The Flora Series, like any hydroponic nutrient, can be used in soil just fine. You just have to understand the difference between using fertilizers in soil versus hydro is all. Soil normally has a lot of nutrients of its own so you don't want to feed at full strength or every watering.
Plus, you can get a lot of salt build-up in soil that isn't as much a problem in hydroponics though it still is a concern with certain media. Aeroponics or DWC, for example, are less likely to have salt build-up issues. So whether you're using the Flora Series, some other hydroponic nutrient, or traditional soil fertilizers, you have to be conscientious of whether you're building up residual nutrients to potentially problematic levels. If so, you need to flush them out periodically.
The Flora Series is a decent all-around nutrient system but it's not what I'd recommend, particularly in soil. While it will work perfectly well (as will any hydroponic nutrient) the more properly balanced and chelated nutrients that use more highly soluble ingredients will not only feed your plants more effectively, but be less likely to create those problematic build-ups.
If you like the Flora Series I recommend trying Advanced Nutrients' higher quality 3 part nutrient system: Grow, Micro & Bloom. You use it just like the Flora but it's better buffered, chelated, and basically just a nicer product.