My reason is for potted citrus trees. It seems that 5-1-1 is pushed as the best but I found that top soil from my gared mix 50% with compost from my compost pile worked really well. In the winter I could go 2-3 months without watering but I used a vacuum cleaner to pull fresh air through the roots daily when the soil was just watered ans soaking wet and as the water was used by the tree I vacuumed it less often until I submerged my tree in a laundry tub till soaked. It was when my trees started hitting 30 gallon pots that I went to 3-2-1.
