By: Dakota Caruso
Using Compost
Compost benefits the soil as a valuable source of food and nutrients for plants and soil organisms. Two effective ways to gain these benefits are to use compost as a soil amendment or a mulch.
As an amendment compost increases nutrient levels, moderates pH, and improves soil structure, drainage and water holding capacity. Applied as a mulch on top of the soil, compost suppresses weeds and regulates moisture and temperature.
How Compost Functions as an Amendment
Compost plays many crucial roles as a soil amendment. With a nutrient profile of 3.5-0.4% nitrogen, 1.5-0.2% phosphorus, and 1.4-0.4% potassium, it’s effective at providing nutrients to plants and food to soil organisms. By increasing biotic activity, compost speeds the turnover of organic matter and further release of plant available nutrients. However, the nutrients in compost are slower to release than inorganic fertilizers. To replace fertilizer use with compost, you’ll need to first build up organic matter in your soil with long term compost application.
It is important to note that having too much organic matter is possible! For operations where crops are harvested and removed, compost applications may not be enough to supply all of the plants’ nutrient needs and other fertility sources may be recommended. However, with perennial plants and minimal harvesting, compost application alone can properly supply most required nutrients. Recommended yearly application rates vary between these two scenarios. Landscapes housing perennial plants need no more than an inch or two of compost each year, while landscapes where harvesting takes place are able to utilize up to three inches. Regular soil testing is the best way to know for sure if your application rate is appropriate for your situation. Testing at the start of your project will also allow you to compare your soil’s organic matter content to ideal levels.
Another benefit of compost is that the increased activity of soil organisms helps to form soil aggregates. These structures are a mix of minerals and organic matter that improve the rooting zone by increasing drainage, aeration, water holding capacity and making the soil more resilient to both drought and heavy rain. Compost will also interact with your soil to make it more resistant to pH changes, and its (often neutral) pH may help shift soil pH toward neutral. Lastly, while compost is a source of nutrients, it will also improve a soil’s ability to hold onto those nutrients.
Using Compost as an Amendment
Choosing a method for amending with compost largely depends on your objectives and the health of your soil. With healthier soils, surface application with minimal disturbance that allow soil organisms to decompose and transport compost throughout the soil is recommended. With more degraded soils, directly mixing compost into the soil may be needed to achieve beneficial results.
With transplants or seeds: When transplanting, simply side dress the hole the plant will be placed in with a handful of compost (see graphic on Page 2). Make sure to mix the compost into the soil around the plant, as the low bulk density of compost makes for weak structural support and may cause rooting issues. This is especially important for trees and shrubs! If direct seeding, simply mix some compost with the disturbed soil where your seeds will be planted.
Tilling in: For healthy soils that can still use improvements or more organic matter, limit your disturbance to the top 4 inches of soil. Deeper mixing may cause more harm than good by destroying soil structure, increasing runoff, and disturbing soil organisms. Deep tillage is only recommended for degraded, overused soils, and sandy or clayey soils with serious nutrient deficiencies and water management issues. Resource 1 (Page 2) has information on how to discern the health of your soil.
How Compost Functions as a Mulch
Using compost as a mulch is simple and effective. Mulch layers will moderate temperature and reduce moisture loss while allowing free drainage, as well as help prevent erosion and runoff. Mulch will also suppress weeds, but be careful to use only hot-composted materials to ensure that you’re not adding any new seeds.
Over time, using compost as a mulch will provide all of the same benefits as using compost as an amendment. As the organic matter decomposes, the benefits to soil structure, pH, nutrients, drainage and water holding capacity will emerge.
Using Compost As a mulch
Applying compost as a mulch is a straightforward process that generally just requires spreading the compost on top of the soil. If used for temperature regulation, mulching when the soil has reached the desired temperature will help prevent dramatic heating or cooling. To reduce heat stress in the summer, mulch before the season becomes hot (in late spring or early summer). Summer mulching will also help reduce moisture loss when plants start having a high-water demand. Using compost as a mulch will also prevent dramatic temperature shifts in the winter, though the mulch should be removed in the spring to allow soils to warm.
An added benefit of late spring mulching is that the compost will suppress weeds and then decompose over the summer, adding nutrients when the plants need it most. The earlier in the season you apply, the more likely nutrients will be freed up for your plants during the growing season. However, the mulch’s slowing effect on the soil warming process should be considered.
For more information on using mulches, see Resource 2 below.
Concluding thoughts
Compost can be made and used many ways, offering benefits to soil and plant health when used wisely, while also keeping food and yard waste out of landfills. While commonly used in gardens or on small-scale farms, compost application is increasingly being integrated into large-scale landscaping and farming operations. Given its benefits as both an amendment and a mulch, it’s not surprising that compost is a go-to management practice for gardeners, farmers, and landscape professionals.
1) An article from Agricology on soil health, including basic concepts to help you to discern the state of your soil:
https://www.agricology.co.uk/field/blog/what-healthy-soil
2) A resource from USDA/NRCS on how to use mulch:
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_006305.pdf