There is no definitive answer when it comes to the best fertilizer for blue spruce trees. However, there are a few things to keep in mind that will help you choose the right fertilizer for your tree.
First, consider the type of soil you have. If you have sandy soil, you'll need a fertilizer that is high in phosphorus. This will help encourage root growth. If you have clay soil, on the other hand, you'll want a fertilizer that is high in nitrogen. This will help promote leaf growth.
Second, think about what time of year it is. If it's early spring, you'll want to use a slow-release fertilizer so that the nutrients are released over time and don't burn the roots of your tree. In late spring or summer, however, you can use a quick-release fertilizer as the tree's roots are more developed and can handle the sudden influx of nutrients.
Finally, make sure to read the labels on any fertilizers you're considering using. Some fertilizers are specifically formulated for blue spruce trees while others are meant for general use with all types of trees. Choose one that meets your tree's specific needs and follow the directions carefully so that you don't end up doing more harm than good!