Roundup herbicide treats some weeds and grasses after they have emerged. The main ingredient in Roundup is glyphosate in isopropylamine salt form. Certain substances effectively neutralise Roundup in the sprayer or in the soil.
If you use Roundup to treat an area that will later be used for planting, you must neutralise the Roundup with water beforehand. The Roundup label indicates that half an inch of water, through natural or man-made means, will effectively neutralise the product.
Do not apply Roundup to the wet dirt area around the weeds and grasses you want to treat, because it will not absorb through the dirt to the root system. Additionally, microbes in the dirt will eventually break down the product. Furthermore, if dirt gets into the sprayer, the dirt will begin to neutralise the product before you even spray it.
Bleach or Ammonia
Use diluted bleach or ammonia to clean sprayers that were used to spray Roundup. Do not use the two products together as they will form chlorine gas, which is toxic.