We were wondering if our leftover coffee grounds were good for our olive tree, but we couldn’t find good information online. So, we did some more research and testing. Here’s what we found.
Coffee grounds are great for olive trees as they contain essential nutrients such as nitrogen, potassium, and phosphorus. They also help promote beneficial soil life such as mycorrhizal fungi. Remember to apply a maximum of 2 cups per tree and mix with other organic matter to avoid soil compaction and mold.
So, while coffee grounds are good for olive trees, which nutrients do they provide, and is their acidity (and caffeine) an issue? Let’s find out.
Benefits of Coffee Grounds For Olive Trees
Nutrients
- Nitrogen
- Phosphorus
- Potassium
- Calcium
- Magnesium
- Boron
- Copper
- Iron
- Zinc
By far the most potent nutrient in coffee grounds is nitrogen (at about 2%). This might not seem like a lot, but it adds up quickly when applied to the soil.
This is good news as nitrogen is the primary nutrient plants need (along with phosphorus and potassium, together making “NPK”).
As a result, coffee grounds make an amazing soil amendment and boost nutrients for olive trees.
Pro-Tip: If you’d like free soil amendment, ask your local coffee shop for used coffee grounds. The coffee shop near me has dozens of bags that they normally toss, and most shops love when people repurpose them. Keep in mind these coffee grounds are probably not organic.
Acidic pH
Unlike most plants, olive trees prefer a neutral to alkaline soil pH of 7.0 to 8.0.
Typically, plants prefer a slightly acidic soil pH because it helps dissolve the nutrient solids in the soil, making them accessible to the plant’s finer roots.
Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.
Donald Bickelhaupt, SUNY College of Environmental Science and Forestry
However, because olive trees do best with a neutral to alkaline soil pH, and coffee grounds are fairly acidic, they should be brewed or composted first to remove the excess caffeine and acidity.
| Used Coffee Grounds | Fresh Coffee Grounds |
|---|---|
| pH of 6.8 | pH of 5.5-6.8 |
The best ways to measure your soil’s pH are with strips or a meter. I prefer using a pH meter since they’re affordable and easy to use. If you’d like to see which meter I use and recommend, see my recommended tools page.
Keep in mind that olive trees can still do well down to 6.0, they just don’t prefer it.
Still, if you find that your olive tree’s soil pH is too acidic (below 6.0-7.0), consider adding alkaline materials to the soil like biochar, powdered lime, or wood ash.
On the other hand, if your soil pH is too alkaline (above 8.0), use acidic amendments such as peat moss, sand, and of course—coffee grounds.
Coffee grounds also help olive trees by increasing the soil’s water retention and temperature regulation (similar to compost).
How to Use Coffee Grounds On Olive Trees
| Under 2 Cups | Over 2 Cups |
|---|---|
| Apply directly to the soil | Add to compost pile (no more than 20-35%) |
| Spread out in a thin layer to avoid mold | Wait 3 months for decomposition |
Before using coffee grounds in your garden, I recommended brewing them first to remove most of the caffeine and excess acidity.
This isn’t a problem if you’re using the leftovers from your daily pot of coffee.
However, if your coffee grounds have gone bad, and you’re looking at disposing of them, it’s best not to use them on olive trees until the coffee grounds have composted and the acidity and caffeine have weakened.
Here’s more information on how to apply them.
Directly to the Soil
If you have under 2 cups of coffee grounds, apply them directly to your olive tree’s soil by spreading them out in a thin layer.
It’s recommended to have a layer no higher than 1/2 an inch and to keep the coffee grounds at least 3 inches from your olive tree’s trunk. Doing this improves aeration and sunlight exposure, reducing the chance of mold building up and affecting your olive tree.
High levels of caffeine can be harmful to beneficial insects such as earthworms and pollinators. Since caffeine is a natural chemical made by plants to repel insects, avoid using too many coffee grounds in vermiculture bins or other beneficial insect areas.
In these cases, it’s safer to compost the coffee grounds first.
In a Compost Pile
If you have over 2 cups of coffee grounds, add them to your compost pile. After 3 months, the coffee grounds should be free of caffeine and decomposed enough to be used in your garden.
Here’s what a gardening expert has to say about using coffee grounds in compost.
Composting coffee grounds is the best thing to do before putting them in the garden. Use no more than 20-35 percent by volume of coffee grounds in a compost pile.
Lisa Ogden, University of Wyoming
Because coffee grounds have a great carbon-nitrogen ratio (20-24:1), they’re amazing to use in compost.
Farmer Joel Salatin recommends compost piles have a carbon-nitrogen ratio of 30:1, so coffee grounds aren’t too far off.
Maintaining sufficient carbon (“brown” materials) in compost piles helps them decompose properly and not get stinky from the nitrogen-rich (“green” materials) such as green leaves, grass clippings, and food scraps.
To help balance the nitrogen coffee grounds in your compost pile, add a handful of carbon materials such as leaves, sawdust, or wood chips.
Place carbon materials on top of compost piles to reduce and eliminate smells and flies.
Other Kitchen and Yard Scraps for Olive Trees
Some other kitchen and yard scraps that are great for olive trees include:
- Eggshells
- Banana Peels
- Citrus Peels
- Potato Peels
- Grass Clippings
- Fallen Leaves
For example, eggshells contain about 95% calcium, while banana and citrus peels contain lots of potassium.
If you’d like to see which primary nutrients the above list provides olive trees (along with a long list of other food and yard scraps), check out my post on homemade fruit tree fertilizer.
Sources
- http://www.santacruzolive.com/planting.php?page=lime
- https://www.gardenclinic.com.au/how-to-grow-article/olives
- https://www.uwyo.edu/barnbackyard/_files/documents/magazine/2017/summer/plantsperk0717.pdf
- https://www.discovery.com/science/Coffee-Grounds-in-Your-Garden
