I have evaluated Cd toxicity in different soils by root-growth tests and tried to calculate IC50 using BioDataFit. Some of my response curves are obviously not complete, they do not reach the "bottom". It is in soils where Cd is effectively immobilised and its toxicity is decreased. However, in these cases, the response, as far as I have determined it, is linear, doesn't look like the "proper" beginning of a dose-response curve. Is it possible that the response curve changes from the typical shape to a linear shape when the effect of the inhibitor is modified by the interaction with soil components, and IC50 cannot be calculated?
curve-fitting, IC50 calculation