th
John Singer
Jackson Community College, Jackson, MI
© 2010 Pearson Prentice Hall, Inc.

Green Plants: Sun-Powered
Food-Making Machines
Green plants contain chloroplasts. These organelles have the ability to convert solar energy into food energy.
6 CO
2
+ 6 H
2
O + solar energy → C
6
H
12
O
6
+ 6 O
2
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Green Plants: Sun-Powered
Food-Making Machines
In an early agricultural society, people grew plants for food and obtained energy from food.
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Green Plants: Sun-Powered
Food-Making Machines
Shown here is the flow of nutrients in a simplified system.
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Farming with Chemicals:
Fertilizers
Fertilizers are used to replace plant nutrients that agricultural activities remove from the soil.
There are three primary plant nutrients, nitrogen , potassium , and phosphorus .
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Farming with Chemicals:
Fertilizers
Nitrogen
Nitrogen from the atmosphere is unavailable for plant use. Legumes contain bacteria in root nodules capable of fixing nitrogen; lightning can also fix nitrogen. Anhydrous ammonia or nitrogencontaining fertilizers are also used to replace nitrogen removed from soil by crops.
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Nitrogen
Farming with Chemicals:
Fertilizers
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Farming with Chemicals:
Fertilizers
Phosphorus
Phosphorus is used by plants in the phosphates of DNA and RNA as well as
ATP. Animal bones and the ore, rock phosphate, are a source of phosphates that can be converted to fertilizer.
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Farming with Chemicals:
Fertilizers
Potassium
Potassium ions (K+) are taken up by the plants and used to balance fluids in cells. They are also involved in formation and transportation of carbohydrates and may be involved in protein synthesis.
The most common form of potassium in commercial fertilizers in KCl.
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Farming with Chemicals:
Fertilizers
Other Essential Elements
In addition to the three major nutrients, three secondary plant nutrients, magnesium, calcium, and sulfur are needed in moderate amounts.
Also, eight micronutrients are needed in smaller amounts.
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Farming with Chemicals:
Fertilizers
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Farming with Chemicals:
Fertilizers
The “ complete fertilizers ” that are purchased by farmers and home gardeners usually only contain the three major plant nutrients. The numbers on the fertilizer bags/boxes, such as 5-10-
5, give the percentage of
N, P
2
O
5
, and K
2
O, respectively.
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20/12

The War Against Pests
Insects have always been a problem involved with our food production. Insecticides are used to kill insects.
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The War Against Pests
DDT was developed shortly before World War II and was at one time considered to be an ideal insecticide. Its widespread use has saved many human lives from disease and at the same time has caused much environmental damage.
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The War Against Pests
DDT is a persistent chemical substance, meaning it does not break down in the environment. It is fat-soluble and concentrates in fatty tissues.
It is biologically magnified and causes the disruption of calcium metabolism. Its use has been documented to cause the decline of fisheating birds.
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The War Against Pests
Organic phosphorus insecticides have been developed that are less persistent than DDT.
Most are more toxic to mammals than DDT.
Malathion is less toxic than DDT.
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The War Against Pests
Carbomates are another family of insecticides.
Most are narrow-spectrum insecticides that are only directed at one or a few insect pests. They break down easily in the environment and do not accumulate in fatty tissue.
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Biological Insect Controls
Parasitic, predatory organisms, microbes, or viruses can be used to destroy target insects.
The release of sterile insects to ineffectively mate with wild insects can reduce a population.
Pheromones can be used to attract and trap certain insects.
Juvenile hormones have been approved by the EPA to control mosquitoes and fleas. They prevent the nonreproducing larvae stage from forming adults.
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Herbicides and Defoliants
Herbicides are used to kill weeds and defoliants cause leaves to fall off plants.
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Herbicides and Defoliants
2,4-D and 2,4,5-T
2,4-D (2,4-dichlorophenoxyacetic acid) was introduced in
1945. It is an herbicide that is effective against newly emergent, broad-leafed plants. 2,4-D can be purchased in garden supply stores.
2,4,5-T (2,4,5-trichlorophenoxyacid) is a defoliant that is effective against woody plants.
2,4-D and 2,4,5-T were mixed as a product called Agent
Orange, which was used in Vietnam to defoliate the jungle and kill crops of the enemy. Unfortunately, 2,4,5-T contained a contaminant called dioxin that lead to birth defects and potential cancers. The EPA banned 2,4,5-T in 1985.
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Herbicides and Defoliants
Atrazine and glyphosate are the most widely used herbicides in the U.S.
Atrazine binds to protein in chloroplasts and shuts down photosynthesis. Atrazine is used on corn crops. Corn is resistant to atrazine.
Glyphosate is a nonselective herbicide that is metabolized by soil bacteria. Therefore, other plants can be grown on the soil shortly after use.
It is sold by the trade name Round-Up ®.
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Herbicides and Defoliants
Paraquat is a preemergent herbicide that inhibits photosynthesis by accepting electrons that would be used to reduce carbon dioxide. It kills weeds before crop seedlings can emerge and it is rapidly broken down in the soil.
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20/22

Sustainable Agriculture
Conventional farming uses pesticides and fertilizers that have negative environmental consequences.
Conventional farming is also energy intensive.
Organic farming does not use synthetic fertilizers or pesticides. It provides lower yields than conventional farming, but is more labor intensive.
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20/23

Some Malthusian Mathematics
Thomas Robert Malthus stated, in 1830, that the human population increases faster than the food supply. Unless population growth is controlled, poverty and war will be limiting factors. Populations grow geometrically and the food supply grows arithmetically .
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20/24

Some Malthusian Mathematics
The irony is that in the developed world, food is abundant and obesity is a concern, while in the developing world, malnourishment is still a fact of life.
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20/25

Can We Feed a Hungry World?
Earth’s population continues to grow. Modern agriculture has greatly increased our food production. At some point, limiting factors will affect the human population. The only questions are when and how? We can reduce the birthrate and/or increase the deathrate through war, famine, pestilence, or destruction of our environment.
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