Where Does Mineral Fertilizer Phosphorus Come From?

Mineral fertilizers are often referred to as “synthetic” fertilizers, implying that they are chemically synthesized and therefore, unnatural. In reality most mineral fertilizers are naturally occurring mineral materials that are treated in ways that enhance their solubility, and hence their availability to plants, or are combined with other minerals that deliver additional plant nutrients, soil conditioning amendments, microbial stimulants, etc. This is certainly true of phosphorus (P) fertilizers.

To start with, some background on P is important to note. Unlike nitrogen, elemental P is not found in nature, but as phosphate (PO₄^-3) in plant and animal tissues and a number of rock phosphate (RP) forms. While plant and animal tissues (manures, bone meal, etc.) are potential P fertilizer sources and commonly used in organic crop production, concentrated mineral P fertilizers for large-scale agricultural and horticultural production come from mined RP sources.

There are numerous known geologic deposits of RP around the world. The top five countries mining RP are China, USA, Morocco, Russia and Jordan, in that order. China’s mining of RP dwarfs second-ranked US production by over five times! In the US, in 2015, there were 10 active RP mines in four states. Of those, Utah and Idaho are notable, along with Florida and North Carolina (Florida accounts for the bulk of US production). Rock phosphate is highly insoluble, being only about 1% soluble in soils having a pH greater than 6.0. Since this is the case in almost all Utah and Western US soils, RP must be reacted with acids to obtain soluble, plant available phosphate.

Most P fertilizers are formed by reacting RP with sulfuric acid to form a soluble P base material. This process is known as the green or wet acid process. The resultant green phosphoric acid contains about 55% P₂O₅ and all P fertilizers built from this foundational form are known as “ortho” phosphates due to the phosphate compounds being primarily in the ortho-phosphate form, or the single phosphate ion form.

Green phosphoric acid can be concentrated to about 79% P₂O₅. As concentration progresses, the ortho-phosphate molecules combine, or polymerize, hence the resulting acid is termed “poly” phosphoric acid. All fertilizers using this as a base for production are called poly-phosphates. Many of the popular liquid fertilizers are developed from this base product. The two acids, green- and poly-phosphoric acids are then reacted with other compounds to form the mineral P fertilizer materials.

Fertilizer P content, unlike that for nitrogen, is expressed as the percent plant available P₂O₅ content, not elemental P, since elemental P as noted previously, is not a naturally occurring form of the element. This is a bit tricky when addressing the chemistry of P fertilizer, but thankfully, P fertilizer requirements are generally given as the quantity of P₂O₅ needed by the plant, thereby rendering the tricky conversion to elemental P, unnecessary.

Plant-available phosphate from fertilizers is determined by a weak citric acid extraction approximating the amount of phosphate that can be utilized from a given material, by plants. This test is used on all P fertilizers to determine plant-available phosphate, though the plant-available content differs from material to material. Therefore, plant-available P is a true apples-to-apples comparison between materials, meaning that no one form is better at delivering its stated amount of available P, to plants.

In an upcoming companion post, I will detail and discuss the more common mineral P fertilizers and aspects of managing fertilizer P in calcareous, high pH soils common to Utah and the Intermountain Western US.

Author

Grant Cardon

More Information

• International Plant Nutrition Institute Fact Sheet on Rock Phosphate:
• The 13th Element: A Torrid Tale of Murder, Fire and Phosphorus by John Emsley. This is a fascinating book on the discovery and history of phosphorus and its many uses (although a bit afield of the fertilizer aspects). I highly recommend it.