Fertilizer classifications include complete and incomplete, broad terms that refer to nutrients contained in the fertilizer.
Complete fertilizers include all three primary nutrients (nitrogen, phosphorus, and potassium); incomplete fertilizers contain only one or two. A example of a complete fertilizer is one with the NPK numbers 5–10–5, showing that all three primary nutrients are present in the formula. Most complete fertilizers are blends of several different chemical fertilizers, and are sold under a brand name. Many are formulated for a particular use, such as citrus trees or azaleas.
Different formulas of complete fertilizer can be used for feeding any type of plant in any situation. They can be used for lawns, houseplants, vegetables, flowers, and trees in any type of soil or container mix. Complete fertilizers are usually in the form of pellets or granules for easy spreading.
An incomplete fertilizer supplies only one or two of the three primary nutrients. It is usually sold by its chemical name as a commodity, rather than as a branded fertilizer. Superphosphate is an example of an incomplete fertilizer. Its formula is 0–20–0, making it an excellent source of phosphorus. Incomplete fertilizer are usually used for specific purposes, such as providing extra phosphorus for lawn seedlings (which need high levels of phosphorus for their first few weeks of life). Incomplete fertilizer are usually less expensive than complete fertilizer, and take more skill to use properly. Incomplete fertilizers are often soluble crystals that look like table salt.
The term “balanced fertilizer” is frequently used in garden literature. Sometimes “complete fertilizer” is intended—a fertilizer that contains substantial portions of the 3 primary nutrients.
At other times, “balanced” refers to a complete fertilizer that contains approximately equal parts of the 3 primary nutrients, such as 5–5–5. This latter recommendation doesn’t make much horticultural sense. Plants don’t use the nutrients in equal amounts; most plants use about equal amounts of nitrogen and potassium and about 1/5 as much phosphorus, but that proportion varies from plant to plant, and with different stages in a plant’s life. Also, different soils contain differing amounts of mineral nutrients. Rich clay soils may need nothing more than occasional nitrogen. In contrast, fast-draining container mixes require substantial amounts of all 3 primary nutrients.