An element is deemed essential to plant nutrition when a plant cannot properly progress through its normal growth and reproduction cycle without it. Over the last century, scientists have determined that 16 elements are vital to proper plant growth. If any one of these elements is absent, a plant will not grow and develop to its fullest potential.

The essential elements are separated into groups based on the roles they play in plant development. Elements termed macronutrients are most important because plants require them at higher amounts to perform important developmental processes. Micronutrients, also called trace elements, are usually found in most healthy soils, but can also be supplied by a grower via supplemental fertilization. Amongst the macronutrients is phosphorus, an element with many key roles to play.

Phosphorus in the Garden

Phosphorus encourages root formation and growth; increases flower and fruit formation and the quality of seed development; and enhances disease resistance. Phosphorus is essential for maintaining healthy, strong plants. It plays a crucial role in the production of the nucleic acids DNA (deoxyribonucleic acid, a molecule that encodes the genetic instructions used in the development and functioning of living organisms) and RNA (ribonucleic acid, which is used to read and interpret the DNA map).

Not only does phosphorus play important roles in the structural development of plants, it also has a hand in the conversion of biochemical reactions within a plant. Most notably, it plays a role in photosynthesis and the production of ATP, or adenosine triphosphate-—the chemical structure that provides the energy needed for numerous other chemical reactions.

Through photosynthesis, a plant harnesses the energy of light in the presence of chlorophyll to combine water and carbon dioxide into simple carbohydrates or sugars. The residual energy created is then held in the ATP and distributed throughout the plant as needed. It is almost like charging a battery: the energy is harnessed, stored and distributed as needed.

Phosphorus Types

Plant-available phosphorus exists in the soil solution as orthophosphate anions dihydrogen phosphate and monohydrogen phosphate. Since orthophosphates are negatively charged ions, they are not attracted by a soil’s cation exchange capacity, but they do still interact quite strongly with iron, aluminum and calcium cations in the soil, often creating products that are insoluble and unavailable for plant uptake.

Phosphorus uptake is directly affected by the soil or rooting media’s pH level, with maximum availability occurring in a pH range between 6 and 7. Plant roots take up orthophosphate ions when they come into contact with them in a soil solution and roots with many lateral root hairs have a significantly increased level of potential phosphorus uptake.

Mycorrhizal fungi can have a dramatic effect on a plant’s ability to unlock and absorb phosphorus. Cool temperatures and low moisture levels can significantly reduce the ability to absorb phosphate ions and can possibly lead to a deficiency. Phosphorus is required by plants in relatively small amounts compared with the other macronutrients—nitrogen and potassium—but when levels reach a critically low state and deficiencies occur, plant growth is still greatly affected.

Phosphorus Deficiencies in Plants

Phosphorus deficiency can be more difficult to diagnose than nitrogen and potassium deficiencies because the symptoms are often much less obvious to the naked eye. Early detection is complicated by the fact that a deficiency may present itself in the form of a slowly developing plant that will looked stunted.

Phosphorus-deficient plants will be slow to mature and are often mistaken for much younger, but healthy, plants. Some plant species, including tomatoes, corn and members of the brassica family, will develop a purplish coloring to the stems and undersides of the leaves. This will often occur in the older growth first as phosphorus is mobile within a plant, meaning it can be transferred to developing growth, where it is most needed.

Phosphorus deficiency can result from insufficient plant-available phosphorus in the soil or rooting media, but can also happen due to soil conditions that are too cold to facilitate uptake by the plant. On the other end of the scale, an excess of phosphorus may show up as a deficiency of the micronutrients iron, zinc or even cobalt. Excessive phosphorus levels can disrupt a plant’s normal metabolism.

What are the Best Sources of Phosphorus for my Garden?

There are both organic and inorganic sources of phosphorus. Soils high in organic matter contain higher levels of organic phosphorus from materials like plant residues, manures, composts and dead microbial tissues.

Commercially sold organic phosphate fertilizers include bone meal, different varieties of composted manure, and colloidal phosphate. Inorganic phosphate fertilizers are manufactured from a material called rock phosphate that is mined and subsequently processed into products that contain a higher concentration of phosphate.

These products include superphosphate, mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP). Hydroponic fertilizers usually mix one of these forms with water and other elemental nutrients to create the easy-to-use, concentrated liquid fertilizers we all enjoy.

When buying a phosphate fertilizer, keep in mind that plants generally require relatively less phosphorus than nitrogen and potassium, so follow directions carefully and be sure to not overdo it. Phosphorus does not readily leach from soil and has a tendency to build up over time. Outdoor soil growers may find it beneficial to have their soil tested prior to any phosphorus fertilizer application.

As a final word of caution, phosphorus and the orthophosphate anions are, for lack of a better word, touchy, and can behave differently than other elements in the soil and in a hydroponic solution. Paying careful attention to both your grow media and environmental conditions can help you get the maximum rate of return.