The internet is flooded with how-to and gardening tip videos and articles promising to make your growing easy. Among the videos that have gathered millions of views are ones showing the uses of raw kitchen waste in the garden. Sounds like a win-win situation, where you reduce the output of waste in your home while feeding your plants. Fertilizer also isn’t that cheap, so there’s that.
Another appealing aspect of these tips is the ease of the presented solutions – you just mix your kitchen scraps with some soil, add water, and voilà; the plant has everything it needs for growth. While finding shortcuts and easy solutions got us, humans, far up the evolutionary ladder, it is not always the best way to go. Especially when it comes to plant cultivation. To avoid falling into the lazy trap, we should take those “an easy way to” tips with a grain of doubt.
Most of the time, when easy solutions promise great success, those promises are not delivered. Even worse, they can create bigger problems. So, to help you separate the wheat from the chaff, we’ll explain today each of the most commonly used kitchen waste materials and whether they can really bring benefits to your plants. This section is only about raw waste, so if you want to find out more about composting, you can do it here.
Passionate coffee drinkers enjoy their brew multiple times a day, and the amount of waste created by the habit can get quite hefty over time. Instead of throwing away the spent grounds, some use them to fertilize their plants. How beneficial is this practice really? Various growers have various experiences, so there is conflicting information. Some encourage this practice, while others jump on to list the negatives. The truth lies somewhere in the middle, and it is mostly about how you apply it and how much.
Coffee has an average pH value of around 5, making it acidic. Acidity is often pointed out as one of the main disadvantages of using spent coffee grounds as fertilizer. However, after you make your morning brew, most of the acidity actually ends up in the brew. Spent grounds have a higher pH value compared to raw or roasted coffee – about 6.5, which is on the slightly acidic part of the scale. Of course, the pH can vary with different types of coffee and brewing methods, but either way, in low amounts, spent coffee shouldn’t be so acidic that it vastly changes the pH of the soil.
Note: Using fresh coffee grounds as fertilizer is not recommended because they can increase the pH value of the soil and cause caffeine toxicity.
Spent coffee grounds are an excellent source of nitrogen (N). Unlike most traditional nitrogen fertilizers, the grounds do not immediately release the available nitrogen to plants. For this element to become available, the grounds first need to be processed by microbes. As such, it provides the plants with this primary nutrient over a longer period of time. However, there is a hidden danger in this process. During the degradation process, the microbes use a portion of the available nitrogen from the soil. This investment can pay out in the future, but only with a moderate application. If there are too many grounds in the soil, the high microbial activity can cause nitrogen deficiency.
Coffee is also a good source of two other primary macronutrients – phosphorus (P) and potassium (K). Unlike nitrogen, these two elements are present in their available form so that the plants can absorb them rather quickly. The spent grounds are also a good source of trace elements – calcium (Ca), magnesium (Mg), copper (Cu), manganese (Mn), zinc (Zn), and iron (Fe).
Plants have developed a wide array of chemical weaponry to fight against pests, diseases, herbivores, and other plants. Alkaloids are just one group in the colorful palette of organic compounds plants produce, but they are one of the most interesting ones, at least pharmacologically speaking. Caffeine falls into this group and is probably one of the most famous members, aside from nicotine, cocaine, and morphine. The plants produce it to defend themselves against various enemies. It displays antimicrobial activity, is highly toxic for many insects, and its bitter taste discourages larger herbivores from ingesting too much plant material.
Although caffeine can be highly beneficial to plants, its presence in soil is not desirable. Due to its antimicrobial activity, it can negatively affect the soil microbiome and decrease fertility. While a good portion of caffeine is released during the brewing process, the remaining concentration can still affect the beneficial organisms. This problem is resolved by re-brewing or simply by not adding too much and not mixing it with the soil.
How to apply coffee grounds
To make the best out of your spent coffee grounds and avoid the cons, spread them on top of the soil, like mulch. This way, the microbes in the soil won’t be overly busy with degradation and thus won’t use as much nitrogen. Also, caffeine is less likely to negatively affect the soil microbiota.
However, to get all the benefits of coffee grounds, it is very important how much you add. Distributing a thick layer around the plant will significantly slow water penetration. The physical qualities of the grounds are such that they quickly absorb moisture, swell, and become compact, creating a layer that obstructs the movement of water deeper into the soil. The grounds also tend to retain moisture once they absorb it. A thick, moist layer of nutrient-rich material invites molds and pathogenic organisms to spread and thrive. So, the best way to use the spent coffee grounds directly is just to scatter them around the plants. As you water the plant, the grounds will break down and release the nutrients. The water will also carry the nutrients in deeper layers of the soil.
However, it should be pointed out that the best way to use spent coffee grounds is to throw it into the compost pile. The process of composting refines the grounds, releases unavailable nutrients and reduces the harmful effects of caffeine. Some also like to mix them with mulch. The presence of bark, hay, and other larger components in the mulch prevents the grounds from clustering and creating that impenetrable layer we mentioned earlier.
Remember: Do not incorporate the spent coffee grounds into the soil. This application method can cause nutrient deficiency and caffeine toxicity.
Fruit and vegetable scraps
Incorporating raw scraps from fruits and vegetables into the soil is generally not a good practice for several reasons. The main one is the availability of nutrients in the scraps. Fruit peels, salad remains, onion flakes, and vegetable pedicles contain a significant amount of nutrients, but they attract unwanted molds due to their high water content. Also, the nutrients are not available to plants until microbes decompose them first. This process can take a while, and practically yields no benefit. Microbial activity isn’t intense enough to degrade the scraps in a short enough time for the plants to get their nutrients.
To avoid the problem of nutrient unavailability, some gardeners put their scraps, most commonly banana peels, in water and leave them to sit there for a few days. The idea is to release the nutrients from the peels and get enriched liquid that is used for watering. Although it can bring some benefits to the plants, this method should never be the primary way of supplying plants with nutrients. It is better to use it as a boost during the key phases of plant growth.
Adding raw food scraps directly to the soil is not recommended. The moisture and an abundant source of nutrients attract various pathogenic microorganisms. Some scraps can also carry harmful organisms on them and introduce these to the previously healthy soil. So, the best way to utilize fruit and veggie scraps is to put them in your compost and let microbes and worms do the job before introducing them to your plants. The composting process will eliminate the pathogens and transform the nutrients into the available form.
Another common kitchen scrap used in gardening is eggshells, and their good reputation is well-deserved without a doubt. Eggshells have high calcium content and can be used almost like lime in terms of moderating soil acidity. Aside from soil pH regulation, they can also provide food to plants. When used as fertilizer, eggshells improve the plant’s growth and development by providing them with various nutrients.
After studying the eggshells of the world’s favorite poultry – chickens, researchers have found that they contain on average:
- 38% calcium
- 8.5% magnesium
- 0.3% sodium
- 0.1% potassium
- trace amounts of iron (0.0014%), zinc (0.001%), and copper (0.00006%)
Note: Nutrient content can vary depending on the diet of the chickens.
See that high calcium content but don’t let it fool you – eggshells are not very eager to let go of it. They do not decompose quickly. Simply chopping them with mortar and pestle and mixing the flakes with the soil won’t do much good. Eggshells supply the plants with the necessary elements through a slow, consistent, long-term release of nutrients, so they cannot be used as a quick treatment for calcium deficiency.
How to apply eggshells
The first step is to disinfect the material. Wash and microwave, or boil the eggshells for 3-5 minutes. This way, you’ll prevent potential pathogenic organisms from contaminating the soil and harming your plants. If you used the boiling method for disinfection, leave the eggshells to dry completely before the next step.
After the eggshells are disinfected and dry, put them in a blender and chop them until they get almost a floury consistency. Shredding them to tiny speckles speeds up the microbial degradation process significantly.
You can mix the powder directly with the substrate, add it to compost, or easily store it in a jar if you don’t need it right away. Our feathered friends also love the calcium boost from eggshells, so you can also add them to your bird feeder.
- Chalker-Scott L. (2016). Washington State University. Using coffee grounds in gardens and landscapes.
- Ajala, E.O., Eletta, O.A.A., Ajala, M.A., Oyeniyi S.K. (2018). Characterization and Evaluation of Chicken Eggshell for Use as a Bio-Resource.