[AmityvilleHams]

Even without obvious fertilizers, don't use soil. It is not worth the risk for so many reasons - stick to paper only and remove the soil completely through a full and very thorough cage clean. It'll be temporary stress, but it is far better to add a small amount of stress now than have to deal with an endless list of serious problems that can come from using soil.

Cat grass shouldn't be live in the enclosure in any form, substrate or otherwise, so that point is still very relevant. You could technically clip off a few very small pieces from a plant grown outside the cage but it wouldn't be all that beneficial for hamsters.

When it comes to soil in the cage and products with similar issues, even with the best ventilation possible it still isn't safe. I'd have to say we as a forum have been the most honest and accurate place especially recently in terms of how unsafe soil actually is and to say the least I would not trust other sources saying it is safe in any way!