In ancient times, it was known that heavy applications of salt to soil reduced or completely destroyed the ability of the soil to support plant growth. The impact of sea water on plants may well have formed the foundation for legends that the Romans cemented their defeat of Carthage in the Punic Wars by sowing salt on the land. Episodes of sowing salt to render land inhospitable to agriculture are found in the Bible and other ancient sources. The use of salt water as a weed killer is therefore based on old wisdom and perceptions as well as modern science.
Reasons to Use Salt Water
As people have become more aware of the impacts of household and yard chemicals on their health, their interest in old methods of removing weeds has increased. There are recipes using salt, vinegar, soap and boiling water to control weeds.
Although some plants have considerable salt tolerance (to the point that they depend on a salt content in water and soil to combat fungal infections and other disease), the notion that salt makes soil more fertile usually stems from the uses plants can make of the chloride part of the compound. The Salt Institute points to plants that tolerate and use salt well, while suggesting how sodium may occasionally function as a potassium supplement. By and large, however, it notes that applications of sodium chloride to plants, whether in liquid or solid form, will result in root damage and stunted growth.
Applying salt water to soil as a weed killer affects both the weed plant and the surrounding soil. The major effect is dessication--a good thing for unwanted weed roots but bad for surrounding soil. It is this dessicating quality that lets us enjoy visits to ocean beaches, walking on sand, which is the ultimate dessicated soil.
Poor Uses of Salt Water Weed Killer
Avoid using salt water to kill weeds that are intermingled with plants you want to keep. Even careful application of salt water to weed roots is inexact science and excess salt will spread through soil to the roots of wanted plants. Further, excess salt can produce short-term bare spots; weeds will not grow, but neither will anything else, until time and moisture dilute and redistribute salt throughout a larger area of soil.
Good Uses of Salt Water Weed Killer
If you like the idea of simple non-commercial weed killers, maximize their effective impact by using them where nothing should be growing. Salt water, a strong vinegar solution or boiling water can be effective in driveway cracks, along cement planter edges and between bricks or other paving stones. Here you have fewer worries about overall soil condition, although you should check any potential drainage problems before applying large amounts of salt water to keep it from leaching into soil where it is not wanted.