This is a fairly good article that describes the activity of fertilzers during the Fall season in Ohio... It is unfortunate that the article specifies Months of the year... the reason this is unfortunate is because people in the colder zones will just look at the months,,, and w/out realizing it,,, they are doing exactly the opposite of what the article is saying... "... Ohio State University research found that the spring color of late-season-fertilized turf remained quite good until late May or early June, when the effects of nitrogen applied the previous fall began to wear off . A 0.75 to 1.0 lb. follow-up application of nitrogen is recommended at this time (late May-June) to maintain an acceptable level of turf quality throughout the summer period. It is important to remember that the nitrogen source for the fall applications be relatively independent of microbial activity to insure adequate nitrogen release due to colder air and soil temperatures in late fall.. This means that urea, sulfur-coated urea (SCU), IBDU, ammonium sulfate and the more high-activity methylene ureas are the ideal most efficient nitrogen sources for the late season applications. Although SCU and IBDU are referred to as controlled-release fertilizers, the rate at which nitrogen is released from these fertilizers is mainly dependent upon soil moisture level and not on the degree of microbial activity. The use of microbially-dependent (e.g., UF-types, polymer-coated ureas and natural organics ) nitrogen sources for late- season nitrogen applications may not elicit the desired fall/winter color response because they do not provide enough available nitrogen for plant uptake when temperatures are low. However, these latter slow-release nitrogen sources would be ideal for spring and summer use. ..."