Question: My St. Augustine seems to lack nitrogen, yet, when I add it, the grass gets worse and worse.
Answer: That sounds like gray leaf spot. If you see pin-head-sized gray-brown lesions on the blades and runners, that's your culprit. It is accelerated by nitrogen fertilizer during the summertime. Do not fertilize St. Augustine between June 15 and September 15 if this disease is present. Turf fungicides will help, but reducing the nitrogen is the biggest single solution.