**2. Phosphorus in soils**

Phosphorus (P; atomic number 15; nonmetal; placed in the 3rd row of the periodic of the elements; discovered by German chemist Hennig Brand in 1669) is a chemical element that occurs as white P and red P, its most common allotropic forms. It is never found as a free element on Earth and generally occurs as phosphates (PO4 3−). P has 22 known isotopes which range from 26P to 47P [1]. Notably, 31P is the only stable isotope of P and is therefore present at 100% abundance. Apatite (comprising minerals pentacalcium triorthophosphate fluoride) partly constitutes inorganic phosphate rock, the chief global commercial source of P, and a nonrenewable resource for the phosphate fertilizers.

Ranked 11th in the list of most abundant elements in the environment, P is neither easily accessible nor evenly distributed in most soils. In addition, the availability of the soil-P to plants/crops is limited. The concentration of P in the soil solution can be very high (10−4 M), very low (10−6 M), or as low as 10−8 M in some very poor tropical soils. Though P is not available in gaseous form, it is available in both inorganic and organic forms in the soil. In general, soil-P is available in both organic form (about 30–65%) and inorganic form (about 35–70%). Notably, plantavailable (soil solution) P, sorbed P, and mineral P are the major pools of inorganic P-form. Contingent to the pH, H2PO4 − , HPO4 2−, and PO4 3− are the major P-forms in the soil solution. P exhibits very low solubility and poor mobility in soil solution, as well as its capacity to form insoluble salts with different mineral elements. Soils with low amounts of organic matter and low water holding capacity are usually nutrient-deficient acidic soils widely reported to exhibit the least mobility of P as well as the least availability to plants (compared to N and K). P is also incorporated into organic compounds, where about 20–80% of soil-P is present in organic matter mainly as phytic acid (inositol hexakisphosphate), which can bind to various anions resulting in the formation of phytate. Most soil-organic P pools (mostly composed of plant and microorganism residues and livestock manures) cannot be absorbed directly by most plants [2]. Hence, P is among the less accessible elements for most plants [3]. H2PO4 − , denoted by inorganic phosphate (orthophosphate; Pi), dominates in the pH range of 3–7 and is the predominant form absorbed by plants. On the other hand, ester derivatives represent the organic P present in organic molecules. Phosphates are not reduced; however, the oxidized form of P is incorporated in the biomolecules [4]. There are several factors (physical, chemical, and biotic) known to regulate P-availability in soils and to the plant roots for its uptake. Major P-availability-modulating physical factors include soil texture and moisture; whereas chemical factors can be the soil solution pH, organic matter, redox potential, P-concentration in the soil solution, P-buffer capacity of the soil, and the concentration of Fe, Al, and Ca. On the other hand, major biotic factors influencing the plant/crop root activity and the P-release in the rhizosphere include the diversity of microorganisms [5–8].
