hau Cesca,
Why rice? well the IMO techniques come from Korea and Korea has a lot of rice being grown every year.
Rice is also very starchy and when you cook it those starches gelatinize and then the bacteria can turn those starch molecules into sugars.
Rice eating organisms happen to be good for soil since they are mostly the right bacteria, fungi and molds which we want in our soil.
Mycorrhizae are not as species specific as many questionable sources will tell you they are.
Most of the really good (made by
mushroom people) mycorrhizae products will have a broad spectrum of species of both exo and endo mycorrhizae so that you will most certainly have several species that will work with your plants and trees.
Now, yes you can use oats, wheat, barley, even corn that has been cooked as gathering mediums for IMO purposes and they will work just fine.
Redhawk