Although nitrogen, potassium, and phosphorus are the most critical nutritional elements for healthy plant growth, there is a wide selection of additive nutrients that can also help increase plant health and productivity. Additive nutrients are not chemically compatible with typical nutrient formulas, and so are usually stored in separate containers. Some additives, such as hormones or plant growth regulators are kept separate because they have a dosage rate that changes throughout the plant's growth cycle, such as additives that are only used during blooming.