Vincent, I think before you add supplements to your soil, you should get it tested. Here is why. Most northwest soils are very high in potassium already, low in calcium and magnesium, and very acidic. Some soils are high in phosphorus. However, different neighborhoods vary, and for some people, different parts of their property can vary.
My trees had leaves like that too. I thought, maybe, they need iron. I sent soil off to be tested. The result was, my soil was very acidic (5.0), low magnesium, low calcium, and iron many times more than the highest level in the test. I should not have been too surprised - there are rust colored flakes throughout the soil. My organic matter was quite high.
Based on that result, I added dolomite lime, and boosted magnesium with Epsom salts. The leaves greened up nicely and growth was better.
Compared to the cost, especially in time and effort, of your trees and other plants, the cost of a soil test is not bad at all. Less than one tree, in many cases. Maybe your soil doesn’t need anything, or maybe it needs more of one thing and not of another. It’s hard to predict, because of construction, land grading, agriculture, glacial activity, historic volcanic activity and geologic events, etc.
simplysoiltesting.com This is the place I used. They gave a detailed report, and recommendations. This is a Washington State company, I’m sure there are lots of other choices too.
Complete test is #32, the basic test is $16.
