Agriculture Study Guide

What are perennial plants? How can they be beneficial in agriculture?

A way farmers could reduce soil erosion in the future is through these. Do not meed to be planted each year. Currently most food crops are annuals, but there are several universities and institutions that are working to develop perennial food crops that could be used in the future. Perennial crops would be advantageous because their root systems would be much more developed than those of annual crops, allowing them to hold the soul in place more effectively. Annual crops have to re-grow their root systems each year, so they are not able to grow as deep into the soil. Photo shows the roots of an annual wheat plant on the left and a perennial grass plant on the right.

Know the advantages & disadvantages of modern pesticides

Advantages: -They save human lives. Since 1945, DDT and other insecticides probably have prevented the premature deaths of at least 7 million people (some say as many as 500 million) from insect-transmitted diseases such as malaria (carried by the Anopheles mosquito), bubonic plague (carried by rat fleas), and typhus (carried by body lice and fleas). -They increase food supplies. According to the FAO, 55% of the world's potential human food supply is lost to pests. Without pesticides, these losses would be worse and food prices would rise. -They increase profits for farmers. Officials of pesticide companies estimate that every $1 spent on pesticides leads to an increase in U.S. crop yields worth approximately $4. (However, studies have shown that this benefit drops to about $2 if the harmful effects of pesticides are included.) -They work fast. Pesticides control most pests quickly, have a long shelf life, and are easily shipped and applied. When genetic resistance occurs, farmers can use stronger doses or switch to other pesticides. -When used properly, their health risks are very low relative to their benefits. Pesticide industry scientists argue that when pesticides are used in the approved regulatory manner, they pose no major risk to farm workers and consumers. -Newer pest control methods are safer and more effective than many older ones. Greater use is being made of chemicals derived originally from plants. They are safer to use and less damaging to the environment than are many older pesticides. Genetic engineering is also being used to develop pest-resistant crop strains and genetically altered crops that produce natural pesticides. Disadvantages: -They accelerate the development of genetic resistance to pesticides by pest organisms. Insects breed rapidly, and within 5 to 10 years (much sooner in tropical areas) they can develop immunity to widely used pesticides through natural selection and then come back stronger than before. Since 1945, about 1,000 species of insects and rodents (mostly rats) and 550 types of weeds and plant diseases have developed genetic resistance to one or more pesticides. -They can put farmers on a financial treadmill. Because of genetic resistance, farmers can pay more and more for a pest control program that often becomes less and less effective. -Some insecticides kill natural predators and parasites that help control the pest populations. Wiping out natural predators, such as spiders, can unleash new pests whose populations their predators had previously held in check and can cause other unexpected effects (Case Study, at right). Of the 300 most destructive insect pests in the United States, 100 were once minor pests that became major pests after widespread use of insecticides. -Pesticides do not stay put and can pollute the environment. According to the U.S. Department of Agriculture (USDA), only 0.1-2% of the insecticides applied to crops by aerial spraying or ground spraying reaches the target pests, and less than 5% of herbicides applied to crops reaches the target weeds. -Some pesticides harm wildlife. According to the USDA and the U.S. Fish and Wildlife Service, each year, pesticides applied to cropland in the United States wipe out about 20% of U.S. honeybee colonies and damage another 15%. Pesticides also kill more than 67 million birds and 6-14 million fish. According to a 2004 study by the Center for Biological Diversity, pesticides also menace one of every three endangered and threatened species in the United States. -Some pesticides threaten human health. The WHO and UNEP estimate that, each year, pesticides seriously poison at least 3 million agricultural workers in developing countries and at least 300,000 people in the United States. They also cause 20,000-40,000 deaths per year, worldwide. Each year, more than 250,000 people in the United States become ill because of household pesticide use. Such pesticides are a major source of accidental poisonings and deaths of young children. -According to studies by the National Academy of Sciences, exposure to legally allowed pesticide residues in food causes 4,000-20,000 cases of cancer per year in the United States. Some scientists are concerned about possible genetic mutations, birth defects, nervous system and behavioral disorders, and effects on the immune and endocrine systems from long-term exposure to low levels of various pesticides. The pesticide industry disputes these claims, arguing that the exposures are not high enough to cause serious harm. Children are much more susceptible to low levels of pesticides and other toxic chemicals, because on an amount-per-weight basis, they eat more food, drink more water, and breathe more air. They also put their fingers in their mouths more often and spend more time playing on grass, carpeting, and soil where pesticides can accumulate.

Know the main ways that food can be grown to promote soil conservation: Alley cropping/windbreaks

Alley cropping is when one or more crops are planted in between rows of trees. The trees also provide shade which retains soil moisture and reduces the need for irrigation. The trees can provide habitats for birds or other organisms that eats pests (depending on the species of tree) and the trimmings of the trees can be used as compost. Windbreaks are similar to alley cropping except that the trees are planted around crops, instead of between them. The trees hold soil in place and block the wind to prevent wind erosion. They also retain soil moisture and provide habitats for pest-eating birds and insects.

Know the main ways that food can be grown to promote soil conservation: No-till farming

Also called reduced-till farming. Normally when farmers prepare to plant, they will till and plow their fields. This means they basically turn over the soil, killing any weeds that are growing and releasing carbon and moisture stored in the soil. This also loosens the soil, making it more susceptible to erosion. No-till farming, in contrast, skips this step. Farmers only dig a small hole in which to plant the seed. This reduces the water lost to evaporation and the fuel required to run tractors and other machinery required for tilling. It usually raises crop yields as well. However, it does require more herbicides since weeds are no longer being killed during the tilling process. It many also require more fertilizers since the weeds are no longer decomposing the soil. N-till or reduced-till farming is becoming more common worldwide. (Approximately 41% of US crop lan was farmed using reduced-tilling methods in 2007).

Know the specific definition and be able to explain the consequences of: Waterlogging

Another problem with irrigation is waterlogging, in which water accumulates underground and gradually raises the water table. Farmers often apply large amounts of irrigation water to leach salts deeper into the soil. Without adequate drainage, waterlogging occurs and saline water then envelops the deep roots of plants, lowering their productivity and killing them after prolonged exposure. At least one-tenth of the world's irrigated land suffers from waterlogging, and the problem is getting worse.

Know the specific definition and be able to explain the consequences of: Salinization

Between 1950 and 2007, the world's area of irrigated cropland tripled, with most of the growth occurring from 1950 to 1978, and little growth occurring since 1990. The 20% of the world's cropland that is irrigated produces almost 40% of the world's food. But irrigation has a downside. Most irrigation water is a dilute solution of various salts, picked up as the water flows over or through soil and rocks. Irrigation water that has not been absorbed into the soil evaporates, leaving behind a thin crust of dissolved salts in the topsoil. Repeated annual applications of irrigation water in dry climates lead to the gradual accumulation of salts in the upper soil layers—a soil degradation process called salinization. It stunts crop growth, lowers crop yields, and can eventually kill plants and ruin the land. The United Nations estimates that severe salinization has reduced yields on at least one-tenth of the world's irrigated cropland, and the problem is getting worse. The most severe salinization occurs in Asia, especially in China, India, Egypt, Pakistan, and Iraq. Salinization affects almost one-fourth of irrigated cropland in the United States, especially in the western states.

Understand why meat production & consumption has increased since 1960

Between 1961 and 2007 world meat production— mostly pork, poultry, and beef—increased fourfold, and is likely to more than double again by 2050 as affluence rises and middle-income people begin consuming more meat and meat products in developing countries. During this same period, average meat consumption per person more than doubled. Industrialized meat production may face certain limits in the future. For example, if we include land used to grow grain fed to livestock, the FAO estimates that 30% of the earth's ice-free land is already directly or indirectly involved in livestock production. And this percentage is likely to increase.

Understand how industrialized agriculture threatens biodiversity

Biodiversity and some ecological services are threatened when forests are cleared and grasslands are plowed up and replaced with croplands. For example, one of the fastest growing threats to the world's biodiversity is the clearing or burning of large areas of tropical forest in Brazil's Amazon basin and in its cerrado, a huge savanna-like region south of the Amazon basin. This land is being burned or cleared for large plantations of soybeans, grown for use as cattle feed, and sugarcane used for making ethanol fuel for cars. Other tropical forests are burned to make way for plantations of oil palm trees increasingly used to produce biodiesel fuel. These activities threaten biodiversity and contribute to climate change by releasing carbon dioxide into the atmosphere. A closely related problem is the increasing loss of agrobiodiversity—the world's genetic variety of animals and plants used to provide food. But we are replacing nature's resilient genetic diversity developed through millions of years of natural selection with human-engineered monocultures. Scientists estimate that since 1900, we have lost three-fourths of the genetic diversity of agricultural crops. For example, India once planted 30,000 varieties of rice. Now more than 75% of its rice production comes from only 10 varieties and soon, almost all of its production may come from just one variety. Rice varieties around the world may drop even more if there is a shift toward genetically engineered golden rice and other genetically engineered crops. In the United States, about 97% of the food plant varieties available to farmers in the 1940s no longer exist, except perhaps in small amounts in seed banks and in the backyards of a few gardeners. In other words, we are rapidly shrinking the world's genetic "library," which is critical for increasing food yields. In fact, we might need it more than ever to develop new plant and livestock varieties that can adapt to climate change. This failure to preserve agrobiodiversity is a violation of the biodiversity principle of sustainability. Wild and endangered varieties of crops important to the world's food supply are stored in about 1,400 refrigerated gene or seed banks, agricultural research centers, and botanical gardens scattered around the world. But increasingly, power failures, fires, storms, and war are causing irreversible losses of these stored plants and seeds. However, a new ice vault built underground in the Norwegian Arctic will eventually contain duplicates of much of the world's stock of seeds and will not be vulnerable to such hazards. Shortages of storage space and money severely limit the number of species that can be preserved, and the seeds of many plants (such as potatoes) cannot be stored successfully in gene banks. Because stored seeds do not remain alive indefinitely, periodically they must be planted (germinated) and new seeds collected for storage. Unless this is done, seed banks become seed morgues. Because of such limitations on seed banks, ecologists and plant scientists warn that the only effective way to preserve the genetic diversity of most plant and animal species is to protect representative ecosystems throughout the world from agriculture and other forms of development.

Know what the different types of pesticides are designed to kill

Common types of pesticides include insecticides (insect killers), herbicides (weed killers), fungicides (fungus killers), and rodenticides (rat and mouse killers).

Know the advantages and disadvantages of genetically modified food

Despite its promise, controversy has arisen over the use of genetically modified (GM ) food and other forms of genetic engineering. Its producers and investors see this kind of food as a potentially sustainable way to solve world hunger problems and improve human health. Some critics consider it potentially dangerous "Frankenfood." Critics recognize the potential benefits of GM crops. But they warn that we know too little about the longterm potential harm to human health and ecosystems from the widespread use of such crops. They point out that genetic engineering mixes genes from widely differing species, which has never occurred in nature or even in selective breeding. They warn that if GM organisms released into the environment cause some unintended harmful genetic and ecological effects, as some scientists expect, they cannot be recalled. For example, there is the potential for GM plant pollen to spread among nonengineered species, threatening crop biodiversity. In a 2006 study by the Union of Concerned Scientists, half of the nonengineered corn and soybean varieties tested by one laboratory contained DNA from GM varieties. Once experimental GM pollens are in the environment, they are impossible to retrieve. Most scientists and economists who have studied the genetic engineering of crops believe that its potential benefits outweigh its risks. They also contend that some of the potential problems associated with GM crops can be eliminated by genetically engineering plants without inserting a gene from another species. This new technique, called chimeraplasty, involves inserting instead a chemical instruction that attaches to a gene, altering it to give desired genetic traits. Critics call for more controlled field experiments, long-term testing to better understand the risks, and stricter regulation of this rapidly growing technology. A 2004 study by the Ecological Society of America recommended more caution in releasing genetically engineered organisms into the environment without adequate testing.

Know what a CSA is and why eating locally/ seasonally can be environmentally advantageous

Eating crops that are produced locally and seasonally can help reduce energy inputs from industrial agriculture. Crops produced out of season can take more energy to produce and are often transported long distances. Eating locally also reduces the amount of energy needed for transporting food. CSA stands for Community-supported agriculture. When consumers join, they pay farmers in advance for a share of their yield. Consumers get fresh local produce in season and farmers get a guaranteed income.

What is artificial selection? What is genetic engineering?

For centuries, farmers and scientists have used crossbreeding through artificial selection to develop genetically improved varieties of crops and livestock animals. Such selective breeding in this first gene revolution has yielded amazing results. Ancient ears of corn were about the size of your little finger and wild tomatoes were once the size of grapes. Traditional crossbreeding is a slow process, typically taking 15 years or more to produce a commercially valuable new crop variety, and it can combine traits only from species that are genetically similar. Resulting varieties remain useful for only 5-10 years before pests and diseases reduce their effectiveness. But important advances are still being made with this method. Today, scientists are creating a second gene revolution by using genetic engineering to develop genetically improved strains of crops and livestock animals. It involves altering an organism's genetic material through adding, deleting, or changing segments of its DNA, to produce desirable traits or to eliminate undesirable ones. It enables scientists to transfer genes between different species that would not interbreed in nature. The resulting organisms are called genetically modified organisms (GMOs) or transgenic organisms. For example, genetic engineers used genes from ordinary daffodils and a soil bacterium to produce golden rice. Compared to traditional crossbreeding, gene splicing takes about half as long to develop a new crop variety, usually costs less, and allows for the insertion of genes from almost any other organism into crop cells. Ready or not, much of the world is entering the age of genetic engineering. Bioengineers are developing, or planning to develop, new varieties of crops that are resistant to heat, cold, herbicides, insect pests, parasites, viral diseases, drought, and salty or acidic soil. They also hope to develop crop plants that can grow faster and survive with little or no irrigation and with less fertilizer and pesticides. Bioengineers have altered citrus trees, which normally take 6 years to produce fruit, to yield fruit in only 1 year. They also hope to use advanced tissue culture techniques to mass-produce only orange juice sacs, which would eliminate the need for citrus orchards. Many scientists believe that such innovations hold great promise for helping to improve global food security. Others warn that genetic engineering is not free of drawbacks.

Know the main ways that food can be grown to promote soil conservation: Strip cropping

Form of polyculture. In strip cropping, a farmer alternates row crops (cotton, corn) with cover crops (completely cover the soil and trap any eroding soil). Both crops are not harvested at the same times so the cover crop will continue to trap soil after the row crop is harvested.

Know the specific definition and be able to explain the consequences of: Desertification

In arid and semiarid parts of the world, the contribution to the world's food supply from livestock and crops is being threatened by desertification. It occurs when the productive potential of soil, especially on arid or semiarid land, falls by 10% or more because of a combination of prolonged drought and human activities that reduce or degrade topsoil. The process can be moderate (a 10-25% drop in productivity), severe (a 25-50% drop), or very severe (a drop of more than 50%, usually creating huge gullies and sand dunes). Only in extreme cases does desertification lead to what we call desert. But when severe desertification occurs, it can expand existing desert area or create new desert in areas that once were fertile land. Over thousands of years, the earth's deserts have expanded and contracted, mostly because of natural climate change. However, human activities have accelerated desertification in some parts of the world. In its 2007 report on the Status of the World's Forests, the FAO estimated that some 70% of world's drylands used for agriculture are degraded and threatened by desertification. Most of these lands are in Africa and Asia, with countries including Nigeria, Iran, Afghanistan, and China suffering from serious desertification. Increasing desertification is also a threat in dryland areas of Brazil and Mexico. According to a 2007 study by the Intergovernmental Panel on Climate Change (IPCC), projected climate change from global warming during this century (mostly the result of human activities) is expected to greatly increase severe and prolonged drought and, consequently, desertification in many arid and semiarid parts of the world. This could result in sharp drops in food production, water shortages for 1 billion-3 billion people, and huge numbers of environmental refugees.

Industrialized agriculture

Industrialized agriculture, or high-input agriculture, uses heavy equipment and large amounts of financial capital, fossil fuel, water, commercial fertilizers, and pesticides to produce single crops, or monocultures. The major goal of industrialized agriculture for any crop has been to steadily increase its yield—the amount of food produced per unit of land. Industrialized agriculture is practiced on one-fourth of all cropland, mostly in developed countries, but has spread since the mid-1960s to some developing countries and now produces about 80% of the world's food. Plantation agriculture is a form of industrialized agriculture used primarily in tropical developing countries. It involves growing cash crops, such as bananas, soybeans (mostly to feed livestock), sugarcane (to produce sugar and ethanol fuel), coffee, palm oil (used as a cooking oil and to produce biodiesel fuel), and vegetables on large monoculture plantations, mostly for export to developed countries. Producing such monoculture crops in the tropics increases yields but decreases biodiversity when tropical forests are cleared or burned for crop plantations. A new form of industrialized agriculture involves widespread use of greenhouses to raise crops. But it requires large inputs of water and energy to move water from one part of the country to another. On a small scale, however, greenhouse production of crops can be water-efficient. Hydroponics is a method whereby plants are grown with their roots in troughs of water inside a greenhouse. Water is pumped from the troughs and sprayed on the plants as artificial rain.

Know how industrial agriculture and traditional farming compare in terms of the amount of energy required to get 1 unit of food energy to the table (& why these values are different)

Industrialized food production takes 10 units of fossil fuel energy to make 1 unit of food energy. By comparison, every unit of energy from human labor in traditional farming provides 1 to 10 units of food energy. Values are different because, with industrial agriculture, energy is required to grow, store, process, package, transport, refrigerate, and cook all plant and animal food.

Know what IPM is

Integrated Pest Management. Has a goal of reducing crop damage by viewing crops and pests as part of an ecosystem. It used all the different methods we discussed to prevent damage from pests, but requires expert knowledge about each crop, its pests and their interactions. Rather than just applying pesticides in case of pests, the farmer constantly evaluates the crop-pest interaction and applies new solution as they are needed. Chemicals can be used, but only as a last resort and only those that are derived from plants. Many countries are experimenting. Indonesia is a success story. Santa Clara County working on it.

Why is eating lower on the food chain a more sustainable way to support the human population?

It allows us to feed more people if more people are eating a plant based diet because 90% of energy is lost at each trophic level.

Slash-and-burn agriculture

One type of polyculture is known as slash-and-burn agriculture. This type of subsistence agriculture involves burning and clearing small plots in tropical forests, growing a variety of crops for a few years until the soil is depleted of nutrients, and then shifting to other plots. Early users of this method learned that each abandoned patch normally had to be left fallow (unplanted) for 10-30 years before the soil became fertile enough to grow crops again. While patches were regenerating, growers used them for tree crops, medicines, fuelwood, and other purposes. In this manner, most early growers practiced sustainable cultivation. However, when too many people use this approach or don't understand its limitations, it can become unsustainable and lead to depletion and degradation of tropical forests, as is now taking place in parts of Africa. In parts of South America and Africa, some traditional slash-and-burn farmers grow as many as 20 different crops together on small cleared plots in tropical forests. The crops mature at different times, provide food throughout the year, and keep the soil covered to reduce erosion from wind and water. This lessens the need for fertilizer and water, because root systems at different depths in the soil capture nutrients and moisture efficiently, and ashes from the burning provide some fertilization. Insecticides and herbicides are rarely needed because multiple habitats are created for natural predators of crop-eating insects, and weeds have trouble competing with the multitude of crop plants.

Know some alternative ways of controlling pests

Pests can be tricked by methods like rotating crops each year will keep specialist pests searching for food. Farmers can also grow crops in a different field each year; again specialist pests will spend more time searching. By changing planting times, even by just a week, farmers can avoid a certain life phase of pest because pest life cycles are very short. Farmers can also provide nearby habitats for the natural enemies of pests. When farmers practice polyculture, they can plant multiple types of plants together so that if one plant attracts a pest, another might be undesirable to the same pest. Farmers can deliberately introduce these natural enemies but this can lead to the enemies becoming pests too. Hormones and pheromones are less common but can be used to repel pests or attract natural predators. Hormones can also be used to disrupt the life cycle of insects to prevent reproduction. These can be expensive to develop and also need to be applied with very specific timing for these reasons they are not all that widely used. On some crops, farmers will spray hot water to kill the pests. This only works on specific corps and if there is an abundant supply of freshwater.

What are the environmental problems with chemical fertilizers?

Pollute local water and air. Nitrated from fertilizers can volatilize (evaporate) into the air which can then form smog in the atmosphere. Excess nitrates runoff into surface water (lakes and rivers) and cause cultural eutrophication. This is when the excess nutrients cause and algae bloom that depletes oxygen in the water as the algae die. Excess nitrates can also leach through soil and contaminate groundwater. If humans pump drinking water from groundwater, these nitrates will then contaminate drinking water. When nitrates enter the human body they are converted to nirities which can bind to hemoglobin. When nitrate binds with hemoglobin molecules, it inhibits their ability to bind with oxygen and thus inhibits hemoglobin's ability to transport oxygen to body tissues. In adults, a small amount of nitrates will not cause a problem, but in babies the same amount can kill them because of their small size. This condition is called blue baby syndrome - the baby will develop blue lips and extremities as those tissues are deprives of oxygen. Eventually this can lead to death. The current EPA standards for nitrates in drinking water is set low specifically to protect infants.

Know who Rachel Carson was & what her contribution to environmental science was

Rachel Carson began her professional career as a biologist for the Bureau of U.S. Fisheries (now called the U.S. Fish and Wildlife Service). In that capacity, she carried out research in oceanography and marine biology and wrote articles and books about the oceans and topics related to the environment. Throughout the 1950s, DDT use was increasing, and in 1958, DDT was sprayed to control mosquitoes near the home and private bird sanctuary of one of Carson's friends. After the spraying, her friend witnessed the agonizing deaths of several birds. She begged Carson to find someone to investigate the effects of pesticides on birds and other wildlife. Carson decided to look into the issue herself and found that independent research on the environmental effects of pesticides was almost nonexistent. As a well-trained scientist, she surveyed the scientific literature, became convinced that pesticides could harm wildlife and humans, and methodically gathered information about the harmful effects of widespread use of pesticides. In 1962, she published her findings in popular form in Silent Spring, a book whose title alluded to the silencing of "robins, catbirds, doves, jays, wrens, and scores of other bird voices" because of their exposure to pesticides. Many scientists, politicians, and policy makers read Silent Spring, and the public embraced it. Chemical manufacturers viewed the book as a serious threat to their booming pesticide business and mounted a campaign to discredit Carson. A parade of critical reviewers and industry scientists claimed her book was full of inaccuracies, made selective use of research findings, and failed to give a balanced account of the benefits of pesticides. Some critics even claimed that, as a woman, Carson was incapable of understanding such a highly scientific and technical subject. Others charged that she was an hysterical woman and a radical nature lover trying to scare the public in an effort to sell books. During these intense attacks, Carson was a single mother, the sole caretaker of an aged parent, and was suffering from terminal breast cancer. Yet she strongly defended her research and countered her critics. She died in 1964—about 18 months after the publication of Silent Spring—without knowing that many historians consider her work an important contribution to the modern environmental movement then emerging in the United States. It has been correctly noted that Carson made some errors in writing Silent Spring. But critics concede that the threat to birds and ecosystems—one of Carson's main messages—was real and that her errors can be attributed to the primitive state of research on the topics she covered in her day. And her critics cannot dispute the fact that her wake-up call got the public and the scientific community focused on the potential threats from uncontrolled use of pesticides, which led to many of them being banned for use in the United States and many other countries.

How can nutrients be returned to/ maintained in soil without using chemical fertilizers?

Rotate crops or plant many types of crops on the same area of land (polyculture). This way, some crops will deplete nutrients and other crops will add nutrients to the soil. Manure can also be added as well as green manure (freshly cut or still growing vegetation that is plowed into the soil to restore nutrients by increasing the O-horizon of the soils). Compost is another option. This is when organic waste material is aerobically decomposed and turned into nutrient rich soil. Composting is easy to do at home as well as on farms and it has the added benefit of reducing your trash production.

What is the green revolution?

Since 1950, about 88% of the increase in global food production has come from using high-input industrialized agriculture to increase yields in a process called the green revolution. The green revolution involves three steps. First, develop and plant monocultures of selectively bred or genetically engineered high-yield varieties of key crops such as rice, wheat, and corn. Second, produce high yields by using large inputs of fertilizers, pesticides, and water. Third, increase the number of crops grown per year on a plot of land through multiple cropping, or multicropping. Between 1950 and 1970, this high-input approach dramatically increased crop yields in most developed countries, especially the United States in what was called the first green revolution. A second green revolution has been taking place since 1967. Fast-growing dwarf varieties of rice and wheat, specially bred for tropical and subtropical climates, have been introduced into India and China and several developing countries in Central and South America. Producing more food on less land has the benefit of protecting biodiversity by saving large areas of forests, grasslands, wetlands, and easily eroded mountain terrain from being used to grow food crops. Between 1950 and 1996, mostly because of the two green revolutions, world grain production tripled. Per capita food production increased by 31% between 1961 and 1985, but since then it has generally declined.

Understand the difference between polyculture and monoculture and why polyculture is more sustainable

Some traditional farmers focus on cultivating a single crop, but many grow several crops on the same plot simultaneously, a practice known as polyculture. Such crop diversity—an example of implementing the biodiversity principle of sustainability—reduces the chance of losing most or all of the year's food supply to pests, bad weather, and other misfortunes. The crops mature at different times, provide food throughout the year, and keep the soil covered to reduce erosion from wind and water. This lessens the need for fertilizer and water, because root systems at different depths in the soil capture nutrients and moisture efficiently. Insecticides and herbicides are rarely needed because multiple habitats are created for natural predators of crop-eating insects, and weeds have trouble competing with the multitude of crop plants. Recent research shows that, on average, low input polyculture produces higher yields than does high-input monoculture. For example, a 2001 study by ecologists Peter Reich and David Tilman found that carefully controlled polyculture plots with 16 different species of plants consistently outproduced plots with 9, 4, or only 1 type of plant species. Therefore, some analysts argue for increasing use of polyculture, along with monocultures, to produce food more sustainably in the future. Both industrialized and conventional production of crops depend on having fertile soil.

Understand how genetic resistance to pesticides works and the consequences of it for agriculture

They accelerate the development of genetic resistance to pesticides by pest organisms. Insects breed rapidly, and within 5 to 10 years (much sooner in tropical areas) they can develop immunity to widely used pesticides through natural selection and then come back stronger than before. Since 1945, about 1,000 species of insects and rodents (mostly rats) and 550 types of weeds and plant diseases have developed genetic resistance to one or more pesticides. Pesticide use has not reduced U.S. crop losses to pests, mostly because of genetic resistance.

Subsistence agriculture

Traditional subsistence agriculture uses mostly human labor and draft animals to produce only enough crops for a farm family's survival, with little left over to sell or store as a reserve in hard times.

Know the main ways that food can be grown to promote soil conservation: Terracing

Used to retain soil on steep terrain. Creates steps on a steep slope. Crops are planted on the horizontal part of each of these steps. This retains water by controlling runoff and therefore erosion. Terracing is a method that has been done around the world for hundreds of years.

Know the main ways that food can be grown to promote soil conservation: Contour planting

Used to retain soil on steep terrain. This is where a farmer plants with the contours of the land rather than in straight rows. Each row acts like a dam to stop soil from eroding from uphill.