It's not recommended that you amend native soil before backfilling the hole. Use the same soil that came from the hole to backfill. If you amend the planting hole with organic material (peat or compost being the most common) it will quickly decompose, causing the tree to settle in the planting hole before establishing & end up being planted too deep in relation to surrounding grade. Trees need to be planted with basal flares above grade. This is especially important in clay.

Fertilizer is only necessary when a particular element is in short supply or unavailable for plant uptake, so w/o a soil test, it's impossible to make a recommendation. If you do decide to fertilize, it's not absolutely necessary to work the fertilizer into the soil, but it does help insure that the fertilizer quickly makes it to a part of the soil where it can be utilized by roots soon after application. It also helps minimize the possibility of being carried away in runoff, particularly in clay soils.

Al

Make that shrubs instead of trees, please.

Al

Most of the clay soils in the PNW are quite nutrient rich - their problems lay more with difficulty in digging or poor drainage. I would second Al's opinion (very commonly held, btw) that individual planting holes should not be amended, nor should any fertilizer be applied at planting time. Amending individual planting holes in clay soil runs a very real risk of creating pockets of poor drainage - bathtubs if you will - as the drainage benefits of the loose, amended soil are lost once the water percolates through and hits the more impermeable clay. Amending entire planting areas rather than individual holes will alleviate this problem.

Shrubs and trees established in the landscape typically need very little in additional fertilization. More fertilizer won't necessarily make them bigger and stronger but unneeded fertilizing can promote lush, leggy growth which is weak wooded and vulnerable to insect and disease problems. The performance of your plants are the best guide for applying additional fertilizers. If they display symptoms indicating a nutrient deficiency, soil tests will indicate what nutrients may be missing.

FWIW, in my well established PNW garden, I fertilize very sparingly, generally only those plants with a reputation for being heavy nutrient users like roses and clematis or container plantings. I do mulch with a good quality compost at least once a year (twice if I can manage it on my schedule) and the compost supplies necessary organic matter to the soils as well as a low grade replenishment of primary nutrients.

Many gardeners fall prey to the heavy advertising campaigns of the fertilizer manufacturers, swallowing their dogma that their garden will just not flourish unless they apply copious amounts of fertilizers (ie., Miracle Gro). This is simply not true - over fertilizing can produce more harm than not fertilizng at all. Keep in mind the old gardening axiom that if you feed and nourish the soil, you will feed and nourish the plants. Continue those applications of organic mulch. It will improve overall soil structure through the addition of organic matter, loosening the clay and improving drainage and create a beneficial environment for soil organisms to breakdown nutrients into a form plants can access.

Thanks for your thoughtful answers. I am happy to hear that being too busy to fertilize this summer was actually a good thing.

Your comments about not amending the backfill soil when planting shrubs and trees will come in handy this spring when I plant a few trees and a few more shrubs.

Thanks