Blue Spruce Trees
In your article, you suggest applying 12-12-1 fertilizer at various times! You do not say how much, and you don't give any sources for 12-12-1 fertilizer.
Look for a slow-release fertilizer labeled for pines or evergreens. Most soils have sufficient phosphorus although you need a soil test to confirm this. You are mostly looking for a nitrogen source and a bit of potassium. Several inches of rich compost would do. Here is an article from Univ of Minn Extension Service: https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/fertilizing-evergreens-conifers/