A genius way of using leftover coffee grounds in your yard is to repurpose the grounds into compost or fertilizer to feed your garden soil. The grounds provide many benefits for your garden — not only do they offer rich additional nutrients for your plants, namely nitrogen, potassium, carbon, and phosphorus, a study from Oregon State University also suggests that coffee grounds act as a natural pest deterrent against unwanted visitors, like slugs. 

In addition to acting like a natural mulch and fertilizer, one study also showed that fresher coffee grounds can lower the pH levels of more alkaline soils, allowing certain vegetables and plants to thrive in more acidic-leaning environments. While it’s more likely that composted coffee grounds likely won’t be acidic enough to skew your soil one way or another, as shown by recent research (and you don’t want to use fresh coffee grounds altogether as it could be a phytotoxin to plants), the fact of the matter is that the added elements that coffee grounds bring to the soil doesn’t always benefit every plant. In fact, some plant varieties downright hate it. For them, you’ll definitely want to skip adding coffee grounds to your compost.

For instance, plants like brassicas don’t do well with excess infusions of elements like nitrogen. Nitrogen also alters the growth of Mediterranean herbs as well as asparagus, and for other plants like lettuce, the excess carbon that coffee grounds provide for the soil can actually reduce growth and viability. Therefore, if your vegetable garden beds are largely made up of these plants with particular soil preferences, you may want to rethink your coffee ground disposal hacks all together (there are some things you should never do with coffee grounds around the home and garden, after all). 

Lettuce (Lactuca sativa) is a finicky crop that’s highly susceptible to changes to its environment, especially those brought on by coffee grounds in compost or fertilizer. A 2018 study on spent coffee grounds on soils measured a 286% increase in carbon levels as well as a 188% increase in nitrogen. While those numbers may help other plants like basil and tomatoes thrive, increasing yield for those nitrogen and carbon lovers, it did the opposite for lettuce growth in the study. That research revealed that lettuce growth was actually inhibited by 233% compared to their control variable that did not have any coffee grounds added to the soil at all.

While exact growing conditions varies by lettuce variety, in general, lettuce prefers loamy soil with good drainage. It does not do well in compacted soils and prefers soil temperatures that remain between 45 and 65 degrees Fahrenheit. As such, it can be grown in a number of hardiness zones, so long as the soil conditions and environments are within the range the plant works well in, something that coffee grounds in your soil can alter for the worse.

Many herbs, particularly those that originate from Mediterranean locales that do well in a variety of hardiness zones including three to 10, don’t fare well in acidic soil, nor in soils that experience an oversaturation of certain elements. Herbs are the prefect type of plant to grow even if you don’t have a green thumb, but herb growth can be quite challenging when you introduce elements that lead to less-than-optimal soil conditions. As discussed, coffee is known to introduce increased nitrogen in the soil, and while that leads to great micro biodiversity for many plants, for Mediterranean herbs, it may have an adverse effect. 

While nitrogen indeed promotes plant growth, it also can inhibit the oil production within herbaceous plants, thus making them less flavorful or aromatic — the exact opposite thing you’d want to happen to your herbs. Excess nitrogen runs many risks generally, like lowering yields and compromising root structures for certain plants. Herbs are among the popular garden plants that don’t tolerant such additions, working against optimal production, so the effect of coffee grounds is adverse to the environment that herbs do the best in. Rather, the majority of herbs prefer moderate compost, well-drained soil, and good sun exposure. They also tend to dislike mulch at the base of the plant.

Asparagus (Asparagus officinalis), a vegetable that does best in zones two to nine, notoriously takes a long time to grow, a multi-year effort that thwarts even the most diligent of growers at times. As a test in patience, it then behooves gardeners to ensure that you’re providing the optimal environment for it to thrive, and that includes maintaining the perfect balance of the soil’s elemental make up. Soil testing is key to growing asparagus, as this crop prefers fertilizer that contributes only 10% nitrogen, 10% phosphorus, and 10% potassium per 100 square feet, according to the University of Minnesota Extension. They are not heavy feeders, and as coffee grounds have been found to contribute an excess of those elements, introducing the common kitchen staple may imbalance the asparagus’ soil environments.

Oversaturated nitrogen levels introduced by overly rich compost or fertilizer can cause a “burn” to the crown of the vegetable, also known formally as fusarium rot. This fungal infection causes a reddish-brown color at the root which is indicative of diminished plant vigor and can eventually result in frustratingly small spear sizes and/or bad yields. And for a vegetable that takes so long to grow, you’ll likely want to reap the best and largest crops you can get!

Brassica crops (Brassica oleracea), like broccoli, cabbage , cauliflower, and kale don’t grow well in soils that have an oversaturation of nitrogen either. For plants like broccoli and cauliflower in particular, excessive nitrogen makes it challenging for the crop to establish itself and grow, and research from Utah State University suggests that adding too much nitrogen at initial planting will actually cause brassicas to exhibit soft and loose heads and splitting vegetables later in the season as well. The presence of excess nitrogen can also lead to hollow steams and overly leafy heads, in addition to splitting concerns.

Even though coffee grounds as fertilizer or in compost can contribute very small amounts of nutrient that brassicas do enjoy — like boron and magnesium — the possibility and dangers of an oversaturation of nitrogen may outweigh the much smaller benefits of these other micronutrients. Organic fertilizer, of which coffee grounds can be a part, is also known to cause salt build up if applied regularly to the soil, a condition that brassicas do not respond well to. Thus, you’re best leaving grounds out of the compost for these veggies; much like asparagus, you want to have as healthy and abundant of yield as possible given how hard it can be to grow even a single head!