Your soil is probably already perfect for wildflowers! Unless your soil is actually sterile, which is rare, it's recommended that you use your soil just as you find it. The test is simple: If anything is growing in the area — even if it's just grasses or weeds — the area should support wildflowers without concern. If you can grow weeds, you can grow wildflowers!
There are some exceptions. For instance, if the planting area had suffered a chemical spill or contamination, if you’re planting on a new construction site where the topsoil was removed, or if the soil is heavily compacted due to drought, heavy traffic, overuse, or neglect. Again, a simple way to assess the soil is by identifying what’s currently growing in it: where nothing grows, you may need to amend your soil, or find a new site for the meadow. Wildflowers are quite adaptable but will not grow on a sterile site. If your soil is heavily compacted clay, or very sandy, adding some organic matter can help improve soil texture.
Wildflowers, as we see on every roadside, are extremely adaptable and do well in poor soils.
Compost or other organic fertilizers are not necessary for growing wildflowers, and in fact, they may make the soil overly rich, inviting weeds and stifling the growth of some wildflowers.
The only absolute requirement is good drainage. Choose a place where water does not stand for longer than one hour after a rainfall.