Get composting the more the better , you want a good ratio of mixture in there , but carrots do not like fresh manure aparantly , the better the mix and the more you turn the quicker it rots , and you need to water it to , you can buy in top soil if need be ,you can get a ton for about £55 delivered , or if your buying by the bag , the best and finest top soil I've come across is from wickes there own brand ,