Biology of Soil - Lesson 6 - Organic Matter in Soil

Biology of Soil - Lesson 6 - Organic Matter in Soil

Target Grade Level / Age Range:


Estimated Time:

45 min.


Students will understand what organic matter in a soil is, its purpose, and the various sources of organic matter. Students will then experiment with the breakdown of organic material in soil.


  • High nitrogen content materials to compost
    • Coffee grounds/filters
    • Banana peels
    • Apple cores
    • Eggshells
    • Orange peels
    • Grass clippings
  • High carbon content materials to compost
    • Paper shreddings
    • Newspaper
    • Woodchips
    • Twigs
    • Wetboard/cardboard
  • Anaerobic compost bin (any container that seals, like an ice cream tub, trail mix cannister, or large peanut butter jar)
  • Aerobic compost bin with cover (wash tub with a cardboard cover, ventilated ice cream tub, or bucket with lid not sealed)
  • Soil thermometer or candy thermometer
  • Scale large enough for compost bins
  • Science notebooks
  • Wooden spoon, stake, or other object to stir compost
  • Optional: gloves, paper towels

Essential Files (maps, charts, pictures, or documents)

Vocabulary (with definitions)

  • Organic matter: biological material in the process of decaying or decomposing
  • Organic: relating to or derived from living matter OR produced or involving production without the use of some synthetic fertilizers or pesticides
  • Manure: animal waste used for fertilizing land
  • Decomposition: the state or process of rotting
  • Composting: the act of decomposing organic material to be used as a soil additive
  • Vermicomposting: the act of composting with the use of worms
  • Biomass: living organisms in soil organic matter
  • Detritus: identifiable dead tissue organic matter in soil
  • Humus: nonliving, non-tissue organic matter in soil

Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)

When students hear the word “organic” they may think about the health food section of the grocery store, but most of science uses the word differently. The broader term (little “o”) organic refers to matter that is carbon-based. Think about things that will decompose or come from nature. These are organic. The marketing term (big “O”) organic refers to production methods that include using only specific types of pesticides and fertilizers.

Organic matter in the soil is very important. We have previously learned that nutrients are held to soil, and that clay can hold more nutrients than silt or sand. Organic matter can hold significantly more nutrients than any of the mineral soil particle sizes. Scientifically, this is known as cation exchange capacity, which refers to the ability of the substrate to hold onto cations (like phosphorus, potassium, sulfur, and other nutrients). Organic matter is what makes our soils dark. Organic matter is what feeds soil microbial life. Therefore, organic material is essential to building soil structure, soil tilth, and maintaining soil health.

Students can experiment with this by composting. In this activity, students will start two compost bins: one aerobic and one anaerobic. They will use this as a way to compare and contrast aerobic and anaerobic microbial activity, as well as recognize what organic material is.

Most compost collections are aerobic. They include about 2/3 high carbon content materials (like paper, woodchips, twigs, leaves, or even cardboard) and about 1/3 high nitrogen content materials (like kitchen scraps – coffee grounds, apple cores, etc.). These materials help feed aerobic microbes. Oxygen is important to these microbes, so it’s important to stir, spin, shake, or agitate the mixture a few times every couple of days. These aerobic microbes work quickly and at a high temperature. Aerobic compost bins can get so hot they can kill weed seeds – something home gardeners like!

Some compost collections are anaerobic. This can work better for folks who aren’t allowed open air compost areas, or don’t want to spend much time or effort composting. Anaerobic microbes work much slower. This compost will smell much worse, will be slimy, and likely pretty acidic. It’s suggested that if folks decide to use anaerobic composting that they still let it aerate for about a month before they add it to a garden or other patch they’d like to fertilize to help neutralize the mixture.

When composting, it’s not necessary to inoculate the mixtures with microbes – they’re already there. Microbial life is present and is extremely rich in soil. To help illustrate this idea, talk about canned food. If microbes weren’t present in everyday life, the anaerobic compost bin would preserve food just as if it was canned. But this is not the case because canned food has been heated up to the point that no organism can live, then sealed. Without the presence of microorganisms the food will not spoil.

So, what’s the real world application of organic materials? There are a few applications in agriculture. First, soil tilth is very important in sustainable agriculture. We have to maintain our soils to continue to use them – and we have to continue to use them. Organic material positively contributes to soil structure, texture, and tilth.  

Common sources of organic material in a row crop field are plant residue from cash crops (think corn and soybean roots, stems, stalks, bean pods, corn cobs, leaves, etc.), cover crops, and manure.

Manure is a very important source of nutrients and organic material. Iowa is the leading producer of pork and eggs. Iowa is in the top 10 producing states for beef and turkey. We have lots of livestock that produce lots of manure. And that’s a good thing, because we also have lots of row crop land! Manure is very high in nitrogen. It is also high in phosphorus and potassium.

Manure is used as a fertilizer for farm ground. But before it is applied, the manure should be tested for nutrient content. The field it is applied to should also be tested for nutrient content. This way the farmer can calculate exactly how much manure should be responsibly applied to that field. To further apply this manure responsibly, liquid manure slurries (like pig manure stored in pits) are also commonly injected into the soil instead of being applied to the soil surface. When the manure is injected, it is protected from the elements, mitigating loss to the environment. This also helps stabilize the nitrogen in the manure so that it doesn’t change form and become unavailable to plants.

People have been applying livestock manure to farm fields for years and years, but cover crops are a newer idea for incorporating more organic matter into the soil. Cover crops are non-cash crops that are seeded in the fall and terminated in the spring. Their purpose is to protect the soil during the idle months when the field is usually bare and susceptible to the elements. This is typically the winter months. Cover crop growth above the soil surface protects the soil from wind and rain and root growth helps hold the soil in place. Cover crops also introduce a different root structure, potentially aiding in soil structure formation and soil compaction mitigation. Cover crops may also help break insect and disease life cycles and hold nutrients and water towards the soil surface.

Though cover crops are gaining in popularity, they can be tricky to implement because they take time and money to seed and terminate, but the crop is not sold. Some farmers can use cover crops to graze livestock on to help hedge some costs. There are also some commercial companies like Hellman’s and Pepsi that are starting programs for subsidizing cover crop programs on land growing soybeans and corn to be used in their products. With more of a push for sustainable food production, we may end up seeing more programs like this in the future.

Main ideas of this lesson:

  • Organic material is anything that was once living and will decompose
  • Organic material feeds microbes, which break down the material
  • Organic material is good for soil health
    • It introduces nutrients to the soil, feeds microbes, and contributes to soil tilth
  • Plant residue, cover crops, and manure are common sources of organic material in agriculture
  • Aerobic microbes break down matter differently than anaerobic microbes
  • All matter and energy is cycled
    • In organic material, organisms live, die, feed other organisms, gases are given off, nutrients are returned to soils, new plants use the nutrients in the soil, and so forth

Interest Approach – Engagement (what will you do to engage students at the beginning of the lesson)

Start class by showing the OK Go This Too Shall Pass music video ( This video shows a Rube Goldberg Machine that uses excessive, silly tactics to do a simple task. It is able to do this by transferring kinetic energy from object to object, tipping scales, rolling balls down ramps, filling water pitchers, etc. Plants are like their own kind of Rube Goldberg Machine, taking the sun’s energy and water, creating sugars which generate potential energy, which allow the plant to grow taller and build more structures, and so forth. Plants are an amazing technology that function based on a waterfall of factors that all have to come together just right to accomplish the task. As the cycle continues, the plant dies, returns to the soil, and starts a chain of new reactions.

What happens when a plant dies? It decomposes, feeds insects, microbes, and vertebrate animals. Those animals create colonies, creating networks of pores in the soil. They excrete waste, which fertilizes the soil. Plant material holds many nutrients, which are returned to the soil to fertilize it. Organic material improves soil structure and soil tilth. Have students draw a diagram in their science notebook outlining some of the steps in a plant’s “Rube Goldberg Machine” from end of the plant’s life cycle through decomposition and eventually to providing nutrients for new plants.


  1. Activity 1: composting
    1. In previous lessons, the idea that plant matter provides organic matter or organic material to the soil is covered. Review with students what organic means.
      1. Clarify that the organic label is different from the scientific term organic. In science, organic means carbon-based, or that it comes from something that was once living.
    2. Using the attached PowerPoint, review examples of organic materials. The PowerPoint asks students to identify which of the items on a list is organic. Go through the list one by one and ask students to give a thumbs up or thumbs down to show if they think the item is organic or not.
      1. The organic items on the list are plant material, manure, animal remains, moss, microbes, and insect remains.
      2. The inorganic items on the list are salt, rocks, water, and metal.
    3. One property of organic materials is that they will break down or decompose over time. To help prove that, tell students they will be engaging in an experiment watching the decomposition of materials.
    4. Composting is the act of collecting organic materials like kitchen scraps and yard waste, helping it decompose, and using it as fertilizer. People have been composting for hundreds of years, with a more intentional and scientific approach in the last 100 years or so. In this class, students will create two compost bins; one with aeration and one without aeration. Students will observe the impact of air on the microbes living in the compost that help break it down.
      1. **Optional: a third compost bin can be created to see what happens if students try to compost inorganic materials like rocks or plastic.
    5. Split the class into two groups. One group will begin the aerobic bin, and one group will begin the anaerobic bin. Tell students building bins that they will need to layer and mix items, put the lid on correctly, and label the bin either aerobic or anaerobic.
      1. Group one (aerobic bin) should be given a container (not airtight, but with a cover), compost starter (like garden soil), various nitrogen-heavy materials (like coffee grounds and other kitchen scraps), and various carbon-heavy materials (like shredded paper, woodchips, or twigs).
      2. Group two (anaerobic bin) should be given an airtight container, compost starter, nitrogen-heavy materials, and carbon-heavy materials. (The more consistent the better between the composting materials in the two groups.)
    6. Give students about 10 minutes to work then bring the class back together. Brainstorm with the class what kinds of other materials they could compost. What items would not compost well?
    7. Tell students to get out their science notebook. Have them journal for a couple minutes about what they hypothesize will happen in the two compost bins and why.
    8. Ask students what they think will happen. Who thought the aerobic bin would work better? Who thought the anaerobic bin would work better?
    9. Tell students they will be observing the two bins each day for the next few days. Students will observe and note each of the following characteristics:
      1. Look of material – color, texture, etc.
      2. Smell of mixture
      3. Temperature of mixture
      4. Weight of mixture
    10. Each of the characteristics will be noted and charted in their science notebooks. Students will also be in charge of mixing, flipping, or stirring the compost during class.
      1. Compost should be mixed every 3-4 days
    11. Students should observe and collect data points on the compost for several days. Continually start class with slide 6 of the PowerPoint outlining what students should observe with the compost.
    12. After a couple of days of observations, revisit what is happening. The aerobic compost bin is likely faring better. It likely smells better, is less slimy, is decomposing faster, looks fluffier, etc. Why is that? Aerobic microbes that break down organic material need oxygen to function. They work better and faster when they have air. However, there are microbes that function without oxygen. They work very slowly, don’t produce as much heat, and create a more acidic, smelly mixture.
      1. What did the temperature do? The aerobic compost was likely warm. This is because of microbial activity decomposing the organic material. It takes energy to break down materials, and heat is energy.
      2. What did the weight do? Was matter created or destroyed? No, all matter in the universe is simply redistributed. Consider that if the mixture does weigh less that carbon was lost to the atmosphere.
      3. Discuss why the anaerobic compost still broke down (to some extent). Why doesn’t canned food break down? Without sanitizing the compost, there were natural microbes in the mixture. When food is canned, all living organisms are killed, so when left in anaerobic conditions, they cannot break down or decay.
      4. Have students write or draw in their science notebook how energy is transferred in compost. Have them diagram the cycle of matter from living plant to decaying plant to microbe, soil, air and eventually to new growing plant.
  2. Activity 2: Organic material in agriculture
    1. Students have learned what organic matter is, but why should they care? What applications are there for it in the real world? Take suggestions from students.
      1. Students might say fertilizer source, or food for microbes. Organic material also helps build healthy soils. (Remember 5% of a healthy soil should be organic material).
    2. Follow along with the PowerPoint to highlight the three main ideas for why we want organic materials in agricultural soils. The first of these mentioned is that it is a source of fertilizer.
      1. Farmers need to apply fertilizer to their land to maintain soil productivity. If they can use organic material as the source, that can work well in some situations.
      2. However, because of the nature of manure, compost, and other sources of organic matter, the exact nutrient content will need to be tested in order to use it responsibly.
      3. Some sources listed in the PowerPoint, like crop residue, green manure, and cover crops can include nutrients, but are seen more as a management practice that helps build soil quality instead of a fertilizer application. However, they do produce organic material that helps improve nutrient content of the soil.
    3. A Food Source for Microbes (slide 10)
      1. Organic material, like plant matter, feeds microbes. Microbes are also organic material. Microbial waste is also organic material. Plants and microbes help build soil structure and contribute to the nutrient content of soil. Microbial life is very important in maintaining a healthy soil. Microbes can break down organic material for us, provide us more nutrients, keep soil structures healthy, and much more – likely that we don’t even know about yet!
    4. A Contributor for Soil Health (slide 11)
      1. Organic matter contributes to a healthy soil. It can help provide aeration to heavy soils and feed the microbes that provide glues that hold together well-structured soils. Soils with high organic material can hold more water and more nutrients. This means less nutrient loss to erosion and leaching through the soil profile.
    5. How is organic matter introduced? (slide 12)
      1. There are many sources of organic matter in agricultural systems. Ask students if they remember any of them from the slide on fertilizer sources. They may mention manure, compost, and crop residue. (slide 13)
      2. There can be many sources of organic material from the natural world – think of all the bugs and wildlife that live and die in a field. However, most Iowa farm fields likely get most of their organic content from manure and crop residue. Farmers have to make decisions about what management systems to employ to care for their soils. In different parts of the state and world those answers may look very different.
    6. There are lots of management decisions to make when considering organic material in farm fields. Walk through some of the decisions in slide 14 of the PowerPoint. There are a few options for incorporating more organic material into agricultural soils, but each have considerations. Time may be an issue. Funding may be an issue. Equipment may be an issue. Depending on the area, the soils, the funding available, and a variety of other factors, different farmers may come to different conclusions on the best thing they can do at a certain time.
    7. Tell students to bring out their science notebooks. Have them summarize some of their learning about organic material in agriculture using the journal prompts on slide 15.
      1. Why does organic material matter to soil?
      2. What are some sources of organic material?
      3. What are some management decisions that can impact organic material?
    8. Bring the class back together and discuss what they journaled. Review each question and take input from the class.
    9. Show slide 16. Tell students to bring their science journals back out. Knowing all that they now do about soil health, what management decisions would they make regarding soil organic material if they were a beginning farmer with few resources?
      1. What if they had more financial resources?
      2. What if they didn’t have livestock?
      3. What if they did?
    10. When students have journaled some ideas for each prompt, bring the class back together and talk through what they journaled. What kinds of decisions would they make?
      1. Some management decisions, like what crops to choose or rotating crops, aren’t as reliant on how much money you have. In the first scenario with the farmer with few resources, rotating crops is an easy way to maintain soil health. Cover crops may only be able to be implemented if a farmer has livestock or if they have excess financial resources. Manure applications will be more feasible if the farmer has livestock as well.

Wrap up with some overall discussion on organic matter. There are several sources and many decisions to make about organic material. Different farmers may make different decisions based on those factors. There may not be a clear cut right or wrong answer.

Did You Know? (Ag facts)

  • Each one percent increase in soil organic matter helps soil hold 20,000 gallons more water per acre.
  • Organic matter in soil will be used by the biome (bacteria, fungi, protozoa, plants, etc.) through respiration. Approximately, 60-80% of carbon from the organic matter will be respired and released into the atmosphere. Carbon in the form of organic matter needs to be constantly returned to the soil.
  • Approximately, 15-35% of carbon in the form of organic matter will be incorporated into the soil in a stable for (carbon sequestration). This organic matter can be
    • char (burned plant matter),
    • locked inside soil microaggregates,
    • protected by mineral surfaces, or
    • protected by cold or wet conditions.

Extension Activities (how students can carry this beyond the classroom)

  • Engineering problem:
    • Up to 40% of all food in the U.S. is wasted. Americans generate lots of waste that fills landfills. Soils degrade if they don’t have adequate nutrition. Are there ways to tackle multiple of these issues by changing systems in the U.S. or globally?
    • Present the above problem to students. Talk through the issues of food waste, soil nutrients, and land use for landfills. Recap discussions about composting and organic material used as fertilizers. Challenge students to create a report in which they address the overall, complex problem, and propose smaller solutions to individual problems within.
      • For example: Small issues are that people buy more food than they can eat. Restaurants serve too large of portions. Food is thrown away when it could be composted. People living in cities may not have access to composting sites or resources. Small solutions may be to start neighborhood composting areas and classes, portion options at restaurants, better food storage or preservation, etc.
  • Math problem:
    • Organic matter in soil will be used by the biome (bacteria, fungi, protozoa, plants, etc.) through respiration. Approximately, 60-80% of carbon from the organic matter will be respired and released into the atmosphere. Carbon in the form of organic matter needs to be constantly returned to the soil.
    • Approximately, 15-35% of carbon in the form of organic matter will be incorporated into the soil in a stable for (carbon sequestration).
    • Assuming that on a corn field that yields 161 bushels per acre there is a total of 6,861 lbs/acre of corn stover (dry weight). If it was a healthy field (70% respiration and 20% carbon sequestration) and the farmer wanted to maintain the level of organic matter, how many pounds of stover could be removed from the field for other uses? If the farmer wanted to increase carbon sequestration in the field, how many pounds of stover should be left on the field?

Suggested Companion Resources (books and websites)



Chrissy Rhodes

Organization Affiliation

Iowa Agriculture Literacy Foundation

Agriculture Literacy Outcomes

  • T1.9-12.b: Describe resource and conservation management practices used in agricultural systems (e.g., riparian management, rotational grazing, no-till farming, crop and variety selection, wildlife management, timber harvesting techniques).
  • T2.9-12.b: Compare similarities between organic and inorganic nutrients (i.e., fertilizer) on plant growth and development; determine how their application affects plant and animal life.
  • T2.9-12.d: Evaluate evidence for differing points of view on topics related to agricultural production, processing, and marketing.

Iowa Core Science Standards:

  • HS-PS3-3: Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.
    • Addressed in introduction activity
  • HS-LS2-3: Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.
  • HS-LS2-4: Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.
  • HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.
    • Addressed in extension activity