Elevating Agriculture to Old Heights

Raised interest in raised beds

Experienced gardeners may recall a meteoric rise in publicity and popularity during the 1970s of raised-bed vegetable production. What many of us didn't know was that farmers of several South and Central American societies practiced "raised-field" agriculture up to 4,000 years ago. Raised fields are what their name implies: cropland built up with soil and vege-tative material to mitigate the negative effects of high water tables and periodic flooding.

Scholars of raised-field agriculture generally concur that the Mayan people of the lowlands of southern Yucatan in Mexico, parts of Belize and Guatemela, developed the system, whose adoption by other agriculture-based societies then proceeded northward. By the first century, raised-field agriculture was the modus operandi of food production in Teotihuacan, near present-day Mexico City. By the 16th century, the elevated rectangular fields, called chinampas, supplied grains, vegetables, and fruits to a quarter million Aztecs in Teotihuacan.

The chinampas system was still used in Teotihuacan in the late 19th century. Functional examples of the system persist today in Xochimilco in Mexico City and southwest Tlaxcala State, Mexico. Similar systems flourished in present-day Peru, Bolivia, and Ecuador well before Columbus' arrival in the New World. Popular use of the system began to decline at the time of the Spanish Conquest, and rapidly disintegrated over the next 20 years. Factors that influenced the decline include salinization, population pressures, inequitable access to technologies which affect labor use, such as plows. In some areas the Spanish completely destroyed the systems, breaking up the waterways and plowing into the chinampas. Recent studies are reviving interest in the chinampas system as a way to sustain food production in specific ecological conditions with minimal imported inputs.

To construct the chinampas system, the Aztecs removed and piled up aquatic vegetation and muck to create horticultural platforms flanked by waterways and drainage canals (zanjas). By dredging one vertical meter of canal debris every one to four years, farmers in Tlaxcala who employ the system today can add an estimated 1,000 kg total N, 10 kg P, and 120 kg K per hectare to their cropland. Trees such as willow and alder, which often grows in symbiosis with a nitrogen-fixing actinomycete (Frankia sp.), are planted around the islands' perimeters. The trees provide shade, increase diversity and anchor the soil in the wet environment reducing erosion.

A typical field measures 150 by 20 meters. The system's environment also provides habitat for fish and waterfowl. Today, alfalfa (originally brought over by the Spanish) is often grown for two to five years in a 3-meter strip that runs lengthwise in a raised field. The alfalfa is usually followed by corn in rotation. Thus, every 15 to 20 years, an entire raised field benefits from the nitrogen fixed by an alfalfa stand (from 30 to 300 kg N per ha per year).

Upland ingenuity also survives

Some 200 kilometers east of Mexico City, another innovative agricultural system developed among the Aztecs as early as 1,000 B.C. Operating at 2,200 meters above sea level, this system featured canals, terraces, and earthen water storage tanks. Certain aspects of the system remain functional today, moreso than the chinampas which were constructed on what is today an urban area, while the cajetes were originally built in what remains a rural area. These include the funneling of rain runoff to a network of relatively small water tanks (cajetes) situated at the base of hillside terraces on cropland. The cajetes serve as catchments and compost pits for soil and organic debris carried by runoff water. The runoff is directed from cropland via the tank network. Nutrient-laden soil and decomposed debris is returned to the fields while trapped water percolates to recharge the water table. In the meantime, the tanks protect the terraces from structural damage due to runoff.

The system's design reflects the concerns of its architects about intense rainfall events, a scenario that harbors the most potential damage to the tank network. Yet the most vital facet of the tank system is neither engineering nor agronomic, but social. All farmers who manage land within a given catchment area must agree to participate as needed to clean out the cajetes and replace the soil on the terraces, and to ensure that runoff from large storms is channelled from the crop area.

Another characteristic of the cajete system is its accent on plant diversity through intercropping, crop rotation, fallowing, and the maintenance of border areas. Fallowing is a means to restore soil fertility and suppress weed species. The border areas provide space for high- value plants such as fruit, fuel, and fodder trees, and medicinal species (see figure 2).

The trees that grow along field borders stabilize terraces, recycle soil nutrients, and serve as windbreaks, but more research is needed on the effects of many other border species on crops.

Maintenance vs. replacement

The essential principle that marks both the chinampas and cajete production systems is maintenance of internal resources as opposed to their replacement from external sources. Each system relies upon biological diversity, organic nutrient recycling, and human energy sources to sustain itself, rather than monoculture, nutrient imports, and fossil-fuel energy.

The maintenance features of traditional farming systems are increasingly well-known to agricultural professionals and scholars. So are the external replacement features of conventional farming systems. Few people advocate the substitution of one system for the other. But many see the potential advantage to all modern-day farming systems of maintaining optimal use of internal resources, and replacing inefficient use of external resources.

Michael Redclift, Raised Bed Agriculture in Pre-Columbian Central and South America: A Traditional Solution to the Problem of Sustainable Farming Systems?

Daniel C. Mountjoy and Stephen R. Gliessman, Traditional management of a hillside agroecosystem in Tlaxcala, Mexico: An ecologically based maintenance system.

Timothy E. Crews and Stephen R. Gliessman, Raised field agriculture in Tlaxcala, Mexico: An ecosystem perspective on maintenance of soil fertility.