How much water?

How much water should be givin to a Plant in a 20L pail of Dirt?

I've always heard a good rule of thumb is until you get about 10-15% runoff.

keep pouring until you see steady runoff coming out. That is the only way to prevent salts from building up in the soil. There is no definitive answer to your question, as it's going to vary on soil conditions. The general rule of thumb is to water until you see a steady stream of run off coming out of the bottom of the pot. Stop at that time, Excess salts from nutrients will be washed out with run off, each time you water if you follow this method.

Uncle Ben wrote a great piece on watering techniques. If you are looking for more information, scare it up in the search bar.

i am on my first grow and i found it easier to just water till you see runoff. Ive learned that the important thing is not how much you water at once but how often you water. water your plant and after u water it pick up the container. make a mentle note of how much it weighs. after a few days when your container is almost dry u will notice a big difference in weight. they get pretty light when they need water!!! GOOD LUCK!!!