Ferns are low-maintenance plants that require very little fertilizer. Too much puts their health at risk, especially if the fertilizer contains excessive amounts of nutrients like nitrogen. Only a carefully chosen fertilizer, applied sparingly, promotes the fern's health and longevity.
Ferns require a balanced fertilizer with an equal ratio of nitrogen, phosphorus and potassium. The three numbers – such as 10-10-10 or 14-14-14 – indicate the levels of these nutrients in the fertilizer.
Because ferns are sensitive to overfertilizing, light application is safest. North Dakota State University Extension Service horticulturalist Ron Smith recommends fertilizing ferns frequently with small amounts of fertilizer for optimal health. Three to four light applications a year should suffice, though increase to five times a year if the fern yellows.
To further ensure that ferns receive the nutrients they need without the stress of overfertilization, Clemson University Cooperative Extension recommends a slow-release formula. Make sure the fertilizer is water soluble, without excess nitrogen that may overindulge the fern.