Compost is often called "black gold," in the garden, and there is one underutilized household staple that can help you make more of it: coffee grounds. Instead of throwing your coffee grounds in the trash, add them to your compost pile.
Coffee grounds benefit compost in more than one way. Just like animal manure, they are rich in nitrogen (about 2 percent in volume, which is considered high) but if you are worried about possible pathogens in manure, coffee grounds are a safer option. Coffee grounds also help maintain a high temperature in the compost, between 135 degrees and 155 degrees F, which potentially kills pathogens and weed seeds.
This article walks you through the best practice of adding coffee grounds to your compost or garden and explains why adding non-composted coffee grounds directly to the soil can do more harm than good.
Acidity of Coffee Grounds
Contrary to popular belief, coffee grounds are not acidic. After brewing, which leached most of the acid out of the grounds, they are close to pH neutral, between 6.5 and 6.8. By working a moderate amount of coffee grounds into the soil, or sprinkling them on top of the soil, you do not alter the soil pH.
How to Use Coffee Grounds Directly in Soil
When you add coffee grounds directly to the soil, just like manure, they don’t act as a nitrogen fertilizer although they are rich in nitrogen. Coffee grounds encourage the growth of soil microbes, which use up nitrogen while they are breaking down the coffee grounds. So temporarily, coffee grounds create a nitrogen shortage. This leaves you with several options:
Work coffee grounds into the soil. To make up for the loss of nitrogen, add nitrogen fertilizer together with the coffee grounds; that way the plants don’t get short-changed on nitrogen.
If you don’t want to add extra nitrogen, compost the coffee grounds before adding them to the soil
Or, mix only a small amount of damp coffee grounds into the soil at any given time and let them fully break down for a few months before adding more. Adding too many uncomposted coffee grounds, about one-fourth the volume, can lead to poor germination and stunted plant growth.
You can also use coffee grounds as mulch. Sprinkle a thin layer of coffee grounds on the soil surface, it should not be thick so it does not create an impenetrable barrier. Also, give them a few months to break down before adding more.
How to Use Coffee Grounds in Your Compost Pile or Bin
Composting coffee grounds is ideal because it gives them time to break down with the rest of the organic material in your compost pile.
Incorporate Coffee Grounds Into Compost
Add the coffee grounds to the compost. They are considered a green composting material so make sure add enough brown composting materials such as leaves to the pile to maintain the recommended ratio of one part green material to two to three parts brown materials. Overall, coffee grounds should not make up more than one-fifth of your compost pile.
Add Coffee Filters, Too
If you are using unbleached coffee filters, you can add those to the compost, too. The filters are a brown composting material and break down fairly quickly.
Turn Your Compost
Turn your compost at the usual intervals. If you don’t turn it, no worries, you still get great compost at the end, it just takes longer.
How to Use Coffee Grounds in a Bokashi Composter
Bokashi composting is different from traditional composting—it is a fermentation process that takes place in a closed container with an air-tight lid under anaerobic conditions. But, you can still add coffee grounds to your Bokashi compost bucket. Here’s how it’s done:
Add Coffee Grounds Into the Compost
Add your coffee grounds and filter to the Bokashi composter. There is no required ratio for green and brown composting materials.
Add Bokashi Compost Accelerator
Add some Bokashi compost accelerator (bran) and close the composter. There are no limits as to how much bran to add, the more the better as the bran contains effective microorganisms (EM) similar to the naturally occurring microorganisms in soil that break down organic material. Bokashi bran is available from several manufacturers.
Coffee Grounds Perk up Compost Pile with Nitrogen. Oregon State University.