Black Walnut tree losing some yellow leafs
A soil test would tell you precisely. The test results tell you how much to apply based on your soil type (sandy, loam, or clay), the crop you are growing, in this case walnuts, and other factors.
Here are two Extension articles that discuss walnut tree nutrition:
The following link is one recommendation based on inches of trunk diameter, and average loamy soil. For a 42 inch diameter tree you would apply 25 lbs actual nitrogen which is equivalent to 114lbs 22-0-0 fertilizer, spread evenly around the root zone, and away from the trunk by a couple feet. Note that this changes slightly based on sandy or clay soil type. Go to the lighter recommendation if you aren’t sure of the diameter or soil type.
The second article goes into great detail for growing walnuts. It states “Depending upon fertilizer application method, time of application, and irrigation uniformity, about 150- 200 lbs actual N (per acre) should be applied annually as a maintenance program.” You would figure the square footage under your walnut tree and reduce the amount to that equivalent( 1 acre=43560sq ft).
If you decide to soil test, you can purchase a Soil Test Self mailer at this link or at Oakland County Extension office.
It takes about 2-3 weeks from receipt at the lab to get your test results and recommendation back. Thanks for using our service.
One other thing- these recommendations are for walnut growers, not homeowners with lawns growing around their trees. Lawns cannot be fertilized with more than 1 lb actual nitrogen per 1000 square feet at one time. For 22-0-0 that is 4.5lbs per 1000sq ft under the tree.