Bark

Tree barks are a source of carbon and contain many beneficial soil building components: enzymes, minerals, active cultures of microbes including mycorrhizal species in sporulate forms. We only use certain species—mostly maple, yellow birch, pine, ash, fir, spruce, and poplar, each in their time and place. We do not use cedar, oak, or hemlock because of high tannin levels or potential plant toxicity. We know that yellow birch contains enzymes which stimulate germination and it smells of wintergreen.