Roundup is pretty good in killing weeds and small vegatation, but the tougher the brush and vine, (poison ivy, virginia creeper, etc), the higher concentration you have to use. If you have small trees, cut them off and use an old paint brush to paint the top of the stump, especially around the bark. Apply liberally.
With Roundup, be careful of spraying on a windy day as spray will drift and land on things you don't want to kill.
2-4-d is really strong stuff. It will kill everything, so if there are things you want to save, be very careful using it. It is hazardous to people as well so be very careful in how you handle it, (i.e. long pants, boots, long sleeve shirt and gloves. If you get ANY on you, wash yourself right away with soap and water. Change clothes when your done and stay out of area until product dries. Keep animals away from area till product is dry as well. If there is water near by, (i.e. pond or creek) beware of overspray.
The problem you'll encounter with all of the above is any seeds that are in the ground. Roundup and 2-4-d only kill the living plants but do not touch seeds. Keep in mind that with Roundup, you can till and plant the ground within seven days for most applications. Therefore, if there are any weed seeds, you've just opened a new patch for them to grow and flourish. Many weed seeds can lie in the ground for up to seven years before they sprout and grow. Therefore, you might have to plan on several applications to totally "kill" the ground. For best results, wait until daytime temps reach the 70's.
The salt idea is not bad at all. It depends on what you want to happen to this area of ground. If you want it to remain clear for a long time and have no plans on planting anything there, (like a garden or flowerbed), it really might be your best and safest way to go. It's Biblical as well as in the Old Testament, when a town was defeated, the victors spread salt on the city to symbolize it was "dead."
You might also consider a "pre-emergent." This product forms a "coating" on the ground that kills weeds and vines as they emerge. The only problem is rain will wash the coating away, so you have to plan on a rain free period of time, (seven to ten days) which can be hard to guess depending on your weather patterns and time of the year. (For example, here in Florida, now is the time for a pre-emergent as the rainy season is still a few months off).
And finally, the most enviormentally friendly way is a good old fashioned hoe and shovel. A grubbing axe can do wonders. Keep all tools sharp. A handy cooler with your favorite beverage makes the job a lot easier
With all of the above, staying with it will ensure best results. A one time spray and walk away will not do the job. You could always buy a goat - they'll eat everything to the ground and provide free fertilizer. Be careful if you get a billy. Do not bend over in front of him.
