It might be that you are witnessing sulfur (S) deficiency. Southeastern U.S. soils are prone to low soil S and many fields suffered from S deficiency this past winter. If your fertilizers have not included any S, then your cool-season grass yields may suffer. Deficiency is most observable during cool, wet weather.

Often yellowing is identified as N deficiency, but the yellowing (chlorosis) in younger growth is more likely shortage of S.  N deficiency is typically more noticeable on older growth.   Additionally, S deficiency gives the field a somewhat yellow patchy appearance, whereas an N deficiency will tend to look more uniform throughout the field (Fig. 1).

Consider applying 15 to 20 lb S/ac for cool-season grasses when soil S test values are below 35 lb/ac. Since S is relatively inexpensive and IFAS does not currently test for soil S, consider requesting some S in your fertilizer blend as a safety measure for your cool-season grasses. Common S sources include ammonium sulfate, potassium sulfate, gypsum, and sul-po-mag. Elemental S is not a quick enough source of sulfate if plant S deficiency is already suspect.