With all cautions the first step I suggest is doing a negative control in the original soil, potted with activated charcoal, AC. AC absorbs chemicals, but has got some direct effect thet increases plant growth, and requires  pota with a single species or specific combinations, as in refs of http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2007.02360.x/full, As far allium it is mainly applied for genoxicity of water soluble compounds  by cytology of metaphases. For other methods you may see http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.404.7099&rep=rep1&type=pdf

Dear Prof. Guido Bongi, Thank you very much for your suggestions and information with very interesting and important articles.