Both pro- and anti-GMO labs used rats that were specially prone to tumors, as a way to provide an accelerated view of the toxicology effects of Roundup, so tumors like below were totally normal and expected, it's the variation of the number of tumors between exposed and non-exposed rats that counts.



Unfortunately, to this day, there are no long term independent studies on Roundup. The last (and only?) one (a French study by Seralini) became infamous and flawed because they used only 10 female and 10 male rats for each test, which was the same number (and the same species) of lab rats Monsanto used for their own (very short term) toxicology study for EU approval, but this number is too small to allow any statistically significant results, so it was basically 2 years of studies paid to the scientists, but completely wasted on the consumer and the funding partners. It didn't mean the results were wrong, they did find more tumors generally, but it needs confirmation with a study that use much more rats, and normal lab rats rather than tumor-prone rats might also work better if it's a long term study.

Glyphosate does degrade mostly into CO2, toxic formaldehyde and some other chemicals, and could migrate to the acquifers in case of heavy rains or phosphorus-rich soil, where it would be more stable and not degrade as quickly, but it usually binds to the soil where soil bacteria will degrade it quickly under normal conditions. It is a bit harmful to wildlife such as birds and mammals, but other papers say the opposite (the usual controversy) and that is hardly a concern on conventional fields anyway, which are no wildlife refuges. Unless your landlord wants to establish an organic farming mandate, glyphosate is probably less toxic than many other herbicides around. Or was, if resistant weeds require rotations with more toxic herbicides and higher doses.

Many studies focus on glyphosate, but Roundup is a special formulation with more than just glyphosate (41%), it for instance contains 15% of the surfactant POEA, which is 2-3 times more toxic than glyphosate, and the combination of the two seems even more toxic than the sum of the components, so only studies specifically about Roundup should be considered.

As for the effects on soil that your landlord is concerned about, it looks like he's right to be concerned, but then that's the case with every single pesticide. Couldn't find any positive link about it, so the better strategy is to read the negative ones and be aware of how Roundup works in order to avoid the negative side effects. That's more than most Roundup users do, and it will show your landlord that you are not a Monsanto shill but have done your homework. Just as bad to me is the fact that all these studies focus exclusively on how Roundup degrades on its own, but never how these compounds degrade and associate when different pesticides or chemical fertilizers are used in combination.

Here's a few of these soil links (never mind the alarmist titles and "scientists" used in place of "God" ;):

Scientist finding many negative impacts of Roundup Ready GM crops
The impacts of glyphosate (Round-UpR) on soil biology and micronutrients
Non-Target Effects of Glyphosate on Soil Microbes
Scientists Reveal Glyphosate Poisons Crops and Soil