The role of potassium is not as well understood, but is important for overall plant development. For this reason, these are the three nutrients that we almost always find present in commercial fertilizers. Commercial fertilizers, whether organic or inorganic, always have three numbers printed on the bag or container. The rest of the product is an inert carrier material. A complete fertilizer will have a number for all three nutrients.
Some fertilizers are incomplete, that is, they contain only one or two of the nutrients.
Inorganic or chemical fertilizers usually have a higher analysis than most organic fertilizers and are less expensive. The nutrients in chemical fertilizers are in a form that is readily available to plants. Organic fertilizers contain nutrients in forms that must be chemically changed in the soil before the plants can use them. These nutrients, although not immediately available, are usable to the plants over a longer period of time. Some chemical fertilizers will contain nutrients in both readily available and slow-release forms.