Re salt - yes in sufficient quantity it will ruin the soil but don't know what sufficient quantity is. In the good old days when nothing went to waste my Mother always added some salt to the water the eggs were boiled in.I think the theory was if the eggs cracked the salt would seal them and they would not boil out of the shell. The salt water was then taken out and poured on the cracks in the paths to prevent weeds (this was offcourse the childrens job). Far as I remember it worked.

Re soapy water as a wetting agent - very interesting point. Some plants just don't respond well to weedkiller as the liquid just runs off them. Thinking about mares tails and perennial sweet peas ( which I always have in danger of invading the road and paths). Maybe if they were first wet with soapy water then sprayed with weed killer it would be more effective. Anybody tried this? I will and will let you know how I get on. Could be on to something here.