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Does Coffee Grounds Make Soil Acidic?

As a gardener, you have likely heard that including coffee grounds in your compost enhances decomposition and adds nitrogen. But what if you add coffee grounds directly to the soil? Does coffee grounds make soil acidic? Well, some gardeners believe that coffee grounds can acidify the soil and burn certain plant roots. Others believe that coffee grounds help acid-loving plants such as blueberries, tomatoes, azaleas, and rhododendrons among others.

coffee beans and soil acidity level


There are tons of recommendations online regarding the use of coffee grounds to attract earthworms, repel moles and voles, prevent weeds, boost soil nutrients and friendly bacteria, aerate the soil, and kill slugs among other uses. While some of these recommendations are legit, others are false myths. One of the most debated issues is whether coffee grounds increase soil acidity levels. Let’s found out the truth in detail.


What is the pH Level PH of Coffee Grounds?

Many gardeners have concerns about whether coffee grounds can acidify their garden soil by lowering its pH level. The pH of coffee grounds ranges from 4.6 to 8.4. Coffee grounds from Starbucks, for instance, have a pH label of 6.8, and testing them shows a pH of 6.2. Most coffee grounds are reported to have a pH value of 6.7. Such a pH level is barely acidic enough to make you concerned.

It is important to assess the impact of any soil amendment method on its acidifying effects. Slightly acidic soil improvement techniques may not change the pH level of most soils, unless you are dealing with very sandy soils. What does this mean? It means that adding organic matter with a pH value of 6.7 to your soil will not make it acidic. Thus, coffee grounds cannot make most soils acidic.


Can You Use Coffee Grounds as Compost or Mulch?

Coffee grounds contain approximately 10 percent nitrogen by weight. Nitrogen is important for seed germination. It also helps plant growth. So should you add coffee grounds to your compost pile for gardening and will it make soil acidic?

Scientific studies on the effect of adding coffee grounds to compost show varying results. In one research study, the grounds created a mildly acidic compost with a pH of 4.6 and an alkaline compost featuring a pH value of 8.4. When the researchers added composed coffee grounds directly to the soil, there was a slight initial increase in acidity, with the pH decreasing shortly after.

Coffee grounds have also shown good results when applied as mulch. They are organic and moderate soil moisture and temperature. Also, coffee grounds bind heavy metals and pesticide residues, thereby preventing them from harming the environment. Besides, coffee grounds increase the availability of essential plant nutrients such as zinc, iron, phosphorous, and nitrogen.


Can You Add Fresh Coffee Grounds to Your Garden?

When you add coffee grounds directly to the soil, they get broken down by fungi and bacteria. They decompose over time to create humic substances that enhance the soil quality. Earthworms pull the grounds into the soil, thereby aerating the soil and improving its structure.

However, fresh coffee grounds contain a high nitrogen content. However, some plants do not respond well to a lot of nitrogen. Fresh grounds may be toxic to some plants. Thus, you should avoid adding coffee grounds directly to the stems and roots of plants.

Also, research studies have shown that coffee grounds suppress germination and inhibit growth in some plants. Some scientists argue that the reason is a result of toxic substances that coffee grounds release when decomposing, which also suppress weeds.

Also, some scientists argue that caffeine is toxic to some plants. If you want to be on the safe side, avoid adding coffee grounds directly to the soil. Also, do not use coffee grounds as mulch on areas of the garden where you are awaiting seeds to germinate.


The Right Way to Use Coffee Grounds in Your Garden

Before you use coffee grounds as fertilizer or in compost, make sure you put them out to dry. Fresh grounds can poison or burn your plants, especially if they are in contact with the stems and roots.

You should also measure the pH of your soil using a pH strip before you decide to use coffee grounds in it.  The best soil pH is between 5.5 and 7. Anything below that is a sign that your soil is devoid of important minerals such as Calcium, and phosphorus.

Therefore, always make sure your soil pH is between these perimeters to ensure healthy plant growth. Once you find out your soil is acidic, avoid using coffee grounds to amend it. Rather, find an alkaline fertilizer to balance the acidity in your soil.

It is also important to note the acidity levels in the beans you use to brew your coffee. Light roasts have a higher acidity than dark roasts. Therefore, use light roast coffee grounds sparingly in your compost or mix them with dark roast to avoid over-acidifying your soil.

The amount of coffee grounds you add to compost will also determine how acidic it gets. The safest percentage of coffee grounds to use in compost is 20 percent. That is enough to improve the structure and nutrition of your soil without destroying its pH. Other materials you can add to compost along with coffee grounds are wood chips, grass clippings, peat moss, and garden leaves.

Make sure you mix coffee grounds with brown compost material to enhance its efficacy. Otherwise, using it with green material will create compost that can burn your plants. You should also avoid using too much coffee grounds in compost because it can spoil the value of your organic manure.


Do Coffee Grounds Help Control Bacteria and Fungi Without Increasing Soil Acidity?

Coffee grounds help suppress fungal wilts and rots including Sclerotinia, Pythium, and Fusarium. Studies done on cucumbers, tomatoes, beans, and spinach show that when coffee grounds are included in compost, and the compost is applied to the soil when growing these plants, the grounds prevent pathogenic bacteria and fungi from establishing. Thus, besides adding nitrogen, they will keep fungi and bacteria away without making the soil too acidic to grow them.



Does coffee grounds make soil acidic? Coffee grounds are barely acid. Thus, they may not add any significant acidity to most soils. However, avoid adding coffee grounds to the soil directly as they may inhibit the germination and growth of certain plants. Preferably, add coffee grounds to your compost alongside grass clippings and shredded leaves. Also, use coffee grounds sparingly and in a thin layer when using them as mulch. That way, you will benefit the soil and plants without risking them to any toxicity.


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