Biological Soil Conservation

An alternative to physical conservation is to use biological techniques. One of the simplest is to plant the main crop along the contour, alternating with a protective crop such as a grass or legume. Water flowing down the slope meets with the rows of crops, is slowed down, and infiltrates the soil. This method is suitable for slopes of 3 to 8.5 degrees. Strips of grass will help to filter out particles and nutrients from the water and over time will build up into terraces. In Indonesian experiments, strips 0.5 to 1 meter wide of Bahia and signal grass were grown along the contours, alternating with 3- to 5-meter-wide strips of annual crops.³⁹ Erosion was reduced by 20 percent, and after four years natural terraces 60 cm high had been formed.

Conservation Farming

Conservation farming is an essentially ecological approach to soil conservation, which is gaining rapidly in popularity. In common with other new practices described in this book, it has several origins and a diversity of interpretations.

Over the years, especially in temperate climates, it has been common practice for farmers to till the soil in fields before seeds are sown in order to loosen and aerate the soil and to destroy weeds—either by hand with a hoe or with animal- or mechanically powered plows. Tillage breaks up heavy clay soils, and in winter the clods are broken down by snow and ice. However, on many soils prone to erosion or drought, tilling can harm soil structure and increase water loss.

In the United States in the 1940s, a few farmers began experimenting with ways to deal with the severe wind erosion affecting the Great Plains. Some started using mulching to control weeds without tilling the soil. However, it wasn’t until the 1960s, with the development of effective selective herbicides, that farmers could begin practicing what is now known as conservation agriculture (Box 13.2).

Conservation farming experiments, ongoing today, were first conducted in Ohio in 1958. In Brazil and West Africa trials began in the 1970s, but the techniques have since become relatively widespread on erosion-prone fields on large farms in South America.⁴¹ The development of herbicide-resistant crops that could be grown with the easier to apply broad-spectrum herbicides has further increased the popularity

Box 13.2 The Nature of Conservation Agriculture

Conservation agriculture includes various systems of reduced or no tillage. The advantages of these practices include the saving of labor used for plowing, protection of vulnerable soils from erosion by preventing topsoil from being blown or washed away, and improvement of soil fertility by keeping soil structure intact and by allowing more beneficial insects to thrive. It also keeps carbon and organic matter in the soil, both leading to a higher microbial content and sequestering carbon and thus reducing carbon emissions from agriculture (see Chapter 16).

Conservation farming requires learning new techniques, however, and can bring new challenges in weed control and drainage management. Increased used of herbicides can lead to negative environmental impacts, and farmers are now beginning to experiment with the use of more mulching and cover crops to control weeds without herbicides. In some situations farmers must also purchase new equipment such as special seed drills, which requires investment.⁴⁰

in the United States and Latin America. In contrast, conservation-tillage practices are much less known in Europe, Asia, and Africa.⁴²

In the Indo-Gangetic Plains of South Asia, where farmers practice a rice-wheat cropping system, minimum-tillage practices are rapidly being adopted thanks to the development and promotion of new techniques by the Rice-Wheat Consortium and the development of new no-till drills.⁴³

There has also been growing interest in southern Africa. Experiments conducted by partnerships between local government bodies and the NGO Concern Worldwide, such as I recently saw being pioneered in western Zambia, are investigating the use of conservation farming as a replacement for the traditional long fallow system of the region. There the woodland is felled and burned before being plowed and sown to maize. Crops are grown for only a couple of years, and the land then takes several decades to return to a state where it can be felled and burned again. The alternative, conservation farming, is not to plow and instead sow the seed in small “pockets” in the soil to which have been added two cupfuls of manure and a bottle top of fertilizer. After harvest, the soil is covered with the stems and leaves of the maize and next year’s seed is sown several months later in the same holes. Despite the need to hoe weeds, the labor is much less than in the conventional systems. Yields are high—some 4 to 5 tons of maize growing new drought-tolerant hybrids. In addition to building carbon in the cropped soil, such a system should allow tree or shrub cover to remain unburned more or less permanently, so increasing carbon sequestration and maintaining soil carbon levels, creating a more stable and sustainable farming system.