Why Do People Use Coffee Grounds To Fertilize Tomatoes (& Does It Actually Work)?

Coffee is a universal morning pick-me-up for a huge percentage of the human population. It's estimated that 2.5 billion cups are consumed daily. But how is it regarded in the tomato world? Do these plants love coffee? Obviously, we don't share coffee dates and double espressos with our beloved climbing friends, but many gardeners do offer them their spent coffee grounds. As for whether this natural fertilizer is a perk or a pfft, though...  like a good latte, the answer is a bit murky.

The common logic is that coffee grounds are high in nitrogen and other nutrients, like potassium and phosphorus, that are an essential part of healthy garden soils. Additionally, coffee is considered an acidic drink and tomato plants enjoy a somewhat acidic growing environment. It's not quite the no-brainer solution you may hope for, however. Like many complicated topics, there are benefits and drawbacks to each side. Before you pop the top on that percolator, let's consider some of the bitter truths surrounding this brewing brouhaha.

Because of the many nutrients in coffee grounds, they actually do make a nice, low-cost natural soil amendment and you'd probably rather toss them in the garden than in the trash. Coffee grounds will indeed add nitrogen to your tomatoes, but not at a rate that's high enough to actually benefit them. Instead, purchasing a fertilizer targeted toward tomatoes is a better choice.

Coffee grounds also offer a good way to keep soil from becoming too compacted due to their coarse consistency and thus can improve drainage. However, if you don't mix the coffee grounds into the soil well or if you use too heavy of a layer, you'll end up with soil that is hydrophobic, meaning it repels water instead of retaining it. You'll need to carefully mix the grounds into the top four inches of your soil and not just toss them on top. In other words, it's never going to be as easy as dumping the grounds on your soil.

Another common belief is that adding coffee to your tomato plants will help acidify the soil by lowering the pH. Tomatoes thrive in about 6.2 to 6.8 pH soil acidity. Coffee grounds contain a pH level of about 6.5, making them close to a neutral pH and a poor choice for this purpose. Instead, amending your tomato plant's soil with a commercial soil acidifier is recommended. You can check your soil's acidity with testing strips or a pH testing meter to be sure you're achieving the target levels.

Instead of tossing your used coffee grounds into the trash or directly onto your tomatoes, a much better use for them is as a main ingredient in your compost bin. This is not only a great way to reuse kitchen waste, but coffee grounds release nitrogen to keep your compost warm, which is essential in breaking down the rest of the organic matter. It's important not to overdo it though. Coffee grounds should only make up 20% of your total compost. If you don't drink enough coffee to achieve this ratio, many coffee shops and restaurants give away their spent grounds for free. While you're at it, you can also toss your coffee filters and any leftover brewed coffee into the compost bin.

Once your richly aged, earthy compost is ready, apply it liberally to your tomato plants and expect to see them flourish. Much as you do after your morning cup of Joe.