Soil Testing
The most effective way to ensure your plants are getting all the nutrients they need is by testing your soil. A soil test is used to determine two things: the available nutrients in the soil, and the soil pH. The soil pH is a measurement of how acidic or alkaline the soil is. With the information that the soil test provides, you can amend your soil as needed to give your plants the necessary nutrients to grow.
It is most effective to perform a soil test a few weeks before you plan to plant. This helps make sure the results remain up to date in accuracy. You can purchase a soil testing kit with directions included online or at your local garden center.