Coffee Grounds. - Do They Benefit The Soil ? - An Experiment

A couple of months ago I went around the coffee shops in town to collect their used grounds.  I had two ideas, one was to make lampshades and the other was to do an experiment to see if seeds grew and germinated in it.  

The lampshades were a total disaster as the family really didn’t like the smell when the bulb warmed the material so they have been composted. The experiment to see if plants grew in the coffee threw up some surprising results though.

I made up seven different soil mixes:

·         100% coffee grounds

·         100% garden compost

·         100% potting compost

·         50/50% garden and potting compost

·         50/50% potting compost and coffee grounds

·         50/50% garden compost and coffee grounds

·         Thirds of coffee, potting compost and garden compost

I put these mixes into the same sized containers and watered them together.  My seed of choice was wheat as I keep a big bag of it for making the wheatgrass and it was to hand.   I put the same amount of seed into each container and covered until germinated.

My expectations for the experiment were that the garden compost would do the best overall as it contained more slow release nutrients. Then the shop bought potting compost would see an initial healthy boost then drop off a bit as the nutrients are depleted.  Then I was thinking the seeds sown in the coffee mixes would suffer as the coffee grounds are very acidic, sometimes too acidic for even azaleas and blueberries to grow in. I thought the seeds in the 100% coffee grounds would just wither and die.

Observations

After the seeds germinated I noticed the wheat in the 100% coffee were sprouting but instead of the seeds settling into the grounds, the roots were actually lifting the seeds off of the surface as though they were trying to pull away. The 50/50 mix of coffee and garden compost was doing the same but then I noticed the seeds in the 50/50 garden and potting mix were also prone to this. The seeds settled in the other mixes so no conclusive seed rejection there.

 I must confess here that I was actually hoping the coffee mixes would fail dramatically as I was under the impression coffee shops were just fobbing us off with toxic waste for our gardens to save them paying to dispose of it.  I was proved right with the 100% coffee mix, from the onset the seeds were drier and the shoots were smaller and more fragile than the rest.  This carried on until the end of the experiment and my suspicions were confirmed, plants don’t like to grow in just coffee grounds. But who would think of growing purely in coffee?  No-one you’d mix the coffee with other things you had in the garden and treat it as a supplement. So it was the rest of the results that were surprising.  The healthiest plants by far after three weeks of growing were in the 50/50 mix of potting compost and coffee grounds closely followed by the other mixes but with the potting compost just behind the 100% coffee. The second best growth came from the third mix of coffee, potting compost and garden compost. 

For experiments to be accurate and consistent I’ll probably have to run the tests a few times to see if they are consistent. I probably will never get around to doing this again so thought I would just leave the plants in the post and see how they got on.  After two months of winter weather the plants are all still alive in the pots and are indistinguishable from their neighbours. You couldn’t pick any that look healthier than the others.  This makes me thing that in the experiment, a lot of the nutrition the small plants had actually came from the seeds themselves, not the potting mixtures. This casts a new light on the results as the 100% coffee could just have been an initial inhibitor for the nutrition and eased off as time went on. 

Conclusion

After three months of trials and observations I have concluded that adding coffee grounds to your garden compost pile could actually be a benefit, but maybe it will be a bit too strong to grow plants in on its own. It’s about variety so mixing the grounds with other garden and kitchen waste, newspapers, cardboard and other woody materials in the composter will give us a balanced medium for growing. I wouldn’t see any issues with scattering the grounds around acid loving plants either.

The results then are pretty inconclusive as the energy from the seeds could have influenced the results.  I’d need to do a few more tests to see if there’s a pattern. No more lampshades though.