The pH of your soil is one of the most important things to the success of your garden. If the pH is either too high or too low plants cannot thrive, mainly due to lack of nutrient availability. Some plants, like blueberries, enjoy a lower pH than most plants and will not thrive in alkaline soils, while other plants, like goji berries, prefer a soil with a higher pH.
You can easily determine the pH of your soil with a soil testing kit. Once you know your soil pH you can make adjustments based on your plants' needs.