(News article for July 3, 2021; edited. This article is part of a series. Click to see the introductory article and ones about nitrogen fertilizers; calcium, magnesium, and sulfur fertilizers; and micronutrients.)

Phosphorus and potassium are two of the plant macronutrients, or elements that occur in plants in relatively large concentrations.

Information about concentrations of phosphorus and potassium, as well as nitrogen, in fertilizers are provided in the fertilizer grade. For example, 15-9-12 is 15% nitrogen, 9% “phosphate” (P₂O₅), and 12% “potash” (K₂O).

You’ll see that the amounts of phosphorus and potassium in fertilizer are not expressed as percentages of elemental phosphorus (P) and potassium (K) but as percentages of P₂O₅ and K₂O. Fertilizers don’t actually contain the compounds P₂O₅ and K₂O, but this is how concentrations of these nutrients are typically expressed in fertilizer analyses, as well as in fertilizer recommendations.

Phosphorus binds to soil particles. It can be lost by erosion, when soil itself washes away, and thereby contribute to algal blooms in bodies of water. However, it doesn’t generally leach out of soil as readily as nitrogen does.

Because phosphorus tends to remain in soil, it may accumulate to very high levels if high-phosphorus fertilizers are used repeatedly. Excessively high phosphorus in soil reduces the availability of some micronutrients to plants. Because of its tendency to bind with iron, excessive phosphorus can contribute to a condition called centipedegrass decline in that turfgrass and cause problems for other plants, such as blueberries, that are sensitive to iron deficiency.

Because phosphorus is relatively immobile in the soil, plants sometimes have a hard time getting enough of it when root systems are small. Cool temperatures at the time of planting can exacerbate this issue. High phosphorus “starter fertilizers” are sometimes of value.

Even though phosphorus generally “sticks around” in soil, it tends to become more tightly bound and less available to plants over time. Laboratory-based soil tests generally estimate plant-available phosphorus, specifically.

Some common phosphorus sources include diammonium phosphate (DAP; 18-46-0), triple superphosphate (typically 0-45-0 or 0-46-0), and superphosphate (commonly 0-18-0).

Potassium is intermediate between nitrogen and phosphorus with respect to its leachability from soil. It isn’t lost as readily as nitrogen tends to be but isn’t as persistent in soil as phosphorus.

Muriate of potash (potassium chloride; 0-0-60) is a common potassium source. Others include potassium nitrate (typically 13-0-44), potassium sulfate (typically 0-0-52), and sulfate of potash magnesia (often called Sul-Po-Mag or K-Mag; commonly 0-0-22).

Many garden fertilizers contain nitrogen, phosphorus, and potassium. These are called “complete” fertilizers. A few of many examples are 13-13-13, 8-24-24, and 15-9-12. The term “complete fertilizer” is somewhat misleading, since nitrogen, phosphorus, and potassium are just three of approximately seventeen (fourteen, if we don’t count the ones obtained from air and water) plant-essential nutrients. However, they are three of the ones of which plants most often need supplemental amounts, in addition to what’s naturally available in the soil. (Some “complete” fertilizers also contain other nutrients, in addition to nitrogen, phosphorus, and potassium.)

When nitrogen, phosphorus, and potassium are needed, complete fertilizers are often convenient and appropriate options. However, there are situations in which using commonly available complete fertilizers would result in over- or under-application of one or more nutrients.

A soil test is needed to know what amounts of plant-available phosphorus and potassium are in the soil and how much, if any, of these two nutrients are needed from fertilizer.

Let me know if you have questions.

Contact Mary Helen Ferguson.