Class 6: Environmental Impacts of Agriculture: Protecting Natural Resources

Class 6 begins with a survey paper by Tilman et al. (2002) that presents the case for sustainable intensification, which we first encountered in Godfray et al. (2010) in Class 1. This approach to improving food security seeks to sustainably increase crop yield so that more food can be grown without expanding cropland. The tricky part is sustainability, which implies that yield increases must not cause environmental problems that jeopardize future agricultural production. The optional reading by Matson (1997) deals with this topic from a more ecological perspective. Both papers provide concise summaries of environmental issues that we need to consider when evaluating options for increasing food production. These include:

  • Changes in nutrient cycles (especially nitrogen, phosphorus, and carbon)
  • Soil degradation
  • Depletion of groundwater reserves and reductions in river flows (see Class 3)
  • Ecosystem changes that can adversely affect the complex communities that sustain crops

The papers by Gruber and Galloway (2008) and Cordell (2009) provide more detail on the nitrogen and phosphorus cycles, respectively. The attached figure from Gruber and Galloway identifies important global nitrogen fluxes, with rough estimates of their magnitudes. One of the environmental concerns discussed in both papers is the accumulation of unprecedented amounts of biologically available nutrients in soil and water reservoirs. This accumulation could have adverse consequences for important ecological communities (e.g. the soil biota that support crop growth). Other forms of nitrogen emitted to the atmosphere can have adverse effects on both crops and humans. The optional readings by Smil (1997, 1999), Galloway and Cowling (2002) and Childers (2011) provide additional information on nitrogen and phosphorus and their environmental impacts.

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Soil degradation stems from a variety of natural and human causes that include agricultural operations, both crop and livestock production. Fertile soil is the source of the water, nutrients, and trace minerals required for successful crops. The impacts of soil quality on crop yield and resilience are poorly understood but well appreciated by farmers who observe the results of soil degradation first-hand. Tilman et al. (2002) briefly review some of the ways that agriculture can adversely affect soil quality, including loss of nutrients and organic matter and damage to microbial communities. These occur through processes such as erosion, leaching, and non-target pesticide toxicity, which can be aggravated by poor management practices. The adverse impacts of agriculture on soil quality can reduced and even reversed through practices such as crop rotation, cover crops, fallow periods, reduced tillage, soil amendments and careful livestock management, sometimes collectively referred to as Conservation Agriculture. Supporting information in S9 provides more discussion of the problems with trying to characterize soil quality on a global scale. The optional reading by McCauley et al. (2005) provides some basic background on soil properties.

Section 4 of the Yudelman (1994) paper follows up on our Class 2 reading from the same paper by considering the environmental impacts of pesticides. The conflict between better environmental quality and increased crop production is apparent in the discussion. This conflict has prompted interest in more environmentally benign methods for managing pests. Some options are discussed in Sections 5 and 6 of the Yudelman (1994) paper (not required reading). A more recent perspective is provided by Peterson et al. (2018), who advocate an ecological interpretation of Integrated Pest Management (IPM), which they define as "a comprehensive approach to managing host stress that is economically and ecologically sustainable." If sustainable intensification is to work on a global scale it will probably need to follow some of the pest management concepts outlined in this article, including greater emphasis on the needs of the host plants and less on elimination of pests.

Overall, the readings for this class outline broad goals for a more environmentally sustainable and productive global agricultural system. Since most of the specific methods for achieving these goals are described anecdotally, for particular study sites, it is difficult to assess how well they will work at large scales and to determine whether they can really achieve the yields required to meet projected food demand. There is a real need for additional research on the effectiveness and scalability of practices such as Conservation Agriculture and Integrated Pest Management. These practices are appealing at first glance but are not always tested under the diverse conditions needed to show that they can provide significant production increases while also satisfying sustainability criteria. We will return to this topic in Section 4.

Required Readings

Sustainable Agriculture



Pest Management

  • Montague Yudelman, Annu Ratta, and David Nygaard. 1998. Pest Management and Food Production: Looking to the Future (Section 4). International Food Policy Research Institute. Washington, DC 20006–1002.
  • R. K. Peterson, L. G. Higley, and L. P. Pedigo, 2018. "Whatever happened to IPM?" American Entomologist. 64, no. 3: 146–150.

Optional Readings

Sustainable Agriculture

Human Impacts on Ecosystems



Soil Properties

  • Ann McCauley, Clain Jones, and Jeff Jacobsen. 2005. "This resource may not render correctly in a screen reader.Basic Soil Properties (PDF)." Soil and Water Management Module. 1, no. 1: 1–12. Montana State University Extension Service.

Discussion Points

  • Considering the papers we have read so far, do you think cropland expansion (extensification) should be ruled out as a major option for increasing food production? Why is expansion continuing so rapidly in tropical regions (see S7) if it is such a bad idea?
  • What is your overall conclusion about sustainable intensification? Is it feasible?
  • Do you think agriculture’s large impact on the nitrogen and phosphorus cycles is a serious problem or an acceptable side effect needed to achieve dramatic increases in food production? Elaborate on the basis for your opinion.
  • After reading the pest management papers for this class as well as Yudelman’s discussion (Class 2) on crop losses to pests, what is your opinion about the best way to manage pests to increase food production?