The use of ground coffee as fertilizer has been gaining popularity in recent years, as it is a great way to add organic matter to the soil. This can improve drainage, water retention, and aeration in the soil, as well as help beneficial microorganisms and worms to thrive. However, there are some drawbacks to using coffee grounds as fertilizer that should be taken into consideration. Coffee grounds are tiny particles that can form lumps, which can become a barrier to water and other nutrients that plants need to absorb.
Additionally, coffee contains caffeine, which has been found to inhibit plant growth. Therefore, it is important to be careful when applying coffee grounds to plants, as it can damage seedling roots if applied too thickly. Fresh (unbrewed) coffee grounds have more acid than used coffee grounds, so they can be beneficial for acid-loving plants such as hydrangeas, rhododendrons, azaleas, lily of the valley, blueberries, carrots and radishes. However, tomatoes should be kept away from fresh ground coffee.
In terms of soil fertilization, coffee grounds have a significant nitrogen content which can help improve soil fertility. However, because they also affect soil microorganisms, plant growth and possibly soil pH, it is not recommended to rely solely on coffee grounds for plant food. Coffee grounds are also popular with worms and can be used in vermicomposting or to encourage worms in the garden. Additionally, studies have shown that coffee grounds can help the soil retain water and slow down weed growth.
Finally, many people swear by using coffee grounds as a repellent for slugs and snails. However, it is important to note that this practice is not backed up by scientific evidence and should be used with caution. Overall, while used coffee grounds can be beneficial for certain plants in certain situations, it is important to remember that they are not a miracle cure-all for all plants and should be used with caution. Coffee grounds can provide nitrogen and other nutrients to the soil but should not be relied upon as the sole source of fertilizer for your plants.