Jennifer Schultz Nelson
Extension Educator, Horticulture
Questions come to me from a wide range of sources—the question that sparked the idea for this week's column came from a friend at the gym this week. Someone she knows uses coffee to fertilize a plant in their office. Supposedly despite being situated in a warehouse with little light, the plant thrives. So her question to me was whether this was true, was coffee some sort of magic elixir for plants? And so I've been trying to find a research-based answer to her question.
Coffee travels a complicated path on its way to our table. There are several species of coffee plant or tree in the genus Coffea, but the two most commonly grown are Coffea arabica and Coffea canephora, also called Coffea robusta. Arabica coffee, produced from Coffea arabica, is considered to have superior flavor to robusta coffee made from Coffea robusta. Robusta coffee is more bitter and has less coffee flavor than arabica.
Along with its superior flavor, arabica coffee commands a much higher price in the market than robusta. Though it has a lesser quality flavor, Coffea robusta is far more disease resistant and has much higher yields than Coffea arabica. Robusta coffee's lower price makes it a common ingredient in mass market coffees, particularly instant coffee.
Coffee as we know it begins as berries on the coffee tree. Coffee production is a labor intensive process because the ripe coffee berries are typically picked by hand. They are sorted by degree of ripeness and the flesh of each berry is removed by a machine. Each berry contains a single seed, or bean, coated with a gelatinous layer of tissue that is removed by allowing the beans to ferment. Following fermentation, the beans are washed with huge quantities of fresh water.
The beverage we call coffee is created by roasting and grinding the seeds of the coffee plant, often called coffee beans, though the coffee plant is not a legume. In extremely simple terms, the degree of roasting determines the flavor of the coffee produced. Darker roasts tend to have smoother more sugary flavors, and lighter roasts retain more of the typical "coffee" flavors that are destroyed in dark roasting. Lighter roasts may also retain more caffeine than dark roasts.
This is the coffee which most of us are familiar with. Typically after brewing, there are grounds left over. Most people end up throwing them in the trash or down the garbage disposal. Many gardeners, myself included, use the grounds directly in our garden or compost piles. The grounds are a source of organic material, great for improving garden soil.
There is a lot of research-based information available on using coffee grounds in the garden. Research has shown that coffee grounds are about 2% nitrogen by volume, making them a great source of nitrogen for composting. Coffee grounds have been suggested as a safer alternative than using manure in compost piles.
Some sources will declare that because coffee grounds contain nitrogen, a major plant nutrient, this makes coffee grounds a great fertilizer. But just because they contain nitrogen, it does not mean that coffee grounds are suitable for use as a fertilizer as is. Some studies have shown that using uncomposted coffee grounds in high concentrations around plants will actually stunt their growth. The grounds need to break down before they are truly a benefit to plants. The grounds need the help of microorganisms in order to release their nitrogen. Research has shown that using coffee grounds in compost piles tends to help the pile reach higher internal temperature. This is an important step in proper composting which helps kill weed seeds and pathogens that may be present in the pile.
A good rule of thumb for using coffee grounds in the landscape is to either mix them in with existing soil, or in a compost pile mix one part coffee grounds to one part leaves or other dry material plus one part grass clippings or other green material.
This is great information, but it doesn't answer my friends question about using actual coffee. I have yet to find any sort of research-based description of using actual coffee as fertilizer. There is some anecdotal evidence on gardening websites and blogs about using diluted coffee, but nothing that has been tested in a research setting.
That said, I think there could be some benefit to using coffee as a fertilizer, just based on what we know is in coffee itself. While coffee grounds have been shown to have a nearly neutral pH around 6.5- 6.8 (neutral is 7), coffee reportedly has a pH of anywhere from 5.2 to 6.9 depending on the type of coffee and how it is prepared. The lower the pH, the more acid, and a drop in one full pH unit is equal to a factor of 10. So if brewed coffee is a pH of 5.5 and the grounds are 6.5, the brewed coffee is 10 times as acidic as the grounds.
Most plants prefer to be slightly acidic to neutral, in the 5.8 to 7 range. Most municipal tap water is kept slightly alkaline (pH greater than 7) to inhibit microorganism growth. So I could see the acidity of coffee helping to keep the pH of a houseplant's soil in the desired slightly acidic pH range.
Nutritionally, brewed coffee does contain measurable amounts of magnesium and potassium, which are known plant nutrients. So it is plausible that there is some nutritional benefit to pouring that last bit of coffee on the plant at your desk.
That said, it would be a good idea to dilute the coffee you plan to pour on any defenseless plants in your office or home, especially if it is the super-concentrated bit that has sat in the pot and is a bit thick at the end of the day. Some anecdotal observers have advised using a solution that looks like weak tea. As far as I can see, really the only risk to using coffee as fertilizer is perhaps killing the poor plant with too much acidity! If any of you try this method, I'd love to hear your results. I plan on trying it myself.