Agricultural practices that humans use are determined by multiple agroecological factors including climate, soil, native organisms, and human socioeconomic factors. Usually, climate and soil resources are the most significant natural factors that determine the crops and livestock that humans produce. Although in some cases, to overcome climate and soil limitations, humans alter the environment with technology (ex. irrigation or greenhouses) to expand the range of food and fiber crops that they can produce. In this module, we will explore how climate and soil influence crop plant selection; crop plant characteristics and classifications; and some socioeconomic factors that influence the crops that humans chose to grow.
After completing this module, students will be able to:
Detailed instructions for completing the Summative Assessment will be provided in each module.
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Climate, soil resources, and the organisms in the environment influence which food and fiber crop plants humans can produce. To overcome environmental resource limitations, humans also alter the environment to produce food and fiber crops.
Plants need light, water, nutrients, an optimal temperature range, and carbon dioxide for growth. In a natural environment, the availability of plant resources is determined by the:
soil fertility, soil depth, and soil drainage
In some environments, nutrients, light, and water, are readily available and temperatures and the length of the growing season are sufficient for most annual crops to complete their lifecycle; we will refer to these as high resource environments for crop production. High resource environments tend to have soils that are fertile, well-drained, deep, and generally level, as well as growing seasons with temperatures and precipitation that are optimal for most plant growth. In general, in environments where competition for resources among plants is low, annual plants with more rapid growth rates tend to dominate (Lambers et al, 1998). Consequently, humans tend to cultivate annual plants with high growth rates in high-resource environments.
By contrast, in low-resource environments plant growth may be limited due to soil features and/or climatic conditions. Soils may be sloped, with limited fertility, depth, and drainage; and/or the growing season may be short due to extended dry seasons and/or long winters (with temperatures at or below freezing). In natural ecosystems, resources can be limited due to competition among plants, such as in a forest or grassland where established plants limit the light, water, and nutrients for new seedlings. And in these environments where resources are limited, plants with slower growth rates and perennial life cycles tend to succeed (Lambers et al, 1998), and perennials are often the primary crops that humans cultivate in resource-limited environments.
Annual plants grow, produce seeds, and die within one year. In general, annual plants evolved in environments where light, water, and nutrients were available, and they could consistently reproduce in one year or less. Where resource availability is high, plants that can germinate and grow rapidly have a competitive advantage capturing light, nutrients, and water over slower growing plants and are more likely to reproduce. To ensure the survival of their offspring, annuals allocate the majority of their growth to seeds (often contained in fruit); and they tend to produce many seeds.
Human selection of annual crop plants typically further selected for large seeds and/or fruit. Some examples of annual crop plants are corn, wheat, oats, peppers, and beans (see photos). What are some other examples of annual crop plants?
Annual crop plants are generally categorized into one of the three seasons that falls in the middle of their plant growth life cycle: spring, summer, or winter. For instance, summer annuals are generally planted in late spring, grow and develop through summer, and complete their lifecycle by late summer or autumn. Winter annuals are generally planted in early autumn and germinate and grow in autumn. Depending on how cold the winter is where they are cultivated, winter annuals may grow slowly in winter or become dormant until spring. In spring, they grow, flower, and produce seed by early to mid-summer (See Figure 6.1, Annual Crop Types). After an annual crop is harvested, in some regions farmers may be able to plant another crop, such as a winter annual crop after a spring annual crop, this is referred to as double-cropping (cultivating two crops in one year). If only one crop is cultivated in a season, the soil may be left exposed until the next growing season. Leaving crop residue on the soil can reduce erosion, but planting another crop with live plant roots and aboveground vegetation provides better soil protection against water and wind erosion. Alternatively, a cover crop may be planted after the harvested crop to protect the soil from erosion and provide other benefits until the next crop is planted. Cover crops are typically annual crops that can establish quickly; you will learn more about cover crops in Module 7.
Biennials are plants that live and reproduce in two years, and at the other end of the life-cycle spectrum are perennial plants that live for 3 or more years. Perennials evolved in environments where resources were limited often due to competition with other plants and their growth rates tend to be slower than annual plants (Lambers et al, 1998). In these resource-limited environments, often plants cannot germinate from seed and reproduce by seed within one year. Therefore, to increase their opportunities for successful reproduction, perennials evolved ways to grow and survive for multiple years to successfully produce offspring. Perennial crops are typically cultivated in environments that may also have a climatic limitation such as a short growing season or dry climate, or where a plant's ability to access resources may be limited due to frequent disturbance such as grazing.
To survive for multiple years, perennials allocate a high proportion of their growth to vegetative plant parts that enable them to access limited resources and live longer. For instance, they often invest in extensive and deep root systems to access water and nutrients, or in tall and wide-reaching aboveground stems and shoots to compete for light, such as bush and tree trunks and branches. Perennials also store reserves to regrow after growth-limiting conditions such as drought, freezing winters, or disturbance such as grazing. Carbohydrates, fat, and protein are stored in stems and roots, or modified stems such as tubers, bulbs, rhizomes, and stolons. In many plant species, these storage organs can produce root and shoot buds that can grow into independent offspring or clonal plants; this is called vegetative reproduction. Although most perennials reproduce both through seed and vegetative reproduction, in resource-limited environments where plant competition is high, the large storage organs and their reserves offer vegetative offspring plants a competitive advantage over starting from seed.
Humans have cultivated and selected perennial crop plants for their vegetative plant parts, storage organs, fruit, and seeds. For instance, the leaves and stems are the primary plant parts harvested from perennial forage crops (crops in which most of the aboveground plant material is grazed or fed to animals). Horticultural perennial crops that are harvested for stems and leaves include asparagus, rhubarb, and herbs. And in some cases, a perennial crop's storage organs are harvested each year, limiting the plant's ability to complete its perennial lifecycle and effectively reducing its cultivated lifecycle to an annual. Examples of such crops perennial crops that are cultivated as annuals include potato, sweet potato, and taro, Tree, shrub, and vine food crops managed as perennial crops are typically cultivated for their fruit and seeds, such as apples, stone fruit (ex. peach, plum), plantains, nuts, berries, and grapes (see photos below).
Annual plants are typically cultivated in high-resource environments and regions with:
climates that have sufficient precipitation and temperatures for plants to complete their life cycle each year
Annual crops produce grain and fruit crops within one growing season. Grain crops are typically a concentrated source of carbohydrates, protein, and sometimes fat, that can be cost-effectively stored and transported long distances, enhancing their market options and utility. Grain and oilseed annual crops are often processed for multiple uses and markets. For instance, oil is extracted from soybean for industrial and human uses, and the remaining meal is high in protein that is used for both human food products and livestock feed.
If conditions are not ideal for annual crops, farmers sometimes use management practices or technologies to improve conditions for crop growth such as irrigation to compensate for the lack of precipitation or black plastic to warm the soil in environments where temperatures may limit plant growth.
Regions, where perennial crops dominate the landscape, tend to have soil or climatic limitations such as steep or hilly slopes that are prone to erosion, shallow or poorly drained soils, soil nutrient limitations; limited precipitation and soil moisture availability, short growing seasons, or temperatures outside of optimal plant growth temperatures. In these environments, farmers may produce annual crops that are adapted to the environment, such as spring or winter wheat that grow during the cooler season or drought-tolerant annuals such as sorghum and pearl millet. Or farmers may use technologies and management practices, particularly for high-value crops, to improve conditions for crop growth such as tile drains, irrigation or season extension technologies.
See illustration and comparison of plant life cycles, the time and forms of reproduction. Can you name a specific crop plant example for each type of plant life cycle?
Because perennials allocate a high proportion of their growth to vegetative structures and regrow for many years, they can: i. protect soil from erosion; ii. return organic matter (carbon-based materials that originated from living organisms) to the soil, providing multiple soil health benefits; and iii. remove carbon dioxide from the atmosphere, potentially sequestering (storing) carbon in the soil or aboveground plant biomass. Forests, for example, sequester carbon above-ground in trees and in below-ground root systems.
Perennial grasses, in particular, have dense, fibrous roots that protect soil from erosion well and are valuable plants for soil conservation. In addition, over the years, some perennial roots and aboveground plant tissues die when environmental conditions limit growth (ex. drought, winter, grazing), and accumulate organic matter and nutrients in the soil. The majority of the most fertile and deep agricultural soils of the world were formed under natural perennial grasslands, whose deep root systems accumulated organic matter in the soil which contributed many beneficial soil properties, as well as carbon sequestration. Some annual crops can also contribute to conserving soil and add organic matter to the soil if a large portion of the crop residue is left on the soil surface, such as corn stalks left on a field after the grain is harvested.
In addition to their lifecycles, crop plants are characterized and classified in multiple ways that are relevant for crop production and management. Common plant features include similar morphology, growth and reproduction; and environmental and climatic adaptions. This module will help you understand more about how crops are adapted to different environments and diversified to interrupt pest lifecycles.
Plants that have similar flowers, reproductive structures, other characteristics, and are evolutionarily related, are grouped into plant families (See Figure 2). Species in the same plant family tend to have similar growth characteristics, nutrient needs, and often the same pests (pathogens, herbivores). Planting crops from different plant families on a farm and the landscape; and rotating crops of different plant families over time can interrupt the crop pest life cycles, particularly insect pests, and pathogens, and reduce yield losses due to pests. Increasing plant family diversity can also provide other agrobiodiversity benefits including, diverse seasonal growth and adaptation to weather stresses such as frosts, and drought; different soil nutrient needs, as well as producing diverse foods that provide for human nutritional needs.
Read this summary of the major world food crop plant families and the value of knowing what family plants are in, The Organic Way - Plant Families [1], then consider these questions.
The Fabaceae/Leguminosae, commonly called the Legume plant family, is important for soil nitrogen management in agriculture and for soil, human and animal nutrition. Legume plants can form a mutualistic, symbiotic association with Rhizobium bacteria which inhabit legume roots in small growths or nodules in the roots (seed images in the video listed below). The rhizobia bacteria have enzymes that can take up nitrogen from the atmosphere and they share the “fixed nitrogen” with their legume host plant. Nitrogen is an important nutrient for the plants and animals, it is a critical element in amino acids and proteins, genetic material and many other important plant and animal compounds. Legume grains crops, also called pulses are high in protein, such as many species of beans, lentils, peas, and peanuts. Most of their plant nitrogen is harvested in grain, although there is some in crop residues that can increase soil nitrogen content. Perennial legume crops are typically grown as forage crops for their high protein for animals. Because they allocate a large portion of their growth to vegetative plant parts and storage organs, perennial legumes also return a significant quantity of nitrogen to the soil, enhancing soil fertility for non-legumes crops grown in association or in rotation with legumes.
Watch the following NRCS video about legumes and legume research.
In addition to characterizing plants by their taxonomic plant family, crop plants are also classified as either cool season or warm season, referring to the range of temperatures that are optimum for their growth. Examples of cool-season agronomic crops include wheat, oats, barley, rye, canola, and many forage grasses are called cool-season grasses, such as perennial ryegrass, timothy, orchardgrass, tall fescue, smooth bromegrass, and the bluegrasses. Warm-season agronomic crops include corn or maize, sorghum, sugarcane, millet, peanut, cotton, soybeans, and switchgrass.
Learn more about the differences in cool and warm season plants and the types of vegetable crops in these categories by reading Season Classification of Vegetables [12].
In addition, plants are classified by the type of photosynthetic pathway that they have.
Plants require light, water, and carbon dioxide (CO2) in their chloroplasts, where they create sugars for energy through photosynthesis. The chemical equation for photosynthesis is:
6 CO2+ 6 H2O → C6H12O6+ 6 O2
Carbon dioxide (CO2) enters plants through stomata, which are openings on the surface of the leaf that are controlled by two guard cells. The guard cells open in response to environmental cues, such as light and the presence of water in the plant.
For a brief and helpful review of photosynthesis and plant anatomy such as the plant leaf structures, see Plant Physiology - Internal Functions and Growth [14].
Water (H2O) enters the plant from the soil through the roots bringing with it important plant nutrients in solution.
Transpiration or the evaporation of water from plant contributes to a “negative water potential.” The negative water potential creates a driving force that moves water against the force of gravity, from the roots, through plant tissues in xylem cells to leaves, where it exits through the leaf stomata. Since the concentration of water is typically higher inside the plant than outside the plant, water moves along a diffusion gradient out through the stomata. Transpiration is also an important process for cooling the plant. When water evaporates or liquid water molecules are converted to a gas, energy is required to break the strong hydrogen bonds between water molecules, this absorption of energy cools the plant. This is similar to when your body perspires, the liquid water molecules absorb energy and evaporate, leaving your skin cooler.
Carbon dioxide (CO2) also diffuses into the plant through the stomata, because the concentration of carbon dioxide is higher outside of the plant than inside the plant, where carbon dioxide concentration is lower due to plant photosynthesis fixing the carbon dioxide into sugars. To conduct photosynthesis, plants must open their leaf stomata to allow carbon dioxide to enter, which also creates the openings for water to exit the plant. If water becomes limited such as in drought conditions, plants generally reduce the degree of stomatal opening (also called “stomatal conductance”) or close their stomata completely; limiting carbon dioxide availability in the plant.
Read more about how water moves through the plant and factors that contribute to water moving into the roots and out of the plant, as well as carbon dioxide movement in Transpiration - Water Movement through Plants [3].
The majority of plants and crop plants are C3 plants, referring to the fact that the first carbon compound produced during photosynthesis contains three carbon atoms. Under high temperature and light, however, oxygen has a high affinity for the photosynthetic enzyme Rubisco. Oxygen can bind to Rubisco instead of carbon dioxide, and through a process called photorespiration, oxygen reduces C3 plant photosynthetic efficiency and water use efficiency. In environments with high temperature and light, that tend to have soil moisture limitations, some plants evolved C4 photosynthesis. A unique leaf anatomy and biochemistry enables C4 plants to bind carbon dioxide when it enters the leaf and produces a 4-carbon compound that transfers and concentrates carbon dioxide in specific cells around the Rubisco enzyme, significantly improving the plant’s photosynthetic and water use efficiency. As a result in high light and temperature environments, C4 plants tend to be more productive than C3 plants. Examples of C4 plants include corn, sorghum, sugarcane, millet, and switchgrass. However, the C4 anatomical and biochemical adaptations require additional plant energy and resources than C3 photosynthesis, and so in cooler environments, C3 plants are typically more photosynthetically efficient and productive.
Since carbon dioxide is the gas that plants need for photosynthesis, researchers have studied how the elevated CO2 concentrations impact C4 and C3 plant growth and crop yields. Although C3 plants are not as adapted to warm temperatures as C4 plants, photosynthesis of C3 plants is limited by carbon dioxide; and as one would expect research has shown that C3 plants have benefitted from increased carbon dioxide concentrations with increased growth and yields (Taub, 2010). By contrast, with their adaptations, C4 plants are not as limited by carbon dioxide, and under elevated carbon dioxide levels, the growth of C4 plants did not increase as much as C3 plants. In field studies with elevated carbon dioxide levels, yields of C4 plants were also not higher (Taub, 2010). In addition, if soil nitrogen was limited, C3 plant response to elevated CO2 concentration was reduced or crop plant nitrogen or protein content was reduced compared to plants grown in high soil N conditions (Taub, 2010). These results suggest that crops will likely require higher soil nutrient availability to benefit from elevated atmospheric carbon dioxide concentrations. For more optional reading information about C3 and C4 plant response to elevated carbon dioxide concentrations, see the following summary of research that is also listed in the additional reading list, Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants [18].
Some additional plant traits that help plants tolerate drought and heat stress include deep root systems (typical of perennials) and/or thick leaves with waxes that reduce water loss and the rate of transpiration. In addition, some plants roll their leaves to reduce the surface area for solar radiation reception and heating, and some reduce their stomatal conductance more (water loss) more than others.
Elevated temperatures projected with climate change can have multiple impacts on plant growing conditions. Climate change may lengthen growing seasons in some regions, although day lengths will not change. As planting dates are altered with longer growing seasons, crops may also be exposed to high temperature, moisture stress, and risk of frost. Elevated temperatures may also increase evaporation of water from the soil, reducing soil water availability. Higher temperatures are not necessarily ideal for yield, even if the temperatures are below a plants’ optimal temperature. At elevated temperatures, plants grow faster which tends to, one, reduce the amount of the time for photosynthesis and growth, resulting in smaller plants, and two, reduce the time for grain fill, reducing yield, particularly if nighttime temperatures are high (Hattfield et al., 2009). High temperatures can also reduce pollen viability, be lethal to pollen. The multiple effects of high temperatures on plant physiological process and soil moisture likely explain why research has found that grain development and yield are often reduced when temperatures are elevated (Hattfield et al., 2009).
Many factors that are projected to change with climate change could influence plant growth. These include carbon dioxide concentration, temperature, precipitation, and soil moisture, and ozone concentrations in the lower atmosphere.
Read the Introduction and Key Message 1 (Increasing Impacts on Agriculture) of the National Climate Assessment [19].
In addition to the climate and soil resources for crop production, many socioeconomic factors influence which crops farmers chose to cultivate, including production costs, domestic and international market demand; and government policies that subsidize agricultural producers, and reduce trade barriers or export costs. As discussed in Module 3, the protein, energy, fat, vitamins, and micro-nutrients of crops for human nutrition are one predictor of the market value of a crop. However some food crops are highly valued and cultivated for their cultural and culinary qualities, such as flavor (ex. chilies, vanilla, coffee, wine grapes); and their high economic value often reflects high production and processing costs, as well as market demand for their unique culinary and cultural properties.
Some crops are cultivated for non-human food uses such as livestock feed, biofuel, fiber, industrial oil and starch, and medicinal uses. Crop processing often creates by-products that can be used for other purposes, adding market value. For example, when oil is extracted from oilseeds such as soybean, the soybean meal by-product is high in protein and sold for livestock feed or added to human food products. And for crops that are cultivated on many acres often with support from government policies, the consistent, abundant supply of these commodity crops has contributed to the development of multiple processing technologies, uses, and markets. To better understand factors that contribute to the production of commodity crops, we will now examine two case studies of corn and sugarcane.
In the following two agricultural crop case studies, you will have the opportunity to apply your understanding of crop plant life cycles, classification systems, and crop adaption to climatic conditions to understand how plant ecological features and human socioeconomic factors influence which crops are some of the major crops produced in the world.
Corn or maize is a summer annual C4 crop in the Poaceae, or grass family that has high nutrient demands. Unless soil conservation practices are used, corn fields do not have live roots protecting the soil from erosion and providing other soil quality benefits after harvest in the fall, winter and spring. The US is the largest corn producer in the world. Soils and climate, particularly in the Midwest, permit high corn yields; and significant investment in agricultural research has produced high-yielding corn hybrids and production technologies, such as fertilizers, pest control practices, farming equipment, and irrigation. Research has also developed diverse uses for the large quantities of corn produced in the US, and the US is also a major exporter of corn.
Read this overview of US corn production and uses from the US Department of Agriculture, Economic Research Service, Corn and Other Feed Grains [20].
The US consumes the most sweeteners of any country in the world. In the US, high-fructose syrup is made from corn, which has displaced some sugarcane production for sugar for the US market. Sugarcane production, however, has continued to increase in Brazil, the biggest sugarcane producer in the world. Sugarcane is a C4 perennial crop in the grass family and it's not grown just for sugar as a food sweetener.
Watch this United Nations video below, about the factors contributing to increased sugarcane production and some of the consequences. Then answer the questions below.
If the video does not play, please see Brazil: The ethanol revolution (United Nations) [21].
Download the FAO Top 50 Commodity Changes Key Spreadsheet [22] which has the ranking and total production of the top 50 commodities for 2000 and 2013. In a spreadsheet calculate the percentage of change in the production of the most recent year's top 15 commodities then answer the below questions. Analysis and critical thinking about the data are encouraged.
FAO Top 50 Commodity Changes Key Spreadsheet [22]
Download the Module 6 Summative Assessment Worksheet [23]
You do not need to submit your worksheets; they will instead act as guides for you to complete the summative assessment quiz.
After completing Module 6, you should now be able to:
You have reached the end of Module 6! Double-check the to-do list on the Module 6 Roadmap [24] to make sure you have completed all of the activities listed there before moving on to Module 7.1!
Sterling, T. M. Transpiration in the Plant and Soil Sciences ELibrary: https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1... [3]
Taub, D. 2010. Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants. Nature Education Knowledge 3(10):21
Lambers, H. S.Chapin and T. Pons. 1998, Plant Physiological Ecology. 2nd edition. Springer-Verlag New York. pg. 340 and 344.
Links
[1] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/OrganicWay.pdf
[2] http://extension.psu.edu/plants/gardening/fact-sheets/vegetable-gardening/seasonal-classification-of-vegetables
[3] https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&topicorder=1&maxto=8&minto=1
[4] http://nca2014.globalchange.gov/report/sectors/agriculture
[5] https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/background/
[6] https://www.e-education.psu.edu/geog3/node/830
[7] https://www.e-education.psu.edu/geog3/node/997
[8] https://pixabay.com/en/vegetables-pepper-tomato-chayote-607305/
[9] https://pixabay.com/en/beans-leguminous-plants-legumes-260210/
[10] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/Weed%20Control%20for%20the%20Home%20Vegetable%20Garden_0.pdf
[11] https://www.usbg.gov/sites/default/files/images/--how_plants_work-_student_discovery_journal-are_plants_like_us.pdf
[12] https://extension.psu.edu/seasonal-classification-of-vegetables
[13] https://commons.wikimedia.org/wiki/File:Tomato_leaf_stomate_1-color.jpg
[14] http://content.ces.ncsu.edu/extension-gardener-handbook/3-botany#section_heading_6937
[15] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&topicorder=5
[16] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&topicorder=5
[17] https://askabiologist.asu.edu/cam-plants
[18] https://www.nature.com/scitable/knowledge/library/effects-of-rising-atmospheric-concentrations-of-carbon-13254108
[19] https://nca2018.globalchange.gov/chapter/10/
[20] https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/
[21] https://www.youtube.com/watch?v=ZitcSUdqmhM
[22] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/Module%206%20Summative%20Assessment%20Spreadsheet%20.xlsx
[23] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/Summative%20Assessment%206.2%20RevisedSumm2017_0.docx
[24] https://www.e-education.psu.edu/geog3/node/538