A purpose-built growing mix for potatoes and sweet potatoes in fabric grow bags, engineered around the two things container potatoes need most: excellent drainage and the right pH.
 
Most potting soils are too alkaline for potatoes. Scab — that rough, corky skin disease that ruins an otherwise healthy harvest — is dramatically worse above pH 6.0 and nearly absent below it. This mix is formulated to hold a pH of 5.0–6.0, the sweet spot where potatoes thrive, and scab doesn't. Gypsum provides the calcium potatoes need for healthy cell development without raising pH. The result is a calcium-sufficient, appropriately acidic mix that grows clean-skinned, healthy tubers.
 
Start a potato bag one-third full, plant your seed potato, and add mix progressively as the vines grow 6-8", burying the stem to produce tubers along its entire length. That means the mix needs to stay loose and open through repeated additions and disturbance. Coco coir holds moisture evenly without compacting. Fine pumice maintains drainage and aeration from the surface to the bottom of the bag. Coarse sand adds weight and grit. Worm castings contribute gentle, steady fertility.
 
Hilling method — how to fill your bag:
Start with the bag one-third full
Plant seed potato 4–6" deep
As vines grow to 6–8" above the soil surface, add mix to bury all but the top few inches of vine
Repeat until the bag is full
At harvest, tip the bag or open the harvest window and work from the bottom up
 
Works best with:
15-gallon harvest window bag — staged harvesting without disturbing the whole plant
20-gallon fabric bag — extra lateral room for sweet potatoes and larger indeterminate varieties
 
Variety note: Indeterminate potato varieties produce tubers along the buried stem and respond best to hilling. Determinate varieties set tubers all at once and don't benefit as much from hilling.
 
Coverage: One 5-gallon bag fills approximately one-third of a 15-gallon grow bag. Plan on two to three additional 5-gallon bags for hilling as the season progresses.
 
Reuse: After harvest, thoroughly remove all tuber debris, as any remaining potato material can harbor disease. Refresh with fresh worm castings and a tablespoon of Complete Organic Fertilizer per 5 gallons. Check pH before replanting; if the mix has drifted above 6.0 after a season, work in a small amount of elemental sulfur to bring it back down. The pumice and sand do not need replacing.