[chopin]

Just use fertilizer. I am pretty sure that will work, too. If not, congratulations, you know now how to defeat the guard.

[mseebach]

The system would need to be resilient to small amounts of fertiliser, otherwise it would trigger on anybody who's been on a farm or done gardening recently. Heck, I use fertiliser when watering plants in my flat. That's not a practical level of false positives.