NPK stands for nitrogen, phosphorus, and potassium, which are three essential nutrients required by plants for growth. NPK fertilizers are formulated to contain a specific ratio of these three nutrients to meet the specific needs of different crops.

Even if you do not see the letters N-P-K, but you see a set of three numbers, for example, 5-10-5, you can correctly assume it stands for 5% nitrogen, 10% phosphorus, and 5% potassium, always in that order. This fertilizer contains 20% nutrients; the remaining 80% is minor nutrients or fillers. Plants need about 16 nutrients; some they get from the air and water, and others are nutrients like iron, calcium, and chlorine from soil.

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Types of NPK Fertilizers

Balanced Fertilizers

A fertilizer listed as "10-10-10" is considered a balanced or "complete" fertilizer because its nitrogen, phosphorus, and potassium proportions are equal. Meanwhile, a fertilizer labeled "10-0-10" is an "incomplete fertilizer."

Incomplete Fertilizers

An incomplete fertilizer is not necessarily inferior to a complete fertilizer. Identifying the right fertilizer for your needs depends on a variety of circumstances. If your soil already has an excess of one of the three nutrients in NPK, you could be harming some of your plants by adding more of it to the soil—in this case, an incomplete or unbalanced fertilizer may be the right choice.

Organic Fertilizers

Organic fertilizer ingredients are soil nutrients from plant, animal, or mineral sources. Products labeled as "organic fertilizers" must specify which nutrients are organic and must be identified as synthetic or natural by percentage. For example, the label might read, "20 percent of nitrogen (6 percent synthetic, 14 percent organic)." Organic fertilizer sources might include manure, alfalfa meal, kelp, blood and bone meal, fish emulsion, cottonseed, sewage sludge, soft rock phosphate, and green sand.