By: Elena Chatrchyan and Adin Burwell
Overview
64% of adult Americans consume coffee every day, leaving behind a lot of spent or used coffee grounds. One-third of American households also grow food either at home or in a community garden, according to the National Gardening Association. You can combine your love of gardening and enthusiasm for coffee by using your spent coffee grounds in your compost, which can then be a source of nutrients for your plants!
How Do I Get Spent Coffee Grounds?
If you already drink coffee, you can use the grounds you produce. If you don’t drink coffee, or need more grounds, many cafes and coffee shops offer free bags of spent coffee grounds.
How Do I Compost My Coffee Grounds?
It is usually best to first compost spent coffee grounds before applying them to a home garden to offer the most benefits to the soil. If not composted first, adding spent grounds at high rates can suppress plant growth. Coffee grounds are high in nitrogen and can be an important addition to a compost pile. However, you should maintain a well-balanced mix of feedstock for your compost pile and incorporate no more than 20% of any particular ingredient as diversity is important for both decomposition and microorganism health. Ideally, you want a ratio of 1:1 to 4:1 “brown” (carbon-rich) to “green” (nitrogen-rich) composting material. These ratios and percentages are just targets – a little more or a little less of any particular feedstock is acceptable. Coffee grounds are considered a green composting material. You can mix your coffee grounds with other green materials, like kitchen scraps. Brown composting materials include wood chips, dry leaves, and paper.
How Can I Amend My Soil With Composted Coffee Grounds?
Due to the limited amounts available to most gardeners, composted coffee grounds are best used in small plantings, raised beds or potted plants. If you want to use coffee grounds alone in larger urban gardens or farms, you will need to find a source of a larger volume of spent coffee grounds. It may be more effective to combine your coffee grounds with other materials to create a larger compost pile, and then apply the finished compost to your field. It is also possible to add small amounts of uncomposted coffee grounds directly to the soil, but is typically most beneficial to compost them before adding to your soil.
Composted coffee grounds offer many benefits to your garden. You can use the grounds as a nitrogen fertilizer, to add organic matter to your soil, or as a food for beneficial microorganisms and earthworms to enrich your soil.
- Nitrogen fertilizer is important for plant growth. As composted coffee grounds decompose, they release nitrogen that is available to your plants.
- Organic matter comes from organisms like plants and animals and is an important indicator of soil health. Organic materials like composted coffee grounds bind soil together and create pore spaces that help with water holding capacity and infiltration. This increases resiliency against flooding and drought, and protects soil from rain impacts and erosion. Organic matter reduces evaporation and smothers weeds when used as a mulch on the soil surface. Finally, organic matter boosts crop performance and may improve pest tolerance.
- Compost has the ability to provide food to microorganisms, which in turn produce plant growth hormones and make nutrients more available to plants. Compost encourages beneficial organisms that feed directly on disease organisms. The presence of beneficial microorganisms is important because the increased biodiversity improves soil resilience and health.
How Do I Apply It?
Like other composts, composted coffee grounds are generally applied in one of two ways: as a mulch or mixed directly into the soil.
As a mulch, finished compost containing coffee grounds and other materials is spread 2-4 inches deep on top of your soil. In established beds, adding compost as a mulch is an effective way to avoid digging that may damage existing plant roots and soil structure. Earthworms and other soil organisms then integrate the new material throughout the soil.
For new garden beds, mixing compost into the soil is recommended. First, make sure the compost is fully broken down, which may take a few months. The compost should have a soil-like appearance and an earthy smell. Mix the compost with the soil to a depth up to 6 inches with a pitchfork or similar tool. Deeper mixing is acceptable, but most plant roots will harvest nutrients from the top 6 inches. Incorporate up to an equal amount of compost to soil. More compost is not necessarily better! Some decomposition products can hinder plant productivity, and many of the benefits of compost can be lost with leaching or settling soil.
Additional Resources
Visit Alberta Urban Garden’s for a video and more information on How to Use and Fertilize Your Garden Soil for Free.
For details on the science behind using coffee grounds check out Washington State University’s Coffee Grounds Factsheet.
For further information or if you have any questions, contact your local Cornell Cooperative Extension office or contact us at soil3@cornell.edu.