Coffee Grinds in the Garden


Hi there: just trying to get the correct answer, do coffee grinds enrich the soil? Always seem to get mixed answers. Thanks so much


Thank you for contacting the Toronto master gardeners.

We know how frustrating it can be to obtain conflicting information. . Some references indicate that the grounds may make the soil slightly more acidic, but others state that the grounds are neutral with respect to pH.  Actually, fresh coffee grounds are acidic but this is only true for unwashed coffee grounds.If you rinse your used coffee grounds, they will have a near neutral pH of 6.5 and will not affect the acid levels of the soil. Coffee grounds are a source of nitrogen; they are approximately 1.45 percent nitrogen, which is necessary for all plants. They also contain magnesium, calcium, potassium, and other trace minerals. While coffee grounds will add nitrogen to your compost, they will not immediately add nitrogen to your soil. The benefit of using coffee grounds as a fertilizer is that it adds organic material to the soil, which improves drainage, water retention and aeration in the soil.

One research study has found that the grounds inhibit the growth of certain types of seedlings, including tomatoes. The reason for this could be that coffee beans contain caffeine, which is said to suppress the growth of other plants to reduce competition for space, nutrients, water and sunlight. How much caffeine actually remains in used coffee grounds is debatable, and some plants will be more sensitive to caffeine than others. It would be advisable to avoid spreading coffee grounds around seeds or seedlings as they may inhibit germination and growth.

Some experts recommend starting with 15 mL (1 tablespoon) of coffee grounds, lightly working this in to the soil once a week to see how the plants tolerate the grounds and adjust the “dose” accordingly.

Like clay soil coffee grounds consist of very fine particles that are prone to clump together, this turns them into a barrier that will resist water penetration and eventually result in plants dying of thirst. The solution is to mix coffee grounds with other organic matter such as compost or leaf mold before using it as a mulch. Alternatively, rake your coffee grounds into the top layer of soil so that they can’t clump together. Variable particle sizes is key to good soil structure.