Recycling food leftovers in order to create less general waste is a good practice which more and more people are adopting. We have often addressed this topic, giving you some handy advice on how to use leftover coffee for sweet and savory preparations, or how to recycle the mocha residues . Today we would like to explore this topic further, to find out if it really is a good idea to use coffee grounds as compost in our gardens or plant pots which we keep on our balconies, which plants we can use them for and in what way..
Coffee grounds as compost: what properties do they have?
Coffee grounds are used as compost for gardening and are considered a precious resource, although there is not yet much research to demonstrate their properties or support this type of use. Before trying out this practice, it is important to remember that coffee grounds increase the level of nitrogen if added to compost, but they don’t do the same thing – or at least not as quickly – if placed directly in the soil. However, there are many advantages to using them: for example they help with drainage, they improve the aeration and help micro-organisms which boost the growth of plants as well as attracting earthworms.
Which plants love coffee grounds?
Coffee grounds work on the soil by lowering its pH value, in other words making it more acid: this is true, though, only when we are dealing with fresh coffee grounds.
Before using the coffee grounds as compost, it is important to know which plants thrive in soil enriched with these leftovers. Acid loving plants, for example, are perfectly suited to this type of earth. This includes for example: Azaleas, lilies, Camelias, Gardenias and Hydrangeas.
Other species, however, do not appreciate coffee-based compost at all: tomatoes, for example, even though they like more acid soil, do not respond well to this treatment. Carrots and radishes on the other hand do, especially if the coffee grounds are added to the soil at the time of planting.
Do coffee grounds improve the growth of your plants?
So, is it a good idea to use coffee grounds as compost ? The answer is “not always”. Indeed, a study carried out by the University of Melbourne, highlighted that using this type of fresh waste, ie. not composted, can actually have a negative effect on the growth of the plants, at least on those used in the research (broccoli, radishes, leaks, sunflowers). The crops reported on in the study were sown in a greenhouse, in different types of soil, and treated in four different ways:
● without any treatment or fertilizer;
● with fresh coffee grounds (5% of the total soil volume);
● with fertilizer;
● with fertilizer and coffee grounds.
Other plants were sown in open fields, without any treatments, with fertilizers, and with four different quantities of coffee grounds.
Both the vegetables in the greenhouse and those out in the open fields showed very slow growth in the soils where coffee grounds had been added. According to the researchers, the reasons could be connected to two factors: the nitrogen content and the phytotoxicity.
We have already touched on the fact that fresh coffee grounds increase the nitrogen content of the soil, but this does not mean that it is readily available for the plants: in fact the absorption happens only after the transformation of the nitrogen by the micro-organisms present. Thus, the actual availability would not be so high.
Scientists attribute the difficulty of the plants in growing to phytotoxicity: the presence of coffee in the soil causes the activation of stress mechanisms, but the precise dynamics of this process are as yet unclear.
Coffee Grounds: it’s better to use them after composting them
In conclusion, we can state that it is possible to use coffee grounds as compost with good results, but it is preferable to compost them first and wait for them to be transformed by the micro-organisms present in them. Only after that, will you get the best results from using them as compost in your gardening, without interfering with the growth of your plants.
Have you ever used coffee grounds as compost in your garden? What kind of results did you get?