Can Coffee Grounds Be Used as a Fertiliser in the Garden?

Coffee is one of the most widely consumed beverages in the world, with an estimated global consumption of 2.25 billion cups of coffee per day in 2022. That translates to approximately 2.5 billion kilograms (5.5 billion pounds) of coffee beans a year!

Using ground coffee beans (as opposed to instant coffee) is a popular way to brew coffee. In the coffee brewing process, hot water is used to extract the flavours, oils, and soluble compounds (including caffeine) from the ground coffee beans, and what remains are the small, gritty particles known as coffee grounds.

Spent coffee grounds are generated in huge quantities, and are often available from cafes and coffee vendors for the asking. Many gardeners are taking advantage of this abundant free resource and collecting coffee grounds to use as a soil amendment to improve their soil, or to add to their compost.

There are a lot of claims about coffee grounds, both good and bad. In this article we’ll discuss whether spent coffee grounds are beneficial for the soil, are safe to use, and the best ways to use them in the garden.

Are Coffee Beans a Good Nitrogen Source?

Coffee trees are broadleaf perennial evergreen tropical small trees or shrubs that belong to the Rubiaceae (coffee or madder) family. Two species are used for coffee production, Coffea arabica (Arabica) which constitutes nearly 75% of the world’s total coffee production globally, and the hardier Coffea canephora (Robusta), which accounts for around 23-24%.

These trees/shrubs produce small, round red fruit known as coffee berries, which usually contain two coffee bean seeds that are extracted and processed to produce coffee.

Since coffee beans are seeds, they contain large amounts of nitrogen-rich proteins needed for seed germination and plant growth. As such, coffee grounds are comprised of around 10% protein, which is a fairly high amount. This makes them useful as a rich source of nitrogen for the garden, much like fertilisers.

Nitrogen Content of Coffee Grounds Compared to Manures

The nitrogen content of coffee grounds is important to us gardeners, because nitrogen, phosphorus and potassium are the three primary macronutrients that plants require in the greatest quantities for their growth. Plants specifically use nitrogen to produce vegetative leafy green growth.

How does the nitrogen content of coffee grounds compare to animal manures?

• In general, coffee grounds contain about 2% nitrogen by weight, though this can vary, and some coffee grounds may contain slightly higher or lower nitrogen levels.
• By comparison, even though the amount of nitrogen in manures is quite variable, cow, sheep and horse manures contain moderate amounts of nitrogen, typically 1.5% to 2.5% nitrogen by weight, which is quite similar to coffee grounds.
• Chicken manures are relatively high in nitrogen compared to other animal manures and typically contain about 2.5% to 4% nitrogen by weight, possibly more.

Manures Contain Organic and Inorganic Forms of Nitrogen

The nutrients present in manure may not be fully accessible to plants during the initial year of application. This is because certain nutrients, like nitrogen, can undergo changes in form, and only specific forms of nitrogen are usable by plants.

• When plant nutrients are bound to carbon they are referred to as being in an organic form. In chemistry, when a compound is ‘organic’, it means it contains carbon atoms.
• If those same nutrients are not bound to carbon, they are said to be in an inorganic form.

Typically, plants can only use the inorganic form of nutrients, that aren’t bound to carbon, but animal manures supply both organic and inorganic forms.

The inorganic forms are immediately available for plant uptake and use, while with the organic forms of nutrients, soil microbes break down and mineralise them into inorganic forms over time, which can take a few years depends on soil moisture and temperature, creating a natural, slow release of nutrients over time. 

Mineralisation is a process in the nitrogen cycle whereby microbes decompose organic nitrogen (N) from manure, plant materials and soil organic matter into inorganic forms, first to ammonia (NH₃), and then to ammonium (NH₄⁺)

Nitrogen in manure is typically released over a period of around two years, and additional nitrogen is not released beyond three years. 

Raw manures supply the soil with two forms of nitrogen:

• Organic nitrogen which gradually broken down in the soil
• Ammonium [NH₄]⁺ as an inorganic form, which is immediately available to plants

Organic nitrogen needs to mineralise into ammonium to be useful to plants. The problem with ammonium as a form or nitrogen is that it can easily convert to ammonia gas (NH₃) through a process known as volatilization, and be lost to the atmosphere if not incorporated into the soil. This is why it’s important to dig manures into the soil as soon as possible after broadcasting them on the soil surface, or covering them up with a thick layer of mulch and watering them in, to reduce gas losses.

There’s also a third form of nitrogen, nitrate (NO₃^–), which is also an inorganic form that’s immediately available to plants, and is a common form of nitrogen in synthetic mineral fertilisers, but is usually only present in negligible quantities in raw animal manures.

The Form of Nitrogen in Coffee Grounds

We’ve already mentioned that coffee beans are seeds that contain large amounts of nitrogen-rich proteins. Proteins are complex substances present in all living organisms that are composed of amino acids joined together in long chains.

These amino acids are organic (carbon-containing) compounds, which means that the nitrogen in coffee grounds is in the bound organic form, that needs to be broken down into the inorganic form to be available for plants for use.

How to Use Coffee Grounds as a Fertiliser

Coffee grounds are a material derived from plants, and are therefore a form of organic matter which can be composted to provide the soil with additional nutrients.

When we add any organic materials such as plant materials into the soil, the microorganisms in the soil use available soil nitrogen to decompose them and release their nutrients back into the soil. This phenomenon is known as nitrogen draw-down, and can lead to a temporary reduction of available nitrogen in the soil, making it less accessible to plants.

When nitrogen draw-down occurs, if there isn’t sufficient soil nitrogen, plants may experience nitrogen deficiency, which appears as an even yellowing of plant leaves, and results in a slowing of plant growth and development. The shortage of nitrogen may last until the decomposition process is complete, after which the nitrogen becomes available again.

How can we prevent temporary nitrogen deficiencies due to nitrogen draw-down?

• Add fertiliser with the coffee grounds – We can reduce the impact of nitrogen draw-down in the garden by adding some fertiliser into the soil along with the coffee grounds . Fertilisers are rich in nitrogen, and will provide the soil microorganisms with the nitrogen that they need to break down the coffee grounds, which will then release the nitrogen of their own into the soil.
• Delay planting – When using coffee ground to improve the soil, allow enough time for them to decompose and release their nutrients into the soil before planting crops that are heavy feeders that require high nitrogen levels.

Using Coffee Grounds as Fertliser in in No-Dig Gardening

To use coffee grounds in no-dig gardening systems,

1. Apply a thin layer of manure to the soil.
2. Next, apply a thin layer of coffee grounds.
3. A layer of compost or worm castings of any thickness, if available.
4. Cover with a layer with mulch around 7-10 cm (3-4″) thick.
5. Water the layers to settle the mulch and help moisten the layers beneath.

Don’t apply thick layers of coffee grounds onto the top of the soil, as they compact very easily due to their fine texture to form a thick crust which create a barrier that can restrict moisture and air movement, to plant roots.

Composting Coffee Grounds

When we buy manures as commercial product for use as a fertiliser, they are always composted first by being left to sit in a pile for around three months. The heat generated during composting helps kill many pathogens and weed seeds, and the composting process changes the structure of the manure, making it more crumbly and easier to use. Fresh manures contain salts that are harmful to plants, and by allowing them to sit these are leached out.

Similarly, we can compost coffee grounds first before using them as a fertiliser. The composting process breaks down organic materials to release their nutrients, and can enhance nutrient availability for plants when the compost is incorporated into the soil.

The Carbon to Nitrogen Ratio of Coffee Grounds

In all composting, the ideal ratio of carbon to nitrogen (C:N) in the compost materials needs to be around 25-30:1, which means 25 to 30 parts carbon to one-part nitrogen by weight. This is because the composting bacteria responsible require these nutrients in those proportions to construct their bodies as they grow, reproduce and multiply.

The carbon-to-nitrogen ratio of coffee grounds is typically around 20:1, which is close to the ideal ratio for composting. Coffee grounds are considered a relatively high-nitrogen “green” material, which makes them a valuable addition to compost piles to balance the dry, carbon-rich “browns” such as dry leaves, straw or wood chips.

Coffee grounds are a great composting material because they retain most of the coffee bean, other than the parts that are extracted in water in the coffee making process. All the compounds that are not water soluble, such as oils, lipids, triglycerides, and fatty acids remain in the coffee grounds along with cellulose and indigestible sugars.

Using Coffee Ground in the Compost

Here are some ways to use coffee grounds in compost systems:

• Sprinkle thin layers of coffee between layers of carbon-rich “brown” materials, such as dried-out leaves and plants, cardboard, wood chips, cardboard pieces, newspaper, etc. Use one bucket of coffee grounds or manure to two buckets of carbon-rich materials. Wet down the layers lightly as composting requires materials to be moist.
• Make a deep, large hole in the middle of an existing compost heap using a wooden stake or other implement, and fill it with coffee from top to bottom, then sprinkle some water over the compost pile to moisten it. This compost will heat up from the centre and break down very quickly. This works well for big compost piles or full compost bins that aren’t breaking down. It’s also possible to make additional narrow holes in the top of the compost pile in a circle around the centre, and either leave these as empty holes for air to enter the compost pile, or fill them with coffee also.

Do Coffee Grounds Make Soil Acidic?

There are many myths surrounding the use of coffee grounds that we can easily dispel with a little science. One of these is the use of coffee grounds to reduce soil pH (acidify soil) for acid-soil loving plants such as blueberries, azaleas and camellias, or the fear making the soil too acidic for other plants by using coffee grounds.

What is soil pH? In chemistry, the pH scale is used as a measure of how acidic or alkaline a substance is, which in this case is soil. The pH scale ranges from 0 to 14, where a pH of 7 is neutral. The lower the pH value, the more acidic a substance is, while the higher the pH value is on the scale, the more alkaline (basic) it is.

If we measure the pH of a cup of coffee, it typically ranges from around 4.85 to 5.10, which makes coffee mildly acidic. Because of this, some gardeners assume that coffee grounds must also be acidic, but the results of research experiments do not support this. 

Fresh coffee grounds are not the same as coffee grounds mixed into the soil. Results have shown that the pH of decomposing coffee grounds ranged from 4.6 (mildly acidic) to 8.4 (somewhat alkaline), and that the pH also changes over time.

The soil, as a living ecosystem, also has the capacity to resists changes in pH in order to maintain stable conditions. The buffer capacity of soil is defined as a soil’s ability to maintain a constant pH level when an acidifier or alkalizer is added to it. Even if coffee grounds are mildly acidic, the soil will counteract the reduction in pH fairly quickly to stabilise soil conditions.

There are more reliable ways to make soil more acidic, by either using elemental sulphur, or mulching with pine needles, both which create acidic conditions over a prolonged period of time due to their actions of the soil ecosystem.

Are Coffee Grounds Safe to Use in the Garden?

As an increasing number of people are using spent coffee grounds in their gardens, there are many myths that have arisen around the safety of using coffee grounds in the garden, and whether they have a harmful effect on plants.

Allelopathy is the biological phenomenon where some plant species releases chemicals from either their leaves, roots, stems, or seeds into the surrounding environment to affect the growth, development, or germination of seeds of other nearby plants. The chemicals they release are known as allelochemicals, and they can have both positive and negative effects on neighbouring vegetation. Ecologically, allelopathy can be understood as a form of chemical interaction between plants, and is considered a mechanism by which plants can influence the composition and structure of plant communities around them.

Research as shown that coffee grounds used in large amounts as composts and mulches enhance the germination of some seeds while inhibiting germination of others, so the allelopathic effects vary from plant to plant.

There is a very limited body of research examining effects of spent coffee grounds on plants, and if we examine the studies with good experimental design, we see can see how coffee grounds perform under real-world conditions.

A two-year study by Texas A&M University AgriLife Research was conducted to assess whether spent coffee grounds could be a good nutrient source or root zone amendment for bermudagrass (couch grass) turf.

In the studies to assess its potential as a nutrient source:

• The findings showed that using spent coffee grounds alone as a topdressing (sprinkled on the soil surface) doesn’t produce the same results as a regular fertilizer.
• However, when spent coffee grounds were combined with an organic fertiliser (poultry litter), it improved turf quality compared to other organic and synthetic commercial fertilisers.

In soil amendment research, the aim was to determine if spent coffee grounds could serve as an alternative root-zone amendment to sphagnum peat moss. Coffee grounds and peat moss added at 10% and 20% by volume were compared with a control of straight sand in growing the grass in columns (tall containers).

• Adding the spent coffee grounds to the layer of sandy soil below the turf), had positive effects, improving water and nutrient retention better than peat moss or sand alone.
• There was some temporary chlorosis (yellow discoloration) of the bermudagrass in the first four to six weeks of establishment, that quickly cleared up.
• However, three months after fertiliser was applied, the growth, color, density and nutrient retention was as good or better in the columns amended with spent coffee grounds than those amended with peat moss or the sand-alone control treatments.
• When subjected to a six-week dry-down period to determine how many days the plants could endure before wilting, the coffee ground-amended plants did not show wilt until later compared to other treatments, and also showed much stronger recovery and vigour after re-wetting.

The research suggested that spent coffee grounds may be a useful ingredient that could be added to organic fertilisers.

It may be worth pointing out that the initial yellowing of the turfgrass would be expected as an effect of the nitrogen draw-down effect, as discussed earlier.

Reviewing the very few studies that exist on the effects of spent coffee grounds on plants, many have flawed experimental designs, using very small samples, testing under conditions that are not representative of soil conditions (using potting mediums) or utilising unusual ornamental plant species chosen for ease of propagation rather than representing plants of interest.

My Own Experience Using Coffee Grounds in an Organic No-Dig Food Forest

In my own garden, which is a backyard no-dig permaculture food forest garden, I have experimented with using coffee grounds.

Over the past two years, I have used approximately 150kg of coffee grounds each year in my backyard urban food forest in an area of approximately 63m² (680 square feet) of garden beds. This was combined with 6 bags of cow manure each year, making it approximately 6x25L = 150L of manure.

The coffee grounds I used were from an enrich360 commercial food waste dehydrator system that a local organic cafe uses to process its coffee grounds and plant-based food waste. The final product is predominately spent coffee grounds with some fruit and vegetable scraps and bread which disappears in the processed end-product, other than the appearance of fine fibrous material seen in the photo at the start of this article.

Despite less than ideal, fluctuating, unstable weather and almost non-existent summers over these two years in this temperate climate, the whole garden is visibly and clearly more lush and vigorous, overflowing with abundant plant growth more so than in previous years. It’s so obvious that others notice the difference.

Why would this be so? Because we’ve effectively doubled the organic nitrogen input into the garden, as well as increase the input of other nutrients, and best of all, it’s all for free.

I haven’t noticed any allelopathic effects, everything that regrows from seeds has continued to do so, and no plants or trees have slowed down or stopped growing. This garden is extremely biodiverse, with hundreds of species , and all seem to be doing extremely well.

These coffee grounds have also been used in compost production, and that compost has also been used in the garden, much to the delight of the plants growing there!

Benefits of Using Coffee Grounds in the Garden

Coffee grounds are plant materials, and therefore contribute organic matter to the soil, which has myriad benefits. They also provide some additional benefits due to the compounds they contain.

Addition of soil nutrients – coffee grounds increase the nutrients in the soil, they’re a good source of nitrogen, which is an essential nutrient for plant growth. They also contain other minor amounts of the macronutrients potassium, phosphorus, calcium and magnesium and very minor amounts of the trace elements iron, copper, manganese and zinc.

Improvement of soil structure – the addition of coffee grounds can improve soil structure by adding organic matter to the soil, which helps with water and nutrient retention, aeration, and drainage.

Promote growth of beneficial soil microorganisms – as part of the soil-food web, beneficial soil microorganisms such as bacteria and fungi feed off organic matter, including those in coffee grounds. Their feeding breaking down the various chemical compounds in then over serval months, releasing the nutrients back into the soil, and increases their populations in the soil.

Soil disease control – coffee grounds contain compounds that help suppress some soil-borne diseases, such as common fungal rots and wilts (Fusarium, Pythium, and Sclerotinia) as well as some bacterial pathogens (E. coli and Staphylococcus). 

Promote earthworm activity – earthworms are attracted to coffee grounds in the soil, as they can consume coffee grounds and deposit them deep in soil, which likely leads to improvements in soil structure, such as increased soil aggregation. They also play a crucial role in soil health by aerating the soil and producing nutrient-rich worm castings which further enriches the soil.

Can Coffee Grounds Be Used in Worm Farms?

The earthworms used in worm farms (vermicomposting), such as Eisenia fetida, more commonly known as red wigglers, tiger worms, or manure worms, are a species of epigeic earthworm (living in the surface layer of soil) that is specifically adapted to thrive in decaying organic matter. They’re different to regular earthworms that can tunnel deeply into the soil, and eat coffee grounds mixed into the soil compost bin materials.

Small amounts of coffee grounds can be added into a worm farm, and by that I mean very small amounts, like a tablespoon full, to see how the compost worms respond and whether they consume it in a few days.

The problem with using spent coffee grounds in a worm farm are:

• Heat – they might begin composting on their own in the worm farm, producing excessive and dangerous levels of heat which may harm the worms.
• Drying – they may dry out, and once they do, they may resist any efforts to make them moist again, making them unattractive to the compost worms which only consume moist food.
• Irritation – they still contain some caffeine, which may be irritating to the worms, and if a lot of coffee ground are placed in a worm farm, they all might crawl out and escape to avoid it.
• Preference – the compost worms may not like to consume the coffee grounds, so the material may just sit there for weeks, untouched.

Never add commercial quantities of food to a worm farm – much like we wouldn’t add several thousand used tea bags into a worm farm, we don’t add a huge amount of coffee grounds either. A household would never produce that much coffee ground waste!

The key to feeding a worm farm is moderation and variety, use small amounts of many different foods that are safe to use in a worm farm. For more information see article – The Complete Guide To Worm Farming, Vermicomposting Made Easy.

My personal preference is to use coffee grounds in the compost bin instead, and to use safer foods in the worm farms to keep the compost worms happy!

References

• Using coffee grounds in the garden. (2018, November 15). Cooperative Extension | the University of Arizona. https://extension.arizona.edu/using-coffee-grounds-garden
• Flores, Garrett T. (2020). FEASIBILITY OF SPENT COFFEE GROUNDS FOR USE IN TURFGRASS SYSTEMS. Master’s thesis, Texas A&M University. Available electronically from https://hdl.handle.net/1969 .1/191630.
• Manure as a nutrient resource (2020, April 13). University of Massachusetts Amherst, Center for Agriculture, Food, and the Environment. https://ag.umass.edu/crops-dairy-livestock-equine/fact-sheets/manure-nutrient-resource
• Manure characteristics. University of Minnesota Extension. https://extension.umn.edu/manure-management/manure-characteristics#nitrogen-817860