well if the soil is too dense compact potatoes won't form. they love sandy soft loamy soil that is deep but no more than a couple feet deep. I found potatoes prefer to spread out in this type of soil. I got a huge harvest out of soil that was about a foot and a half deep in an 8 foot by 8 foot bed. still picking potatoes out from last year. they also like the roots to be cool and if it gets too warm this will have a negative effect. too much manure/compost and they get scabby, too much water and they will be hallow or water filled.