There are so many factors that are involved that make it very complex. When you fertilize in the Spring, your not doing it for that Fall, your doing it for the next year or years down the road. I would actually argue that fertilizing a tree that Spring will encourage more vegetative growth instead of seed setting. It can help certainly, but is the extent to which it would help worth the work, etc? No one really knows the answer to that question. There are far to many other factors that can influence the mast crop that a proper test is just not realistic, IMO. I do fertilize my fruit trees however, but your talking about a much smaller root system on them and not a tree that has been established in a location for 80 years or more sending down deep tap roots. I guess my philosophy is that it is much more beneficial to simply get SUNLIGHT to oaks than to be fertlizing. But I do think we all know that an oak in a lawn getting fertlizer, lime, and sunlight will often times be a great producer.