I got huge spores in the substrate without organic matter. My substrate was only the mixture of coarse sand and field soil (3:1) and left the open sunlight with an indicator plant. regular irrigation was given at one day interval with normal water. I believe you will also get sporulation. Wish you success.
if arbuscular mycorrhiza is what you are after, my experiments have shown that a sand only +5-10% field soil works best, although it is harsh on the plants. I would also suggest using 1/4 peat/sand + ~10% field soil or go with 1/3 field soil and sand as our colleague suggested. This ensures that you shouldn't have issues with plant survival.