Dry cereal straw comes in bales, which can vary in size and shape. As a general rule, grey straw is appropriate for biomass heat while yellow straw is not. Cereal straw should be weathered for at least a week before baling, and not baled at high density. Straw burns best if it is rained on first, then turned and left to dry up to 16% and stored under cover. This is called rain washing, which restores the potash back into the soil. Cereal straw from sandy soil burns better and with less ash than straw from clay soils.