Ask if it is Roundup Ready corn.. they just blanket spray the fields here in the US with glyphosate, as the corn has been GMO'ed to be immune from the chemical. That might be something you would like to consider as well. I am not advocating one way or the other.. just that you might think about it as it's your property.
I think it would be very surprising if the user of the land didn't in fact use any kind of fertiliser. Could they have spread some slurry (farmyard variety!) without your having noticed? The farmer who rents my land has given up his dairy herd - losing money with milk production - so has no need for grazing now. In recent years he has grown potatoes, then maize. I think the potatoes ended up as a well-known variety of frozen chips (!) and the maize is used by the few farmers round here who still keep livestock. The sileaged version of maize has replaced the grass sileage which was more commonly used in the past. Apparently the maize has higher nutrient content as well as the necessary "bulk" for cattle, and farmers nowadays are very careful to keep what they call "food to flesh" records, and I guess it's more economical too - with less of a problem ref harvesting in poor weather. In another part they are now growing winter barley which is also sileaged - a new-ish variety of barley apparently.
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I am not advocating one way or the other.. just that you might think about it as it's your property.