Too much salt can, in fact, kill beneficial microbes. Sodium chloride (commonly called salt) is used as a preservative in food because of its ability to prevent bacterial growth. Too much salt, however, can be just as harmful to plants as too little; indeed, it can be lethal. When the soil becomes saturated with water and salt does not get washed away, the excess salt can kill off the microscopic organisms that are essential for healthy soil. Salt blocks water absorption by crops' roots, which can cause them to die from dehydration. This is especially problematic for small fruits and vegetables that cannot be harvested before they collapse from lack of water. Salt also promotes the growth of fungi and other pathogens that can destroy crops. In addition, when salt accumulates in soil at levels higher than what's recommended by your local agricultural department, this can lead to salinization- the permanent alteration of soil quality due to excessive sodium concentrations- which can have negative effects on future crops. The amount of salt needed to kill off beneficial microbes and harm plants varies depending on the type of soil and crop, but generally, you should avoid adding more than 1/4 cup of salt per quart of water up until about two weeks before harvest.