Phosphorus is one of nutrients essential for plant growth and an adequate supply of P is required for optimum growth and reproduction. Phosphorus is classified as a major nutrient, meaning that it is frequently deficient for crop production and is required by crops in relatively large amounts. Phosphorus consumption by plants is less than nitrogen; it accounts for 0.2-1.0% of dry matter mass. Most of it is accumulated in reproductive organs and organs, where the processes of synthesis of organic matter occur intensively.
Phosphorus is found in soils both in an organic form and an un-organic (mineral) form and its solubility in soil is low. The types of phosphorus compounds that exist in the soil are mostly determined by soil pH and by the type and amount of minerals in the soil. Mineral compounds of phosphorus usually contain aluminum, iron, manganese and calcium. In acidic soils phosphorus tends to react with aluminum, iron and manganese, while in alkaline soils the dominant fixation is with calcium. The optimal pH range for maximum phosphorus availability is 6.0-7.0. There is equilibrium between solid phase phosphorus in soil and the phosphorus in the soil solution. Plant roots absorb phosphorus from the soil solution. In comparison to other macronutrients, the phosphorus concentration in the soil solution is much lower and ranges from 0.001 mg/l to 1 mg/l. When plant roots remove phosphorus from the soil solution, some of the phosphorus adsorbed to the solid phase is released into the soil solution to maintain equilibrium. In general, plants take up phosphorus in the form of orthophosphate ion: either HPO4-2 or H2PO4-. The proportion in which these two forms are absorbed is determined by the soil pH, when at higher soil pH more HPO4-2 is taken up.
The mobility of phosphorus in soil is very limited and therefore, plant roots can take up phosphorus only from their immediate surroundings. Since concentration of phosphorus in the soil solution is low, plants use mostly active uptake against the concentration gradient (i.e. concentration of phosphorus is higher in the roots compared with the soil solution).
Once inside the plant root, P may be stored in the root or transported to the upper portions of the plant. Through various chemical reactions, it is incorporated into organic compounds, including nucleic acids (DNA and RNA), phosphoproteins, phospholipids, sugar phosphates, enzymes, and energy-rich phosphate compounds. It is in these organic forms as well as the inorganic phosphate ion that P is moved throughout the plant, where it is available for further reactions. Phosphorus plays a vital role in virtually every plant process that involves energy transfer.
Phosphorus is essential in photosynthesis and respiration where it plays a major role in energy storage and transfer. It is a vital component of the RNA and DNA structures, which are the major components of genetic information. An adequate supply of P is essential to the development of new cells and to the transfer of the genetic code from one cell to another as new cells are formed.
Phosphorus is also a component of phytin, a major storage form of P in seeds. About 50 percent of the total P in legume seeds and 60 to 70 percent in cereal grains is stored as phytin or closely related compounds.
An inadequate supply of P can reduce seed size, seed number, and viability.
Seeds have the highest concentration of P in a mature plant, and P is required in large quantities in young cells, such as shoots and root tips, where metabolism is high and cell division is rapid. P aids in root development, flower initiation, and seed and fruit development.
Adequate P allows the processes described above to operate at optimum rates and growth and development of the plant to proceed at a normal pace. P is needed in large quantities during the early stages of cell division and when it is limiting the initial overall symptom is slow, weak, and stunted growth. There is a reduction in leaf expansion and leaf surface area, as well as the number of leaves. Root growth is also reduced by P deficiency, leading to less root mass to reach water and nutrients. P is relatively mobile in plants and can be transferred to sites of new growth, causing symptoms of dark to blue-green coloration to appear on older leaves of some plants. Under severe deficiency, purpling of leaves and stems may appear. Other effects of P deficiency on plant growth include delayed maturity and poor seed and fruit development.
Phosphorus (P) is the second limiting nutrient required for plant growth and development, after nitrogen. It is relatively immobile in the soil and P availability is dependent on several factors, such as:
- Soil texture: soils high in clay content fix/absorb more P than those with less clay.
- Calcium carbonate content: more P is converted to the less available calcium phosphate compounds in soils containing more calcium carbonate.
- Soil temperature: low soil temperature will reduce P availability by slowing the movement of P from the soil to the root and by reducing the mineralization of organic matter to plant available inorganic P.
Organic fertilizers are an excellent source of P for crop production thanks to the high content of organic phosphorus which is not subject to insolubility issues. P in soils can exist in many inorganic and organic forms that are unavailable to plant uptake, but specific microorganisms play a critical role for the solubilization of recalcitrant P forms in soils. Phosphate solubilizing bacteria (PSB) are the main contributors of plant nutrition in agriculture and play a pivotal role in making soluble phosphorus available to plants. The principal mechanism in soil for solubilization of P is lowering of soil pH by microbial production of organic acids ultimately resulting in P availability in soil. Also, mycorrhizal fungi, which develop a symbiotic relationship with plant roots and extend threadlike hyphae into the soil, can enhance the uptake of phosphorus, as well especially in acidic soils that are low in phosphorus. To achieve the aim of sustainable agriculture, the application of microbes with multiple P sources utilizing abilities provides a new approach able at the same time to improve soil quality and phosphorus uptake.
Phosphorus fertilizer use efficiency in agricultural soils can be enhanced or reduced by a producer’s choice of fertilizer placement, timing, and rate. Proper management of P fertilizer, manure, and soil is essential to prevent agricultural phosphorus from degrading water quality.
Applying P fertilizer at rates higher than production requirements is unwise from both environmental and
economic viewpoints. There is no agronomic justification for building P soil test levels higher than crop sufficiency levels. Therefore, once the crop sufficiency levels have been reached in your fields, P applications should be made only as dictated by soil testing. Besides, applying the right source of phosphorus and the best management practices for fertilization is essential to reach the modern standards of sustainable agriculture.
