Chapter 13

Mulching

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

Many traditional farmers, such as those in China (Chandler 1981, King 1926, McCalla and Plucknett 1981), India (Raychaudhuri 1964), Mexico (Coe 1964 ), Papua New Guinea (Brass 1941, Waddell 1972), Peru (Poma de Ayala 1987), ancient Rome (Columela 1988, Spurr 1986, White 1970), Spain (Bassal 1955, Al-Awam 1988, Bolens 1972), and South and Central America (King 1926, Siemens 1980, Turner and Harrison 1981) have added considerable quantities of organic material to the soil. Youtai (1987) wrote that the value of manures was established in China before the fifth century B.C. According to Spurr (1986), at some times in ancient Rome the quantities of manure used in agriculture were higher than those used in Italy today. One of the tasks farmers had during the time of the Incas in Peru was to carry "much" manure (mucho estiercol) to their maize and potato fields in July (Poma de Ayala 1987). Denevan (1987) noted that the soils of Peruvian terraces were "now maintained by the application of manure and compost and by periodic fallowing, and both were likely true prehistorically." Von Hagen (1959) quoted Cieza de León as describing Inca practices in the 1500s as follows: "They bring back the droppings (guano) of birds to fertilize their cornfields and gardens, and this greatly enriches the ground and increases its yield, even if it once was barren. If they fail to use this manure, they gather little corn."

Llama trains from the Peruvian coast transported guano from birds for fertilizer to the highlands during the time of the Incas (Gade 1975 and Julien 1985), and the Chimu, another ancient Peruvian civilization, used guano from islands off the coast in their agriculture (Ravines 1980). Peruvian Indians not only fertilized with guano, manure from corralled llamas and other cameloids, but also with anchovies, green manure, and ashes, according to Del Busto (1978). Fish such as anchovies or sardines were often planted with a grain of maize to provide moisture and fertilizer at the time of planting (Cieza de León 1985, Del Busto 1978, Mateos 1956). North American Indians in New England also used fish as a source of fertilizer (Barrerio 1989). Human waste was highly regarded in Peru at the time of the Incas as a fertilizer for maize, and was dried, pulverized, and stored for planting maize (Weatherwax 1954). Cobo (Mateos 1956), a Spaniard writing in the seventeenth century, suggested that the Spanish could learn how to fertilize from the Peruvian Indians.

Orlob (1973) noted that many ancient Indian, Islamic, Roman, and medieval Spanish writers repeated over and over a common remedy for diseases of trees and shrubs; namely to remove soil around the plant and replace it with manure, other organic matter, and/or ashes. Raychaudhuri (1964) gives numerous examples of such treatments used in ancient India. Reading the books written centuries ago by Columela (1988), Alonso de Herrera (1988), and Ibn al-Awam (1988), I also found that application of manure was their most common recommendation for disease management. These authors characterized many diseases as "red leaf" or "yellowing." Many of their "diseases" were probably nutrient deficiencies that may have responded well to these recommendations. Ibn al-Awam (1988), an Arab living in Southern Spain, also wrote that various types of manure were useful in curing similar problems of many crops, such as bananas, apples, peaches, citrus trees, figs, grapes, palms, cedars, and wheat. He gave recommendations for restoring fertility to soil by using various combinations of crop residues, straw, manure, and ashes. Manure was sometimes added to raised beds. Ibn al-Awam also described methods of treating and composting animal manure and human waste. Bassal (1955), an Arab writer of eleventh century Spain, extolled the virtues for agriculture of "much manure". Numerous additional ancient Greek, Roman, Arab, and medieval authors also praised the virtues of manure for agriculture. Composting of manure and use of manure pits was recommended by several ancient authors (Al-Awam 1988, Cato 1934, Columella 1988, Ibn Luyun (Equaras Ibañez 1988), Varro 1934). Thus, the ancients knew the value of organic matter not only as an essential agronomic practice, but also for the management of plant diseases.

Types of Organic Matter

Organic matter is 1) either brought to fields from elsewhere or 2) consists of crop residues or green manure crops, that are incorporated into the soil. Palti (1981) distinguished between organic amendments (incorporated into the soil) and mulches (which are spread or left, e.g. stubble, on the soil surface). Organic amendments used by traditional farmers consisted of manure, composts, aquatic plants, mud from rivers, streams, and canals, and crop and plant debris. Watson (1983) lists the many types of organic matter used in ancient Arab agriculture. Manure from many different domestic and wild animals, in addition to human waste, was used. Animal products such as blood, urine, and powdered bones, horns, and ivory were incorporated into the soil in addition to vegetable matter such as straw, husks, amurca, leaves, rags, shavings, and other plant debris.

Organic matter applied to fields in Asia was described by King (1926) as follows:

For centuries, however, the canals, streams and the sea have been made to contribute toward the fertilization of cultivated fields, and these contributions in the aggregate have been large. In China, in Korea, and in Japan all but the inaccessible portions of their vast extent of mountain and hill lands have long been taxed to their full capacity for fuel, timber, and herbage, for green manure and compost material; and the ash of practically all the fuel and all the timber used in the home finds its way ultimately to the fields as fertilizer. In some cases organic material had to be transported long distances, and this was time-consuming and expensive.

King (1926) further observed:

In China enormous quantities of canal mud are applied to the fields, sometimes at the rate of even 70 and more tons per acre. So, too, where there are no canals, both soil and subsoil are carried in to the village and there they are, at the expense of great labor, composted with organic refuse, then dried and pulverized, and finally carried back to the fields to be used as home-made fertilizers. Manure of all kinds, human and animal is religiously saved and applied to the fields in a manner which secures an efficiency far above our own practices. Statistics obtained through the Bureau of Agriculture, Japan, place the amount of human waste in that country in 1908 at 23,950,295 tons, or 1.75 tons per acre of her cultivated land.

Long-term soil fertility is often enhanced by the use of organic sources of nitrogen, although much of the nitrogen supplied by organic sources is not immediately available. Bouldin et al. (1984) state: "The organic fraction of manure has many of the properties of the ideal N fertilizer -- it is not subject to leaching or denitrification losses, it is not toxic to plants, and it mineralizes N at a rate dependent on the same climatic conditions as plant growth." Huge quantities of organic manure are produced in the US. King (1990) estimated that 160 million metric tons were produced in 1979. Although King stated that 90% is returned to the land, much of it is not properly utilized. Describing the situation relative to the use of animal manure in the in the US, Bouldin et al. (1984) wrote: "At least 50% of the manure N is not recycled through the farming system, and there is reasonable evidence that no more that 25% of the manure N from most feedlot, dairy, and poultry operations is recycled."

Organic Amendments and Plant Disease Management

There is an extensive literature on the positive effects of organic amendments on plant pathogens (Baker and Cook 1974, Cook and Baker 1983, Lewis and Papavizas 1975). Most introduced biological control agents against soilborne plant pathogens are added to the soil with organic matter. A classic example of the positive effects of adding copious quantities of organic matter to the soil is given by Baker and Cook (1974) and Shea and Broadbent (1983). In Australia, Phytophthora cinnamomi causes a severe root rot of avocados. Growers who add large quantities of chicken manure to the avocado soils have little problem with P. cinnamomi. Growers in the same area with little organic matter in their soils have severe problems with avocado root rot. Borst (1986) and Coffey (1984) reported that mulches reduced damage caused by Phytophthora cinnamomi to avocado.

The addition of large quantities of organic amendments does not always control soil pathogens. In a few cases amendments may increase disease, at least in the short term (Cook 1986, Garrett 1960, Kaiser and Horner 1980), as some soilborne pathogens may survive on organic amendments. Also, some organic decomposition products are phytotoxic (Linderman 1970), so amendments are not always necessarily beneficial.

As Huber and Watson (1970) point out, "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) suggest that organic amendments generally produce 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 effect of organic amendments on soil pathogens, but it is clear that considerable additional research is still needed to clarify the overall value of such amendments for managing plant diseases and their future role in agriculture. There are many examples of soilborne pathogens managed by the addition of organic matter, but in many cases huge amounts of organic matter are needed to provide effective management.

Nematode Management

Organic amendments are useful in managing nematodes (Castillo 1985, Rodriguez-Kabana 1986, Rodriguez-Kabana et al. 1987, Rodriguez-Kabana and Morgan-Jones 1988, Sayre 1971). When chitin, available in some areas as crustacean, fish and other animal wastes, is added to soil, there is an increased parasitism of nematode eggs by fungi (Rodriguez-Kabana 1986, Rodriguez-Kabana and Morgan-Jones (1988). Muller and Gooch (1982) list 125 papers referring to the use of various organic amendments for management of nematodes. Rodriguez-Kabana (1986) and Rodriguez-Kabana et al. (1987) have reviewed the effect of adding organic amendments to the soil for nematode management, and this appears to be a promising area for further research, especially in developing countries where the cost of nematicides is prohibitive. Cook and Baker (1983) suggest : "The addition of copious quantities of organic matter has reduced damage from root-knot nematodes in some cases, perhaps because populations of trappers and hyperparasites increase or because the nematodes are attracted to the organic matter rather than to the roots." The addition of cow dung to yam mounds before planting in Ghana increased yields and significantly reduced nematode numbers (Scutellonema bradys), according to Adesiyan and Adeniji (1976).

The Use of Organic Amendments in China

Wittwer et al. (1987) estimated that organic sources furnish about half of the nutrients applied to crops in China. McCalla and Plucknett (1981) stated:

The story of organic fertilizer use in China is a fascinating one. Probably nowhere else in the world have organic fertilizers attained the same level of importance. For centuries, Chinese farmers have labored to gather and utilize human and animal wastes, crop residues, and other organic and inorganic wastes. Many of the practices described here –– and taught elsewhere as modern scientific farming -- have been a most efficient system of waste recycling.

Cook and Baker (1983) wrote that about 80% of The People's Republic of China's fertilizer requirements are met with organic sources such as composted crop residues, green manures, human waste, and livestock manure. Over 100 tons of compost is often used on a single hectare of land. They state:

Perhaps the best large-scale demonstration of effective biological control by cultural practices is the widespread multiple-cropping organic system used in the People's Republic of China. The agriculture of that country, which feeds nearly one fourth of the earth's population, clearly demonstrates that farming can be both intensive and sustainable and, if stabilized for years or perhaps for centuries, can provide a biological balance and disease suppression similar in effect to the disease suppression that can occur with prolonged monoculture of some crops.

McCalla and Plucknett (1981) estimated that in 1974 there were 9.5 million metric tons of nitrogen produced in China from organic sources. They described in detail the collecting, transporting, and processing of organic fertilizers. Sources of fertilizers in China included crop residues, green manure, pond, river and canal silt and sediments, soil from uncultivated areas, burned soil, plant ashes, chicken manure, weeds, and aquatic plants. Dazhong and Pimentel (1986) analyzed a seventeenth century farming system in China, and concluded that the system was "generally sustainable" and maintained soil nutrients and organic matter.

The combined practices of flooding fields for rice and using organic matter for fertilizers "are apparently key factors in the general absence of soilborne diseases in China." (Kelman and Cook 1977). Fertilizers that are primarily organic contribute to root health not only by improving soil structure, but also by suppressing or eliminating inoculum of soilborne plant diseases (Kelman and Cook 1977). Cook (1986) suggests: "Improvements in root health, by making the root system more efficient, can contribute as much if not more to growth and yield of a crop as can high rates of fertilizer." In describing Chinese agriculture, Youtai (1987) states:

Agricultural production methods practiced over thousands of years in China have proved that the application of organic fertilizers or manure is the most effective means to improve soil structure, raise the productivity of the land, and achieve a "sustainable agriculture" even with intensive cultivation. The Chinese-based "organic agriculture" has not only won international recognition in recent years but has a long history of confirmation in practice as well.

Organic Amendments in Mexican Chinampas

Large quantities of organic material were used in the chinampas of Mexico (Coe 1964) and are described in more detail in Chapter 14. Mud rich in nutrients from the bottom of the canals was dredged up by hand and spread on the chinampa surface. This practice maintained the canals and enriched the chinampas. In addition, aquatic weeds, animal manure, and (in the time of the Aztecs) human waste were also spread on the chinampas.

As previously described in Chapter 3, Lumsden et al. (1987) studied chinampa soils relative to root disease occurrence. When they compared relative levels of damping-off disease incidence caused by Pythium spp. on seedlings grown in soils from the chinampas with those grown in soils from modern systems of cultivation near Chapingo, Mexico, they found that disease levels were lower in the chinampa soils. When they introduced inoculum of Pythium aphanidermatum into the chinampa soils the fungus was suppressed. From their studies they concluded that the copious quantities of organic matter added to the chinampa soils stimulated suppression of Pythium due to biological activity in the soil of organisms antagonistic to the fungus. Zuckerman et al. (1989) also studied suppression in Mexican chinampa soils, but of plant parasitic nematodes rather than fungi. They found that the high organic content of the soil is probably responsible in part for the relatively few nematodes in chinampa soils, but they also found nine organisms with antinematodal activity.

Organic Amendments in Other Traditional Systems

The Arab Ibn Luyun of Almeria, Spain (Equaras Ibañez 1988), wrote the following in 1348: "The straw of faba beans, barley, and wheat sweetens the earth and improves it greatly, and they say that it is used also against "tizón" of grapes. It is applied in December onto plants that have signs of the tizon, and it eliminates the disease." What disease the "tizon" of grapes might have been is unknown, but the organic material appeared to have beneficial effects.

Farmers of Papua New Guinea grew sweet potatoes on especially prepared mounds. Waddell (1972 ) stated that over 20 kg of sweet potato vines, sugar cane leaves, and other vegetation were placed in the mounds. As the material began to decompose, the mound was closed with soil and subsequently planted with sweet potato cuttings. Waddell noted that diseases did not appear to be a serious problem in these plantings.

Incorporation of organic material into mounds and raised beds in Africa by traditional farmers is common (Miracle 1967). Fresco (1986) and Miracle (1967) mentioned burning organic matter, that had been incorporated into mounds. 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). Many other examples of the benefits of adding organic matter to the soil by traditional farmers are found in the literature.

Traditional Use of Green Manure Crops

The value of green manure crops in agriculture has been known for centuries. Cato (1934), a Roman who lived 234-149 B.C., wrote that lupines, beans, and vetch fertilized the land. Varro (1934), writing between 116-27 B.C., suggested that some plants, although they gave no benefit the year they were ploughed under, gave benefits the following year. Varro wrote:

Some crops are to be planted not so much for their immediate return as with a view to the year later, as when cut down and left on the ground they enrich it. Thus is is customary to plough under lupines as they begin to pod -- and sometimes field beans before the pods have formed so far that it is profitable to harvest the beans - - in place of dung, if the soil is rather thin.

Green manure has been used for centuries in China, and its present and past use has been mentioned by Cook and Baker (1983), King (1926), and McCalla and Plucknett (1981).

The practice of using green manure crops has important benefits, as they add organic matter to the soil, and play a role in the suppression of soilborne pathogens while improving the physical condition of the soil. Nutrients may also be added to the soil, especially from leguminous green manures. A wide variety of plants have been used as green manure crops (Karunairajan 1982). Crotalaria spectablis is often used as a cover crop and subsequently plowed under as a green manure. Root-knot nematodes (Meloidogyne spp.) enter the roots of Crotalaria, but do not survive. Thus, Crotalaria also acts as a trap crop and can be useful in the management of nematodes.

Palti (1981) noted that a great variety of effects on diseases, positive and negative, result from the use of green manures, and that it is important to take into account the effect of green manure crops on the C/N ratio as soluble soil nitrogen may be locked up in the microorganisms decomposing the organic material. Some crops may be more susceptible to soilborne pathogens if there is a serious deficiency of nitrogen.

Wilken (1987) wrote that cover crops and green manures are not widely used in Central America and Mexico by traditional farmers. However, he did note one rather unusual system in Ostuncalo, Guatemala. The soils in the area are volcanic, have a high sand content, and are low in nutrients and organic matter. The farmers grew in their fields trees called sauco (Sambucus mexicana) and pruned them annually, so only stumps were left. Leaves and small branches were chopped up and incorporated as a green manure into the fields, where potatoes, maize, and beans were grown. According to Wilken, farmers of the area claimed that good crops depend on this practice. Carter (1969) described the use of velvet bean (Stizolobium spp.) as a combination green manure and mulch by Kekchi Indians in the lowlands of Guatemala. (See Chapter 12).

Contributions Possible from Human Waste

Numerous ancient Arab, Chinese, Greek, Roman, and Spanish authors extolled the benefits of human manure, and some gave specific instructions on how to process it and get a product that was odorless and useful as a fertilizer. One Spanish author (De el Seixo 1793) even claimed that crop yields resulting from the use of human manure were "monstrously" large. Human waste still is widely used in many traditional agriculture systems.

Witter and Lopez-Real (1987) recently calculated that the potential contribution of human waste in the United Kingdom could be nearly 40% of the current demand for nitrogen fertilizer. At present it accounts for less than 3% of the demand. Sewage sludge could also be important, but contamination by industrial pollutants, especially heavy metals, is a major problem. In developed countries, without subsidization, sewage sludge cannot compete economically with chemical fertilizers. They suggested that contamination seems to be a less important problem in developing countries. The lack of effective treatments for human pathogens, and cultural taboos, limit the use of human waste in many developing countries. Unless human waste is properly treated to eliminate pathogens, the many human diseases that such pathogens can cause are a serious concern. However, Witter and Lopez-Real reported that composting is an effective treatment for sewage sludge and night soil, and can produce a hygienic and aesthetically acceptable product. Hoitink and Fahy (1986) have reviewed the literature on the suppression of plant diseases by composted municipal sludge.

Summary

F. H. King had been a professor of soil science at the University of Wisconsin before he went to China, Korea, and Japan in 1907. His observations on agriculture in those countries make fascinating reading. In today's world, where energy shortages and concern about the environment are increasing, his observations (King 1926) are highly pertinent:

It could not be other than a matter of the highest industrial, educational and social importance to any nation if it could be furnished with a full and accurate account of all those conditions which have made it possible for such dense populations to be maintained upon the products of Chinese, Korean, and Japanese soils. Many of the steps, phases and practices through which this evolution has passed are irrevocably buried in the past, but such remarkable maintenance efficiency attained centuries ago and projected into the present with little apparent decadence merits the most profound study.

An important conclusion can be made from this chapter. Historically, many sustainable agricultural systems incorporated large quantities of organic matter into soil. Numerous highly respected authorities in the discipline of plant pathology suggest that the incorporation of large quantities of organic matter into soil generally results in reduced soilborne disease in addition to other important agronomic benefits, and the practice should be recommended whenever feasible.