Key Concepts

After completing this chapter, you will be able to:

1. Explain how agricultural production is essential to the survival of large numbers of people and domestic animals.
2. Define the term “domestication,” and relate it to humans, their agricultural crops, and livestock.
3. List the most important plants and animals in agriculture and describe the management systems used in their cultivation.
4. Identify the most important environmental effects of agriculture and describe the damage that they cause.
5. Explain how organic agriculture uses a more ecological approach to the cultivation of crops, resulting in less environmental damage.

Introduction

Agriculture can be defined as the science, and art, of cultivating the soil, producing crops, and raising livestock. Even relatively simple agricultural practices can greatly increase the production of food, compared with the hunting and gathering of wild animals and plants. Prior to the development of agriculture, which first appeared around 10,500 years ago, perhaps 5-10 million people were able to subsist through a hunting and gathering lifestyle. Today, the world supports an enormous population (more than 7.8 billion in 2020), and almost all depend on the agricultural production of food (fishing and hunting also provide some food). Clearly, the development of agricultural practices and technologies, and their improvements over time, are among the most crucial of the “revolutions” that have marked the socio-cultural evolution of Homo sapiens.

Agriculture was probably first practiced in the Fertile Crescent, a region of southwestern Asia that includes parts of what are now Iran, Iraq, Israel, Jordan, Lebanon, Syria, and Turkey. Similar developments likely occurred at about the same time in China (~12,000 years ago), although the archaeological evidence is less clear. Other cultures discovered the benefits of agriculture somewhat later, in part through the domestication of local species of plants and animals (for instance, in parts of Central America, western South America, and New Guinea). In other regions, however, domesticated species were mostly imported from elsewhere, as occurred in Australia, Europe, and North America.

In any event, beginning with the cultivation and then domestication of a few useful plants and animals, agricultural technology has advanced to the point where it is now able to support enormous populations of humans and our mutualist species (see Chapter 10).

Modern agriculture involves a number of distinct management practices. In the case of crop plants, they include: selective breeding, tillage, the use of fertilizer and pesticides, irrigation, and reaping. Each practice helps to increase the yield of biomass that can be harvested for food or other uses. The practices are typically used in various combinations, which are undertaken as an integrated system of ecosystem and species management to achieve a large production of crops. However, the management practices also cause important environmental damages.

We previously examined agricultural production and economics in Chapter 12. In this chapter we investigate environmental damages that are associated with agriculture, with particular attention to effects that occur in the United States. We will examine the intensive cultivation of crop plants and livestock, as well as softer management practices that are used in organic agriculture.

Crop Plants

Almost all of the important agricultural crops have been domesticated. Domestication refers to the progressive modification of crops through the selective breeding of cultivated races (or cultivars), which are now genetically, anatomically, and physiologically different from their wild ancestors. Crop plants have been selectively bred to increase their yield and response to management practices and to enhance their palatability. In some cases, thousands of years of domestication have resulted in crop plants that bear so little resemblance to their wild ancestors that they are now incapable of maintaining themselves in the absence of management by people. For example, several millennia of selective breeding of maize (corn; Zea mays) have resulted in its cob becoming tightly wrapped within leafy bracts. As a consequence, its seeds are no longer able to scatter from the cob, so they cannot germinate and develop new plants unless assisted to do so by humans.

A few crop plants have not yet been domesticated. One example is the lowbush blueberry (Vaccinium angustifolium), which has been cultivated for only a few decades. In this case, the habitat of wild plants (in the genetic sense) is being managed to increase their abundance and fruit production as a perennial crop. Because not much selective breeding has been conducted, the blueberry is not yet a domesticated plant. Most crop plants are grown as food, while others are sources of fiber, fuel, or medicine. Important domesticated food plants include the following:

• Small grains: barley (Hordeum vulgare), maize (corn, Zea mays), millet (Panicum miliaceum), oats (Avena sativa), rice (Oryza sativa), sorghum (Sorghum vulgare), wheat (Triticum aestivum and T. durum)
• Legumes (pulses): broad bean (Vicia faba), garden bean (Phaseolus vulgaris), garden pea (Pisum sativum), lentil (Lens culinaris), peanut (Arachis hypogaea), soybean (Glycine max)
• Sweet fruits: apple (Malus domestica), banana (Musa sapientum), grape (Vitis vinifera), grapefruit (Citrus maxima), mango (Mangifera indica), orange (Citrus sinensis), peach (Prunus persica), pear (Pyrus communis), plum (Prunus domestica), raspberry (Rubus idaeus), strawberry (Fragaria virginiana and F. chiloensis), sweet cherry (Prunus avium), watermelon (Citrullus lanatus)
• Vegetable fruits: cucumber (Cucumis sativus), pumpkin (squash, Cucurbita pepo), red pepper (Capsicum annuum), tomato (Lycopersicon esculentum)
• Roots and tubers: beet (Beta vulgaris), carrot (Daucus carota), garlic (Allium sativum), onion (Allium cepa), parsnip (Pastinaca sativa), potato (Solanum tuberosum), radish (Raphanus sativus), sweet potato (Ipomoea batatas), turnip (Brassica rapa)
• Vegetables: asparagus (Asparagus officinalis); broccoli, cabbage, cauliflower (all varieties of Brassica oleracea); celery (Apium graveolens); lettuce (Lactuca sativa); spinach (Spinacia oleracea)
• Edible oils: canola (or rape, Brassica napus), oil palm (Elaeis guineensis), olive (Olea europaea), peanut, soybean
• Sugar crops: sugar beet (Beta vulgaris), sugar cane (Saccharum officinarum)
• Herbs and spices: chili pepper (Capsicum annuum), mint (Mentha spp.), pepper (Piper nigrum)
• Beverages: cocoa (Theobroma cacao), coffee (Coffea arabica), cola (Cola acuminata), hops (Humulus lupulus), tea (Camellia sinensis)
• Recreational drugs: cannabis (marijuana, Cannabis sativa), coca (Erythroxylum coca), opium poppy (Papaver somniferum), tobacco (Nicotiana tabacum)

Other domesticated plants are cultivated as sources of fiber, which is used to manufacture thread, woven textiles, cordage (such as rope), and paper. Important fiber plants include cotton (Gossypium hirsutum), flax (Linum usitatissimum), and hemp (Cannabis sativa). Some species of trees, such as pines (Pinus spp.), poplars (Populus spp.), Douglas fir (Pseudotsuga menziesii), and spruces (Picea spp.), are grown in plantations (called agroforestry) as sources of fiber. However, these species have not been selectively bred to the degree that they would be considered domesticated.

A few plants are grown for the production of bio-energy, such as maize, sugar cane, and other carbohydrate-rich crops that are fermented to manufacture industrial ethanol used to power motor vehicles (as a mixture with gasoline known as gasohol).

Other crops are grown as sources of rubber (especially para rubber, Hevea brasiliensis), for medicinal purposes (such as digitalis, Digitalis purpurea), as chewing gum (chicle, Achras zapota), as dyes (indigo, Indigofera tinctoria), or for other relatively minor uses.

Image 13.1. Intensive Monoculture. Agricultural lands are often intensively managed to develop a monoculture, which is a cropping system comprised entirely of a single crop. This field is a corn monoculture in Iowa. Source: Image from Pixabay is licensed under CC0 1.0.

The parts of plants that are used for food include seeds (beans, wheat, and other grains and pulses), flowers (broccoli), fruits (melons, grapes, tomato), leaves (lettuce, cabbage), stems (asparagus, celery), and roots, tubers, and other underground tissues (onion, potato, radish). In many cases, the edible parts are tissues that evolved to store energy for the plant, such as swollen leaves and stems, and tubers. In other cases, the edible parts are energy-rich tissues that are involved in sexual reproduction, such as fruits and seeds. An important aspect of the domestication process is the selective breeding of crops to exaggerate their desirable traits, which usually results in cultivars that are very different from their wild ancestors.

Production of Crops

The above lists suggest a rich diversity of crop species. We must remember, however, that the inventory of cultivated plants is only a tiny fraction of the number of species that are potentially useful as foods or for other purposes, but have not yet been investigated for their usefulness (there are more than 350-thousand species of vascular plants, but only a tiny fraction of them have been investigated for their usefulness).

Overall, people eat several thousand species of plants, of which about 200 have been domesticated. Of these, only 12 species account for about 80% of global food production (Diamond, 1999). They are:

• Five cereals: wheat, maize, rice, barley, and sorghum
• One pulse: soybean
• Three root or tuber crops: potato, manioc, and sweet potato
• Two sweeteners: sugar cane and sugar beet
• The soft fruit banana

Of these top-12 crops, the cereals account for about half of the calories that are consumed.

As we examined in Chapter 12, the cultivation of agricultural crops is an extremely important economic activity. The three most widely grown crops in the United States are corn, soy, and wheat. In 2020, over 90 million acres of corn were harvested, and the production of corn has been steadily generating more income since 2016. The same holds true for soy, which harvested over 82 million acres in 2020, and has increased revenue by over 10 billion dollars since 2016. Wheat production, however, has decreased from 47 million acres harvested in 2016 to 36 million acres harvested in 2020, but despite this decrease in land use, wheat has generated about the same amount of income since 2016 (USDA 2020). Table 13.1 shows the production of several crops grown in the United States in 2016 versus 2020 as well as the income generated from these crops.

Table 13.1. A comparison of crop production and revenue in the United States between 2016 and 2020. Source: Data from the USDA Statistics by Subject (2020).

Crop	Acres Harvested 2016	Acres Harvested 2020	Revenue Generated 2016 ($)	Revenue Generated 2020 ($)
Corn	94,004,000	90,819,000	51,304,297,000	61,039,005,000
Soybean	82,706,000	82,318,000	35,195,882,000	46,068,982,000
Wheat	43,848,00	36,746,000	9,179,190,000	9,324,496,000
Oats	979,000	1,004,000	153,824,000	186,241,000
Barley	2,565,000	2,133,000	949,876,000	753,314,000
Lentils	905,000	514,000	366,564,000	129,220,000
Rice	3,097,000	2,987,000	2,421,111,000	3,069,000,000

Management Systems

Various management practices and systems, which vary greatly in their intensity, can be applied to the cultivation of any crop plant (or to livestock). The most intensive systems may involve cultivating a monoculture (only one crop species) using a series of such practices as tilling the soil, planting, applying fertilizer and pesticide, and a harvest when the crop is ripe. Intensive agricultural systems are typically used on relatively large farms and they rely on specialized, fossil-fueled machinery (known as mechanization). Intensive systems may also be used on smaller farms in order to achieve higher production on a limited area of land.

The use of intensive agricultural systems is common in relatively developed countries, such as the United States. It also occurs in plantation-style agriculture in less-developed countries, where commodities are grown mostly for an export market. In contrast, subsistence farming, as is commonly practiced by poor people in less-developed countries, involves little or no use of fertilizer or pesticide and no mechanization. So-called organic agricultural systems used in developed countries also eschew the use of synthetic fertilizer and pesticides (this approach to farming is examined in detail later in this chapter).

Key practices for growing crop plants in intensively managed systems include the following:

• Selective breeding of crop varieties for higher yield, greater response to management practices, adaptation to local climatic or soil conditions, and resistance to disease or herbicide
• Tilling the soil so that seeds can establish and to reduce competition from weeds
• Planting the crop at an optimal spacing, usually as a monoculture, to increase productivity and the ease of harvesting
• Applying inorganic fertilizer or organic matter (including animal dung) to enhance the nutrient supply
• Irrigating to enhance the availability of water
• Controlling weeds by mechanical means (such as tillage) or by the use of herbicide
• Controlling invertebrate pests using pesticide (most commonly insecticide or nematicide), by introducing diseases or predators of the pests, or by managing the habitat to make it less suitable for them
• Controlling fungal pathogens by using fungicide or by managing habitat to make it less suitable
• Harvesting the crop biomass as efficiently as possible
• Developing crop-rotation systems that maintain site quality and help prevent the build-up of pests and pathogens
• Using mechanized systems to till the soil, plant seed, apply fertilizer and pesticide, and harvest the crop
• Cultivating some crops, such as tomato and cucumber, in greenhouses
• Developing so-called organic systems that maintain high crop yields while reducing or eliminating the use of synthetic fertilizer and pesticide

As we noted previously, intensive management systems vary greatly among crop species and among regions, and it is far beyond the scope of this chapter to describe such systems in detail. Nevertheless, we can get an idea of what an intensive system can involve by examining case studies dealing with selected crops (see U.S. Focus 13.1, 13.2, and 13.3). Practices used in organic agriculture are examined later in this chapter.

Image 13.2. Mechanized Farming. The image above of a cotton-picker illustrates how modern farming uses heavy machinery to increase the efficiency of agricultural practices in the U.S. Source: “John Deere 9960 cotton harvester/picker” by USDA is licensed under CC0 1.0.

Livestock

Livestock are raised primarily as sources of food. The most important domesticated livestock in the U.S. are the cow (Bos taurus), horse (Equus caballus), pig (Sus scrofa), sheep (Ovis aries), and goat (Capra hircus). The most prominent birds are chicken (Gallus gallus), duck (Anas platyrhynchos), and turkey (Meleagris gallopavo). The most important fish are Atlantic salmon (Salmo salar) and rainbow trout (Oncorhyncus mykiss) – the cultivation of fish is known as aquaculture. Ranching of non-domesticated livestock, such as bison (Bison bison) and elk (Cervus elaphus), is also increasing.

Livestock in developed countries are mostly grown under intensive management systems. In large part this involves rearing animals on “factory farms,” although beef cattle may spend much of their lives foraging on rangeland, as do sheep. Key practices for growing livestock under intensive management include the following:

• Selective breeding of varieties for higher yield and greater response to management practices
• Developing “tame” or converted (seeded) pastures to supply fresh fodder, and hayfields for hay feeding, silage production, or bedding
• Feeding livestock with concentrated foods that are manufactured from fish, slaughterhouse offal (meat “wastes”), pulses, and other products, together with mineral supplements
• Using antibiotics and other medicines to prevent or treat diseases
• Using growth hormones to increase production in certain animals (particularly cows)
• Killing natural predators of free-ranging livestock, such as bear, cougar, coyote, and wolf
• Confining livestock in dense feedlots or factory farms, with feeding to satiation and other intensive husbandry practices
• Developing organic systems that maintain high yields of livestock, while reducing or eliminating such intensive practices as close confinement and the routine use of medicines and growth hormones

Again, it is beyond the scope of this chapter to describe intensive management systems for livestock in detail. We can, however, examine a case study to get an idea of what the systems may involve (U.S. Focus 13.3).

Environmental Impacts of Agriculture

Declining Site Capability

Agricultural site capability (or site quality) refers to the ability of an ecosystem to sustain the productivity of crops (Chapter 12). Soil fertility is an important aspect of this – it is related to the amount of nutrients present and to factors affecting their availability, such as drainage, tilth, and organic matter in the soil. Site quality can be degraded by agricultural practices, which may result in the erosion of topsoil, loss of organic matter and nutrients, and a buildup of weed populations. These result in decreased crop yields, which may then require intensive management practices (such as fertilizer and herbicide application) to try to compensate for the damage. Allowing site quality to degrade is a non-sustainable use of agricultural land.

Nutrient Loss

As plants grow, they take up nutrients from the soil (Table 13.2). When a crop is harvested, the nutrients contained in their biomass are removed from the site. The ability of the soil to supply nutrients may then diminish if the removals exceed the rate at which nutrients are regenerated by atmospheric deposition, nitrogen fixation, and the weathering of soil minerals.

In fact, nutrient depletion is a common problem with agricultural systems, and it is most often treated by applying inorganic fertilizer to the land. However, careful attention to the conservation of organic matter and nutrient content of soil can greatly alleviate nutrient depletion and may even eliminate the need to add inorganic fertilizer (we examine this later in the context of organic agriculture).

In any event, the use of fertilizer in agriculture has increased greatly in the U.S. The U.S. measures its fertilizer consumption in pounds of nutrient (nitrogen, phosphate, or potash) applied per acre. In 1960, the U.S. consumed 46.2 nutrient pounds per acre which rose to 111 lb/acre in 2009 and 115 lb/acre in 2015 (EPA, 2019). Fertilizer application rates vary globally: in 2012, an average of 116 kg of fertilizer was applied per hectare of agricultural land in the U.S., compared with 443 kg/ha in China, 234 kg/ha in Japan, 124 kg/ha in France, and 66 kg/ha in Canada. In countries with the highest rates of fertilizer application, agricultural land is relatively valuable and property taxes are high, which creates an economic incentive to use intensive management practices to increase the productivity.

Table 13.2. Annual Uptake of Key Nutrients by Selected Crop Species. Data are in kg/ha. Sources: Data from Hausenbuiller (1985) and Atlantic Potato Committee (1993).

Crop	Nitrogen	Phosphorus	Potassium
Alfalfa	500	40	450
Soybean	375	30	134
Orchard grass	335	50	350
Bluegrass	225	25	165
Potato	211	40	321
Wheat	210	25	150
Maize (corn)	200	40	200
Barley	170	25	140
Oats	170	25	140
Hardwood forest	95	9	30

Often, the rate of fertilizer application is intended to satiate the needs of the crop so that its productivity is not limited by the availability of nutrients (Table 13.2). This may result in an excess of nutrients in the soil, which may cause several environmental problems:

• Pollution of the groundwater with nitrate
• Eutrophication of surface waters
• Acidification caused by the nitrification of ammonium to nitrate (which is followed by the leaching of nitrate)
• Emission of nitrous oxide to the atmosphere
• A need to use herbicide to control the weeds that flourish under fertile conditions Fertilizers are manufactured from non-renewable resources, and large amounts of energy are used in the process. The principal sources of fertilizer are:
• Urea and ammonium nitrate, which are manufactured by combining nitrogen gas with hydrogen obtained from methane (or natural gas)
• Phosphate fertilizer made from mined rock phosphate
• Potassium from mined potash

Organic Matter

Soil organic matter is a crucial factor that affects fertility and site capability. Organic matter has a strong influence on the capacity of soil to hold water and nutrients, and on its aeration, drainage, and tilth (see Chapter 12). Typical agricultural soil has an organic concentration of 1-10% (it can exceed 90% in the peaty substrate of drained wetlands, but this soil is uncommon in agriculture). This is considerably less than what occurs in the soil of natural prairie or forest. Those natural ecosystems have a surface layer of litter and humus, and within the mineral soil itself the concentration of organic matter is at least 15-30% higher than occurs in agricultural soil (Acton and Gregorich, 1995). Therefore, when an area of prairie or forest is converted to an agriculture use, there is a large decrease in the amount of organic matter on the surface and within the mineral soil (Figure 13.1). A depletion of organic matter is widely regarded as an important problem that affects the sustainability of agricultural production.

Figure 13.1. Changes in Organic Matter in a Prairie Soil. The data are for the surface 30 cm of soil and reflect historical management practices of annual tillage and fertilizer application. The data since 1990 show the potential effects on soil organic matter of conventional tillage without fertilizer, no-tillage without fertilizer, and no-tillage with nitrogen fertilizer at 50 kg/ha-y. Source: Modified from Acton and Gregorich (1995).

Several practices are recommended for increasing the amount of organic matter in soil. They include adding crop residues, livestock manure, or other organic-rich materials, such as composted municipal waste or sewage sludge. The use of no-tillage or low-tillage systems is also helpful, because ploughing encourages the decomposition of organic matter (no-tillage involves sowing seeds by drilling them directly into the ground without prior cultivation). The use of no-tillage in combination with fertilizer application is also effective at increasing organic matter in prairie soil (Figure 13.1). The regular addition of manure and organic matter is useful, as both increase soil quality, through promoting and increasing microbial activity and improving water retention (USDA, 2021).

Soil Erosion

Soil is eroded by wind and by the runoff of rain and melted snow. Although erosion is a natural process, its rate can be greatly increased by agricultural practices, and this may be a serious environmental problem. Erosion represents a loss of soil capital, which can impoverish site capability and can cause deep gullying of fields, a damage that is almost impossible to rehabilitate. Erosion also damages aquatic ecosystems by increasing sedimentation and turbidity, which are destructive of fish habitat. Wind-eroded soil can also be a local nuisance (for example, as a source of dirt inside homes and by soiling laundry hung out to dry), and in severe cases it can literally bury machinery and buildings (as occurred on the prairies during the “dust-bowl” years of the 1930s).

Agricultural practices that increase the rate of soil erosion include the following:

• Cultivating land on moderate to steep slopes
• Ploughing furrows up and down slopes rather than contouring along them
• Leaving fields without a cover (such as stubble or a cover-crop) during the winter

Worldwide, nearly 80% of agricultural land is at severe or high risk of erosion which has resulted in a loss of 30% of the world’s cropland in the last 40 years. The rate of erosion varies greatly among countries. The greatest losses in cropland have occurred in Asia, Africa, and South America. Surprisingly, the U.S. and Europe have some of the lowest average soil erosion rates of cropland. Nonetheless, the rate of erosion is still too high for the land to recover naturally which has placed great stress on some of the U.S.’s most productive croplands. For instance, Iowa has lost half of its fertile soils due to erosion (Pimentel & Burgess 2013). The United States Department of Agriculture has implemented a conservation program involving more sustainable farming methods to slow the rate of soil erosion. The results of these efforts can be seen in Table 13.4 and Figure 13.2 (USDA, 2019).
Figure 13.2. Soil Erosion from Water and Wind on Cultivated Cropland, 1982-2012. The figure above shows an overall decrease in soil erosion across the U.S. as a result of soil conservation efforts, such as more sustainable tillage practices. Source: USDA Economic Research Service.

Fortunately, there is an increasing use of soil-conservation practices (such as those listed below), and this is reducing the risks of erosion by wind or water:

• Leaving crop residues and/or stubble in the field
• Using longer crop rotations
• Incorporating a forage crop into the rotation
• Growing winter cover-crops (such as winter wheat)
• Planting perennial shelter-belts (such as trees)
• Strip-cropping
• Not cultivating beside streams (leaving a riparian buffer)
• Ploughing along contours rather than across them
• Using no-till or low-till planting
• Maintaining perennial pastures (instead of using erosion-prone land for crops)

Table 13.3. Estimated reductions in fuel use and emissions from adoption of conservation tillage. This table compares five types of tillage techniques in their fuel use per acre, fuel use reduction, and emission reduction. Source: CEAP-Cropland Conservation Insight, 2016.

Compaction

The frequent passage of heavy machinery, or the yarding of a dense livestock population, can compress the air spaces in soil, a condition known as compaction. Soil compaction is a serious problem that results in waterlogging, oxygen-poor conditions, impaired nutrient cycling, poor root growth, and decreased crop productivity. Compaction can be largely avoided by avoiding any unnecessary passages of heavy machinery over fields, using large tires to spread the load, and reducing the density of livestock kept in outdoor stockades.

Salinization

The buildup of soluble minerals in the surface soil, or salinization, is a major problem in drier regions. The most important salts are usually sulfates and chlorides of sodium, calcium, and magnesium, which in severe cases are visible as a whitish crust on the surface. Salinization occurs when there are high concentrations of salts in the soil, and the rate of evaporation exceeds the water input from precipitation. These conditions bring salts to the surface, where they are deposited as the water evaporates. Irrigation practices can also be a cause of salinization if insufficient water is added to allow dissolved salts to drain to below the rooting depth of the crop. Saline soil is toxic to most crops, largely because of interference with the uptake of water, along with ion imbalance and toxicity.

Salinization affects more than one fourth of all irrigated lands throughout the entire U.S. (Bauer, 2014). Soil salinization is especially problematic in California where 4.5 million acres of irrigated croplands are affected in some way by salinization (Kafka & Knapp, 2009). The Imperial Valley and the Western San Joaquin Valley in California are the most affected in the state and throughout the country.

The control of salinization requires practices that keep the height of the water table low or prevent dissolved salts from rising. These practices include the diversion of surface flows, installation of a subsurface drainage system, using longer crop rotations (including deep-rooted forage species), practicing conservation tillage, and increasing organic matter in the soil. Salt-tolerant crops may be grown on moderately salinized soil, such as barley (Hordeum vulgare) or forage plants such as alfalfa (Medicago sativa) and slender wheatgrass (Agropyron trachycaulum).

Desertification

The increasing aridity of drylands, or desertification, can make agriculture difficult or impossible. Desertification may be affecting more than one billion people in over 100 countries throughout the world (UN Decade, 2020; Figure 13.3). Desertification is a complex problem, caused by both climate change and other anthropogenic influences. The latter include unsustainable land-use practices in drylands, such as over-grazing, intensive cultivation, deforestation (often to obtain fuelwood), and improper irrigation. These practices can cause the loss of topsoil and vegetation cover and a degradation of agricultural capability. These effects are greatly intensified by drought.

Figure 13.3. Areas at Risk of Human Induced Desertification. Many areas worldwide (mostly in arid and semi-arid climates) are at risk of desertification due to a combination of agricultural intensification and climate change. Source: USDA.

Desertification is largely a problem of regions that are already marginal in terms of the amounts of precipitation and soil moisture that are available to support agriculture. The best-known cases occur in less developed countries, such as dryland regions in Africa, south and central Asia, and Latin America. However, it is also an important problem in the interior regions of North America and Australia. In North America, 74% of the land is affected in some way due to desertification (Eswaran et al., 2001). This region experienced widespread loss of topsoil by wind erosion during the drought of the 1930s, and it is still vulnerable to this degradation. The best agricultural land-use in this semi-arid region is livestock grazing on unbroken perennial range. The development of “tame” pastures and the cultivation of annual crops are more likely to cause desertification.

Pollution Caused by Agriculture

Groundwater and surface waters can become polluted by runoff containing fertilizer, pesticides, and livestock sewage. Inputs of nutrients and organic matter from fertilizer and sewage can cause severe ecological damage to surface waters through eutrophication and oxygen depletion. These changes, coupled with the presence of pathogenic and parasitic organisms, can result in waters becoming unsuitable for drinking by people, perhaps even by livestock, or for use in irrigation.

The worst problems involve the disposal of the enormous quantities of manure that are produced by livestock kept in feedlots and factory farms. It is estimated every day, 60 to 80 pounds of manure are produced for every 1,000 pounds of body weight of livestock animals (USDA Natural Resources Conservation Service, 1995). This manure, unlike human feces, is not treated or disposed of properly and instead is applied directly to agricultural lands, which often leeches and pollutes nearby environments, especially nearby water sources. This pollution is especially great near industrialized feedlots, which concentrate hundreds of livestock animals into small sections of land. The most important agricultural pollutant of groundwater is nitrate, which originates with manure applications to farmland and the use of fertilizer. This problem occurs because the nitrate ion (NO₃^–) leaches readily with water that percolates through the soil to groundwater (nitrate is highly soluble in water and is not retained by ion-exchange reactions in the soil). Nitrate pollution is a hazard for people who use groundwater as a source of drinking water. Although nitrate itself is not very toxic, it is converted by microbes in the human gut to nitrite, which when absorbed into the blood strongly binds with hemoglobin (forming a compound known as methemoglobin), thereby reducing the capacity to carry oxygen. Children are especially vulnerable to this effect; the so-called “blue-baby syndrome” refers to oxygen-starved infants who have been poisoned by nitrate in their drinking water or food.

Nitrate pollution of groundwater is a widespread problem. The U.S. EPA has set the drinking-water standard for nitrate at 10 m/L in natural groundwater sources; nitrate concentrations are usually less than 2 mg/L (Nolan et al., 1998). Nitrate pollution is highest in the central U.S., due to higher concentrations of agricultural sources and higher vulnerability of aquifer groundwater sources. It is estimated that 24% of wells throughout the U.S. exceed the set drinking-water standards for nitrate (Nolan et al., 1998). In fact, nitrate exceeding hundreds of ppm (as NO₃-N) has been found in groundwater in agricultural regions as a result of the application of manure and/or fertilizer. This important and extensive problem can only be resolved through more prudent fertilizer application and by prohibiting the disposal of untreated manure onto agricultural land. Manure should undergo sewage treatment, just like most human sewage does (Chapters 24 and 28).

The dumping of raw manure can also pollute groundwater and surface waters with fecal coliforms and other intestinal pathogens and parasites. These are health hazards to anyone using a polluted waterbody or aquifer as a source of drinking water or even for swimming. As set by the EPA, the maximum contaminant level for total coliforms in drinking water is zero, because even very low levels of coliforms could indicate waterborne disease outbreaks (EPA, 2021). Even with these strict regulations, one study found nearly 70% of 146 household-supply wells were contaminated, with fecal coliforms found in 25% (USGSC, 1998).

Groundwater and surface waters can also be contaminated by agricultural pesticides, whose use has increased enormously (Figures 13.4, 13.5). Moreover, some commonly used pesticides are highly leachable in soil, with important examples being atrazine, dinoseb, metolachlor, metribuzin, and simazine. Once a pesticide reaches groundwater, it may persist for a long time. Atrazine, for example, persists for at least five years.

Contamination of groundwater by pesticide is especially concerning in the U.S., where over half of the population rely on groundwater for drinking water. In the rural and agricultural areas of the country, where the threat of pesticide contamination is highest, around 95% of the population rely on groundwater for drinking water (USGS, 2021).

Figure 13.4. Adoption of Genetically Engineered Crops in the U.S. Since “Round-up ready” crops were created in 1996 by Monsanto (now owned by German chemical company Bayer), the percentage of agricultural land using GMO crops has risen steadily. Because some of these crops are resistant to the chemical herbicide glyphosate (found in the commercial herbicide Round-up), use of chemical herbicides to control weeds has greatly increased. Source: USDA.

Figure 13.5. Estimated Agricultural Use of Glyphosate. Since “Round-up ready” crops were created in 1996 by Monsanto (now owned by German chemical company Bayer), the percentage of agricultural land using GMO crops has risen steadily. Because some of these crops are resistant to the chemical herbicide glyphosate (found in the commercial herbicide Round-up), use of chemical herbicides to control weeds has greatly increased. Source: USGS.

Conversion of Natural Ecosystems

Extremely large areas of natural habitat have been converted into agroecosystems that are used for the production of food (Figure 13.6). This change has resulted in huge losses of natural ecosystems, and in some regions, grievous damage has been caused to indigenous biodiversity (see Chapter 14).

Figure 13.6. Global Land Use for Food Production. At least 50% of all habitable land worldwide is currently being used for food production. Source: “Global land use for food production” by OurWorldinData.org is licensed under CC BY-SA 3.0.

Agricultural conversion is the leading cause of deforestation in the world today, particularly in subtropical and tropical countries. From 1982 to 1997, more than 8 million acres of forest land in the U.S. has been converted to agricultural lands (Alig et al., 2010). Most of this land has been converted to pasture use, with the rest being used as croplands or rangelands. This deforestation throughout the U.S. leads to forest fragmentation, which threatens biodiversity and negatively influences ecological processes in the remaining forest lands.

The damage to biodiversity occurs because agricultural ecosystems typically provide poor habitat for native species. This is because agroecosystems are simple in their physical structure (especially compared with natural forest) and are strongly dominated by non-native plants and animals. For example, natural forests cover about one-third of the land in the U.S., providing for many essential and unique ecosystems (Alig et al., 2010). These forest systems range from wildland forests to urban forests, providing for diverse levels of biodiversity of native plant and animal species. When that natural forest is converted to, for example, cultivated pasture for livestock, it becomes dominated by only a few forage plants that are not native to the U.S. Those non-native plants include barnyard grass (Dactylis glomerata), meadow grass (Alopecurus pratensis), ryegrass (Lolium perenne and L. multiflorum), timothy (Phleum pratense), alfalfa (Medicago sativa), red clover (Trifolium pratense), and white clover (Trifolium repens). The pasture would also support other plants, including species considered to be “weeds,” but almost all would also be aliens.

Although highly productive in an agricultural context, such pastures are extremely degraded from the ecological perspective. The same is true, more or less, of other agroecosystems in North America – they provide poor habitat for native species.

The conversion of natural ecosystems into agroecosystems is an ongoing process in all countries. Private organizations such as the Nature Conservancy of the U.S. and other land trusts are attempting to purchase the best surviving tracts of natural habitat in order to protect them from being destroyed. To some degree, governmental conservation agencies are also working to do this. However, limited funds are available for this purpose, and the losses of natural habitat are proceeding rapidly (see Chapter 14).

Emissions of Greenhouse Gases

Deforestation and other conversions of natural habitat to agricultural use result in enormous emissions of carbon dioxide into the atmosphere (see Chapter 21; Figure 13.7). Natural ecosystems store a large amount of carbon in their plant biomass and soil. Because agroecosystems store much less organic carbon, a consequence of agricultural conversion is a large emission of CO₂ to the atmosphere. Since 1850, such conversions have resulted in almost as much CO₂ emission as has occurred through the combustion of fossil fuels. Prior to 1750, the atmospheric concentration of CO₂ was about 280 ppm, but by 2019 it had reached 410 ppm, a 46% increase (Lindsey, 2020). This change in atmospheric chemistry is an important problem because the increasing CO₂ may be helping to intensify Earth’s greenhouse effect. In fact, increased CO₂ is responsible for about 81% of all U.S. anthropogenic greenhouse gas emissions (EPA, 2020).

Figure 13.7 Global Greenhouse Gas Emissions by Sector. In 2016, nearly 20% of all greenhouse gas emissions generated by humans were from agricultural activities. Source: “Global greenhouse gas emissions by sector” by OurWorldinData.org is licensed under CC BY-SA 3.0.

In addition, the use of nitrogen fertilizer results in high concentrations of nitrate occurring in the soil. This encourages the process of denitrification, which results in the emission of nitrous oxide (N₂O) to the atmosphere (Chapter 5). N₂O is also a greenhouse gas, having 310 times the warming potential of CO₂. The atmospheric concentration of N₂O has increased from about 0.27 ppm in 1750 to 0.33 ppm in 2018, which accounts for 6.5% of anthropogenic greenhouse gas emissions (EPA, 2020).

Organic Agriculture

In organic agriculture, crops are grown using relatively “natural” methods of maintaining soil fertility, and pest-control methods do not involve synthetic pesticides. Compared with conventional agricultural systems, less environmental damage is associated with organic agriculture, and it tends to result in more stable crop yields, which may even be higher in some cases. However, operating costs are often higher in organic agriculture because more field labor is required to manage weeds without the use of chemical herbicides. Overall, compared with more intensive agricultural systems, organic practices better sustain soil quality, energy and material resources, and ecological integrity.

Organic Agriculture and Soil Fertility

A major focus of organic agriculture is the maintenance of soil fertility by enhancing natural pathways of nutrient cycling as well as soil tilth. In natural ecosystems, microorganisms continuously recycle inorganic nutrients (such as nitrate, ammonium, and phosphate) from dead organic matter, most of which is plant litter. The microbes metabolize the complex organic forms of nutrients, converting them to simple inorganic molecules. The fixation of atmospheric N₂ by microorganisms is also an important source of nitrogen input in organic agriculture (this involves legume-bacterial mutualisms as well as free-living bacteria; see Chapter 5). Overall, the release of inorganic nutrients is typically slow enough that they can be effectively taken up by crop plants, so relatively little is lost to groundwater or surface water.

In conventional agriculture, most inorganic nutrients are added directly as synthetic fertilizer. In contrast, organic methods of maintaining fertility rely on the management and enhancement of soil organic matter, from which decomposition makes inorganic nutrients available to crops.

Organic matter is also critical to maintaining tilth, a vital soil property that helps to:

• Bind nutrients and release them slowly for efficient uptake
• Hold water so that it can be used more effectively by plants
• Give the soil an aggregated structure with good aeration and easy penetration by roots

In contrast, tilth becomes degraded in conventional agriculture because frequent ploughing increases the decomposition of soil organic matter, even while there are relatively small inputs of new organic matter from crop debris, and heavy machines compact the soil.

Organic farmers enhance the organic content and fertility of cultivated soil in three major ways.

1. They add livestock manure and urine (often these are first composted) to the soil because these materials contain useful organic matter and nutrients. However, as mentioned earlier, this practice must be controlled because excess applications can pollute groundwater and surface water with nutrients and pathogens and cause local air pollution with ammonia and odors.
2. They add green manure, which is living plant biomass that is incorporated into the soil by ploughing. The most fertile green manure is legume biomass, such as that of alfalfa and clover, because these fix atmospheric nitrogen gas, making them a good organic source of nitrogenous fertilizer. Organic farmers often grow legumes in a crop rotation to maintain soil nitrogen.
3. They add compost, or partially decomposed and humified organic material, to the soil. Composting is a partly aerobic process by which microbes and soil animals fragment and decompose organic material, eventually forming complex, high-molecular-weight humic substances. These are resistant to further decay and are extremely useful as a soil conditioner and organic fertilizer.

It is important to understand that growing plants take up the same inorganic forms of nutrients (such as nitrate, ammonium, and phosphate) from soil, regardless of whether they are supplied by organic practices or with manufactured fertilizer. The important difference is in the role of ecological processes versus manufacturing – organic methods rely on renewable sources of energy and materials, rather than non-renewable ones. Overall, the longer-term effects on soil fertility and tilth using organic practices are much less damaging than those associated with conventional agriculture.

Pest Management

All agroecosystems have problems with pests. In conventional agriculture, they are usually managed using pesticides (but often within the context of integrated pest management; see Chapter 26). Although pesticides can reduce the effects of pests on the yield of a crop, their use may cause environmental damage. Instead of synthetic pesticides, organic farmers rely on other methods of pest management, such as the following:

• Using crop varieties that are resistant to pests and diseases
• Using biological pest management by introducing or enhancing populations of natural predators, parasites, or diseases
• Changing habitat conditions to make them less suitable for pests, for example, by growing crops in mixed culture rather than monoculture, by rotating crops or using a fallow period to avoid a buildup of pest populations, and by using mechanical methods of weed control such as hand-pulling and shallow inter-row ploughing
• Undertaking careful monitoring of pest abundance, so control tactics are used only when necessary
• Using pesticides that are based on natural products, such as an insecticide based on the bacterium Bacillus thuringiensis (B.t.) that may be considered acceptable in organic agriculture, or one based on pyrethrum extracted from a daisy-like plant, but not their synthetic analogues, such as genetically engineered B.t. or synthesized pyrethroids

Organic farmers, as well as the consumers of their produce, must be relatively tolerant of some of the damage and lower yields that pests may cause. For example, most consumers of organic produce are satisfied with apples that have some blemishes caused by the scab fungus (Venturia inaequalis), and aesthetic that does not affect the nutritional quality or safety of the apples. In conventional agriculture, this cosmetic damage is managed using fungicide, to provide consumers with apples having a blemish-free appearance that they have been conditioned to expect in their food.

Antibiotics, Growth-Regulating Compounds, and Transgenic Crops

Intensive livestock rearing may involve keeping animals together under crowded conditions in poorly ventilated environments, often continuously exposed to their own manure and urine. Animals kept in such unsanitary conditions are vulnerable to infection, which may retard their growth or kill them. In conventional agriculture, this problem is managed partly through the use of antibiotics, which may be given to sick animals or as a prophylactic treatment by adding them continuously to the feed of an entire herd.

Ultimately, humans are exposed to small residues of antibiotics when they eat the products of these animals. Although this low-level exposure has not been conclusively demonstrated to pose an unacceptable health risk to people, the issue is nevertheless controversial. One potential problem lies in the development of antibiotic-resistant pathogens, which can result in antibiotics becoming less effective for medical purposes.

Organic farmers might use antibiotics to treat an infection in a particular sick animal, but they do not continuously add them to livestock feed. In addition, many raise their animals under more open and sanitary conditions than those used in conventional agriculture. Animals that are relatively free of the stresses of crowding and constant exposure to manure are more resistant to diseases and have less need for antibiotic treatment.

In addition, some industrial systems for raising livestock use synthetic hormones, such as bovine growth hormone, to increase the growth rate of animals or the production of milk. Inevitably, these hormones persist as trace contaminants in animal products that are consumed by humans. Although no risk to humans has been conclusively demonstrated from these exposures, there is controversy about the potential effects. Organic farmers do not use synthetic growth hormones to enhance the productivity of their livestock.

Another recent innovation in agriculture is the use of so-called transgenic crops, which have been genetically modified by the introduction of genetic material (DNA or RNA) from another species (see Environmental Issues 7.1; Figures 13.4, 13.5). The intent of this bioengineering is to confer some advantage to the crop that cannot be developed through selective breeding, which relies only on the intrinsic genetic information (the genome) that is naturally present in the species.

Varieties of several important crops are transgenic and have been patented by the private companies that developed and market them. For example, a transgenic variety of canola is resistant to glyphosate, which allows that herbicide to be used as part of the management system. Transgenic varieties of potato and maize produce the insecticide that is naturally synthesized by B.t. and so are resistant to some insect pests. Transgenic crops are increasingly being grown in conventional agriculture in the U.S. and elsewhere, but they are not generally used in organic agriculture.

Organic versus Conventional

Many people believe that organically grown food is safer or more nutritious than food grown by conventional agriculture. This belief is mainly influenced by the knowledge that non-organic foods may have trace contamination with antibiotics, growth hormones, and pesticides, and the idea that this poses a health risk. This topic is highly controversial, but it is important to understand that scientific research has not conclusively demonstrated that organically grown foods are generally safer or more nutritious than those from conventional agriculture.

From the environmental perspective, the most important benefits of organic agriculture are the reduced use of non-renewable sources of energy and materials, better health of the agroecosystem, and enhanced sustainability of food production. However, organic agriculture also uses more land for comparable productivity, due largely to the use of fewer synthetic fertilizers (Figure 13.8).

Figure 13.8. Environmental Impacts of Conventional vs. Organic Agriculture. The data above come from a meta-analysis of 164 published life cycle analyses (LCAs) across 742 agricultural systems. Source: “Environmental impacts of conventional vs. organic agriculture” by OurWorldinData.org is licensed under CC BY-SA 3.0.

However, it appears that organic agricultural systems will not become more widely adopted until a number of socio-economic conditions change. First, more consumers must be willing to pay the often slightly higher costs of organically grown food and to accept a lower aesthetic quality in certain products. Second, vested agricultural interests in business, government, and universities must become more sympathetic to the goals and softer environmental impact of organic agriculture. These institutions must support more research to advance organic agriculture and promote its use. Finally, the practitioners of conventional agricultural systems must deal more directly with the environmental damage that is associated with their activities, especially the use of manufactured pesticides and fertilizers. If this were done, it would probably eliminate or even reverse the existing price differential between food produced by organic and by conventional agricultural systems.

Conclusions

Agriculture is a huge and necessary enterprise because it provides food for billions of people. A variety of crops are grown in various parts of the world, many of them domesticated, but only a few key ones account for most of the food production. These are barley, maize, manioc, potato, rice, sorghum, soybean, sweet potato, and wheat. Much environmental damage is associated with agriculture, including pollution, degraded land capability, and the destruction of natural habitats. In addition, livestock are not treated well in the industrial agro-food system, often being subjected to unnecessarily inhumane conditions while being reared, transported, or slaughtered. Much of the damage associated with agriculture can be avoided by using more organic means of production and processing. This is the major environmental advantage of organic foods, along with a perception of health benefits by many consumers. Although organic foods are usually somewhat more expensive to purchase, the price differential is more than offset by the environmental benefits from improved stewardship of land, the conservation of resources, and decreased pollution.

Questions for Review

1. What are the processes by which plants and animals become domesticated? How do these processes work?
2. Make a list of the most important food crops, both plant and animal, that are grown in the United States. For comparison, make a list for any selected country not in North America.
3. Make a list of the most important environmental effects of agriculture. Which of them do you think could be avoided relatively easily, and which not?
4. How is the production of agricultural crops important to you? How does agriculture contribute to the size and functioning of the United States’ economy?

Questions for Discussion

1. Could humans be viewed as a domesticated species? Explain your answer.
2. Agricultural activities cause serious and widespread environmental damage in terms of pollution and losses of natural habitat. Why do these damages seem to attract less attention than those associated with forestry, oil and gas extraction, and other industrial activities? Are agriculture-related damages being treated seriously enough?
3. Consider the practices that are used in raising livestock on factory farms and in transporting and processing them in slaughterhouses. Do you think that these animals are being treated in an ethically acceptable manner?
4. U.S. agriculture is highly mechanized and depends on the use of large amounts of fossil fuels and non-renewable materials such as steel and plastics. Do these circumstances pose risks for the longer-term sustainability of agriculture in the United States?

Exploring Issues

1. Select a crop plant or animal that is cultivated in the province where you live. Use the website of your provincial agricultural department to find out the practices that are recommended for growing the crop and what its production costs and economic value are.
2. Make a list of key agricultural practices (such as tillage, planting, fertilizer application, and pest control), and compare how they are done in conventional and organic agriculture. Based on your comparison, to what degree do you think organic agriculture causes fewer environmental damages than conventional practices?
3. A committee of the House of Representatives is examining organic agriculture in the U.S. The committee has asked you to compare the environmental effects of conventional and organic agricultural practices. What information about crop production and ecological impact would you assemble for the committee?

References Cited and Further Reading

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Alig, R., Stewart, S., Wear, D., Stein, S., and Nowak, D. 2010. Conversions of Forest Land: Trends, Determinants, Projections, and Policy Considerations. Advances in Threat Assessment and Their Application to Forest and Rangeland Management, USDA Forest Service. https://www.fs.fed.us/pnw/pubs/gtr802/Vol1/pnw_gtr802vol1_alig.pdf.

Atlantic Potato Committee. 1993. Atlantic Canada Potato Guide. Atlantic Provinces Agricultural Services Co-ordinating Committee, Fredericton, NB.

Bauer, S. 2014. Salinization in field in San Joaquin Valley, California. USDA, Agricultural Research Service. United States Geological Survey, Science for a Changing World. https://www.usgs.gov/media/images/salinization-field-san-joaquin-valley-california.

CEAP-Cropland Conservation Insight. 2016. Reduction in Annual Fuel Use from Conservation Tillage. Conservation Effects Assessment Project, Natural Resources Conservation Service, United States Department of Agriculture. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcseprd1258255.pdf.

Certified Humane. August 2020. Animal Care and Slaughter Standards. Certified Humane. https://certifiedhumane.org/our-standards/

Clay, J. 2003. World Agriculture and the Environment. A Commodity-by-Commodity Introduction to Impacts and Practices. Island Press, Washington, DC.

Clements, D. and A. Shrestra. 2004. New Dimensions in Agroecology. Food Products Press, New York, NY.

Conford, P. (ed.). 1992. A Future for the Land. Organic Practice from a Global Perspective. Green Books, Bideford, UK.

Diamond, J. 1999. Guns, Germs, and Steel. The Fates of Human Societies. W.W. Norton, New York. Environment Canada. 1996. The State of Canada’s Environment, 1996. Government of Canada, Ottawa, ON.

EPA. 2021. Drinking Water Requirements for States and Public Water Systems, Revised Total Coliform Rule and Total Coliform Rule. United States Environmental Protection Agency. https://www.epa.gov/dwreginfo/revised-total-coliform-rule-and-total-coliform-rule#:~:text=Total%20Coliform%20Rule%20(TCR),-The%20Total%20Coliform&text=EPA%20set%20the%20MCLG%20for,coliforms%20and%20Escherichia%20coli%20(E.

EPA. 2019. Report on the Environment: Agricultural Fertilizer. United States Environmental Protection Agency. file:///C:/Users/court/Downloads/ag-fertilizer%20(2).pdf

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Howard, A. 2007. The Soil and Health: A Study of Organic Agriculture. The University Press of Kentucky, Lexington, KY.

Kaffka, S. & Knapp, K. 2009. Soil Salinization. University of California Agricultural Issues Center. https://www.cdfa.ca.gov/agvision/docs/soil_salinization.pdf

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