. In: J.W.T. Bottema and D.R. Stoltz (Eds.). Upland
Agriculture in Asia. Proc. Workshop held in Bogor, Indonesia, April 6-8, 1993. pp. 195-224.
Howeler, R.H. 1995. Agronomy research in the Asian Cassava Network – Towards better production
without soil degradation. In: R.H. Howeler (Ed.). Cassava Breeding, Agronomy Research and
Technology Transfer in Asia. Proc. 4th Regional Workshop, held in Trivandrum, Kerala, India.
Nov 2-6, 1993. pp. 368-409.
553
Howeler, R.H. 1996a. Cassava agronomy research in Asia, 1987-1992. In: R.H. Howeler (Ed.). Cassava
Production, Processing and Marketing in Vietnam. Proc. Workshop held in Hanoi, Vietnam. Oct
29-31, 1992. pp. 255-290.
Howeler, R.H. 1996b. The use of farmer participatory research methodologies to enhance the
adoption of soil conservation practices in cassava-based cropping systems in Asia. In: S.
Sombatpanit, M.A. Zöbish, D.W. Sanders and M.G. Cook (Eds.). Soil Conservation Extension -
From Concepts to Adoption. Proc. Int. Workshop on Soil Conservation Extension, held in
Chiang Mai, Thailand. June 4-11, 1995. pp. 159-168.
Howeler, R.H. 1998. Cassava Agronomy Research in Asia. – An Overview 1993-1996. In: R.H.
Howeler (Ed.). Cassava Breeding, Agronomy and Farmer Participatory Research in Asia. Proc.
5th Regional Workshop, held in Danzhou, Hainan, China. Nov 3-8, 1996. pp. 355-375.
Howeler, R.H. 2001. The use of Farmer Participatory Research (FPR) in the Nippon Foundation
Project: Improving the sustainability of cassava-based cropping systems in Asia. In: R.H.
Howeler (Ed.). Cassava’s Potential in Asia in the 21st Century: Present Situation and Future
Research and Development Needs. Proc. 6th Regional Workshop, held in Ho Chi Minh city,
Vietnam. Feb 21-25, 2000. pp. 461-489.
Howeler, R.H. 2008. Results, achievements and impact of the Nippon Foundation Cassava Project.
In: R.H. Howeler (Ed.). Integrated Cassava-based Cropping Systems in Asia. Proc. of the
Workshop on the Nippon Foundation Cassava Project in Thailand, Vietnam and China, held in
Thai Nguyen, Vietnam. Oct 27-31, 2003. pp. 161-223.
Howeler, R.H. and S. Guzman. 1985. Practicas de conservación de suelo en explotaciónes
agropecuarias en ladera (Soil conservation practices in crop/livestock enterprises on sloping land).
In: Memorias III Congreso Colombiano de Cuencas Hidrograficas, held in Cali, Colombia. Aug 6-
10, 1985. pp. 208-239.
Howeler, R.H. and Thai Phien. 2000. Integrated nutrient management for more sustainable cassava
production in Vietnam. In: Progress in Cassava Research and Extension in Vietnam. Proc.
Vietnamese Cassava Workshop, held in Ho Chi Minh city, Vietnam. March 16-18, 1999. pp. 12-54.
(in Vietnamese with English abstract, tables and figures)
Howeler, R.H., H.C. Ezuma and D.J. Midmore. 1993. Tillage systems for root and tuber crops in the
tropics. Soil & Tillage Research 27: 211-240.
Jantawat, S., V. Vitchukit, S. Putthacharoen and R.H. Howeler. 1991. Cultural practices for soil
erosion control on cassava. In: M. Scnepf (Ed.). Proc. Intern. Workshop on Conservation
Farming on Hillslopes, held in Taichung, Taiwan, R.O.C. March 20-29, 1989. pp. 201-205.
Jantawat, S., S. Putthacharoen and R.H. Howeler. 1992. Soil and crop management practices for
sustainable production of cassava on sloping lands. In: Evaluation for Sustainable Land
Management in the Developing World. IBSRAM Proc. no. 12, Vol. 3. pp. 63-64.
Jantawat, S., A. Tongglum, S. Putthacharoen, P. Poolsanguan and R.H Howeler. 1994. Sustaining
environmental quality: The erosion control challenge. In: Proc. 25th Conference of Intern.
Erosion Control Assoc. pp. 521-526.
Leihner, D.E., M. Ruppenthal, T.H. Hilger and J.A. Castillo F. 1996. Soil conservation effectiveness
and crop productivity of forage legume intercropping, contour grass barriers and contour
ridging in cassava on Andean hillsides. Expl. Agric. 32: 327-338.
Magrath, W.B. 1990. Economic analysis of soil conservation technologies. In: J.B. Doolette and
W.B. Magrath (Eds.). Watershed Development in Asia. Strategies and Technologies. World
Bank Techn. Paper No. 127. Washington D.C. USA. pp. 71-96.
Magrath, W.B. and P.L. Arens. 1989. The cost of soil erosion on Java – A natural resource
accounting approach. Environment Dept. Working Paper. No. 18. World Bank, Washington
D.C. USA.
554
Magrath, W.B. and J.B. Doolette. 1990. Strategic issues in watershed development. In: J.B.
Doolette and W.B. Magrath (Eds.). Watershed Development in Asia. Strategies and
Technologies. World Bank Techn. Paper No. 127. World Bank, Washington D.C., USA.
Margolis, E. and O.R. Campos Filho. 1981. Determinaçao dos fatores da equaçao universal de
perdas de solo num podzolico vermelho amarelo de Gloria do Goita. Anais do 3rd Encontro
Nacional de Pesq. sobre Cons. do Solo en Recife, Pernambuca, Brazil. July 28-Aug 1, 1980. pp.
239-250.
Marsh, B. 1971. Immediate and long-term effects of soil loss. Proc. Australian Soil Conservation
Conference. 1971.
Milliman, J.D. and R.H. Meade. 1983. World-wide delivery of river sediments to the ocean. J.
Geology 91: 1-21.
Nguyen Huu Hy, Nguyen The Dang and Tong Quoc An. 2010. Soil fertility maintenance and
erosion contol research in Vietnam. In: R.H. Howeler (Ed.). A New Future for Cassava in Asia.
Its Use as Food, Feed and Fuel to Benefit the Poor. Proc. 8th Regional Workshop, held in
Vientiane, Lao PDR. Oct 20-24, 2008. pp. 263-274.
Nguyen The Dang, Tran Ngoc Ngoan, Dinh Ngoc Lan, Le Sy Loi and Thai Phien. 2001. Farmer
Participatory Research in cassava soil management and varietal dissemination in Vietnam –
Results of Phase 1 and plans for Phase 2 of the Nippon Foundation Project. In: R.H. Howeler
(Ed.). Cassava’s Potential in Asia in the 21st Century. Present Situation and Future Research
and Development Needs. Proc. 6th Regional Workshop, held in Ho Chi Minh city, Vietnam.
Feb 21-25, 2000. pp. 383-401.
Pham Van Bien, Hoang Kim and R.H. Howeler. 1996. Cassava cultural practices in Vietnam. In:
R.H. Howeler (Ed.). Cassava Production, Processing and Marketing in Vietnam. Proc.
Workshop held in Hanoi, Vietnam. Oct 29-31, 1992. pp. 58-97.
Phommasack, T., O. Sengtaheuanghung and K. Phanthaboon. 1995. The management of sloping
land for sustainable agriculture in Laos. In: A. Sajjapongse and C.R. Elliot (Eds.). Asialand:
The Management of Sloping Lands for Sustainable Agriculture in Asia. (Phase 2, 1992-1994).
Network Doc.#12. IBSRAM, Bangkok, Thailand. pp. 87-101.
Phommasack, T., O. Sengtaheunghung and K. Phanthaboon. 1996. Management of sloping lands for
sustainable agriculture in Laos. In: A. Sanjjapongse and R.N. Leslie (Eds.). The Management of
Sloping Lands in Asia (IBSRAM/ASIALAND) Network Doc. #20. IBSRAM, Bangkok,
Thailand. pp. 109-136.
Putthacharoen, S., R.H. Howeler, S. Jantawat and V. Vichukit. 1998. Nutrient uptake and soil
erosion losses in cassava and six other crops in a Psamment in eastern Thailand. Field Crops
Research 57: 113-126.
Quintiliano, J., A. Margues, J. Bertoni and G.B. Barreto. 1961. Perdas por erosão no estado de S.
Paulo. Brigantia 20(2): 1143-1182.
Roose, E.J. 1977. Application of the Universal Soil Loss Equation of Wischmeier and Smith in West
Africa. In: D.J. Greenland and R. Lal (Eds.). Soil Conservation and Management in the Humid
Tropics. John Wiley and Sons. New York, NY. USA. pp. 177-187.
Ruppenthal, M., D.E. Leihner, N. Steinmuller and M.A. El-Sharkawy. 1997. Losses of organic
matter and nutrients by water erosion in cassava-based cropping systems. Expl. Agric. 33: 487-
498.
Sheng, T.C. 1982. Erosion problems associated with cultivation in humid tropical hilly regions. In:
Soil Erosion and Conservation in the Tropics. Proc. Symp., held in Fort Collins, Co, USA. Aug
5-10, 1979. ASA, SSSA. Madison, Wisc., USA. pp. 27-39.
Suwarjo and Abujamin. 1983. Crop residue mulch for conserving soil in uplands of Indonesia. In:
El-Swaifi, S.A., W.C. Molderhauer and A. Lo (Eds.) Soil Erosion and Conservation. Soil
Cons. Soc. of America. Ankeny, Iowa, USA. pp. 607-614.
555
Tian Yinong, Lee Jun, Zhang Weite and Fang Baiping. 1995. Recent progress in cassava agronomy
research in China. In: R.H. Howeler (Ed.). Cassava Breeding, Agronomy Research and
Technology Transfer in Asia. Proc. 4th Regional Workshop, held in Trivandrum, Kerala, India.
Nov 2-6, 1993. pp. 195-216.
Tongglum, A., V. Vichukit, S. Jantawat, C. Sittibusaya, C. Tiraporn, S. Sinthuprama and R.H.
Howeler. 1992. Recent progress in cassava agronomy research in Thailand. In: R.H. Howeler
(Ed.). Cassava Breeding, Agronomy and Utilization Research in Asia. Proc. 3rd Regional
Workshop, held in Malang, Indonesia. Oct 22-27, 1990. pp. 199-223.
Tongglum, A., P. Suriyapan and R.H. Howeler. 2000. Cassava agronomy research and adoption of
improved practices in Thailand. Major achievements during the past 25 years. Paper presented
at the 6th Regional Cassava Workshop, held in Ho Chi Minh city, Vietnam. Feb 21-25, 2000. (in
press)
Wargiono, J., B. Guritno, Y. Sugito and Y. Widodo. 1995. Recent progress in cassava agronomy
research in Indonesia. In: R.H. Howeler (Ed.). Cassava Breeding, Agronomy Research and
Technology Transfer in Asia. Proc. 4th Regional Workshop held in Trivandrum, Kerala, India.
Nov 2-6, 1993. pp. 147-174.
Wargiono, J., Koeshartoyo, H. Suyamto and B. Guritno. 1998. Recent progress in cassava agronomy
research in Indonesia. In: R.H. Howeler (Ed.). Cassava Breeding, Agronomy and Farmer
Participatory Research in Asia. Proc. 5th Regional Workshop, held in Danzhou, Hainan, China.
Nov 3-8, 1996. pp. 307-330.
Wischmeier, W.H. 1960. Cropping management factor evaluations for a universal soil-loss equation.
Soil Science Soc. America Proceedings 23: 322-326.
Zhang Weite, Lin Xiong, Li Kaimian, Huang Jie, Tian Yinong, Lee Jun and Fu Quohui. 1998.
Cassava agronomy research in China. In: R.H. Howeler (Ed.). Cassava Breeding, Agronomy
and Farmer Participatory Research in Asia. Proc. 5th Regional Workshop, held in Danzhou,
Hainan, China. Nov 3-8, 1996. pp. 191-210.
Zheng Xueqin, Lin Xiong, Zhang Weite, Ye Kaifu and Tian Yinong. 1992. Recent progress in
cassava varietal and agronomic research in China. In: R.H. Howeler (Ed.). Cassava Breeding,
Agronomy and Utilization Research in Asia. Proc. 3rd Regional Workshop, held in Malang,
Indonesia. Oct 22-27, 1990. pp. 64-80