Chemically, baking soda is a salt called sodium bicarbonate. As a salt, sodium bicarbonate is considered phytotoxic, even in small or very diluted portions. Baking soda naturally has phytotoxic effects on the crabgrass. The baking soda can cause it to dry out and burn up more easily in the sun. Typically, baking soda is either sprinkled directly onto the crabgrass or is mixed with water to dilute the sodium bicarbonate. It is then sprayed onto the crabgrass problem areas.
The sodium bicarbonate kills the crabgrass chemically with its alkaline composition. With an alkaline nature, the application of baking soda to plants causes the pH levels of the plants to change. As the sodium bicarbonate works its way into the plants, it throws off the pH balance of the plant to make it more acidic. The increased acidity of the plant causes it to not absorb vital macro nutrients like calcium. The lack of macro nutrients in the crabgrass causes it to wither, dry and eventually die.
The phytotoxicity of the baking soda makes it problematic to treat crabgrass in areas where there is also grass. In these cases, the deadly sodium bicarbonate must be further diluted and applied directly and carefully to the crabgrass areas only.