This article is a bit disturbing, because I have been composting coffee grounds for years with seemingly good results. Then again, I wasn’t doing an actual controlled experiment.

Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth

There are frequent anecdotal recommendations for the use of locally produced spent coffee grounds in urban agriculture and gardens, either through direct soil application or after composting with other urban organic wastes. This study investigates the scientific basis for direct application of spent coffee grounds (SCG) and the influence of different: i) plant pH and nitrogen preferences, ii) soil types, and iii) application rates. We specifically consider impacts upon plant growth, soil hydrology and nitrogen transformation processes.

We grew five horticultural plants (broccoli, leek, radish, viola and sunflower) in sandy, sandy clay loam and loam soils, with and without SCG and fertilizer amendments. The same horticultural plants were grown in the field with 0, 2.5, 5, 10 and 25% SCG amendment rates. Plant biomass growth was related to soil pH, soil moisture, nitrogen concentration and net mineralization, as was weed growth after harvesting.

All horticultural plants grew poorly in response to SCG, regardless of soil type and fertiliser addition. Increasing SCG amendment significantly increased soil water holding capacity, but also decreased horticultural plant growth and subsequent weed growth. There was evidence of nitrate immobilization with SCG amendment. Growth suppression was not explained by soil pH change, or nitrogen availability, so is more likely due to phytotoxic effects.

Fresh SCG should not be used as a soil amendment in ‘closed loop’ urban food production systems without consideration of potential growth suppression. Further research is required to determine the optimal composting conditions for SCG and blends with other organic wastes.

I’m not a very scientific composter. I just throw stuff together in an old recycling bin, then when it fills up I move it to another bin, and then a third (despite a few raised eyebrows from neighbors, I rescued all these bins on my old street after they were abandoned for at least a week, and filled with litter and dog crap). At that point it’s usually a nice crumbly organic mix that I can use. I don’t follow any of the rules. I don’t mix green and brown items in the right proportions, I let it get wet and dry according to the rain, I compost perennial weeds, and my pile almost certainly doesn’t get hot enough. I probably wouldn’t give the stuff away because there are weeds in there, but I don’t worry about putting it back on my garden, which is where the weeds came from. If they want to keep pulling carbon dioxide out of the air and turning it into organic matter for me, I am fine with that. I would rather be outside pulling weeds than doing almost anything inside.