I was told to add coffee grounds to my blueberries. After I did the leaves on two of them turned dark red and they all fell off. Leaves have returned and loads of flowers, but the berries seem to not be developing well. It is also about a month after my berries should have been picked. I am in Largo Fl. Is this due to coffee grounds and why?
Reddening of the leaves of blueberry plants can be caused by a lack of phosphorus. Although coffee grounds contain phosphorus, they are also highly acidic. If your blueberry plant were already in acidic soil as it should have been, you could have made it even more acidic rendering phosphorus insoluble and unavailable to the roots.
Nitrogen (N), Potassium (K), and Sulfer (S) are major plant nutrients that appear to be less affected directly by soil pH than many others, but still are to some extent. Phosphorus (P), however, is directly affected. At alkaline pH values, greater than pH 7.5 for example, phosphate ions tend to react quickly with calcium (Ca) and magnesium (Mg) to form less soluble compounds. At acidic pH values, phosphate ions react with aluminum (Al) and iron (Fe) to again form less soluble compounds.
That's why it is suggested that if you're adding a lot of coffee grounds to your compost pile, you also add some agricultural lime.