Our black gold: Spent coffee grounds
Until a few years ago, we used to collect horse manure from the local racetrack for our composting needs. While it worked reasonably well, it came with a few too many issues: it smelled awful, and more importantly, we had no real idea what was in it. Racing horses are often medicated, and traces of those substances may have been passed on to our soil. So, we changed course.
Enter Josh, aka “Wormboy”. He’s a local legend who collects spent coffee grounds from nearby cafés and delivers them to us at the garden - for free!!! We now use only two ingredients in our compost: our own green garden waste and Josh's coffee grounds.
Whenever Josh drops off a fresh batch, usually around 200 litres, we mix it with roughly 500 litres (or more) of garden clippings, leaves, and other green waste. The exact ratio isn’t set in stone, but the result is consistently excellent. Within a day or two of tossing the pile, it heats up to 60°C and often more. That’s hot enough to kill off weed seeds and pathogens, and after about 6 to 8 weeks, we’ve got a finished, dark, rich compost ready for our garden beds.
Coffee grounds are surprisingly good for compost. Their NPK (nitrogen-phosphorus-potassium) value sits at roughly 2-1-3, which gives us a compost blend that leans towards nitrogen (Academic reference). Nitrogen is a nutrient that’s often in short supply because it can be lost from soil into the air or washed away by rain. Phosphorus (P), on the other hand, tends to stay put and accumulate. Too much P can actually block plants from taking up other key nutrients like zinc. So, the fact that coffee grounds provide a good nitrogen boost without piling on phosphorus is a win for our soil health.
On top of all that, using coffee grounds is a small but meaningful step in building a circular economy. Instead of sending waste to landfill, we’re turning it into something valuable. Yes, the grounds often arrive looking a bit mouldy, but that’s completely fine. The fungi just mean the breakdown process has already begun. It’s smart to wear a face mask when handling them because of the risk of spore inhalation. But the compost doesn’t mind, and neither do our plants.
Below are a few photos from a recent delivery. Big thanks again to Josh.
