Coffee grounds can play a valuable role in the garden where used correctly.
Spent coffee grounds are increasingly recommended by professionals and gardeners as a sustainable way to improve your garden soil and provide nutrients to your plants. Claims include improved soil structure, an ideal carbon to nitrogen ratio, improved fertility and provision of nitrogen. However, the scientific literature has not sufficiently assessed the impacts on soil and plant production.
Recent research conducted by Dr Stephen Livesley and Sarah Hardgrove from the University of Melbourne, has shown that fresh (uncomposted) spent coffee grounds, applied directly to gardens, can significantly decrease plant growth and development. This article reviews the case for using spent coffee in the garden and describes recent scientific findings and their implications.
The problem of coffee waste...
An estimated 6 million tons of spent coffee ground waste is produced annually worldwide, predominantly in cities. Australia’s contribution is estimated at 75,000 tonnes annually, and with coffee consumption rates in Australia increasing at a rate of 4.3% per year, the volume of waste is also set to increase. Using spent coffee grounds as an urban soil amendment provides an attractive and sustainable way to take advantage of this under-utilised urban waste product.
Spent coffee grounds can possibly provide similar plant growth and soil property benefits as other organic amendments such as manures, biochar, vermicasts and compost. These amendments provide nutrients (particularly nitrogen), increase plant growth, improve soil water and nutrient holding capacities, improve soil structure and water infiltration, increase buffering capacity against leaching of nitrates and changes in pH, increase biological activity and resilience against pathogens.
Under natural conditions, plants have adapted and evolved to the soil and climatic conditions of their local environment. As the soil properties and nutrient content varies, so too will plants vary in their nitrogen requirements and pH preferences for optimal growth. These differences affect our decisions about application rates of fertiliser for various crops. In addition, soil texture affects the movement of water and nutrients throughout the soil and their availability to plant roots. The aim of this recent research was to investigate the impacts on plants that vary in their nutrient requirements and pH preferences.
In a greenhouse pot trial, broccoli, radish, leek, viola and sunflower (chosen for their varied nutrient-pH preferences) were grown in sand, loam and sandy clay loam substrates. Four treatments were applied: no treatment control, spent coffee grounds (5% volume), fertiliser and spent coffee grounds plus fertiliser.
Concurrently, a field trial grew the same plants under six treatments: control, fertiliser, and spent coffee grounds at 2.5%, 5%, 10% and 20% volume application rates (in the upper 10cm of soil).
In the greenhouse trial, all plants grown in coffee-amended soil treatments showed poor growth compared to the control and fertiliser-amended soil treatments. The left hand picture below shows the five plants under the four treatments, from top to bottom: control, fertiliser; spent coffee grounds; spent coffee grounds plus fertiliser. In the field trial similar results were obtained.
Water holding capacity
The results for the greenhouse trial showed a general trend for increased water holding capacity for poorly structured soils, but less of an impact for well-structured soils. The results also suggested that these improvements take time to emerge. Other research also shows that benefits do not clearly emerge until 6-12 months after application, depending on species, soil type and temperature.
The pH of the soil was also tested to investigate whether the acidic nature of spent coffee grounds may be impacting plant growth. In the field trial, the pH of the soil amended with spent coffee grounds was similar to the pH amended with fertiliser. As such, pH from the added coffee could not adequately explain the plant growth response.
What explains this decreased plant growth?The two possible explanations for this plant growth response were biological nitrogen immobilisation (nitrogen drawdown) and phytotoxicity.
Nitrogen drawdown occurs when the amount of nitrogen required by decomposer microorganisms is higher than the amount of nitrogen that is available for plants from the soil. As organic matter decomposes, nitrogen is supplied to the plants via its roots from the surrounding soil water solution. However, the vast majority of the nitrogen in soils is not readily available to plants. In order for it to become available, it needs to be transformed by microorganisms within the soil water solution, a process known as mineralisation. Through mineralisation, organic nitrogen changes to inorganic nitrogen and becomes plant available. In its plant available form, as nitrate (NO3-) and ammonium (NH4+), it can be easily translocated throughout the plant and soil.
Conversely, as microorganisms require nitrogen as fuel for their own metabolic processes, they can also draw it out of the soil water solution, leaving the soil apparently deficient in nitrogen and, therefore, unavailable for plant uptake: a process known as biological immobilisation. These processes can operate dynamically as the chemical nature of the soil changes over time and new material is added. The carbon to nitrogen (C/N) ratio is a useful guide here, as an organic amendment with a higher C/N ratio will mineralise more slowly than one with a low C/N ratio and is more likely to lead to biological immobilisation as nitrogen is taken up by the microbes involved in decomposing the new large, carbon substrate pool. The C/N ratio of the coffee in this research was 23. It is generally accepted than a C/N ratio occurring within the range of 20-30 will not lead to microbial immobilisation.
A mineralisation study was also conducted as part of this research to test whether the plant growth differences were the result of nitrogen drawdown. The findings clearly showed that the nitrogen drawdown could not explain the plant growth response.
The most likely explanation for decreased plant growth is a toxic stress response as a result of the applied coffee grounds. The exact mechanism of phytotoxicity remains unclear, however caffeine, tannins, polyphenols and lignin content have all been implied in previous scientific research.
Should I add coffee grounds to my garden?
This research suggests that when fresh, uncomposted coffee is added to gardens at volume application rates of 2.5% and higher it will likely decrease all plant growth and development. As such, it is probably a better idea to add coffee grounds to your compost to allow for decomposition of toxic components, and for the improved water holding capacity benefits to emerge. It is recommended that no more than 20% volume of spent coffee grounds be added to your compost.
Alternatively, if you have a patch that is lying fallow for 6 months or more and you want to reduce the rate of weed growth, then fresh spent coffee grounds incorporated into the soil or applied as a thin layer of mulch, is a pretty good idea.