Any mathematical geniuses amongst us. I am wanting to know roughly how many litres of soil are in a kilogram ? and I'm talking your standard potting mix here. Much appreciated to the person with the answer to my question. I am planting in 21kg plastic buckets and intend on using Canna Tera Professional potting soil which comes in a 50 litre bag. I need to fill 10 of these buckets so I need to know how many litres I will need to do so.