Not Coffee Grounds: The Kitchen Scrap You Could Be Using For Thriving Leafy Greens

Coffee grounds have had enough time in the spotlight — often touted as the go-to kitchen scrap for gardeners to recycle, coffee grounds are far from the only ingredient that's worth saving for your plants when you cook. If you live in a household where dried lentils often feature in meals, the water they leave behind after soaking is another kitchen scrap you can reuse as garden fertilizer, particularly when it comes to heavy feeders like leafy greens.

A 2024 research project by students in Oman investigated whether lentil water could act as a natural liquid fertilizer for tomato plants, and it found that the plant treated with lentil water grew taller, developed a thicker stem, and produced greener leaves than the one that wasn't. This may work because, as the students discovered, nutrients naturally leach out into the water as the lentils soak — including nitrogen, phosphorus, and potassium. The seasoned gardeners reading this are already nodding their heads because they know what this trio of nutrients is: the primary macronutrients that plants need for healthy growth. They're the nutrients behind the NPK acronym written on fertilizers (N for nitrogen, P for phosphorus, and K for potassium). While the project focused on tomato plants, leafy greens are another good candidate for this trick because they also need a high amount of nutrients to support their growth.

Of all the vegetables that you may be growing in the garden, leafy greens are one group that could really benefit from the occasional lentil water soak. They are considered heavy feeders, so they need a steady supply of nutrients throughout the growing season. In fact, successfully growing anything leafy such as lettuce, spinach, kale, Swiss chard, arugula, and bok choy is dependent on giving the plants a particularly good supply of nitrogen, the nutrient that allows them to keep producing their tasty leaves, making this a more practical use for your lentil water than simply pouring it down the sink. It might even help leafy greens that grow in late summer extend their harvest. 

Despite the fact that your leafy greens will likely appreciate the nutrients in lentil water, you should still keep your expectations realistic. Research on using lentil water as a fertilizer is still very limited, so more studies are needed before its effectiveness can be confirmed across different plants and growing conditions. And be mindful not to use lentil water in your garden if salt, seasonings, or stock have been added, as the excess sodium can damage your plants' roots. Still, little habits like this can give everyday kitchen waste a second purpose whilst offering plants a little extra nourishment, so it's certainly a trick worth trying.