Last week we discussed the benefits and how to use spent coffee grounds in the garden. Today I though we could see if cold coffee can be of use in the garden.
If your like me Sunday afternoon after brewing a pot of organic coffee at home and often have some left over. Can your garden get some benefit from the cold black coffee like it does from spent grounds?
There are two benefits people commonly use cold coffee for. The first is as a weak organic fertilizer and the second is to help adjust the pH around acid loving plants like blueberries.
In order to test this we sent samples kindly provided to us by Starbucks to Maxxam Analytical for analysis. We had both espresso and coffee tested for the available NPK, trace elements and finally the pH
Based on these results coffee has an NPK of 0.000848 — 0.0099 — 0.0976
while espresso has an NPK of 0.00806 — 0.0665 — 0.8433
These represent the immediately available NPK. Other than the Potassium in the espresso these are relatively low numbers. You will get more nutrients if you use compost or mulches made up of coffee grounds and comfrey with fall leaves or wood chips.
Both coffee and espresso do have low levels of trace elements that are are essential and beneficial to plant growth including Boron, Calcium, Copper, Magnesium, Manganese and Sulphur.
In summary the NPK and trace elements are in very low concentrations so can you use coffee to lower the pH of soil?
pH is a scale where 7 denotes a neutral like pure water 14 is very basic and 0 is extremely acidic.
Some plants like blueberries, roses, and grapes enjoy soil that has a pH range of 4.5 -6.5.
The reported pH of the coffee was 5.10 and the espresso has a reported pH of 5.31. Coffee can have a range of pH including some sources reporting lower pH. however the average cup of coffee has a pH similar to our results.
Similarly unpolluted rain water often has a pH of around 5.0 to 5.5. As the water moves through the air it absorbs carbon dioxide creating carbonic acid. With all the rain over thousands of years one would assume most soils would have a pH around 5.0? Well no soils have a buffering capacity that neutralizes these the weak acid in rain water fairly quickly.
With a pH of 5.10 cold coffee has very weak acids in it. Most soils especially if they have clay content have enough buffering capacity to quickly neutralize the acids in cold coffee.
Coffee is essentially an hot water extract and the acidic compounds found in it are high water solubility. If the weak acids are not neutralization right away they will likely move away from the area you wish to influence as water moves through the soil.
Based on these results using cold coffee in your garden likely has limited benefits. Any changes to the pH of your soil will be limited and will not persist long enough to be of benefit to acid loving plants. Cold coffee also only adds small amount of NPK and trace elements.
Even if you used a longer coffee extraction you’re likely not going to get significantly higher concentrations of nutrients or acids in the coffee. The nutrients in coffee grounds are tied up in complex molecules requiring months to break down with the assistance of the beneficial organisms in your soil.
Although free and local resources like pine needles and used coffee grounds have pH reported near 3.5 and 5.4 respectively they don’t transfer that acidity to the soil effectively enough to lower the pH.
In the absence of other options I have had to purchase a product in order to keep the pH around my blueberries and other acid loving plants in their optimal range. I have been using organic sulphur. Beneficial organisms use the sulphur to slowly make sulphuric acid keeping the pH of the soil lower for longer at a much more stable rate.
It is important to slowly lower the pH over time. If you change the pH too fast you can damage the soil, beneficial organisms and the plants themselves. I follow the recommended application rate of my product. If you would like more information on increasing soil acidity visit Robert Pavlis’ website addressing common Garden Myths.
Next time I am going to take a look at a fantastic plant that you can use as a fertilizer; Comfrey.
I would like to give a special thanks to Starbucks and Maxxam analytical for helping with the samples and their analysis.
Do Coffee Grounds Acidify Soil?
Do Pine Needles Acidity Soil?
Acid loving plants:
Essential and Beneficial Elements:
Starbucks Coffee Nutrition Information:
Acids General Information:
Lowering the pH using Sulfur
Understanding how to adjust soil pH