The "nulsituatie" is the reference or baseline determined in a soil survey, in order to be able to measure subsequent contamination of the soil. I'm not sure if "baseline condition" is the best translation (and it does sound a bit buzzword-y), but it's one possibility:

"Soil health can also be evaluated by establishing a baseline condition for the use-dependent properties (indicators). After a period of years, the use-dependent properties are measured again and compared to the baseline."

http://soils.usda.gov/technical/handbook/contents/part624.ht...

"Description of Degree of Impact ... Negligible - Very slight change from baseline condition. Change hardly discernible, approximating to a 'no change' condition"

http://www.mayococo.ie/EIS/Volume II - Main Text/Chapter 3.5...

You will also find "zero situation" on the Web in this context, but only in translations from Dutch!

So, your sentence could perhaps be rendered as something like:
"In the past various surveys have been carried out to determine the baseline condition of the soil."