If you are under the impression that dirt is dirt, you are mistaken. This is especially true if you are planning on growing a garden. Having good top soil is going to make a big difference in your garden. There is a balance that needs to be reached. Having soil that is too sandy will drain well, but it will also dry out very quickly. This will cause plants to wilt. If you have soil that is clay rich it will hold the water very well. Unfortunately that is all it will do is hold the water, there is little drainage. This will cause your plants to become waterlogged and will not grow well. The ideal top soil to have is a sandy loam. This will hold the water so that the plants will stay hydrated and allow for proper drainage.
Sometimes by simply looking at your top soil you can tell what type of soil you have. If you are not sure scoop up a hand full and squeeze it in your fist very tightly. Watch the soil as you open your hand again. If it maintains its compact form, you likely have clay soil. If at the other extreme it crumbles and falls through your fingers quickly you have sandy soil. The ideal scenario is for the soil to hold its shape for a minute and slowly crumble into moist large chunks, chunks that will break apart easily.
If you are not lucky enough to have sandy loam top soil, you can amend your soil so that you do. The extreme method of getting the ideal top soil is to have the old top soil dug up and hauled away. Then you could replace it with good top soil. You would likely be able to find someone to deliver a truck load or two.
You really don’t need to replace your top soil. You will be able to work with what is there to turn it into the ideal top soil. The goal is to get the soil to where your plants are thriving. Adding organic material to any type of soil is beneficial. The condition of your soil will determine the outcome of your garden.