I really think it's false advertising to call this product "soil based". You have to look at the actual strains listed, and when you do, they are all either Lactobacillus sp. or Bifidobacterium sp. As far as I can tell from my research, NONE of these reside in the soil. Therefore, it's silly to call them soil-based.
If you look at the strains listed for Prescript-Assist, practically all of them do live in the soil. Interestingly, the one that I credit as the most potent, Bacteroides, does not live in the soil, but it's still not available in any other probiotic that I know of.
So be wary when you read "soil-based". It's just another marketing term that companies are using while including the identical strains as a regular probiotic.