While some gardeners believe adding coffee grounds to soil can help cucumber plants thrive, the evidence suggests this is not an effective approach:
Coffee grounds provide only trace amounts of nutrients that cucumbers need to grow. They are not a significant source of nitrogen, phosphorus, potassium or other essential nutrients.
Once coffee grounds are brewed, they are only mildly acidic (pH 6.5-6.8) or even close to neutral. Unbrewed grounds are more acidic, but most people don't use fresh unused coffee. The acidity is not enough to significantly alter soil pH for cucumbers.
In very specific controlled research conditions, coffee grounds have been shown to suppress some fungal diseases on cucumbers. However, replicating those conditions in a home garden setting is unlikely.
Better Ways to Fertilize Cucumbers Instead of relying on coffee grounds, experts recommend using these methods to provide nutrients for healthy cucumber plants:
Amend soil with well-aged compost or worm castings before planting cucumbers. This provides organic matter and nutrients.
Feed plants with compost tea or a side-dressing of slow-release organic fertilizer halfway through the growing season.