Chapter 12

Mulching

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

Until I began writing this book, I thought I knew what mulching was. In reviewing the literature, I found that mulching means different things to different people. It has been simply defined as "application of a covering layer of material to the soil surface" (Rowe-Dutton 1957) or "any covering placed over the soil surface to modify soil physical properties, create favorable environments for root development and nutrient uptake, and reduce soil erosion and degradation" (Wilson and Akapa 1983). Webster's dictionary (1960) shows a temperate zone bias and defines mulch as "leaves, straw, or other loose material spread on the ground around plants to prevent evaporation of water from soil, freezing of roots, etc." Covering seems to be a key word in most definitions.

Wilken (1987) and Gindrat (1979) distinguished between crop residues, which are developed in situ, and mulches, which include fresh and dried plant material and composts brought to the field (Figure 12.1). However, it should be noted that crop residues are frequently used as mulches. Pathogens are often killed by the heat generated in the production of composts (Hoitink and Fahy 1986). Palti (1981) distinguished between organic amendments (incorporated into the soil) and mulches (what is spread or left, i.e. stubble, on the soil surface).

Unfortunately, mulches provide a good environment for the multiplication and survival of slugs, which sometimes cause serious losses to crops such as beans when mulched. In Costa Rica the same slugs that attack beans also vector a serious human nematode pathogen (Beaver et al. 1984). Mulches may also provide nutrition and a suitable environment for certain plant pathogens. The effect of mulches incorporated into the soil on the C/N ratio is important, as soluble soil nitrogen may be locked up in the microorganisms decomposing the organic material. This may cause a serious nitrogen deficiency, and make some crops more susceptible to soilborne pathogens.

Materials Used for Mulches

The list of materials used as mulches by traditional farmers is very long (Rowe-Dutton 1957, Wilken 1987, Wilson and Akapa 1983). Cereal straw and stalks are perhaps the most commonly used mulches, but other examples are crop debris, sawdust, leaves, grass, corn stover, manure, weeds, reeds, Spanish moss, and various aquatic plants. In modern or commercial agriculture, the list is even longer and includes manufactured products such as various plastic materials, aluminum foil, asphalt paper, glass wool, and paper.

Some authors refer to "live mulches," which are similar to "green manures" (Akobundo 1984, Karunairajan 1982). Live mulches are intercropped with the crop of interest for their mulch value, whereas green manures are also crops grown for their mulch value, but plowed under before planting the crop of interest (personal communication -- M. B. Callaway). The use of green manures is discussed further in Chapter 13 on organic amendments.

Benefits of Mulches

Wilson and Akapa (1983) wrote: "Mulches also decrease soil moisture evaporation, increase infiltration rate, smother weeds, lower soil temperature, and enrich soils." Mulches are especially valuable for protecting seedlings from the impact of rain, hail, and the wind. Mulches can be especially important in tropical areas with heavy rainfall, as they improve water absorption and are important in water conservation. Mulches reduce rain splashing, an important means of dissemination for numerous bacterial and fungal pathogens. Soil temperatures are lower under mulches in warm tropical areas. Valverde and Bandy (1982) cited figures indicating that mulches reduced temperatures in the upper 10 cm of soil by 2Æ C during hot days and by 5Æ in the afternoons. Such temperature changes can have have significant effects on the ability of soilborne plant pathogens to cause disease.

Wrigley (1988) cited a number of benefits from mulching coffee with non-living crop residues. He suggested that mulches reduced soil temperatures, protected against rain, conserved rainfall, increased soil nutrients, increased soil organic matter, produced conditions ideal for root growth, reduced weeds, reduced soil acidity, and increased coffee yields. The main disadvantage Wrigley cited for the use of mulches was high labor costs.

Wilson and Akapa (1983) stated that in the tropics crop response to mulching is almost always positive. In Nigeria, Okigbo and Lal (1982) tested 22 different mulch treatments and found that a rice hull mulch increased maize yields by 0.7 t/ha and cassava yields by 12 t/ha. They observed: "As mulches minimize soil erosion, crop yield can be sustained without a requiring bush fallow rotation." They also pointed out the value of leguminous mulches in providing nitrogen to the soil. At the International Center for Tropical Agriculture (IITA) in Nigeria, research indicated that mulch tillage helped to control erosion and weeds and also improved the soil organic matter content (Rockwood and Lal 1974). In Peru, maize benefitted from mulching, but soybeans, peanuts, and cowpeas showed no yield advantage when mulched (Bandy and Sanchez 1986).

Lal (1975) reviewed work on mulching practices in the tropics. Sanchez (1976) also discussed the general use of mulches in the tropics, and Nair (1984) discussed their use in agroforestry systems. One of the major problems with the use of mulches is that large quantities of material are often needed and, unless crop residues produced in situ are used, material has to be brought in from sources outside of the field. This was done in China for centuries, but at a tremendous cost in human labor (King 1926, McCalla and Plucknett 1981, Witter and Lopez-Real 1987).

Effects of Mulches on Plant Diseases

A variety of effects on diseases, positive and negative, result from the use of mulches. Many authors suggest that mulches may reduce the incidence and severity of plant diseases. Rowe-Dutton (1957) made a comprehensive review of the literature on disease management using mulches for a variety of vegetable crops. Much of the information she cited is anecdotal rather than experimental. Mulches contribute to disease management in various ways. Reduction or prevention of soil splashing is an important function of mulches in the management of some plant pathogens (Fitt and McCartney 1986, Gilbert 1956, Galindo et al. 1983b, Moreno and Mora 1984, Rowe-Dutton 1957). According to Fitt and McCartney (1986): "Rain splash is the second most important natural agent, after wind, in the dispersal of spores of plant pathogenic fungi." Intercropping cassava with maize, melons, or other crops reduced soil splashing by rain and significantly decreased the severity of cassava bacterial blight (Xanthomonas campestris pv. manihotis) in Nigeria (Ene 1977). Mulches served the same purpose as intercropping. Muimba-Kankolongo et al. (1989) found that mulches reduced the incidence of a cassava stem tip dieback of unknown etiology in Zaire. Mulches may prevent direct contact of the foliage, fruit, or vines with the soil and thus prevent diseases transmitted from the soil. Moisture is preserved during dry periods by mulches, so they help provide a constant water supply to plants. The severity of the disease blossom end rot of tomatoes can be reduced by mulching (Rowe-Dutton 1957).

Mulches do not always reduce disease. Experiments in Costa Rica by Mora and Moreno (1984) showed that both incidence and severity of Stenocarpella leaf spot of maize (caused by Stenocarpella macrospora) was increased by soil treatments that included mulching, compared to those including the removal of crop residues. Bandy and Sanchez (1986), working in the Peruvian Amazon, found that mulches of the grass Panicum maximum were detrimental to rice yields, as the rice plants remained green for a longer period and thus were more susceptible to fungal attacks (no fungus was specified).

Cook et al. (1978) have reviewed the literature on the effect of crop residues on plant disease. They list three ways that crop residues left in the field can affect plant diseases:

1. For many plant pathogens, residues provide food and a place to live and reproduce.

2. Residues affect the physical environment occupied by the host and pathogens.

3. As organic soil amendments, residues intensify the microbial activity of the soil and this, along with a variety of decomposition products (some phytotoxic or fungitoxic), may affect pathogens, susceptibility of the host plants, or both.

Organic mulches might have similar effects. Some information is found regarding the use of mulches as a covering layer by traditional farmers, but much more information is found on the use of various organic amendments incorporated into the soil. When organic mulches are so incorporated, there is considerable evidence that they usually have a suppressive effect on plant pathogens and nematodes (Cook and Baker 1983). Organic amendments have been found to be useful in managing nematodes (Castillo 1985, Muller and Gooch 1982, Sayre 1971). According to Hoitink and Fahy (1986) composts prepared with tree barks and used as a mulch may release inhibitors of plant pathogens such as Phytophthora spp. and some nematodes. Some organic decomposition products are phytotoxic (Linderman 1970, Patrick et al. 1964), and thus amendments are not always beneficial. As Huber and Watson (1970) observed: "the physical, chemical and biological interactions in soil are so complex and varied that it is a challenge to determine the specific effect responsible for disease control."

Cook and Baker (1983) suggested that organic amendments generally produced enhanced competition among soil microorganisms for nitrogen, carbon, or both, and that this may result in fewer soil pathogen problems. Baker and Cook (1974), Cook and Baker (1983), Garrett (1960), Gindrat (1979), Lewis and Papavizas (1975), Muller and Gooch (1982), Papavizas (1973), and Patrick et al. (1964) have reviewed the effects of organic amendments on soilborne pathogens, but it is clear that additional research is still needed to clarify the overall value of such amendments for managing plant diseases and their future role in agriculture. Gindrat (1979) listed many examples of soilborne pathogens managed by the addition of organic matter, but noted that in many cases huge amounts of organic matter were needed to provide control. There are many soilborne plant pathogens, and the management of each may depend on different site-specific environmental and soil factors.

Traditional Mulching Practices

Traditional farmers, especially the Chinese (Youtai 1987), have used mulches in their agriculture for millennia. It is often difficult to distinguish between mulches, crop residues, organic fertilizer, and green manures, because in much of the literature authors often do not specify whether these organic amendments were used as a covering layer on the soil or incorporated into the soil. Additional examples, besides those that follow, of traditional mulching practices can be found in the chapters on biological control, organic amendments, raised beds, and terraces. Brass (1941) describes the use of raised beds and mulching in the highlands of New Guinea by traditional farmers as follows:

They (raised beds) are made, instead, to get at the rich black swamp deposits and virgin alluvial material of subsurface levels, which, when spread over the impoverished topsoil, brings a new lease on life to the land. But the procedure, as observed, is first to cover the ground with a mattress of cut grass, then to heap the excavated materials on this in a bed 12-15 inches thick.

The Aztec Indians of Mexico used mud from canals, aquatic plants, and manure to spread on the surface of raised beds called chinampas. These practices maintained the canals between the chinampas and enriched the chinampa soils (Coe 1964). In Tlaxcala, Mexico, water hyacinth is collected from ditches and canals and used as a mulch. In addition to its value as a mulch, the process of collecting aquatic weeds and muck cleans the ditches and canals (Wilken 1987). Additional examples regarding the incorporations of organic materials into ridges, mounds, and raised beds are given in Chapter 14.

In Costa Rica, poró (Erythrina poeppigiana) is a commonly used shade tree for coffee. Trees are pruned 1-3 times a year. The pruned branches provide a mulch and return nitrogen to the soil. Recently, Beer (1988) concluded that poró, when pruned 2-3 times per year, can return, as a litter layer, the same quantity of nutrients as are applied to coffee plantations in Costa Rica via inorganic fertilizers, even at the highest recommended rates of 270 kg N, 60 kg P and 150 kg K/ha/yr. In addition, trees contributed 5,000-6,000 kg organic matter/ha/yr. The total leaf litter from both coffee and poró was between 5,000 and 20,000 kg/ha/year. This amount is within the range of leaf litter fall reported for tropical forests. Although the nutrient contribution by nitrogen fixation is important, Beer (1988) concluded that, especially in fertilized plantations of cacao and coffee, leaf litter productivity is a more important contribution of the leguminous shade trees than nitrogen fixation. Litter also provides organic material to the soil and shades out weeds. In Ethiopia, when nutrient deficiencies are noted in coffee, leaves and branches of Erythrina burana are cut and buried around the coffee bushes. Subsequently, farmers claim higher production for several years (Teketay 1990). Wilken (1987) described the use of leaf litter as a mulch in Guatemala, where the nutrient content of the leaf litter is sometimes enriched by placing it in stables beneath animals. The large quantities of organic matter provided by leaves and leaf litter may have important effects on biological control of soilborne pathogens, but no information was found in this regard.

In the South Pacific farmers also use leaves of Erythrina spp. as a mulch. Weeraratna (1990) wrote that farmers there also use grass, weeds, banana leaves, and parts of the coconut palm -- fronds, husks, wood chips, and shredded logs -- as mulches for taro. The application of leaves of Erythrina spp. at the rate of 30 tons/ha as a mulch to taro increased yields by 65%. In Uganda, the Ganda farmers maintain bananas in the same field for up to 50 years without rotation by the use of careful pruning, weeding, and mulching (Fallers 1960). Karani (1986) also describes the mulching of bananas in Uganda.

A number of authors (Cieza de Leon 1959, de Acosta 1987, Parsons and Psuty 1975, Ravines 1978, and Soldi 1982), have described the sunken field agriculture used on the coast of Peru and Chile, one of the driest deserts in the world. Crops were grown in excavated depressions, called hoyas, close enough to the water table so that plants had adequate moisture. A significant percentage of the land in some coastal valleys of Peru was in sunken fields. Fish were often planted with a grain of maize to provide moisture and fertilizer at the time of planting in these sunken fields (Mateos 1956, Del Busto 1978 and Cieza de León 1985). The use of mulches in sunken fields in Peru was described by Cobo (Mateos 1956), a priest who wrote in the seventeenth century. Indians collected the rotten leaves of a tree called guarango and then covered the soil of the sunken fields with a thick layer of the leaves. The purpose of the thick mulch, according to Cobo, was to prevent or remedy the accumulation of salts harmful to agriculture. The thick mulch also probably provided a benefit as an organic amendment. Flores Ochoa (1987) described the cultivation of sunken beds called quochas in arid areas as high as 3,840 m above sea level in Peru.

Slash/Mulch Systems

Several early Spanish chroniclers in tropical America described the use of a slash/mulch system for maize, beans, and other crops (Patiño 1956, 1965). Patiño (1965) suggested that Indian civilizations living in humid tropical forests invented the practice. In the sixteenth century Pedro Cieza de Leon in his "Cronica General de Peru" (cited by Patiño 1965) described an Indian practice: "on hillsides they cut the vegetation and plant their roots and other food crops into it." Patiño (1965) also cited Miguel Cabello Balboa in 1577 as reporting another native practice on the coast of Ecuador: "they do no more than broadcast maize seed in the hillsides and cut the vegetation over it and collect the harvest." This certainly is an early description of the slash/mulch method. Patiño cited other descriptions of the practice, including that of Francisco José de Caldas in 1801, from the Choco province of Colombia:

In those places where it rains continuously such as in the Province of Choco, and the entire west coast of the country, they don't burn; but the excessive humidity combined with great heat makes the land there very fertile. They plant in these areas with no other operation except to cut the small bushes and trees, broadcasting at the same time the grain, after which they cut the vegetation covering the maize.

Conklin (1961), Finegan (1981), and West (1957) referred to the system as slash/mulch. The areas described above, such as the Choco of Colombia, have extremely wet climates. For example, Quibdó, Colombia, located in the Province of Choco, receives over 10 meters of rain annually (West 1957). Patiño cited other descriptions of the slash/mulch system (called tapado in Spanish), not only from Colombia, but also from Panama and Costa Rica. After its introduction to the Americas by the Spanish, rice was also planted in the tapado system. West (1957) wrote a fascinating description of the extensive use of the slash/mulch system on the Pacific coast of Colombia (Province of Choco) and in Ecuador:

Throughout most of the Pacific lowlands, however, the heavy precipitation and lack of a dry season precludes the effective use of fire. Instead a peculiar system, which might be called "slash-mulch" cultivation, of probable Indian origin, has evolved. Seeds are broadcast and rhizomes and cuttings are planted in an uncleared plot; then the bush is cut; decay of cut vegetable matter is rapid, forming a thick mulch through which the sprouts from the seed and cuttings appear within a week or ten days. Weeds are surprisingly few, and the crops grow rapidly, the decaying mulch affording sufficient fertilizer even on infertile hillside soils.

West described the cutting of vegetation in the Choco as a community affair or "minga" with ten or fifteen men and women cutting the bush together with their machetes. Maize, cassava, and plantains were crops planted in the Choco using the slash-mulch system. A primitive variety of maize called "chococito," especially adapted to the slash-mulch system, was commonly planted in Ecuador, Colombia, and Panama (Patiño 1956).

A slash-mulch system on the hot, wet coast of Colombia near Tumaco was described by Finegan (1981). In this high rainfall area, farmers slashed the vegetation, since they could not burn it. Maize, cassava, sugar cane, beans, fruit, taro, sweet potatoes, yams, tannier, and trees for wood were planted in the fields. Certain plants were used as "site indicators" for determining the degree of soil fertility, drainage conditions, and the amount of shade present in a potential slash/mulch field.

Carter (1969) described the use of velvet beans (Stizolobium spp.) as a mulch by Kekchi Indians in the lowlands of Guatemala. The luxurious growth of the velvet beans may reach a height of 2.5 meters in six months. The Kekchi slashed the growth with machetes and chopped it up finely. A mulch 8-10 cm thick of the decayed velvet bean matter was left on the soil, after which maize was planted. Carter claimed that plots planted to velvet bean did not revert to grassland or forest, and some plots had been used consecutively for 14 years of dry season farming with little indication of diminishing fertility. The above observations, if confirmed, indicate the possibility of sustaining soil fertility in the lowland tropics with the velvet bean or other cover crop systems for long periods of time with a minimum of inputs.

Frijol Tapado System

Traditional farmers in many areas of Costa Rica grow beans (Phaseolus vulgaris) using a slash/mulch system called in Spanish "frijol tapado", which in English means "covered beans." According to Patiño (1965), Professor Tulio Ospina of Colombia was the first to use the name "siembra de tapado" for the practice. The procedure consists of broadcasting bean seeds into carefully selected weeds, then cutting and chopping the weeds with a machete so the broadcasted bean seeds are covered with a mulch of weeds (Araya and Gonzalez 1987, Cavallini 1972, Galindo et al. 1982, Jimenez 1978, Patiño 1965, Skutch 1950, von Platen et al. 1982, von Platen 1985). A semi-determinate type of bean, between a bush and a climbing bean, is planted. The beans grow through the mulch and eventually cover it. This combination of mulch and bean plants effectively prevents weed growth and appears to conserve soil moisture. In addition, the mulch prevents soil splashing, which was found in a Costa Rican study by Galindo et al. (1983a, 1983b) to be the most important source of inoculum of Thanatephorus cucumeris causing a severe bean disease called web blight. The fields selected for tapado are generally occupied by broadleaf weeds and certain grasses, which will not regrow after they are cut. Thus the weeds do not compete with the beans for light, nutrients, or moisture. The disease is effectively managed by traditional farmers who use the traditional practice of frijol tapado, even in areas where climate is optimal for web blight development. Skutch (1950) described the practice as follows:

The bean seed is broadcast through the low, dense vegetation, which is then cut down with machetes and chopped up (picado) so that it lies close to the ground. The bean vines sprout up through the mulch of stems and leaves, finally covering them over. No cultivation of the crop is necessary or feasible.

Web Blight of Beans

Web blight of beans is caused by the fungus T. cucumeris (asexual stage -- Rhizoctonia solani ). The disease was described in detail by Thurston (1984). In the humid lowlands of the tropics, web blight is possibly the single most destructive disease of beans. Beans are traditionally grown in cooler, temperate areas in Latin America, but because of population pressures, farmers migrate from high to low-altitude areas and often take beans with them. In warm and humid tropical areas T. cucumeris can cause rapid defoliation of beans and sometimes complete crop failure. In 1980, an epidemic of web blight occurred in the Guanacaste region in the northern part of Costa Rica, resulting in a 90% reduction in bean yields (Galindo 1982). This loss occurred on beans planted under clean cultivation. As with many tropical diseases, precise information on yield losses is difficult to obtain, but the disease has been characterized as severe in Mexico (Crispin and Gallegos 1963), Costa Rica (Echandi 1965), and elsewhere in Latin America (Cardenas-A. 1989, Schwartz and Galvez 1980).

The main sources of inocula that can initiate infection are mycelial fragments and sclerotia (fungal resting bodies). Basidiospores (airborne sexual spores produced by Basidiomycete fungi) can also cause infection (Cardenas-A. 1989, Echandi 1965, Galindo et al. 1983). The study by Galindo et al. (1983) found sclerotia and mycelia free in soil or in the form of colonized debris to be the main source of inoculum in the hot, humid areas of Costa Rica. Inoculation of beans occurred mainly by splashing of rain drops containing infested soil. Large numbers of small sclerotia were produced on rain-splashed soil and debris adhering to bean tissues and on detached tissues on the soil surface. These sclerotia provide new sources of inoculum, which again can be splashed onto beans. Weber (1939) suggested that sclerotia may also be disseminated by wind. In the study by Galindo (1982) in Esparza, Costa Rica, infections caused by basidiospores were observed as previously reported by Echandi (1965). However, the lesions observed were not numerous, remained restricted in size, and apparently caused little damage.

Web Blight Management by the Tapado System

The frijol tapado system was compared in Costa Rica with another mulch system (a 2.5-cm thick layer of rice husks, a cheap by-product commonly found in the area) using both web blight susceptible and tolerant bean cultivars (Galindo et al. 1983b). Bean yields were increased significantly by mulching with rice husks or by frijol tapado. Rice husks and frijol tapado were equally effective in avoiding splashing of infested soil and in managing web blight , and both treatments gave better control of web blight than the fungicide pentachloronitrobenzene (PCNB). PCNB is a highly effective chemical against R. solani and can be applied as a soil or foliar treatment.

In the absence of web blight, the yields in fields under the frijol tapado system are generally lower than those in fields planted in drilled rows with clean cultivation. For this reason, some in Central America oppose continuation of the frijol tapado system; however on small farms in Costa Rica, most of the beans currently produced are grown using the frijol tapado system. Small farmers persist in using the system because of its low risk, its small investment in labor (primarily to cut weeds), and because there is always some yield even when prolonged periods of rain produce conditions that allow T. cucumeris to destroy beans under the clean cultivation system. Von Platen et al. (1982) noted that covered beans could be planted on steep hillsides without erosion problems. Also, once planted, tapado fields required little if any maintainence, so farmers could safely leave a planting while they went to harvest coffee or engage in other off-farm activities. Tapado fields required less labor and, although they have a low productivity per land unit, they have a high return on a labor per work-day basis. Furthermore, the tapado beans would suffer less from possible prolonged droughts, as compared to the clean cultivation system, and thus the risk of decreasing bean harvests is reduced.

Studies by Galindo et al. (1983a, 1983b) on management of web blight by mulching indicated that in the area of Costa Rica where they conducted their research, and during that time period, basidiospores played a minor role in disease spread. Cardenas-A. (1989) studied web blight of beans in Colombia and found that at higher, cooler elevations basidiospores played an important role in the disease epidemiology. Mulching was of no value in management of the disease under the conditions of his experiments (in Darien, Colombia, 1,400 meters above sea level). Cardenas reported that the maximum and minimum temperatures there were 23.6Æ and 16.5Æ C, while Galindo (1982) reported that the maximum and minimum temperatures in his study area (Esparza, Costa Rica) were 30Æ and 20Æ C, respectively. Rainfall was also much higher in the experimental site in Costa Rica. These climatic differences probably help to explain the different results obtained. This is a striking example of the need for site-specific studies when using mulching as a plant disease management practice.

Other Traditional Practices for Web Blight Management

Galindo et al. (1982) noted that in Costa Rica frijol tapado fields were generally planted in hilly areas. Farmers selected hills that received full sunlight early in the morning, thereby reducing the periods of high humidity, which favor the web blight disease. Over 50 farmers were interviewed in Tabasco, Mexico by Rosado May and Garcia-Espinosa (1986) relative to their strategies for management of web blight of beans. In this area yield losses of up to 95% of bean production due to web blight had been recorded. The tapado system was used in association with maize, and farmers also increased the distance of planting for better disease management. Two farmers claimed that they saw no web blight in fields where the "good" weed Euphorbia heterophylla (painted leaf) had been prevalent. Sadly, all farmers interviewed indicated that they were expecting a chemical solution to the web blight problem.

The frijol tapado system is an excellent example of a traditional system that is easily managed, requires low inputs, is sustainable over time and environmentally sound, and, for the farmers that practice it, provides a secure source of food and income that fits in well with their off-farm activities. A challenge remains to modify the system to make it more productive without losing its advantages.

Summary

The use of mulches provides many agronomic benefits, such as lowering soil temperatures, protecting against erosion, providing nutrients and organic matter to the soil, improving soil texture, and reducing weeds. In addition, mulches may manage plant diseases by reducing soil splashing of primary inoculum, influencing the moisture content and temperature of the soil, and enhancing the soil microbiological activity that suppresses soilborne plant pathogens. Mulches may also have negative effects on crops, as decomposition byproducts may be phytotoxic, mulch residues may harbor pathogens or other pests, or the microenvironment, when changed by mulches, may be more favorable to disease development. Nevertheless, the positive benefits of mulches have been generally found to greatly outweigh these potential negative aspects. Effective use of mulches, relative to the control of plant diseases, varies according to location, crop, and pathogen.

Traditional farmers have used mulches for millennia. One of the major problems with the use of mulches is the large quantities of material that are often required. Unless the crop residues or weeds that are produced in situ can be utilized, material has to be transported from outside the field, often at great expense and at a tremendous cost in human labor. Some societies have such labor available, many others do not. In the hot humid tropics where plant growth is rapid and luxurious, there should be many opportunities to increase the use of green manures and natural vegetation as mulches. Considering the potential value of mulches for erosion control, nutrient enhancement, temperature control, weed suppression, and disease management, it appears that there should be far more research on this important subject, using some of the resources expended on agricultural development in tropical developing countries.