I'm not sure what your local market is like, but since pretty much all common houseplants and garden plants need plenty of nutrients, most soil mixes have things added to them.
The key here is that you watch your plants carefully; if their health continues to decline, I would assume that the potting mix is to blame and find an alternative. You can buy nutrient-free peat online, or you could try looking around locally for nutrient-free ingredients. I have heard of people being able to find long fiber sphagnum moss and perlite which are free of nutrients at their local store.
Also, although I have not tried this myself, you could try washing the nutrients out of the soil. The method is explained here:
http://www.growsundews.com/rinsing_p...us_plants.html
Here's a thread on the subject for more information:
http://icps.proboards.com/index.cgi?...ay&thread=4305
If you do go with washing the soil, you will need a lot of pure water, either distilled, reverse osmosis, or rainwater.