Module 5: Soils and Nutrients
The purpose of this module is to give you as a learner a basic grounding in the nature of soils and soil nutrients. Module 5.1 provides the foundation for understanding soils, soil nutrients, and their connection to food. We will also focus on ways that soils are vulnerable to degradation that impairs their role in food production. In module 5.2 we will deepen understanding of how soil management can protect soils in their role of supplying nutrients to crops and protecting other valuable resources such as surface water. To accomplish this we will focus on nitrogen (N) and phosphorus (P) as key nutrients for food production in module 5.2.
- Identify soil nutrients and soil function as key resource in need of protection for food production and food systems.
- Describe spatial and geographic variation in soil resources and soil fertility.
- Distinguish between preexisting aspects of biogeochemical cycling and human-induced processes that affect biogeochemical cycling.
- Attribute different soil fertility outcomes in food systems to the coupled natural and human factors and feedbacks that produce them.
After completing this module, students will be able to:
- Describe the basic properties of soil that distinguish it from mere "dirt."
- Explain how soil serves as a medium for plant growth.
- Explain how the five soil-forming factors interact to produce soils.
- Explain the term "biogeochemical cycling."
- Explain common limiting factors to plant growth that limit food production around the world.
- Explain how nutrient and carbon depletion from soils and soil erosion create conditions of low food productivity.
- Assess how farming practices affect soil fertility.
- Analyze modern fertilizer use as the emergence of a strong human system impact on nutrients in soils that replenishes soil nutrients but can create nutrient pollution.
- Analyze how natural/human system feedbacks operate to limit the actions of poorer food producers around the world.
- Incorporate sustainability challenges related to soil nutrient management into an analysis of food systems.
Context for Use
This two-part module is designed for one week of classroom sessions, either as two or three classroom sessions or as a blended format with out-of-classroom reading and work followed by a classroom session to introduce and begin the summative assessment. An all-online format could also be used, although we piloted the module in the blended format. Module 5 is designed to introduce students basic knowledge of soils, especially related to the way that soils and soil nutrients support food production, and also to introduce the topic of how nutrient management in soils for food production can affect the environment, which follows up on the issue of watershed pollution from food production presented in Module 4. The module will have strong overlap with other introductory courses on soils and sustainable agriculture, along with the following modules on crops and agroecosystem management, except that we try to always reinforce the connections with food production and management within human systems. The module could provide a brief introduction to soils and sustainable soil management for some type of broad survey course on sustainability issues or rural development, if soils were a particular focus in such a course.The module is designed for learners in their first two years of undergraduate education or students in other disciplines looking for a very brief introduction to soil science with a highly applied perspective.
Description and Teaching Materials
- Online reading of the course pages in the two modules. The first part of the module focuses on basic soil concepts and definition and how soils function in food production. The second part focuses on two important nutrients for food crops (nitrogen and phosphorus) and the issues of shortage and surplus associated with these nutrients.
- Knowledge check activities focused on basic concepts and definitions from soil science, and using real data from soils to assess how soil organic matter affects water holding capacity so that crops extend the reserve of water they can draw on for dry weather. The latter knowledge check or Activate Your Learning" activity links to concepts of soils and agroecosystem resilience in Modules 7 and 10.
- A formative assessment tool that asks students to use a global soil mapping resource to assess changes in soil properties along rainfall gradients.
- An outside reading on the important role of organic matter in creating conditions that help to sustain food production.
- A summative assessment that asks students to use data from publications to analyze how the nutrient balance of soils for nutrients like nitrogen and phosphorus affects their food production potential and their tendency to pollute river watersheds.
The module can be completed by students in a variety of online and classroom options. Students can complete the readings and knowledge checks before class, and then prepare for and begin the summative assessment in-class after addressing questions about the module material. A completely online format is also possible, especially if instructors are available on discussion boards, chats, or other formats to address questions and introduce students to the summative assessment. Some modifications for online group work may be necessary if an all online option is chosen. The module could also be used for self-study or as a short soils refresher course. In an all-classroom format, each of the two module sections would be used to structure a class, with class time in the first session used to address basic soil properties, definitions, and geography, with the formative assessment as an assignment started in class. The second classroom session would focus on nutrient cycling and the ways that humans intervene in nutrient cycling to boost and sustain food production. The second class would also be used to begin the summative assessment as an assignment among pairs of students.
Teaching Notes and Tips
What works best for the module
It is important for instructors to establish the rhythm of whatever format (in-class, blended, 100% online) is being followed in the class and stress that students need to complete reading and other work on time so that they can participate in class or online discussions and not fall behind. The formative and summative assessments can be fun and highly relevant for students since they are interacting with real data from soil maps for the formative, and real watershed and food production issues for the summative assessment. In the summative assessment, it can be powerful for students to appreciate that a simple balance approach that tells us whether a soil is gaining or losing nutrients can help to answer basic sustainability questions. The opportunity should not be missed to communicate this central message, even if there are complexities around nutrient cycling when approached by more advanced researchers or practitioners. Instructors need to be available to clear up misconceptions that may arise in learning basic soil properties. We worked hard after the pilot to simplify and make the soils concepts more accessible, but for students with a limited background in geosciences or agriculture, misconceptions can crop up easily that are generally easily resolved by an instructor with some background in geosciences or environmental sciences. As in the other modules, we found it good for students to work in pairs on the summative assessment. These teams and individual efforts should be closely watched and supported to deal with questions. If at all possible, the summative assessment can be checked using an in-class discussion as students are beginning to make their analysis of the nutrient balances, or after it is handed in as a follow-up. The overall amount of work in the course was reduced after the pilot to allow this.
What students found difficult
- In a semester-long course, this module may come at the early to mid-semester point when course demands are mounting. Because there is a substantial amount of detail in soil concepts, it may be easy for students to fall behind. Checking students' knowledge or even going over knowledge checks in class may be a good way to avoid students "missing the boat" in this module. We noticed in the pilot that there were junctures where students began to become frustrated with the amount of work and the constantly changing topics of a survey course, and this module has the potential to do so. However, we did try to simplify it after the pilot.
- For many students it may be helpful to boil down soil properties and soil management that foster better or sustained food production to catch-phrases, if this helps them remember. For example for nutrients, "a fair amount, but not too much" is a good catch phrase, or those that are given in headings on soil properties in Module 5.1, such as "near neutral is best." Another example is understanding the way in which nitrogen is a "leaky" or "flighty" nutrient, which should also reinforce ideas from Module 4.
This module is intended as a first introduction to soils concepts for students with and without previous experience in geosciences or environmental sciences. Many of the topics and issues addressed link to concepts in other areas of the course, such as Module 4 on water, and the subsequent modules on crops and systems approaches to sustainability. An effective role for the instructor is to make sure that these connections are appreciated. This should help to increase students' level of confidence with the material, since they will see similar concepts in different ways later in the course, and also build their ability to apply an interdisciplinary, systems approach to food production and food systems.
References and Resources
Brady, N.C. and Weil. R.R. 2016. The Nature and Properties of Soils. Columbus: Pearson. Very readable and visually engaging textbook that gives an extremely comprehensive treatment of soil science.
Dybas, C.L.,2005 "Dead Zones Spreading in World Oceans" Bioscience 55(7): 552-557 - freely available article in Bioscience journal, available for reading and download at http://bioscience.oxfordjournals.org/content/55/7/552.full.pdf+html
Scoones, I. 2010. Dynamics and Diversity: soil fertility and farming livelihoods in Africa, case studies from Ethiopia, Mali, and Zimbabwe. Earthscan. USDA-SARE. 2009. Building soils for better crops. E-book available online for free at: http://www.sare.org/content/download/841/6675/Building_Soils_For_Better_Crops.pdf?inlinedownload=1