Those tests usually use chemical fertilizer potency as the benchmark.

If magnesium is deficient then the leaves will turn yellow. Your tree will benefit from a dose of Epsom salts which is magnesium.

And reducing watering to less often but deeper will help too.

Use Tui Epsom Salts Magnesium Sulphate. Also make sure your tree is free from weeds which will compete for water and nutrients and mulch around it to help conserve soil moisture as well as keeping weeds down. Jun 19, Yellow leaves are often a sign of needing iron chelates – I water that onto the leaves. If you have alkaline soil – any iron chelates in the normal.

Jun 12, Not Enough Nitrogen.

It helps ailing trees a lot.

Yellow leaves are often an indicator of insufficient nitrogen. Test soil around the root base for NPK (Nitrogen, Phosphorus, and Potassium) with a simple home test kit, available online and at some hardware stores.