by Kim Dieter

Created on: April 16, 2010   Last Updated: July 29, 2010

Readily available in many households, coffee houses and restaurants, coffee grounds are the byproduct of a good cup of coffee.  Instead of adding waste to the garbage can and landfill, coffee grounds may have another use as a fertilizer or soil amendment for plants.  Fertilizers or amendments improve plant growth by adding required plant nutrients to the soil.  There are several factors to consider when using coffee grounds as a plant fertilizer. 

In a study commissioned by Sunset magazine, Starbucks coffee grounds were analyzed for use as a soil fertilizer.  The study found that coffee grounds added significant amounts of phosphorus, potassium, magnesium and copper to the soil. The coffee grounds were added to the soil in quantities up to 35% of the soil volume.  The grounds were rototilled to a depth of 6 to 8 inches.  All four nutrients are required plant nutrients and by adding coffee grounds, additional fertilizers to provide these nutrients may not be required.  Coffee grounds also contain nitrogen.  The nitrogen will slowly release into the soil.  However, additional fertilizer to provide nitrogen will probably be required.

The study found an approximate fertilizer grade of 2.28-.06-.6. for coffee grounds. This grade shows the percent nitrogen (N), phosphorus (P) and potassium (K) in the fertilizer.  The coffee grounds contained approximately 2.28% nitrogen, .06% phosphorus and .6% potassium.  Another well known organic fertilizer, steer manure, has an approximate grade of 2.1-.3-.3.  Coffee grounds provide more potassium than steer manure along with a similar amount of nitrogen.

The study commissioned by Sunset magazine found a pH level of 6.2 for coffee grounds.  Coffee grounds are slightly acidic. On a pH scale, 7 is neutral and numbers lower than 7 indicate an acid.  However, other studies have found a pH as low as 5.  If coffee grounds are added to alkaline soil, soil with a pH greater than 7,  the grounds may beneficially lower the pH.  Many plants prefer a pH of about 7.  However, if coffee grounds are added to an acidic soil, the pH may drop well below 7.  Plants such as blueberries and azaleas grow well in a low pH soil but many other plants do not.   Alkaline soils are more commonly found in the western United States while acidic soils are more common in the eastern United States.

Coffee grounds are often free.  They are a natural, organic material and make a good fertilizer to provide four required plant nutrients.  The grounds should be tilled into the soil.  However, coffee grounds may increase soil acidity and in some cases this will be detrimental to some plants. 

 References:

 http://www.sunset.com/garden/earth-friendly/starbuck s-coffee-compost-test-00400000016986/

 http://www.ces.ncsu.edu/depts/hort/consumer/factshee ts/trees-new/text/fertilizing_tables.html

http://www.ars.wisc.edu/spooner/factsheets/fertilize rs.pdf

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