That's technically called "phytotoxicity": broadcast fertilization with very nitrogen-rich products will produce leaf scorch, plant stunting, necrosis and a host of other ill effects. If you relieve yourself in a bucket seeded with potatoes, you are basically drenching them in an urea-rich solution.
Different crops have different susceptibility to high nitrogen levels at different growth stages, for example corn gets far more susceptible as it grows and nitrogen fertilization should be carefully administered after V3. Also urea moves in the ground according to moisture "bands" in the potting medium (in your case) and will naturally concentrate causing scorch when applied in excess, with high draining soils such as sand being the worst.
So stop relieving yourself in the potato bucket.
PS: very broadly speaking (soil, weather etc conditions need to be taken into account) potatoes require NPK at a 3-5-5 ratio. Being tubers too much nitrogen can be counterproductive by stimulating too much foliar growth.
PPS: according to Jenny Uglow in her Little History of Gardening, in old European country estates gardeners and other workers were expected to relieve themselves in the compost heap, not in the flower beds.
