Optimum soil temperatures for cool season root growth occur within the 50 to 64 F range.  However, root growth function continues until soil temperatures reach freezing (32 F). Late season fertilization treatments can still be made, but the use of proportionally more quick release nitrogen would be recommended.  Once soil temperatures fall below 50 F, microbial activity drastically drops off, which could influence the release rates of slow-release fertilizers that depend on microbial (temperature) breakdown for release. Slow release nitrogen sources can still be used but those that are not temperature dependent, like slow release carriers that release by dissolving in the presence of water might be more efficient.

In general this is the best time for late season fertilization when the turf is still green but not growing (no clippings removed).  The rate is usually around 1 pound actual nitrogen per 1000 sq.ft. for cool season turfgrasses