You can manage your material stock level by filling the Min. level field in the Materials. This value determines minimum total area of stocks you want to keep on the storage to achieve maximum fluency of the production. If minimum stock level is lower than real stock level, it is immediately indicated in the grid highlighted in red color. This informs you, that you need to order and store additional material stocks.
Following images show state which is under minimum level of stocks in the Materials and also in the Plans.