The Essential Element
Elements deemed as essential plant nutrients are classified as such due to the fact that a plant cannot properly progress through its normal growth and reproduction cycle without them. Over the last hundred plus years, scientists have studied plant growth and behavior and have found 17 elements to be the most important to plant growth. In the absence of any one of these elements, a plant will not grow and develop to its full potential. The essential elements are separated into groups based upon the level of importance they play in plant development. The elements termed macronutrients are the most important because the plant requires them at higher amounts or cannot perform important developmental processes without them. Amongst these essential elemental plant nutrients is an element, a macronutrient, called phosphorus (P).
Phosphorus plays many key functional roles in plant development, including: encouraging root formation and growth, increasing flower and fruit formation, maintaining quality of seed development and enhancing resistance to disease. In essence, phosphorus is essential for the overall health and strength of all plants. Phosphorus is a crucial component in the production of nucleic acids DNA (deoxyribonucleic acid- the “map” of the genetic code every cell needs to develop) and RNA (ribonucleic acid – used to read and interpret the “map”) because the chains of these substances are linked together by phosphorus bonds. Not only does it play important roles in the structural development of a plant, it also has a hand in the conversion of biochemical reactions within a plant. Most notably, the role it plays in photosynthesis and the production of ATP. ATP or adenosine triphosphate is the chemical structure that provides the energy allowing for numerous other chemical reactions within a plant to occur. 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 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 accordingly. The energy in ATP is highly mobile within a plant and is concentrated mostly in new leaves and growth.
Plant available phosphorus exists in the soil solution as the orthophosphate anions H2PO4– (dihydrogen phosphate) and HPO42– (monohydrogen phosphate). Since orthophosphates are negatively charged ions, they are not attracted by a soil’s cation exchange capacity. However, they do still interact quite strongly with iron (Fe), aluminum (Al) and calcium (Ca) cations in the soil, which create products that are insoluble and unavailable to a plant’s roots. Phosphorus availability is directly affected by the soil/rooting media pH, with maximum availability occurring between a pH range of 6.0 to 7.0. Plant roots uptake orthophosphate ions when they come into contact with them in a soil solution. Roots with heavy production of lateral root hairs have a significantly increased level of potential phosphorus uptake. Mycorrhizae fungi can have a dramatic affect on the roots’ ability to unlock and absorb phosphorus in the soil or rooting media. Soils with cool temperatures and low moisture can significantly reduce the roots’ ability to uptake phosphate ions and can possibly lead to a phosphorus deficiency within the plant. Phosphorus is required by a plant in relatively small amounts vs. the other macronutrients nitrogen (N) and potassium (K). However, when the levels reach a critically low state and deficiencies occur, plant growth is very much affected.
Phosphorus deficiency can be more difficult to diagnose compared to deficiencies of nitrogen (N) and potassium (K) 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 in its growth. Phosphorus deficient plants will be slow to mature and are often mistaken for much younger, healthy plants. Some plant species, including, tomatoes, corn and members of the brassica family, will develop a purplish coloring to the stems and under side of the leaves. This symptom will often occur in the older growth first as phosphorus is mobile within a plant (meaning it can be translocated to the newest developing growth where it is most needed). Phosphorus deficiency can come as a result of insufficient plant available phosphorus in the soil or rooting media. It can also occur due to soil conditions that are too cold to facilitate uptake by the plant. When phosphorus levels reach an excessive point within a plant, it may show up as a deficiency of the micronutrients iron (Fe), zinc (Zn) or even cobalt (Co). Excess phosphorus can also lead to a disruption of a plant’s normal metabolism.
Sources of phosphorus, i.e. phosphate, fertilizers range from organic to inorganic. Soils that are high in organic matter contain higher levels of organic phosphorus from materials like residual plant residues, manures or composts and dead microbial tissues. Commercially organic phosphate fertilizers include bone meal and different varieties of composted manure, such as poultry 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, monoammonium phosphate (MAP) and diammonium phosphate (DAP). Fertilizers sold commercially, such as hydroponic fertilizers, usually utilize one of these forms and mix them together with water and other elemental nutrients to create the easy to use liquid concentrate fertilizers that we all enjoy. Keep in mind when buying a phosphate fertilizer, plants generally require much less phosphorus compared to nitrogen (N) and potassium (K), so follow directions carefully and be sure to not over do it. Also, make sure that phosphorus does not leach from a soil. It has the tendency to build up over time, as it has in much of our farmlands. Outdoor soil growers may find it beneficial to have their soil tested prior to any phosphorus fertilizer application.
Phosphorus and the orthophosphate anions are touchier and can behave differently than other elements in the soil and in solution. Paying careful attention to both soil and environmental conditions can help a grower receive the maximum rate of return from both fertilizer and produce. And remember, especially when it comes to phosphorus, it isn’t only how much is applied that matters, but the timing and the placement as well.
