Although not necessarily required for earthbag building, stabilizing soil improves water resistance and compressive strength, and speeds drying time. Since 1978, Istanbul Technical University has been researching a technique called alker technology or alker construction. Basically it’s just soil stabilized with lime and calcined gypsum (plaster of Paris). The following summary is taken from their websites.
Traditional earthen construction: preprocessing, grinding, sieving, setting the soil aside, 30% clay
Alker construction: preprocessing is not necessary, 10% clay
Ingredient/% by Weight
Traditional earth construction
15-21 days strengthening
20 min. strengthening
no curing area
no water sprinkling
no rain protection
No building system is perfect. Although more moisture resistant than plain earth, Alker stabilized soil will absorb moisture, so your design must take this into account. Either raise the lower wall (ex: higher foundation), add waterproofing sealant and/or protect walls with wide roof overhangs. Note the rapid set time — you have to place the soil within 15 minutes. Plus, there is the added cost of materials.
Note the similarity to Cast Earth at Wikipedia that explains the cast earth formula is proprietary knowledge, but the alker method is not. The major advantage of cast earth is greater working time that enables continuous pouring of an entire house, and then rapid strengthening so forms can be removed the same day.