5.10 Consequences of Agricultural Practices

• Slash-and-burn: farmers cut and burn vegetation to clear a plot, and the ash fertilizes the soil for a few years. It's part of shifting cultivation. With long fallow periods and few people, the forest regrows; with shorter fallows and more people, soils wear out and forests shrink.
• Terraces: flat steps cut into hillsides hold soil and water so steep land can be farmed. The rice terraces of the Philippine Cordilleras and those in Peru's Andes are famous examples.
• Irrigation: bringing water to dry land, from ancient canals in Mesopotamia to center-pivot sprinklers that make green circles on the Great Plains.
• Deforestation: clearing forests for fields and pasture. In the Brazilian Amazon, cattle ranching is the largest driver of clearing, with soy farming also important.
• Draining wetlands: wetlands are drained to create farmland, as in the Netherlands' polders and much of the U.S. Midwest. Drained land is fertile, but drainage destroys habitat and removes natural flood and water filters.
• Shifting cultivation and pastoral nomadism: moving fields and herds also reshape the land, usually lightly unless population pressure rises.

• Pollution: fertilizer and animal waste wash into rivers. Extra nitrogen and phosphorus feed algae blooms, which use up oxygen when they die (eutrophication). Runoff carried by the Mississippi River creates a dead zone in the Gulf of Mexico most summers. Pesticides can also harm wildlife and drinking water.
• Land cover change: forests, grasslands and wetlands turn into fields and pasture, which reduces biodiversity and releases stored carbon.
• Desertification: land in dry areas loses its plant cover and soil and becomes more desert-like. Overgrazing, overcultivation and cutting trees for fuel combine with drought. The Sahel, south of the Sahara, is the classic example.
• Soil salinization: when irrigation water evaporates in hot, dry places, it leaves salts behind, and poor drainage lets them build up until crops can't grow. Parts of Pakistan's Indus basin and Central Asia are badly affected.
• Soil erosion: plowed, bare soil blows or washes away. In the 1930s, plowing up native grassland on the southern Great Plains, followed by drought, produced the Dust Bowl.

From the 1960s, the Soviet Union diverted the Amu Darya and Syr Darya rivers to irrigate cotton in Central Asia. The Aral Sea, once one of the world's largest lakes, lost most of its area. Fishing towns were left far from the water, and dust carrying salt and farm chemicals blew off the dry seabed, harming farmland and people's health. A national policy choice caused regional environmental collapse and local hardship. A dam built in 2005 has helped part of the northern sea recover.

Farmers can reduce damage with contour plowing (plowing across a slope, not up and down it), terracing, no-till farming (planting without turning the soil), cover crops, crop rotation, buffer strips of grass along streams, and drip irrigation, which delivers water straight to roots.

Farming practices also have social effects. They shape diets, such as the move toward more meat and processed food as incomes rise. They shape women's roles, since women do much of the farm work in many places (5.12). And they shape the economy, as farms shift from feeding families to producing for sale.