If you are doing a soil grow then yes you can use worm castings. By cheers I take it your not in the US and I do not know the L to gallon conversion LOL, but I add 2 big handfulls of WC to a 5 gallon bucket of water and mix well then water the plants every 2 weeks. You can also add 1 handfull to a 3 gallon pot along with a teaspoon of bloodmeal when you pot. I use 3 gallon pots that are inside of a 5 gallon bucket.WC are about the best nute you can add to a soil grow because they are loaded with microbes.The microbes break the soil down so the plants can take up nutes easy. I would not use wc with seedlings they can burn the plant but keep them around and use when the plant has atleast 4 true nodes then use the wc.