A basic test tells you your soil's pH, which is a measure of how acidic or alkaline it is, on a scale from 0 to 14. Most ornamental plants, including the majority of annuals, perennials, and flowering shrubs, do best in soil with a pH between 6.0 and 7.0. In that range, nutrients that are naturally present in your soil are accessible to plant roots. When pH drifts too far in either direction, those nutrients get chemically "locked out" — meaning they're technically in the soil, but your plants can't absorb them. It's a bit like having a full pantry with the door stuck shut.
Acid-loving plants like hydrangeas, azaleas, rhododendrons, and hollies prefer a lower pH, generally in the 4.5–6.0 range. If you're growing any of these and your soil is too alkaline, they'll struggle to absorb iron and other key nutrients, which often shows up as yellowing leaves even on plants that are otherwise well cared for.
DIY soil test kits are widely available and easy to use. You can pick one up at Ace Hardware, Home Depot, or Lowe's, or order one online through Amazon. Follow the kit instructions, jot down your results, and bring those numbers into Step 4.