We’ve always used our old coffee grounds to fertilize tomatoes and other plants in the greenhouse, but is this actually a good idea?
Coffee grounds contain nitrogen, phosphorus, and potassium so they are good for tomato plants. However, with only 2% nitrogen content, they are no substitute for a professionally manufactured nitrogen-rich fertilizer that can help you better control the pH level of your soil.
Let’s dive down into the subject of using coffee grounds as fertilizer for tomatoes in more detail, and discover how best and often to use them, how much to use – and what alternatives you have…
Do Coffee Grounds Make Good Fertilizer for Tomato Plants?
Used coffee grounds contain nitrogen, phosphorus, and potassium, which are all great for your tomato plants. Your plants need these minerals in order to thrive, so surely coffee makes the ideal fertilizer?
Well, yes, coffee is a good fertilizer for tomatoes, but only up to a point. Tomato plants need a nitrogen-rich fertilizer, and whole coffee contains nitrogen, so it doesn’t have nearly enough.
In fact, coffee grounds are around only 2% nitrogen, and the amounts of the other minerals are variable.
By all means, add coffee grounds to the soil around your plants, but don’t use them as a substitute for a genuinely nitrogen-rich fertilizer.
How Often Should I Put Coffee Grounds on My Tomato Plants?
While some gardeners say you need to add fertilizer to your tomato plants only twice a year, others suggest sprinkling coffee around your plants a couple of times a week. Who’s right?
As we’ve established, coffee is good, but it’s not that good. In that case, we reckon that it can be applied more frequently than two times a year.
However, coffee grounds can alter the pH of the soil (more about this later), and tomatoes can be pretty fussy about this.
Coffee grounds have other benefits. Worms like a regular caffeine fix, so will be delighted to come across coffee-flavored soil. Also, coffee grounds can act as a bug deterrent, and we’ve heard that slugs, snails, and ants don’t like it, either.
Can You Put Too Much Coffee Grounds on Tomato Plants?
And now we hit upon another gardening debate: are coffee grounds acidic or alkaline? Tomatoes prefer their soil to be slightly acidic, so we have to be careful that coffee grounds don’t tip the balance. But which way would they tip it?
Actually, recent experiments by Washington State University show that coffee grounds aren’t as acidic as we’ve traditionally believed. In fact, they tend to range from “slightly acidic” to “mildly alkaline”, so pretty much just to either side of neutral.
However, there’s another issue with coffee grounds applied directly to soil, and that goes back to how coffee is extracted.
As confirmed coffee fans, we all know that the extraction process is what draws the flavors from the grounds and into our cup.
As the water passes through the coffee grounds, it collects the flavors on the way. The slower it passes through the grounds, the more taste and aroma it gathers.
Because we want our coffee to taste as rich and as deep as possible, we tend to grind our beans to a pretty fine consistency, because this is harder for the water to pass through, so it moves more slowly. Do you see where this is going now?
If coffee grounds are thrown down in clumps, they can form a hard mat. This can be so tricky for water to pass through that it’s practically hydrophobic. This is less than ideal for your thirsty plants.
The solution? Sprinkle the coffee finely and don’t let it clump. We’ve also heard that if you microwave the grounds first, it dries them out and makes them easier to sprinkle.
Are Coffee Grounds Better Than Traditional Fertilizer for Tomato Plants?
While we love the idea of reusing old coffee grounds, they don’t contain enough nitrogen to make them a feasible alternative to a proper tomato fertilizer.
There are a lot of good commercial organic fertilizers out there. Fertilizers containing seaweed are especially good for young tomato plants, during their first few weeks.
So what can you do with the coffee grounds? One of the best things to do is to dispose of your used grounds in the compost. They’ll break down alongside the other matter, without any fear of them clumping.
Worms do enjoy a bit of coffee, so they’ll be pleased with this caffeinated compost cocktail.
If you use a coffee machine, most filter papers are biodegradable, so you can simply chuck the whole thing in the compost once you’ve made your brew.
If you want to know more about which waste items are the best for your plants, take a look at this short film from Epic Gardening. This takes a look at how food scraps from your kitchen can be used as fertilizers, including coffee grounds.
Do Tomato Plants Like Coffee Grounds and Eggshells Mixed?
A mixture of coffee grounds and crushed eggshells is a time-honored way to fertilize tomato plants, but does it actually work?
Yes, it does! Egg shells are great for tomatoes and peppers, and prevent nasty diseases like blossom end rot. This is because crushed eggshells release much-needed calcium into the soil.
A lack of calcium can lead to diseases, so introducing this rich (and free) source is always a good idea. Plus, slugs and snails really don’t like those pointy, crunchy shells.
Why add the coffee grounds to the eggshell mix? Mainly to add some extra nitrogen, and like the shells, to guard against pests like snails.
When they’re mixed together, crushed egg shells and used coffee grounds form a pleasing blend. The grounds make the egg shells easier to sprinkle, while the sharp shells stop the grounds from forming that impenetrable mat we mentioned earlier.
Why Do Some Tomatoes Not Like Coffee Grounds?
It’s nothing personal. It’s all to do with the acidity or otherwise of the soil. It’s always best to know what pH your soil is before planting, so you can adjust your fertilizers accordingly.
However, it’s really hard to tell what pH your used grounds are (coffee manufacturers don’t add that to the packaging, funnily).
If you’re concerned about changing the balance of your soil, use a commercial fertilizer, because it will clearly state what its pH value is and what types of soil it’s good for.
As we mentioned earlier, be aware of how you scatter your coffee grounds on the earth, so they can’t form a hard mat before they break down.
If your tomatoes are especially thirsty or if you live in a dry climate, clumped grounds can prevent water from reaching the roots.