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. (C) 2016 Elsevier GmbH. All rights reserved.