Composting is essentially decomposition under controlled conditions. It uses soil organisms to break down organic matter, resulting in CO2 being released to the atmosphere, and ultimately resulting in organic matter that’s highly concentrated in nutrients. It is performed at various scales for many reasons, such as municipalities composting food and yard waste to reduce landfill expenses, farmers composting deceased animals to reduce rendering costs, gardeners turning their autumn leaves into valuable mulch, and a coffee drinker feeding coffee grounds and filters to earthworms. In this lab we will examine the Soils Teaching Lab worm farm, and visit a model industrial scale composting facility at the Agronomy North Farm where dining center food waste and campus greenhouse waste is turned into compost to be later used for agricultural research experiments.
- Discuss vermicomposting and review how worm farms are managed.
- Visit the K-State composting operation at the Agronomy North Farm.
- Learn about managing a thermophilic composting operation.
- Discuss how organic waste from the KSU campus is turned into compost.
- Conduct a test for compost maturity
- Active worm farm
- Compost thermometer
- Basic Compost Maturity Test, Woods End Laboratories, Inc.
- Optional: Digital Color Reader – Multi-Mode Unit, Woods End Laboratories, Inc.
- Compost Maturity Test Directions, Woods End Laboratories, Inc.
Using the recommended reading and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts addressed in the lab. They will also serve as the basis for the post-lab quiz and are useful study notes for exams.
- Define vermicomposting.
- Define thermophilic composting and describe how it results in the building up heat.
- Name and describe the three stages of thermophilic composting.
- Organic matter is decomposed during composting, resulting in a decrease in the overall C content and relatively low C:N ratios. Describe how the C:N ratio changes over time, and identify the gas that is released as C is emitted to the atmosphere.
- What is the range of C:N ratios that would be expected in mature compost?
- Why does compost need to be turned during thermophilic composting?
Composting is the practice of creating soil organic material from decomposing organic matter under aerobic conditions outside the soil. The product, compost, is highly desirable, commonly used in potting soil mixes, as mulch, as slow-release fertilizer, and as a soil amendment. Compost can be produced under ambient temperatures through vermicomposting (composting using litter-dwelling worms) or via slow decomposition.
Vermicomposting is growing in popularity. If managed properly, worm farms are odorless, and the worms will rapidly turn fruit and vegetable waste and some paper products into nutrient-rich worm castings. Worm farms can be as small and simple as a plastic bin tucked under a desk, or large, industrial-scale operations. The Soils Teaching Lab worm farm is built with multiple levels. Note the increasing level of decay as you go from the top tray to the bottom. Common food wastes and some paper wastes make great substrates for worms. We will discuss what foods are ideal for worms, and what things should be avoided later.
A more common method of composting is thermophilic decomposition. Decomposing organic matter in thermophilic composting undergoes a dramatic buildup in heat due to both the mass of the decomposing material and the insulating properties of the pile itself. Thermophilic composting involves three distinct stages: the mesophilic, thermophilic, and curing stages. During the mesophilic stage, easy-to-decompose organic materials are metabolized, with a resulting gradual increase in temperature to more than 40°C. The next stage is the thermophilic stage, in which the temperature increases to between 50 and 75°C. Materials like cellulose that decompose more slowly and under more difficult conditions decompose during this stage. Regular mixing is required during this stage so that aerobic decomposition can be maintained. Without adequate oxygen, the humification process will slow, and the overall time required for decomposition will increase. The final stage is the curing stage. During this stage, the temperature drops to near-ambient conditions. As decomposition and humification progresses, carbon is removed from the organic matter and released to the atmosphere as CO2. This removal of carbon decreases the C:N ratios and increases the percentages of N, P, and other nutrients in the compost.
At the K-State compost facility at the Agronomy North Farm, food waste from the campus dining halls and plant and soil waste from the campus greenhouses are broken down into compost that is then used for erosion, greenhouse, and field experiments. Figure 11.1 shows the C:N ratios, nitrogen content, and phosphorus content of both fresh and cured compost. The figure shows cattle feedlot manure for comparison. As carbon is removed through respiration and released to the atmosphere as CO2, the C:N ratio decreases, and nitrogen and phosphorus content increase to levels comparable to the manure.
Composting using thermophilic decomposition organic waste is becoming very popular because of its many benefits. Some examples include:
- Organic materials are safely stored with minimal odor release.
- Compost is easier to handle than raw materials because of a 30 to 60% reduction in volume
- Nitrate depression during the decomposition of organic materials with high initial C:N ratios is limited to the compost pile and thus avoids any effect on plant growth.
- High temperatures during the thermophilic stage kills most weed seeds and pathogens.
- Toxic organic compounds like pesticides are destroyed by the time the compost is mature.
- Many soil-borne plant diseases are suppressed through microbial antagonism
- Because compost is made from recently photosynthesized carbon, it is carbon-neutral and thus a much more environmentally friendly choice than peat for potting mixes.
Activity 1: Vermicomposting Demonstration.
We will start with a group discussion of vermicomposting, and a demonstration of the Soils Teaching Lab worm farm. Participate in the discussion, and answer the following questions.
Activity 2: Compost Facility Field Trip.
You will be traveling to the Agronomy North Farm compost operation. The compost operation is managed by Dr. DeAnn Pressley and Brent Wehmeyer. They will lead a tour of the compost facility. During the tour, pay close attention, and answer the following questions.
The last thing you’ll do at compost facility is collect samples from the compost piles to test for maturity. Collect one sample from a compost pile selected by your instructor using a clean trowel, then place it in a Solvita® test jar. Gently tap the jar on the ground or a bench top to allow it to compact to the compost’s natural density. Add enough compost to reach the fill line. Then open the foil packs for a CO2 test probe and a NH4+ test probe and place those probes in the jar so that the color can be viewed from outside the jar. Close and tighten the lid to allow the CO2 and NH4+ gasses to react with the test probes. Mark the start time of the test on the jar lid, then allow the test to proceed for four hours at room temperature (20-25°C or 65-75°F). Your instructor will complete the test for you, and will send you a picture of the final test probes next to the color chart for reading the test probes. Your instructor may also provide you with more precise results analyzed using a Digital Color Reader to measure the color of the test probes.
A quiz for this lab will be available online. Please access it as directed by your instructor.
Solvita® is a trademark of Woods End Laboratories, Inc.