Conventional produce is grown using methods based on technology from the industrial revolution and the development of agricultural chemistry. Nineteenth-century scientists such as Jean Baptiste Boussingault and Justus von Liebig demonstrated that plants obtain nutrients from minerals dissolved in water. This is the basis of hydroponics (Figure 1.2), a system where plants are grown without soil in a solution of minerals. Previously it was believed that plants “ate” humus (also called compost or organic matter). Compost is the residue from decomposed organic matter derived from plants and animals. It is now widely believed that both minerals and compost contribute to the growth of healthy plants.
Vegetables and fresh-cut flowers were traditionally grown in fields close to their markets; the growers had a wide variety of produce. Farm fields became less diverse with the advent of good road networks and refrigeration technologies that allowed for produce to be shipped to distant parts of the country. The increase in competition from regions with mild climates and long growing seasons caused other growers to specialize in crops that they could grow more economically in their area.
Although the use of conventional methods may initially result in higher yields than the old-fashioned methods, there are problems with regard to long-term use since it decreases soil fertility and causes water pollution. The drawbacks to this method were not immediately apparent, but we now know that over time the heavy use of chemical fertilizers and pesticides harms the microbes in the soil, which ultimately reduces yield by causing infertility. The microbes in the soil are directly related to the availability of nutrients for plant growth. Soil infertility leads to weak plants that are more susceptible to microbial pathogens or insect infestation and thus require the application of more pesticides.
Many pesticides contain chemicals that are toxic to wildlife and humans and that persist in the environment for years. Insecticides influence the evolution of the target insect, which becomes resistant to the chemical. This necessitates the continual introduction of new chemicals into the environment (Figure 1.3). Entry of these chemicals into the groundwater endangers our fresh drinking water supplies. Some insecticides can cause damage to the nervous system in humans.
Excess nutrients such as nitrate or phosphate may also run off into the surface water because of soil erosion from heavy irrigation or rainfall events. This causes problems with excessive algal growth, which reduces habitat for fish. A region in the Gulf of Mexico is called the Dead Zone because it becomes overgrown with algae every summer as a result of nutrients that enter the Gulf from the Mississippi River. As the algae decompose, oxygen in the water is used up and the fish die. The Dead Zone is about the size of the state of Rhode Island.
An additional problem is the use of heavy equipment, which causes compaction of the soil and results in the loss of good soil structure necessary for plant growth. To avoid this problem and also to lessen the possibility of erosion, some growers decline to till the soil. Usually, they instead add large amounts of herbicides to kill weeds that would otherwise be dug up by tillage. Some herbicides have been reported to affect the reproductive system in animals.
Proposed Geographic Origins of Edible Domesticated Plants
Near East, Egypt, and the Mediterranean – apple, cherry, date, fig, grape, olive, pear, plum, cabbage, broccoli, cauliflower, caraway, leek, carrot, hazelnut, melon, garlic, lettuce, pea, onions, beet, chard, pistachio, almond, dill, parsley, poppy, lentil, carob, and flax
Mountains of Central and Western China – apricot, peach, cucumber, adzuki bean, water chestnut, wasabi horseradish, ginger, cinnamon, and bamboo
Southeast Asia and the Pacific Islands – grapefruit, orange, lemon, lime, tangerine, banana, mango, clove, nutmeg, black pepper, turmeric, cardamom, sugarcane, coconut, mung bean, taro, and eggplant
Central America – pepper, green bean, squash, pumpkin, sweet potato, sunflower, vanilla, corn, scarlet runner bean, lima bean, pineapple, guava, and jicama
South America – avocado, tomato, potato, chocolate, peanut, cashew, beans, squash, yam, and papaya
The relatively recent domestication of the sunflower, blueberry, strawberry, cranberry, and bramble fruits (raspberry and blackberry) were bred from wild plants of the eastern United States.
Source: Jack R. Harlan, Crops & Man (Madison, Wisc.: American Society of Agronomy, Crop Science Society of America, 1975).
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