Chapter 14

Raised Beds

Thurston, H. David. 1992. Sustainable Practices for Plant Disease Management in Traditional Farming Systems. Westview, Boulder, CO. 279 pp.

The management of wetlands for agriculture by raised beds or raised fields has been practiced extensively by indigenous peoples of the Americas and by Chinese farmers for at least 2,000 years. Darch (1983), Denevan (1970), Denevan et al. (1987), and Parsons and Denevan (1967) described more than 170,000 ha of raised field remnants found in South America. Extensive systems of raised fields known as chinampas were found in Mexico and were also common in Central America (Adams et al. 1981, Barrera et al. 1977, Gomez-Pompa, 1978, Siemens 1980, Siemens and Puleston 1972, Turner 1974, Turner and Harrison 1981).
North American Indians used raised beds in their agriculture before European arrival in several areas of the US (Fowler 1969, Riley and Freimuth 1979). Raised beds were also common in Africa (De Schlippe 1956, Jurion and Henry 1967, IITA 1988, Miracle 1967) and Asia (FAO 1980, King 1926, Harwood and Plucknett 1981, Hsu 1980, Williams 1981). The practice is quite ancient, as raised bed farming was developed in China in the fifth century B.C. (Wittwer et al. 1987). Raised beds in the Wahgi Valley of New Guinea have been dated as older than 350 B.C. (Lampert 1967). Ibn al-Awam (1988), a twelfth century Arab writer who lived near Seville, Spain, recommended raised beds for green beans, chicory, radishes, onions, melons, lettuce and eggplant. Manure was often added to the raised beds. His recommendations are valid today: "Eggplants placed in soil mounded into raised beds, grow robust and very advanced." Regarding raised beds for lettuce, he wrote: "This manner of planting -- in raised beds -- is very good, as the plants receive water uniformly from below, unlike plants grown on level ground."
In 1778 the Spanish writer Francisco Vidal y Cabasés described a new method for cultivating wheat on raised beds recommended by the Englishman (Jethro) Tull. Vidal y Cabasés wrote that using the raised bed system wheat could be grown for many years without fallow; that the system gave excellent yields while using less fertilizer (manure); and that there was less soil compaction, better distribution of water, and in heavy rains better drainage. His claims of better yields were backed up with data provided by a Señor Thomé of the Royal Society of Leon, Spain. In 1775 and 1756, using rye, Señor Thomé compared the raised bed system with normal practice in two different locations. In one location he obtained 530 pounds of grain from raised beds, versus 174 pounds in the same extension of land planted on the level. In another locations yields were 207 pounds on raised beds versus 73 pounds on the level. In spite of these dramatic results, I found no evidence that the system was used again for grain in Spain, probably because of the heavy labor requirements for making large hills and the difficulty of planting wheat on them. Thus, the benefits of raised beds were known centuries ago, but European civilizations of the 1700s were, I suspect, discouraged by their high labor requirements.
Denevan and Turner (1974) defined a raised field as " an agricultural feature created by transferring earth to raise an area above the natural terrain." Denevan (1970) differentiated the following types of wetland cultivation used by indigenous American peoples:

l. soil platforms built up in permanent water bodies
2. ridged, platformed, or mounded fields on seasonally flooded or waterlogged terrain
3. lazybeds or low, narrow ridges on slopes and flats subject to waterlogging;
4. ditched fields, mainly for subsoil drainage
5. fields on naturally drained land, including sandbars, river banks, and lake margins
6. fields diked or embanked to keep water out.
"Raised beds", "ridged fields", "drained fields", and "cambered beds" (Webster and Wilson 1980) are also terms found in the literature to describe raised fields. Hills, ridges, and mounds, types of broken raised beds, historically have been routinely used for many root and tuber crops. Ibn Luyun (Equaras Ibañez 1988) mentioned caballónes (raised beds or mounds) used for trees in the thirteenth century in Almeria, Spain. De Schlippe (1956) described several types of ridges and mounds cultivated by the Zande people of tropical Africa. The different ridging practices of the Kofyar people of the Jos Plateau in Nigeria were described by Netting (1968). For some of their crops the Kofyar used enclosed ridges, called tie ridges, that trap water. After a rain their fields appear to be checkered with pools. Ochse et al. (1961) described the construction of ridge terraces, erosion dikes, and bench terraces, which are used in "modern" tropical agriculture.
Ridge tillage is becoming popular in the US. It is considered to provide significant erosion control benefits. The practice also helps to overcome some of the soil temperature, soil compaction, and weed problems found in many modern systems of agriculture (National Research Council 1989a). Recently, many home gardeners in the US have become interested in raised beds, and books describing their use are available (Carr 1978, Chan 1985, Jeavons 1982, Mittleider 1986).
Relatively little is reported on the utility of raised fields for plant disease management, but there is little doubt that, in addition to their obvious irrigation, drainage, and agronomic value, disease management is often an additional benefit.

Chinampas

Probably the best known raised field system is the chinampas or "floating gardens" of the Valley of Mexico, which the Spanish conquistadors erroneously thought floated (Figure 14.1). When the Spanish arrived in Mexico in 1521 and entered the capitol of the Aztec civilization located on an island in Lake Texcoco, they were amazed by the immense areas in chinampas. Chinampas apparently interfered with the Spanish conquest. As Squier (1858) noted: "The lands around the lake were highly cultivated, as appears from the references made by Villagutierre to 'great fields of maize', surrounded by fences and deep ditches, which were impossible for the Spanish horsemen to leap." Early descriptions of chinampas were made in the 1500s by De Acosta (1987) and in the 1700s by Javier Clavigero (1974) and Torquemada (1969).
Despite Spanish attempts to drain Lake Texcoco for flood control, which diminished greatly the area in chinampas (Mateos 1956), some are still farmed near Mexico City at Xochimilco (Armillas 1971, Gomez-Pompa, 1978, Jimenez-Osornia and del Amo 1988). Only 1,000 ha of chinampas remain, one-tenth of the area the Aztecs possibly had under cultivation 2,000 years ago (Salas 1988). The productivity of the chinampas was cited as a major factor that allowed the Aztecs to grow from a small tribe to a powerful group that dominated most of Mexico.
Chinampas were probably first developed by the Maya and then later utilized by other Indian cultures in Mexico and Central America (Adams et al. 1981, Chen 1987, Siemens 1980, Siemens and Puleston 1972, Redclift 1987, Turner and Harrison 1981). Adams et al. (1981) stated "New data suggest that Late Classic period Maya civilization was firmly grounded in large-scale intensive cultivation of swampy zones." Many investigators do not believe that slash and burn agriculture could have provided sufficient food for the large Maya populations, which constructed Tikal, Palenque, and other Mayan centers. Only a more intensive food production system, such as raised field agriculture, could have produced food sufficient for the large populations that existed then. Over 165 square miles of raised fields have been documented in Maya areas (Rice 1991). Pollen data indicated the possibility of raised fields on the Hondo River (between Mexico and Belize) going back to 1800 B.C. Radiocarbon dating of worked timbers in the area dated them at 1110 B.C. plus or minus 230 B.C. (Puleston 1978). A system similar to chinampas was being used by Indians on the swampy shores of Lake Maracaibo, Venezuela when the Spanish arrived (Simon 1892).

The chinampas currently found at Xochimilco, constructed in shallow Lake Texcoco, are generally rectangular in shape (90 m by 4.6-9.0 m) and separated by canals (Coe 1964). The surface of the chinampas is usually a meter or so above the water level in the canals. Two operations build up the chinampas. First, mud rich in nutrients from the bottom of the canals is dredged up using a hand tool and spread on the chinampa surface. This maintains the canals and enriches the chinampas. In addition, aquatic weeds and animal manure (and in the time of the Aztecs, human waste) are spread on top. A wide variety of crops were grown by the Aztecs on the chinampas and many diverse crops are still seen today (Muñoz 1986). Maize is planted directly in the chinampas, but other crops are first planted in seedbeds prepared by spreading a layer of mud over vegetation, cutting it into small rectangular blocks called chapines, and planting a seed in each chapín. A covering layer of straw is usually placed over the seed bed for protection (Figure 14.2). The chapines are subsequently transplanted to the soil of the chinampas, thus giving the crops a good start. The chinampas are perpetually moist, and cropping can continue year round, even through the dry season. Although little information on yields was found, according to Armillas (1971) yields of the system are very high. In 1978 Venegas (cited by Redclift 1987) calculated maize yields of 4-6 tons/ha, while Gomez-Pompa (cited by Chen 1987) reported yields of 6-7 tons/ha on chinampas in Tabasco .
Chapin (1988) describes the unsuccessful attempts to reintroduce chinampa technology into the lowland tropics of Mexico. He is highly critical of these attempts and correctly identifies many of the technical, social, and political reasons for failure. Nevertheless, his strong implication that chinampas are not a valid model for increasing food production in tropical areas seems superficial and premature. Chinampa construction and maintainence require considerable labor, and this meant that the discovery of immense oil reserves in Tabasco, which increased greatly the cost of scarce labor, poor planning and coordination of government projects, and lack of marketing opportunities and planning contributed more to failure than a faulty conceptual model.
The chinampa system made continuous cropping possible by sophisticated water control, multiple cropping, high levels of organic material and nutrients periodically added to the system, and transplanting of healthy, selected seedlings (chapines) with strong root systems (Gómez-Pompa 1978, Jiménez-Osornio and del Amo R. 1988). These practices contribute towards good disease management. The diversity of crops grown on traditional chinampas may have also contributed to the success of the system by inhibiting disease spread.
Lumsden et al. (1987) studied chinampa soils relative to disease. They compared relative levels of damping-off disease caused by Pythium spp. on seedlings grown in soils from the chinampas versus those grown in soils from modern systems of cultivation near Chapingo, Mexico. They found that disease levels were lower in the chinampa soils. When inoculum of Pythium aphanidermatum was introduced, the fungus was suppressed by chinampa soils. From their studies they concluded:

In the chinampa agroecosystem, apparently a dynamic biological equilibrium exists in which intense management, especially of copious quantities of organic matter, maintains an elevated supply of organic nutrients and calcium, potassium and other mineral nutrients which stimulate biological activity in the soil. The elevated biological activity , especially of known antagonists such as Trichoderma spp., Pseudomonas spp., and Fusarium spp., can suppress the activity of P. aphanidermatum, other Pythium spp. and perhaps other soilborne plant pathogens.

Zuckerman et al. (1989), in a cooperative study between scientists from Mexico and the US, also studied suppression in chinampa soils, but of parasitic nematodes rather than fungi. The authors pointed out that the high organic content of the soil is probably responsible in part for the relatively few nematodes found, but nine organisms with antinematodal activity were also found. Their results were summarized as follows:

Soil from the Chinampa agricultural system in the Valley of Mexico suppressed damage by plant parasitic nematodes in greenhouse and growth chamber trials. Sterilization of the chinampa soils resulted in a loss of the suppressive effect, thereby indicating that one or more biotic factors were responsible for the low incidence of nematode damage. Nine organisms were isolated from chinampa soil which showed antinematodal properties in culture. Naturally occurring populations of plant-parasitic nematodes were of lower incidence in chinampa soils than in Chapingo soil.

The chinampas are examples of traditional agricultural systems that used copious quantities of organic matter; thereby benefitting from natural biological control. Chapters 3 and 13 give additional details on the use of organic amendments and biological control.

Waru Waru

The story of the reconstruction of raised beds originally made by pre-Incan civilizations around Lake Titicaca has received widespread publicity. Erickson (1985), Erickson and Candler (1989), Garatcochea (1985, 1987), Lennon (1982), National Research Council (1989b), and Sattaur (1988) have described over 80,000 ha of raised fields or raised beds called waru waru or caballónes at an elevation of 3800 m near Lake Titicaca bordering Peru and Bolivia. Denevan (1985) suggested that some may be 2,000 years old. When a few of the raised beds were rebuilt according to specifications obtained from Erickson's archeological studies, potato yields in the 1983-84 crop season were 15 tons/ha from caballónes, while the regional average was 4.8 t/ha (Garatcochea 1987). Erickson and Candler (1989) reported potato yields for 1983-1986 of 8-14 tons/ha with an average of 10 tons/ha. Average potato yields in the Department of Puno were only 1-4 tons/ha. More recently, Straughan (1991) described yields of 20 t/ha from potatoes on raised beds reconstructed near Lake Titicaca in Bolivia. Yields were seven times those in nearby altiplano fields. In addition to better water management, the raised beds contributed to flood and frost control. Fish thrived in the canals between raised beds and ducks were also raised. The Bolivian and Peruvian governments now help farmers to reconstruct raised beds and some 50 ha have been reconstructed in Peru. Thus, farmers in the Andes are learning a forgotten technology from their traditional ancestors.

Tablones

Besides the chinampas, many other examples of raised fields are found today. The Maya in the Panajachel River delta of the highland Lake Atitlan basin, Guatemala, grow a large number of crops, especially vegetables, on raised beds called tablones, described by Mathewson (1984) and Wilken (1987). Both authors suggested that the tablones are probably pre-Colombian. Separated by irrigation trenches, they varied in height from 20-65 cm. During construction, a trench was made in the center of the tablón. Weeds, ground litter from nearby coffee plantations, and animal manure were placed in the trench, covered, and allowed to decompose. Later, muck from the irrigation ditches was sometimes added to the tablón. No mention was made of the possible presence or absence of root pathogens in this system, but the matter is worth investigating.

Flooding and Raised Beds

In Southern China, after two or three crops of paddy rice are grown in a field, the land is often ridged for growing various vegetables and ginger (King 1926, Williams 1981). A similar system was described in Taiwan (Su 1979). The anaerobic conditions prevalent under flooding for rice culture destroy many soilborne pests and pathogens (Cook and Baker 1983). A similar system is used in West and Central Africa. According to the International Center for Tropical Agriculture (IITA 1988), there are 85 million ha of inland valleys in sub-Saharan Africa, and 80% of the inland valley fields found in this region practice an annual cycle of mounding for vegetables, cassava, or sweet potatoes during the dry season and flat tillage for rice in the wet season. In the construction and destruction of mounds, organic matter and soil nutrients are recycled through incorporation of crop residues and weeds, and through flooding, many pests and pathogens in the soil are destroyed. Miracle (1967) also described the planting of rice in valley bottoms by the Tabwa people of Zaire. After the rice crop was harvested, mounds were made, which were planted to sweet potatoes or maize. Subsequently the mounds were flattened and rice was grown again. It is noteworthy that traditional farmers in widely separated areas of Asia and Africa developed similar systems that included flooding and raised beds, and these systems appeared to be effective against soilborne organisms. Chapter 11 discusses the effect of flooding on plant pathogens in greater detail.

Raised Beds in Asia

Raised beds, ridges, and mounds are commonly used in Asia for agriculture, especially in high rainfall and swampy areas (Chandler 1981, Harwood and Plucknett 1981, Herklots 1972, King 1926, Marten and Vityakon 1986, Milsum and Grist 1941, Villareal 1980). The importance of raised beds in traditional Asian agriculture is described by Herklots (1972) as follows:

Throughout the monsoon countries of South-east Asia this is the almost universal technique adopted where water is abundant or too abundant. In the vast silted plains of the river valleys of Thailand, of the Mekong delta of Cambodia and of the Pearl River of South China and even of the smaller rivers of Taiwan raised vegetable beds are a feature of the countryside, especially near the large cities.

Hsu (1980) discussed the introduction to the Han Court in China of ridge farming (tai-t'ien) over 2,000 years ago. In many areas of China the use of raised beds is still common (FAO 1980). Ruddle and Zhong (1988) describe the "dike-pond system" found in the Pearl River Delta of Southern China. This intensive agriculture-aquaculture system has been evolving for the last two millennia and includes fish ponds and mulberry and sugar cane dikes. The cultivated land on the dikes also produces fruit trees, vegetables, and ornamental plants and flowers. The system includes an area of 800 square kilometers and supports a dense population of 1.2 million. Luo and Han (1990) call this raised bed system the "deep ditch-high bed system" and note that the ditches produce rice, water hyacinth, snails, and fish while the beds are intercropped with various vegetables, flowers, and fruit. There are about 48,000 ha of such raised beds in the Pearl River Delta. This integrated system provides an outstanding example of long-term agricultural sustainability.
Milsum and Grist (1941) described the formation of raised beds by Chinese gardeners in Malaysia. Raised beds in moist situations were as much as 0.6 m in height, but in dryer sites they were lower. Rotations were common; they noted that farmers rarely allowed a bed to have more than two successive crops. Extensive vegetable and flower growing on large raised beds is common in swampy areas near Bangkok, Thailand (Figure 14.3). Large quantities of organic material, consisting of manure, aquatic plants, mud from canals, and crop and plant debris, were incorporated into the soil in many Asian raised bed systems (FAO 1980, King 1926, McCalla and Plucknett 1981). The combination of raised beds, flooding, and copious quantities of organic matter added to the soil have contributed to better disease management and a sustainable agriculture.
Serpenti (1977) described the "garden islands" of the Kimain people of Frederik Hendrik Island, West Irian. Developed in swamps, these were raised beds similar to the Mexican chinampas. Regarding the use of the beds, Serpenti wrote: "Experience has taught the people exactly what the ground-water level should be at every stage of growth of every crop. Time of planting is chosen so that the changes in the ground-water levels combine most favorably with the level of the beds." Organic matter, consisting of grass, earth with high humus content, and ashes of sago palm leaves, was incorporated into the beds.
The Intha people living on Lake Inle in Burma and raise a variety of vegetables on floating islands made of grasses and reeds and fertilized with muck from the lake bottom (Jeffrey 1974). Peruvian Indians occasionally grow potatoes in reed boats on Lake Titicaca (personal communication -- John S. Niederhauser).
Root and Tuber Crops

An early account of the planting of cassava on mounds was that of Gonzalo Fernando de Oviedo (1986), who described the cassava planting practices of Caribbean Indians in 1526. The mounds they made were 1.8 m in circumference and "knee high". Six to ten stem pieces were planted per mound. Yams and sweet potatoes were also planted in mounds. Many authorities have reported that root and tuber crops such as cassava, yams, sweet potatoes, and taro were usually grown by traditional farmers in Asia, Africa, and the Americas in raised beds, mounds, and ridges (Barrau 1958, Coursey 1967, Denevan and Treacy 1987, IITA 1988, McLoughlin 1970, Miracle 1967, Nyoka 1983, Okafor and Fernandez 1987, Okigbo and Greenland 1976, Omohundro 1985, Waddell 1972, Wang 1983, Yen 1974a). According to Curwen and Hatt (1953), when Captain James Cook "discovered" New Caldonia in 1774, he found taro planted in ridges. Yen (1974a) reported taro growing on mounds in New Caldonia in 1965. Taro is found today planted on ridges or raised beds in most areas of Asia (Harwood and Plucknett 1981, Wang 1983, Yen 1974a).
Jones (1959) states that as early as 1854 the missionary David Livingstone saw cassava planted in raised oblong beds near the Belgian Congo. At this time Europeans thought cassava was indigenous to Africa whereas, in reality, it had been introduced into Africa with the slave trade by the Portuguese (Jones 1959). In Africa almost all yams are planted on mounds, ridges, or raised beds (Coursey 1967). Probably the most complete description of the use of mounds and raised beds by traditional African farmers is that of Miracle (1967). Organic material was commonly incorporated into mounds and raised beds. In several cases described by Miracle, organic matter placed in mounds was burned. Fresco (1986) mentioned burning organic matter incorporated into mounds for cassava in Zaire. Mounds and raised beds were used not only for root and tubers crops but also for vegetables, maize, and a variety of food crops. Prinz and Rauch (1987) described raised beds used in West Cameroon, which were adapted from a form of traditional mounds. Hahn et al. (1987) wrote that in West Africa, except where yams are grown on flat ground, they are usually planted on hand-made mounds, which may be 1 to 2 m in height and 2 to 3 m wide. Parsons and Denevan (1967) noted that many of the ancient South American raised fields probably were used for producing cassava, although positive evidence is lacking. Furthermore, Denevan and Turner (1974) suggested that root crops probably dominated raised bed agriculture in South America. Ibn al-Awam (1988), a Spanish Arab writer of the twelfth century, recommended that caballónes (raised beds) be made for radishes and onions.
In his fascinating ethnobotanical treatment of the sweet potato Yen (1974a) stated: "Within the range of methods of physical preparation of the soil for sweet potato cultivation, there is one physical effect which is universal -- the elevation of soil surface above normal for the area." Ridges, mounds, and raised beds are used for most of the sweet potatoes grown by traditional farmers. Yen (1974a) provided several examples of sweet potatoes growing on mounds, ridges, or raised beds. The Ifugao near Bontoc in the Philippines grew sweet potatoes in rotation with rice on circular mounds in their irrigated terraces.
The "mound builders" of New Guinea (Barrau 1958, Brass 1941, Lampert 1967, Yen 1974a) provide another example of traditional farmers who have developed a sustainable system of agriculture by cultivating sweet potatoes on mounds, producing high yields for long periods of time, with no apparent disease problems. Waddell (1972) described one of the systems in detail. Although the sweet potato gardens covered almost two-thirds of the cultivated land in his study area , other types of farming were practiced, such as mixed gardens, kitchen gardens, and cash-crop gardens. Sweet potato culture was primarily on large mounds called "modó." The mounds averaged 3.8 m in diameter and 0.6 m high. Smaller mounds were also made. The modó mounds permitted continuous cultivation without fallow. Sites in the study area were known to have been in continuous cultivation at least since 1938 (when Europeans first encountered these people). When a new mound was prepared, approximately 20 kg of old sweet potato vines, sugar cane leaves, and other sources of vegetation were placed in the center. When this material began to decompose, the mound was closed with soil and subsequently planted with sweet potato cuttings. According to Waddell (1972), the two to three harvests obtained per year totaled 19 tons/ha of sweet potato roots. The only reference to disease in this excellent, detailed study is the following : "It (sweet potato) is also less susceptible to disease than taro (Colocasia esculenta), which has suffered greatly in recent years from the depredation of the taro beetle (Papuana spp.) and the virus Phytophthora colocasiae in various parts of the Pacific." The above error regarding the nature of Phytophthora (an oömycete fungus - not a virus) perhaps illustrates the level of knowledge and interest that most anthropologists, archaeologists, economists, ethnobotanists, geographers, and sociologists have developed regarding disease problems. Diseases are seldom mentioned or considered in their published studies of traditional or indigenous agriculture. One should not be unduly critical, however, since few plant pathologists study or cite work in those disciplines.
In their recently published compendium on sweet potato diseases, Clark and Moyer (1988) stated: "The rows may be prepared as level beds or raised beds, depending on local requirements. Raised beds are generally preferable to flat beds when it is necessary to improve drainage." Although most sweet potatoes in the US are grown on raised beds (personal communication -- C.A. Clark), the compendium did not mention the effect of raised beds on plant disease.

Maize Mounds and Ridges

The use of hilling, mounds, and ridges in maize culture appears to be an ancient practice in the Americas (Barrerio 1989, Weatherwax 1954, Wilson 1987). It was common throughout these continents. Clavigero (1974) stated in the 1700s that maize was hilled in Mexico because "it is better nourished and can resist sudden winds." When plants have reached a height of about 60 cm, a considerable amount of soil is mounded up around the base of the plant. In Guatemala, the practice is called calzando, "putting boots" on the corn. This practice is still common in higher elevations of Mexico, Central America, and parts of the Andes. Estrella (1986) reported that maize was planted with beans in mounds (caballónes) in Ecuador in 1573.
According to Carrier (1923) and Mt. Pleasant (1989), North American Indians, such as the Iroquois nation, also planted maize in mounds. The soil in the mounds was loosened at the time of planting. As the maize grew, soil was scraped around the roots and the mounds were carefully weeded. Crop residues and weeds were returned to the mounds, thus providing organic matter for the maize. Mounds were used repeatedly and thus became rather large. Remnants of such large mounds in abandoned fields were frequently found by early colonists in North America. Ridged fields, which had probably been used by Indians for maize culture, have been found in Mississippi, Georgia, Texas, Illinois, Michigan, and Wisconsin (Fowler 1969).
Reichel-Dolmatoff (1965) noted that in many parts of the San Jorge Valley in Northern Colombia one can observe hundreds of hectares covered with parallel ridges separated by furrows, the remnants of ancient raised beds used for maize and other crops. Whether these practices of growing maize in hills and ridges and on mounds had any effect on plant disease is unknown, but the better physical conditions for roots and better drainage probably reduced soilborne and crown diseases.
Knight (1978) described a system called "nkule" used in the grasslands of Tanzania. Grass was collected in piles, and soil was hoed on top of it. The grass under the mound was subsequently burned. A similar practice is used in Zaire (personal communication -- Diane Florini). Maize and cucurbits were then planted on the mounds. Fresco (1986) and Miracle (1967) described the use of like practices in the Congo Basin.
Some mound systems are rather complicated and ingenious in that they make use of existing resources in a treeless environment and take advantage of nutrients provided by decomposing grass and, after the mounds have been formed, leguminous plants. Strømgaard (1988) described a system used by the Aisa-Mambwe in Zambia illustrative of this complexity. First, circular mounds (fundikila) were made by cutting up grass sod and piling it into mounds with the turf side inwards.

The compost-mounds, fundikila, made towards the end of the rainy season, are either left to rot for the remainder of the dry season or grown with beans and a little cassava in the mounded 1-year garden, ntumba. Next year the mounds are thrown down, and in the garden, icalo, maize and later millet are broadcast between the growing cassava of the first year. At the start of the following rainy season, in November, the Mambwe normally throw the garden up into mounds for beans and groundnuts with some millet and a little cassava grown between the mounds; in this stage the garden is called ichitikula. If mounds are not made, maize and cassava are grown on the flat, mpepe. The ichitkula is followed by mpepe, as above, after which the peasant either decided to throw up the garden into mounds for the third time, ichituklulu, with beans, or continue the flat mpepe, this time with groundnuts.

Raised Beds for Plant Disease Management

In addition to the obvious benefits of better water management, raised beds, ridges, mounds, and hills are undoubtedly also used because of their value in reducing a high incidence of various root rots in poorly drained soils. Many investigators have found that flooding and associated oxygen stress predispose or increase the susceptibility of plants to infection by various species of Phytophthora, Pythium, and Aphanomyces (Barta and Schmitthenner 1986, Cook and Baker 1983, Kuan and Erwin 1980, Matheron and Mircetich 1985, Mueller and Fick 1987, Stolzy et al. 1965, Wicks and Lee 1985, Wilcox and Miretich 1985). The literature on plant water stress as a predisposing factor to plant disease has been reviewed by Ayres and Boddy (1986), Cook and Baker (1983), Kozlowski (1978), Schoeneweiss (1975, 1986), and Stolzy and Sojka (1984). Raised beds would often prevent or reduce such predisposition to disease due to flooding.
Several species of Phytophthora cause serious root rots of cassava in tropical areas (Booth 1977, Oliveros et al. 1974). Planting in well-drained raised beds or ridges was found to be an effective practice for reducing cassava root rots caused by Phytophthora spp. (Booth 1977, Lozano and Terry 1976). Yields of cassava planted near Caicedonia, Colombia without ridges were reduced to 7 tons/ha. by Phytophthora spp. After farmers planted in ridges, following CIAT recommendations, cassava yields rose to an average of 23 tons/ha. in a 20,000 ha area (personal communication -- J. C. Lozano).
When cow dung was added to yam mounds before planting in Ghana yields were increased and nematode numbers (Scutellonema bradys) were significantly reduced, according to Adesiyan and Adeniji (1976).
For potato production, exceptionally large hills (0.7-0.9 m high) were commonly made by traditional farmers in some parts of the Andes Mountains. I have seen such large hills in several countries in the Andes. Padre Bernabé Cobo (Mateos 1956) wrote in the seventeenth century that Peruvian Indians preparing land with the Andean foot plow (the taclla) made caballónes (large hills); he added "y muy grande algunos" (that some hills were very large). These large hills were made for planting potatoes. In my experience, tuber infection caused by the fungus Phytophthora infestans (causal agent of late blight of potatoes) was rare in the Colombian Andes. The soil of the large hills probably filtered out the fungal spores before they could reach the tubers (Thurston and Schultz 1981). Goodall et al. (1987) and Coffey (1984) found that mounds aided in the management of Phytophthora cinnamomi, a root rot pathogen of avocado trees. Thus, planting in large hills, mounds, and ridges appears to contribute significantly to disease management .
Some examples of the contribution of raised beds or ridges to the management of pathogens are found in the more modern literature. The losses caused by Erwinia caratovora, causal agent of bacterial softrot of Chinese cabbage, is reduced by the use of raised beds or ridges in the US (Fritz and Honma 1987) and China (Williams 1981). Ridging is recommended for the management of bottom rot of lettuce (Rhizoctonia spp.) in New York (Pieczarka and Lorbeer 1974). Heart rot and root rot of pineapple (Phytophthora nicotianae var. parasitica) was reduced in raised beds, according to Baker (1938) as cited by Shea and Broadbent (1983). According to Abawi (1989), in Popayan, Colombia, Rhizoctonia solani is less severe during the wet rainy season if beans are planted on raised beds, which facilitate good drainage. Abawi and Pastor-Corrales (1990) wrote that growing beans on raised beds or ridges reduced diseases that are favored by high soil moisture, such as southern blight (Sclerotium rolfsii), Rhizoctonia root rot (Rhizoctonia solani), and Pythium root rot (Pythium spp.). Raised beds are used extensively in California and contribute to the management of the red stele disease of strawberry (Phytophthora fragariae) and various root rots of lettuce. Raised beds are recommended for management of leather rot of strawberries (Phytophthora cactorum) in Ohio (Madden et al. 1919). Knowles and Miller (1965) recommend that safflower be planted in raised beds in California for the management of Phytophthora drechsleri. Johnston and Springer (1977) found that planting in ridges in New Jersey contributed to the management of Phytophthora capsici (Phytophthora blight of pepper). Arneson (1971) found that raised beds helped to manage white rot of peanut (caused by Sclerotium rolfsii) in Nicaragua. Abawi, Crosier, and Cobb (1985) recommended raised beds for management of Pythium root rot of snap beans in New York. Despite such reports, raised field technologies receive surprisingly little if any attention in most plant pathology texts, and the terms "raised beds" and "ridges" are almost never found in the indices of books about plant pathology.

Summary

Raised fields, raised beds, ridges, and mounds were used widely for millennia by traditional farmers in geographically separated areas of tropical America, Asia, and Africa. Raised bed systems of agriculture with striking similarities evolved in these areas. Drainage, fertilization, frost control, and irrigation were among important considerations in these systems, but planting in soil raised above the soil surface is also an important disease management practice for soilborne pathogens. I have described specialized raised bed systems such as chinampas, tablones, and waru waru. Almost all of the systems included the incorporation of organic matter into the raised beds. How much the management of plant diseases and other pests entered into the evolution of these systems is unknown. Raised beds are used extensively today in Asia, often after a rice crop, as flooding the soil for rice culture destroys many soilborne pests and pathogens, and subsequently vegetables and other crops can be grown on the semi-sterilized soil in the raised beds with fewer disease problems. Similar practices are used after rice in tropical Africa. Today, mounds, ridges, and raised beds are used worldwide by indigenous farmers for root and tuber crops, and their use reduces root rot problems. I have listed a number of the diseases managed by raised beds in modern agriculture. The widespread use of raised beds in agriculture today testifies to their value.
Most knowledge regarding raised beds in traditional agriculture comes from anthropologists, archaeologists, ecologists, geographers, and ethnobotanists rather than from plant pathologists or other agricultural scientists. Cooperation and communication among disciplines to make the principles and knowledge regarding the merits of raised beds available to farmers worldwide would be highly desirable. Some of the raised bed systems, such as the chinampas, are rapidly disappearing (due to urban encroachment). Establishing "natural preserves" where such traditional systems can be maintained would be a contribution to the enlightenment and well-being of future generations. Determining the value of these practices should become part of the research agenda of governments and international agencies in developing countries.