Speculation abounds that coffee grounds repel cats, kill slugs, prevent weeds, aerate and acidify the soil, provide nitrogen, and attract earthworms. This fact sheet examines the science behind the use of coffee grounds in gardens and landscapes and provides recommendations for home gardeners to use coffee grounds appropriately.
Chemical composition of coffee grounds
Not everything contained in a coffee bean makes it into a cup of coffee. Nitrogen-rich proteins needed for seed germination and growth comprise over 10 percent of the content in coffee grounds (Tokimoto et al. 2005). Since coffee is extracted in water, most of the hydrophobic compounds, including oils, lipids, triglycerides, and fatty acids, remain in the grounds, as do insoluble carbohydrates like cellulose. Structural lignin, protective phenolics, and the wonderful aroma-producing essential oils also remain in the grounds following the brewing process. Even small amounts of caffeine may remain in the grounds.
Decomposition of coffee grounds
Left outdoors over the course of several months, bacteria and fungi break down the various chemical components of coffee grounds. Nitrogen-rich compounds including proteins and caffeine break down quickly.
Some larger bioconsumers, including earthworms (Figure 2), use coffee grounds as a food source (Bollen and Lu 1961). The fact that earthworms pull coffee grounds deeper into the soil may account for noted improvements in soil structure such as increased aggregation following the application of coffee grounds.
Humic substances, which are important chemical and structural soil components, are produced through coffee ground degradation (Ouatmane et al. 2002). Carbon-to-nitrogen ratios change as well, generally starting out high (e.g. 25-26:1) and decreasing over time to about 10:1 (Morikawa and Saigusa 2008; Ouatmane et al. 2000). The latter is an ideal ratio for plant and soil nutrition.
How coffee grounds affect soils
Coffee grounds used as mulches or amendments have mostly positive effects on soils (Yamane et al. 2014). Coffee grounds will moderate soil temperature and increase soil water (Ballesteros et al. 2014) like any other good mulch material. Coffee grounds bind pesticide residues (Bouchenafa-Saïb et al. 2014; Fenoll et al. 2014) and toxic heavy metals such as cadmium (Azouaou et al. 2010; Kim et al. 2014), preventing their movement into the surrounding environment.