Question
Would one very old landscape tie beside a garden patch leach chemicals into the soil in high enough concentrations to make the veggies grown there dangerous?

Answer
Dear Petra:

Railroad ties are soaked in creosote which is a known carcinogen therefore, even though this is an "old landscape tie" you risk the possibility of garden vegetables planted near it being contaminated.  If you must use this particular tie, then consider this: lining the garden bed with a heavy mil plastic, like bamboo barrier, making sure there is adequate overlap at the seams and with the plastic extending below the ties several inches, like a curtain. 6-8 inches is ideal. This should minimize any leaching directly into the planting bed and the root zone of the plants but unfortunately won't stop leaching into the surrounding soil.

Regards,

Mack Jean
AllExperts
Master Gardener
Tennessee