Chapter 6: Putting Compost to Use Composters experience many benefits in the early stages of composting from the satisfaction of diverting numerous materials from the waste stream to the excitement of finding beneficial organisms in a pile. For many, the ultimate reward comes at the end of the process when dark, rich compost is added to a garden or yard resulting in vibrant and vigorous plant growth. Since soil health is the foundation for all plant life, the addition of compost yields impressive outcomes. Compost improves multiple aspects of soil structure, chemistry, and biology and fortifies healthy, sustainable systems in gardens, lawns, farms, and other landscapes. Specific Benefits of Compost-Amended Soil By adding organic matter, compost improves the physical structure of soil, allowing root systems to fully develop, as well as increasing air exchange and oxygen availability necessary for healthy root structures. The strengthened physical structure of compost-amended soil and the increased germination rate it provides help to stabilize slopes and minimize erosion. Compost improves drainage when incorporated into heavy clay or silt soils. Compost ensures that plant-appropriate water retention is achieved in a range of soils and, in particular, increases plant-available water by retaining moisture in the root zone. Compost helps capture and hold nutrients in the root zone, whereas many of the nutrients provided by the application of synthetic fertilizers are often leached out of the soil with watering or rain. Plants benefit from both macro- and micro-nutrients present in compost. Soil ph is balanced by compost. Compost improves the cation exchange capacity of soil, helping it bind nutrients and making them more available to plants. The microbial community in compost provides a slow release of nutrients into the soil and to plants. Compost-amended soil will hold supplemental macronutrients, increasing their availability to plants over a longer period of time and creating sustained, steady growth. Soil amended with compost provides a habitat for a diverse population of beneficial soil organisms from nitrogen-fixing bacteria to mycorrhizal fungi and earthworms that contribute to sustained soil health. Rich, biologically-active soil encourages vigorous plant growth and overall plant health, reducing impacts of and susceptibility to disease, pests, and damage from drought and freezing temperatures.
Benefits of Vermicompost Vermicompost tends to contain higher plant-available nitrogen, greater microbial activity, and more nutrients overall than traditional composts. This results in even greater impacts on plant growth and vigor. Ideally, both types of compost are used to achieve the broadest range of benefits. For example, in soil amended with both composts, the traditional compost may provide more of the structural improvements to the soil, while vermicompost contributes greater quantities of nitrogen for plants. The C:N ratio, available nutrient levels, and other factors can vary significantly depending on the composting method, materials, containment method, and curing process. The effects of vermicompost on plant growth and disease and pest resistance is a growing topic of research, and it is expected that more detailed evidence of the benefits of vermicomposting will be emerging in the near future. Deciding to Harvest Compost An important step in the composting process is deciding when it s ready to be harvested. This is particularly important for those anxiously waiting to use the rich soil amendment in their garden, yard, pots, or orchard. For traditional backyard composting, the timeline from starting a pile to plant-ready compost ranges from about 3 months to 1 year, depending on many factors. If optimal conditions are maintained, the pile is turned often, and no new materials are added, a backyard composter may obtain finished compost in 2 months. However, these conditions are typically difficult to achieve in a backyard composting scenario. Similar factors influence the rate of decomposition in vermicomposting, with harvestable compost available at around 3 months if no new bedding material (carbon sources like shredded paper, napkins, etc.) have been added in the last 2 months. Traditional Backyard Compost: Steps to Harvesting 1. Stop adding new materials (store or divert these to another compost pile or worm bin) 2. Monitor and maintain moisture and aeration levels 3. Determine when compost is ready for curing (more on that in the following section) 4. Cure compost for 3-4 weeks (or more) 5. Decide to sift compost or not depending on level of coarseness and intended use 6. Collect compost and use Refer to Chapter 3 on vermicomposting for information about the different options for harvesting vermicompost. Depending on the method chosen, you may only feed one part of the bin, add food and bedding to the next bin layer, or only add foods they will consume quickly. Determining When Compost is Done As the composting process progresses, the original materials will become increasingly unrecognizable, transforming into a dark, crumbly material that has a pleasant, earthy smell. There are a number of signals that collectively indicate a compost pile is ready to enter the curing phase. Factors to look for in finished compost: Consistent, dark brown color Consistent, crumbly texture. Larger woody pieces may still be present, but most materials are no longer recognizable. Earthy smell throughout the pile. To test this carefully, place a sample of the compost in a zip-top bag and seal it for several hours. Then open the bag to see if any of the trapped air contains gases that
indicate instability. If the trapped air smells like rotting matter, the decomposition process needs more time. If the air still smells earthy, the compost is ready to be cured. The temperature in the core of the pile is within a few degrees of the ambient temperature. If you turn the pile, there is no longer the large spike in temperature that was witnessed after earlier turnings. Once your compost demonstrates most or all of these qualities, it is ready to enter the curing phase. Curing Compost Curing simply refers to a period of rest in which the compost becomes more stable as the heat dissipates and the biological population changes. The rate of decomposition slows but does continue. Azobacteria balance the carbon-nitrogen ratio and convert the nitrogen to a form accessible by plants. Additionally, the presence of compounds harmful to plants like ammonia and phytotoxins are greatly reduced. Uncured, immature compost can reduce the amount of nitrogen available to plants as microorganisms use it while continuing to break down material. This can result in stunted plant growth, and, in some cases, weakened plants may die. Curing is an important part of the composting process because it ensures that the compost is safe for immediate use on plants and that it will confer maximum benefits. To cure compost, you may let it sit in its original pile or bin or move it to a new location or container. The curing compost should still have access to oxygen to keep aerobic organisms active. Some experts recommend keeping the pile covered to prevent the excessive loss of nitrogen as gas into the air and the loss of other nutrients as water flows through the pile. Curing may take 3 weeks or more, depending on the initial make-up and stability of the compost. Applying Compost Now for the fun part! There are several options for applying finished compost, depending on the qualities of the compost and the qualities of the soil and plants where it is to be added. As a general guideline, about 20% compost (by volume) works well for a wide range of soils and plants. Some plants do well in higher volumes of compost (think of the tomatoes, squashes, etc. that sprout up in 100% compost), while others particularly those that thrive in quite poor native soils do not require such rich growing medium. As in the rest of the composting process, experimentation is encouraged! Following are some suggested uses for compost as well as application guidelines: Garden soil Dig 1-4 in. of compost into the top 6-8 in. of soil. If starting seeds or young plants, be sure you are using a mature, cured compost. The upper ranges (3-4 inches) of compost application are recommended for poor or newly reclaimed soils, while the lower ranges (1-2 inches) are recommended for soils that are in relatively good condition for planting. However, regular application of compost is recommended for garden soils (at least once per year) since plants are using nutrients. Some plants are considered heavy feeders which may require more compost support. These include tomatoes, squashes, and corn, among others. Potting mixes Compost is an important component in potting or seed-starting mixes. Use 20-30% compost by volume. If growing from seeds or planting immature starts, use cured and sifted (finely-textured) compost. Container-grown plants can also get a boost by top dressing with compost. Apply a thin layer of compost to the top of the soil and water in.
Backfill for trees and shrubs Grass Compost as mulch Compost tea Use 20% by volume, blending thoroughly into the backfill mix. Till 1-2 in. of compost into the top 6-8 in. prior to planting grass or laying sod. Finely screened compost can also be broadcast evenly over established turf to improve the microbial population of the soil beneath. It will also provide organic matter and micronutrients and can improve the moisture-holding capacity of the area. Instead of incorporating compost into the soil, it can also be used as a mulch on top of soil. A good use of unsifted compost that still contains some larger woody pieces is to use it as mulch around established trees and shrubs. Larger, deeper-rooted plants will not be impacted significantly by any further decomposition of the carbon-rich pieces, but will appreciate the nutrient and microbial boost and water retention properties of the compost. Finer compost can be used to mulch around smaller or less-established flowers, fruits, and vegetables. Both traditional compost and vermicompost can be used to make compost tea by a portion of finished compost in a sock, nylon foot, or cotton bag and soaking it for a few hours in water. The compost-infused liquid can then be applied as a foliar spray or soil drench. More information about the benefits and best practices for making and applying compost tea will be covered in class. More on Mulching Mulching enhances the health of soil and plants. It helps moderate soil temperature, hold moisture, slow erosion, and adds nutrients as the mulch slowly breaks down. Mulching also inhibits weeds that can steal nutrients and water from more desirable plants. Some of the benefits of mulching with organic matter: Moderate soil temperature, protecting roots from extremes of cold in the winter and heat in the summer. Slow evaporation of moisture from the soil. Improve beneficial microbial population in the soil. Prevent excessive weed growth. Help control erosion. Enhance the aesthetic value of landscaped areas. Can be made inexpensively from materials generated at home, such as grass, leaves, and ground brush. Sometimes the terms mulch and compost are used interchangeably, but they are really two separate, but complementary, materials and processes. Mulch and mulching can occur in nature, and do all the time. Sweep a forest floor with your hand and you ll find a layer of decomposed leaves and plants, black, moist, and bounding with microbial and macrobial life. This is nature s mulching process at its finest. Composting, on the
other hand, is when we pile up organic material and manipulate conditions in an attempt to create optimum conditions for microbes (preferably aerobic ones) to convert the material into a useable soil amendment. Compost can be used as mulch and often is with great success. But common mulches, such as woodchips and ground brush do not work well mixed into soil because the large pieces of woody material can draw nitrogen away from plant roots as the pieces decompose. Amending soil and mulching serve two different functions, and creating materials for each involves two different processes. Mulch materials made from brush or bark are simply ground to size, and while some types of mulch may be semi-composted, a good soil amendment is typically composted and cured to a point where it s stable and mature. Compost may be applied as mulch on the surface, or incorporated into the soil. Mulch is always applied on the surface. Following is a list of some of the many materials suitable for use as mulch: Leaves Compost Grass Best semi-composted or shredded first. Shredding can be achieved by running them over with the lawnmower. Good mulch for flowers and vegetables. Weeds may germinate in this mulch, but it holds excellent benefits for the soil. Excellent mulch for flower and vegetable beds. Straw Commonly used in fruit and vegetable gardens. An excellent way to utilize straw that has gotten wet or has already seen one use. Woodchips/ ground brush Often can be acquired for free from tree services. Due to high carbon content and potential nitrogen draw from soil, best used for stronger, larger, well-established plants such as shrubs and trees. Black plastic and gravel may fall into the category of mulches, but organic mulches are preferred. Composting, in conjunction with mulching and other green practices like cover cropping, integrated pest management, and intercropping, allows us to create vibrant and sustainable landscapes that best support human and non-human life.