Suppose there exists a magic-based civilization. Their mining techniques are medieval and their understanding of organic chemistry and biology are on the Enlightenment era level. However, they are capable of creating portals, so they can establish stable supply of water and any needed materials.
This civilization has access to large supplies of niter (potassium nitrate), chalk, and peat. Will a combination of those, with minuscule additions of fishbone meal for phosphorus and steppe soil for microbial contamination purposes, be enough to provide an adequate starting base for a soil desert greening project?