The soil temperatures will soon be reaching the 53-55 F. degree mark. At this point the grass plants will be able to start using the fertilizer more effectively. In the northwest this usually coincides with the advent of Spring. The day length is at least equal to the night length and therefore this warming happens rather consistently. If you have been using a fertilizer sparingly throughout the cool months I am sure have seen some results. This is due to the shallow surface roots being able to warm instantly due to the solar energy. Now we are talking about the top 2″ and deeper – the effective root zone of the grass plant. By applying fertilizer when the entire plant can respond you are maximizing your dollars spent and your time to do it.