What is found in soil but can be an actual pollutant if dumped on the soil? Then think of the many ways this item is used by humankind?
Soil could be alkaline or acidic and might consist of naturally occurring salts/minerals as well as acids depending upon the region. Salt could be one of the answer as Benjamin explained above, soil in the agricultural region with close vicinity to sea is generally alkaline and unsuitable for many plants. We use vinegar and fat in cooking for our food but dumping those in soil pollutes it. However, there are some natural acids and oils that are normally present in small amount in soil.
Hope this helps.