These are all vegetable materials and have their respective characteristics. The characters of sawdust differ more or less depending on the types of tree and care should be taken since some sawdust contains elements injurious to the growth of crops. This method is widely introduced in Canada and other regions where large-scale lumber industry exists. They are the material easiest to be obtained for Japanese farmers and can be disposed after use. Phosphorus and potassium all leached out from rice hulls to some extent, but their pH value is stable, showing slightly acid.
Rice hulls are porous, too.
As they don't absorb water readily at first, they should be matured for some time or mixed with Kuntan, perlite or other substrates having a good water retaining capacity. It was in 1946 that soilless culture was started in Japan. The United States forces in Japan began this culture in Chofu, Tokyo. The acreage was 20 hectares and said to be the largest scale in the world at that time. The system adopted was gravel culture.