Pretty much everyone in my family loves tomatoes, so it’s no surprise that they’re one of the first things we decided to grow. Last year, our cherry tomatoes did incredibly well (as you can see from the image above). All we did was use the seeds from a store-bought tomato! But this year, we were wondering if we can use our used coffee grounds as a fertilizer for our tomato plants. So, I did some research to find out. Here’s what I found.
Used coffee grounds can help tomato plants by adding nutrients such as nitrogen and provide acidity to the soil. However, there are some concerns about the residual caffeine. Since caffeine can kill beneficial bacteria and insects, the safest way to use coffee grounds is to let them decompose in a compost pile first.
So, while coffee grounds are good to apply to your tomato plant’s soil, exactly which nutrients are in coffee grounds and how do you apply them? Let’s take a further look.
How Do Coffee Grounds Help Tomato Plants?
Coffee grounds provide good levels of nitrogen, phosphorus, magnesium, and copper, which are all important nutrients needed to maintain a healthy tomato plant. Because tomato plants prefer a slightly acidic soil pH of 6.0-6.8, and used coffee grounds have a pH of 6.5-6.8, they make a great pairing.
While I already explored if coffee grounds were good for citrus trees, I wanted to dive deeper into using them for tomato plants.
Here are all the ways coffee grounds can benefit your tomato plant’s soil:
- Increases nutrients
- Balances pH
- Stabilizes soil temperature
- Increases water retention
- Reduces harmful fungi
- Improves germination
- Increase growth rate
- Promotes earthworm activity
- Natural and organic insecticide
- Reduces weeds
When it comes to nutrients, coffee grounds provide a good volume and variety. Here are the primary nutrients they add to the soil:
Since nitrogen and phosphorus are two of the three main nutrients for plants (the other being potassium, together making “NPK”), coffee grounds are a good way to fertilize your tomato plants. While nitrogen levels can vary, it’s often the most valuable nutrient in coffee grounds.
“Nitrogen-rich proteins needed for seed germination and growth comprise over 10% of coffee grounds. In fact, the carbon-to-nitrogen ratio of coffee grounds can be as low as 11:1, an ideal ratio for plant and soil nutrition.”Linda Chalker-Scott, Ph.D.
There’s also evidence that shows coffee grounds are good for earthworms, which further benefit the soil’s health.
Additionally, research suggests the natural bacteria and fungi that are found on decomposing coffee grounds reduce and even prevent harmful fungi from taking hold in the soil of tomato plants.
Finally, coffee grounds can be used as a natural insecticide. This is because caffeine is a natural defense mechanism created by plants to repel insects. Normally, insecticides have harmful effects since they also kill beneficial worms and pollinating insects, but coffee grounds are a much safer alternative.
For example, there are some mentions of coffee grounds repelling insects such as ants, snails, and slugs. However, there are still debates about the science on this as some believe the caffeine is gone after brewing and others say plenty is remaining in the coffee grounds.
Generally, if you’d like to take the safer option on this one, consider letting your coffee grounds decompose in a compost pile first before using them on your tomato plants.
Are Coffee Grounds Acidic?
Tomatoes are an acidic fruit, so it’s no surprise that they like slightly acidic soil. Tomato plants prefer a soil pH of 6.0-6.8, while a neutral pH is measured at 7.0.
Now, when it comes to coffee grounds, the science is a bit mixed on its pH. In short, unused coffee grounds are acidic, but most of the acid gets washed out with brewing. This means the used coffee grounds have only a slightly acidic pH of 6.5-6.8.
Since the pH of used coffee grounds is right within range of what tomato plants prefer, they prove to be a great addition to the soil.
However, pH levels can change over time in the soil. Because of this, it’s a good idea to check your soil’s pH every few months.
For an easy way to check the soil’s pH, consider using a pH meter. To see which pH meter I recommend, you can check out my recommended tools page.
How to Apply Coffee Grounds As a Tomato Plant Fertilizer
Coffee grounds can either be applied directly to the soil, mixing it with a 1-2 inch layer of mulch, or composting it and then applying 1-2 inches of finished compost. Using too many coffee grounds can result in it becoming compact, which blocks water and air from reaching the soil. It can also grow mold.
There are three ways to apply used coffee grounds as a fertilizer or soil amendment for your tomato plants:
- Apply it directly to the soil
- Mix with mulch
- Compost it
Whichever method you choose, the biggest guideline is: don’t let the coffee grounds (or compost) touch the tomato’s plant directly. The same goes for mulch. You should leave at least a 2-inch clearance from the steam and any lower leaves. This is because coffee grounds and compost can introduce mold and their high levels of nutrients can chemically burn the plant.
However, the exact method you should use to apply coffee grounds for your tomato plant comes down to how many coffee grounds you’re using.
How Many Coffee Grounds Should You Use on Tomato Plants?
If you have the occasional pot of coffee or espresso brick, it’s okay to apply it straight to your tomato plant’s soil. However, if you have a daily coffee pot of used grounds, or you’re getting larger amounts from coffee shops or the like, then you’ll want to mix it with mulch or compost it first.
Since a large volume of coffee grounds can become compact on the top of the soil and block air and moisture from reaching the soil, the two best ways, in this case, is to apply coffee grounds is by mixing them with a 1-2 inch layer of mulch or compost it first.
Generally, as long as the coffee grounds are sufficiently broken up by the mulch and not clumping in large amounts, you can’t really overdo it with this method.
Some good mulches to use for tomato plants are:
- Pine needles
On the other hand, if you’re going the safer route and first adding your used coffee grounds to your compost pile, know that they should be completely decomposed after 2-3 months. From here you can apply your compost to your tomato plant’s soil in a 1-2 inch layer.
When adding used coffee grounds to your compost, make sure of three things:
- The coffee grounds don’t exceed 20% of the total compost
- Allow 2-3 months for them to decompose before applying it to your garden
- When applying the compost, don’t it directly touch the plant’s stem or roots
While coffee grounds are great for soil and compost, they can be too much of a good thing. Like anything, use coffee grounds in moderation. Their high levels of nitrogen and potential caffeine can imbalance the soil or compost if used in excess. Stick to no more than 20% of the total volume of your compost pile.
For a standard 3x3x3 foot compost pile, this means no more than 650 cups of used coffee grounds. As you can see, it would be pretty hard to go over this limit. I’m not the best with math, so if you’d like to check it, here are my notes:
3x3x3 feet = 27 cubic feet. 20% of this is 5.4 cubic feet.
Since 1 cubic foot = 120 cups, 5.4 times this is about 650 cups.
When Should You Apply Coffee Grounds to Your Tomato Plants?
If you have a small volume of coffee grounds, you can use it to fertilize your tomato plants when you first plant them in the garden and 1-2 times per month after. If you’re composting the coffee grounds first, you can add coffee grounds to the compost pile in the fall so it’s ready to use by the spring.
Even though tomatoes are perennials in their native environment, the frost in many climates means tomatoes are typically grown as annuals. Additionally, determinate varieties (like our cherry tomatoes) only fruit once or twice before dying.
Because of this, you can use coffee grounds to fertilize tomato plants anytime they’re growing. Typically, most plants prefer fertilizer in their growing season, just before they fruit, but again, any time will work for tomato plants.
Keep in mind not to apply too many coffee grounds as this will affect how often you can apply them. Make sure to check your soil’s pH periodically to determine if the pH is becoming too acidic. As mentioned, the easiest way to do this is with a pH meter (there’s a link above in the pH section if you’d like a recommendation on which one to get).
While this was a lot of information, it was all useful and I found out that the occasional application of used coffee grounds is great for tomato plants. Overall, they help many aspects of the soil’s health from providing good nutrients to feeding beneficial bacteria and worms.
However, keep in mind that a daily pot of coffee grounds can be too much (depending on the size of your garden). In this case, mixing it with mulch or in a compost pile would work best.