1.1.1 Soils, topography and climate.
Viet Nam is a humid tropical, long and narrow country, located between latitudes 8.5° and 23.5° North and longitudes 102° and 110° East. The narrowest part is only 40 km wide. Toward the east lies the sea and towards the west the Truong Son Mountain range. The terrain is highly varied and tends to slope down towards the sea. This results in marked differences in soils and climatic conditions between regions (Fig. 1),
Figure 1: Agro-climatic map of Vietnam
Adapted from Agro-climatic map of SE Asia by Huke 1982
Source: Hoang Kim, Pham Van Bien and R.H. Howeler 2003
http://www.fao.org/docrep/009/y1177e/y1177e00.htm
Because of this topographic and climatic heterogenuity the country has been divided into seven more or less homogeneous agro-ecological zones i.e. the north Mountainous Region (which includes the Northern Highlands and Midlands), the Red River Delta, the North Central Coast, the South Central Coast, the Central Highlands, the Southeastern Region and the Mekong Delta
The soils of Viet Nam are closely associated with its topography. The mountainous and hilly areas of the northern and central part of the country are mainly Ultisols with some Oxisols in the more tropical regions of the south (Fig.2)
Figure 2: Soil map of Vietnam
Adapted from FAO World Soil Map by R.H. Howeler
Source: Hoang Kim, Pham Van Bien and R.H. Howeler 2003
http://www.fao.org/docrep/009/y1177e/y1177e00.htm
Large areas of Inceptisols are found mainly in the Mekong and Red River deltas as well as along smaller rivers and near the coast. The more recently developed Entisols are found mainly along riverbanks and along the coast. By overlying the cassava production map on the soils map, it is possible to estimate the area of the various soil orders on which cassava is grown. Thus, it was estimated that in Viet Nam about 66% of cassava is grown on Utisols, 17% on Inceptisols, 7% on Oxisols, 4% on Alfisols, 3% on Entisols and 2% on Vertisols (Howeler, 1992). The soil pH generally varies from 4.5 to 6.0. (Pham Van Bien et al., 1996)
In North Viet Nam about 68% of the cassava growing area has a rocky soil, while 18 and 12% have clayey and sandy soils, respectively. Rocky soils are prevalent in Ha Son Binh and Ha Bac provinces. Cassava is grown mainly in areas with ondulating and hilly topography. About 89% of cassava in North Viet Nam is grown on these kinds of soils (Pham Van Bien et al,1996).
In South Viet Nam most cassava soils are sandy in the Central Coastal area and in the Southeastern Region, while rocky soils predominate in Gia Lai-Kon Turn and are also common in Dac Lac province. The cassava growing areas in the provinces of the Central Highlands have a similar topography. In the Central Coastal Region and in the Southeastern Region, cassava is grown mainly on white-grey soils or coastal sandy soils. These regions are flat, soils are poor in nutrients and not suitable for rice cultivation. The cassava growing area in this type of soils accounts for more than 70% of the total cassava area of the south. In the Mekong Delta, cassava is not very important. Due to these marked differences in cassava soil characteristics, research in the north should concentrate on erosion problems and soil fertility enhancement, whereas in the south research on cassava soil improvement and conservation by using intercropping systems is of highest priority (Pham Van Bien et al.,1996).
The climate varies substantially between regions. The northern part of the country has a subtropical climate with low winter (15°C) and high summer (29°C) temperatures. Most rain falls during the summer months of May to September, but during the winter months of January-March there are many rainy days with almost constant drizzle, resulting in a low number of sunshine hours (Fig. 3).
Figure 3: Cassava cropping times in eight agro-ecological regions of Vietnam
Source: Hoang Kim, Pham Van Bien and R.H. Howeler 2003
http://www.fao.org/docrep/009/y1177e/y1177e00.htm
In the south, however, the climate is tropical with relatively small fluctuations in monthly temperatures (25–29°C). The rainy season is about one month delayed compared with the north, but total rainfall is similar. The dry season in the south is more intense, due to an almost complete lack of rain during 5–6 months. In the Central Coastal Region total rainfall is high, but it is poorly distributed with very heavy rainfall from September to November and a long dry season of 7–8 months.
1.1.2 Cassava production areas
Cassava production in Vietnam has mainly been allocated in the Central and Southeast with an increase in planted areas in 2001-2006. In 2006, cassava planted area has reached 474.8 thousand hectares, in which about 65% of total area was allocated in the Central and Southeast. It can be seen that the cassava production in Vietnam has been gradually shifted to the Central and the Southeast areas in the recent years, especially in Gia Lai, Kon Tum, Dak Nong and Dak Lak provinces in the Central highlands; Tay Ninh, Dong Nai, Binh Phuoc, Binh Thuan provinces in the Southeast; and Quang Nam, Quang Ngai, Binh Dinh and Phu Yen provinces in the South Central Coast. The three regions all have increased the areas of cassava production with different levels. Prominent among three regions is the Southeast with a significant increase in 2001 (see Figure 4).
Figure 4. Area and Output of Cassava in the Central provinces, Vietnam
1.1.3 Socio -economic conditions
Cassava farm size in Viet Nam is small and does not vary much among locations. But, cassava farms in the Southeastern Region are on average double the size of those in North Viet Nam. Cassava area per farm for all of Viet Nam is small as well, and average 0.27 ha, with extremes for the Southeast (0.85 ha) and the North Mountainous Region (0.20 ha). When farms are classified according to size, 31.6% of the sampled farms are smaller than 0.6 ha, 35.5% are between 0.6–1.05 ha, and the remaining 33% are larger than 1.05 ha (Pham Thanh Binh et al., 1996).
1.1.4 Cassava cultural practices
Land preparation: Cassava land preparation methods vary among agro-ecological regions. On the sloping soils of the mountainous regions of both North and South Viet Nam, most cassava fields are plowed once, while in the Red River Delta and the North and South Central Coastal Regions they are plowed twice. Especially in some cassava areas under intensive cultivation, such as in Son La, Hoa Binh and Quang Binh provinces, many farmers plow their cassava fields up to three times. After plowing, most of the farmers harrow their land once in the south, once or twice in the north, but three to four times in Son La, Hoa Binh and Quang Binh provinces. However, more than half of all farmers in both North and South Viet Nam do not harrow their fields at all, but plant directly after plowing or manual land preparation. Nearly all tillage for cassava is done by hand or with animals. Only in some provinces of South Viet Nam (Lam Dong, Tay Ninh, Binh Duong, Binh Phuoc and Dong Nai) cassava land cultivation is done with tractors.
According to cultural traditions of each region, farmers plant cassava on ridges or on the flat. In both North and South Viet Nam about 50% of the cassava area is planted on ridges and 50% on the flat. However, large differences exist among provinces, with farmers in some provinces, like Hai Hung, Son La, Hoa Binh and Quang Nam, planting predominantly on ridges, while those in Gia Lai, Kon Turn, Dac Lac, Ha Giang, Tuyen Quang and Bac Thai planting predominantly on the flat (Pham Van Bien et al., 1996). However, in production and experimental plots the two different cassava planting methods did not show any significant difference in yield. The ridge planting seems to be more suitable in heavy soils or on light grey and sandy soils which are poor in nutrients. On relatively good soils, flat planting may produce higher yields due to better land use.
Planting time: The planting time is variable, depending on local climatic conditions and cropping patterns (Fig. 3). Generally, farmers plant cassava either at the beginning (first or second semester) or at the end (4th semester) of the rainy season. In the provinces of the North Mountainous Region and Red River Delta, the planting is done from February to April, when soil moisture increases due to spring rains. However, the low temperature in February is a limiting factor, so in many areas cassava planted in March produces better growth and higher yields. In the North Central Coastal Region, planting usually starts at the end of the year (November–December), while in the South Central Coast and the Central Highlands cassava planting is generally done in the first semester; the harvest takes place from August to October before the height of the rainy season, which may cause flooding. In the Southeastern Region, cassava is generally planted in the second semester, at the beginning of the rainy season (April–May), but in Tay Ninh province most cassava is planted at the end of the rainy season (October).
Stake preparation and storage: Almost in every region farmers use their own cassava stems for planting. Only a few buy their stakes, mainly the medium- and large-scale farmers . Stake quality also influences cassava growth and yield. At the cassava harvest, long stems of 10 to 12-month old plants, free of diseases and insects, are selected to cut stakes. The duration of stake storage varies from <1 to >12 weeks, depending on the harvesting time and the time of planting the next cassava crop. In the provinces of the North Mountainous Region and the Central Coastal Regions many farmers plant stakes taken from cassava stems harvested during the previous month. But in parts of the regions, cassava stakes are normally stored 5–12 weeks before planting. If it is necessary to store stakes for a long time, most farmers place stems, tied in bundles, in a vertical position in a shaded area under trees, or cover the stems with leaves to keep them fresh and viable. Under conditions of reduced sunlight and low temperature of the north, some farmers store stems horizontally in an open field and still maintain good quality. However, the majority of farmers in both North and South Viet Nam store stems in a vertical position in the shade (Pham Van Bien et al., 1996).
Planting method: Most farmers in Viet Nam plant stakes horizontally (76% in the north, 68% in the south). This planting method is suitable mainly on poor soils with thin surface soils. This method maintains adequate moisture in the stakes for sprouting and for root development at the early stage. Vertical planting is used by farmers of a few provinces, such as Quang Binh, Quang Nam, Da Nang and Lam Dong, where the planting is done at the end of the wet season with heavy rain and high moisture content in the soil. In many regions, mainly in the Central Coastal Region and the Central Highlands, farmers plant stakes in a slanted position to combine the advantages of the two previous planting methods. In a slanted planting position, cassava has a relatively good root development, it can take full advantage of the soil depth and generally produces high yields, especially in areas where the surface soil is deep. Slanted planting is done mainly in Quang Nam, Da Nang (71.0% of farmers), Quang Binh (72.5%), Nghe An, Ha Tinh (45.9%) and Lam Dong (40%).
Plant population and spacing: Cassava is planted at various spacings depending on the region. Most of the farmers use a spacing between rows of 50–100 cm and between plants in the row of 50–100 cm. In some provinces, farmers adopt a wider spacing (in the North Mountainous Region and the Central Highlands) due to a thicker surface soil layer. In contrast, in some provinces, such as Ha Bac, Tay Ninh, Quang Ngai and Song Be, a closer spacing (50 × 50 to 100 × 50 cm) is used because of poor soils. Therefore, the cassava plant density in these provinces increases up to 20 000–25 000 plants/ha, whereas in Ha Giang, Tuyen Quang, Cao Bang, Bac Can, Lang Son, Dac Lac, Lam Dong and Khanh Hoa a plant density of less than 15 000 plants/ha is generally used.
Fertilization: Cassava grows rather well on poor soils but to produce high yields the crop requires a large amount of nutrients. To maintain high yields it is necessary to maintain the fertility of the soil. Otherwise, with time, soils will become poorer and cassava yields will decrease. In some provinces of the Red River Delta and the Central Coastal Regions farmers apply 5–7 tons of manure per hectare. But in other areas the amount of manure applied is generally much lower (less than 2 t/ha). Fertilizer N is applied to the cassava fields at a rate of 0–50 kg N/ha; highest rates are applied in the provinces of the Red River Delta, the South Central Coast and the Southeastern Region, especially in Tay Ninh. The average K application rate for the whole country is only about 19 kg K2 O/ha. However, a higher rate of 30–90 kg K2 O/ha is used in Ha Bac, Hai Hung, Ha Son Binh and Tay Ninh provinces. A rate of 50–100 kg K2 O/ha is generally needed to replace the K removed in the root harvest. Due to the abundant availability of single super phosphate in the north, P is applied to cassava at a fairly high rate of 15–30 kg P2 O5/ha in some provinces of the North Mountainous Region, in the Red River Delta and in the North Central Coast. A similar P rate is also used in Tay Ninh province of the Southeastern Region. However, in other parts of the North Mountainous Region, the South Central Coast and the Central Highlands, P application rates are very low.
In general, chemical fertilizers are applied to cassava fields at low rates. Due to a lack of resources, farmers usually apply fertilizers only to other crops. When the cassava price fluctuates and their income from cassava production is not stable, they can not afford to apply large amounts of fertilizers to cassava. Thus, the application of nutrients to cassava soils in the form of animal manure and through intercropping with grain legumes can play a significant role in increasing cassava yields and maintaining soil fertility.
Weeds, diseases and pests. Weeds cause a decrease in cassava yield by competing for light, water and nutrients, especially in the rainy season and at the early growth stage. Weed control is done up to four times, mainly by hand, using a hoe. Most farmers weed two-three times during a crop season. Due to the high temperature all through the year in the south, the number of weeding in that region is slightly higher than in the north. The last weeding is done when cassava is about four months old and the crop canopy completely shades the ground. No herbicides are used to control weeds in cassava fields.
Diseases and insects are not very important in cassava and no serious damage to cassava production has been reported. Most farmers do not report the presence of any insect pests in cassava. Only mites are reported to damage young cassava plants in the Central Coastal Region of the north, but the area affected is limited. The principal cassava diseases are cassava bacterial blight (caused by Xanthomonas manihotis) and Cercospora leaf spot (caused by Cercospora sp.).No good control means are available except the use of clean planting material and resistant varieties. A study in 1968 in Viet Nam reported 19 diseases on cassava caused by different pathogens, but none are of economic importance.
Intercropping: In the north, cassava is generally planted in monoculture. After many years of cassava monoculture, soil productivity is often reduced due to erosion and nutrient exhaustion, resulting in a decrease of cassava yields. Cassava-based intercropping systems in the north occupy less than 10% of the area, while in the south, this area reaches 30–40%. In Binh Thuan, Ninh Thuan, Gia Lai, Kon Tum, Dac Lac and Dong Nai provinces, the area under intercropping with cassava is as high as 70–90%.
Generally, maize, groundnut, black bean and mungbean are used as intercrops with cassava. Besides these, cashew nut, fruit trees, vegetables, soybean and winged bean are also intercropped with cassava, but to a lesser extent (Hoang Kim, Magdalena Buresova 1986; Buresova, M.; Kim, H.; Quyen, T.N., 1987; Nguyen Tien Dung et al, 2004, Nguyen Huu Hy et al 2007a; Nguyen Huu Hy et al. 2007b; Nguyen The Dang 2007); Although we can show the good effect of cassava-based intercropping systems on soil conservation, most farmers are concerned only with the economic aspects of the intercropping systems.
Harvesting: Harvesting time is an important factor affecting cassava yield. If the farmer harvests too early, cassava is still young, and the starch content and yield are low. In areas where cassava is grown for fresh human consumption, farmers harvest from six to seven months after planting up to complete maturity (at 11–12 months). When cassava is consumed as boiled fresh roots, farmers in some areas harvest cassava at less than 6 months after planting. However, the majority of cassava is harvested after 10–12 months, especially in the south, where the processing of cassava into different products requires a high starch content in the roots (Pham Van Bien et al., 1996).
1.1.5. Cassava yield and output
There has a great achievement in cassava yield and output. During the 1980s and 1990s cassava production in Vietnam was in decline. But in the past six years, cassava production increased from 1.99 million tones in 2000 to 7.71 million tones in 2006. (Figure 5) Cassava in Vietnam has rapidly changed its role from a food crop to an industrial crop, with a high rate of growth during the first years of the 21st Century. Vietnam has become the second largest exporter of cassava products, after Thailand. Cassava is now one of seven new agricultural export products, which has caught the attention of the government and local authorities.
Figure 5. Cassava production and yield trends in Vietnam and Asia’s principal cassava producing countries, 1961-2006.
Source: Reinhardt Howeler and Keith Fahrne, 2008
This was achieved through both area expansion, from 237,600 ha in 2000 to 474,800 ha in 2006 (14.3% annual growth), and marked increases in yield, from 8.36 tons/ha in 2000 to 16.2 tons/ha in 2006 (13.4% annual growth). New high-yielding cassava varieties (Table 4) and more sustainable production practices have increased the economic effectiveness of cassava production. In year 2006/07 about 300,000 -350,000 ha of new varieties, mainly KM94, KM140, KM98-5, KM98-1, SM937-26, KM98-7 were grown, this corresponds to about 75 % of the total cassava area in whole country.
Table 4. New high-yielding cassava varieties in Vietnam (1998-2008) 1/
Cassava yields and production in several provinces have doubled, stimulated by the construction of new large-scale cassava processing factories. The cassava output in each region/province has been corresponding to the planted area and yield which greatly depends on the application of new high-yielding cassava varieties in each province.
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