**4. Behavior and chemistry of phosphorus in soil**

When P-fertilizers are applied to the soil, they react with cations (Ca, Fe, and Al) and form insoluble P-compounds. Consequently, P-use-efficiency declines, and crops can recover only 15–20% of applied P [52]. In mineral soils, P is highly immobile [53]. P-cycling in soil occurs through numerous processes, viz.,

mineralization-immobilization, precipitation, adsorption-desorption, dissolution, and plant uptake [46, 54]. The fixation of P is common in acid soils because P precipitates as oxides and hydrous oxides of Fe and Al or gets adsorbed on clay surface or surface of oxides and hydrous oxides of Fe and Al [55, 56]. Similarly, in calcareous soils, these phenomena (precipitation and adsorption) lead to the formation of insoluble Ca compounds [57, 58]. The adsorption process takes place by ligand exchange, which favors monodentate phosphate complexation [54, 59]. Based on adsorption, P-forms can be classified as labile and nonlabile [60]. Labile forms are related to weak adsorption because of low desorption time. They supply P to the soil solution. Thus, under equilibrium conditions, P-dynamics in the soil [54] are represented as:

$$\text{Nonlabel} \\ \text{P} \leftrightarrow \text{Labile} \\ \text{P} \leftrightarrow \text{solution P}. \tag{1}$$

In the mineralization process, the soil organic P is converted into inorganic P by the microbes present in the soil. This is also known as P solubilization. Organic P hydrolysis is performed by microbes producing enzymes (phosphatase, phytase) and organic acids (formic, oxalic) [61]. When the available P is low in the soil, microbial immobilization of P begins, i.e., inorganic P gets transformed into organic P [62].

With the increase in soil age, the concentration of nonlabile forms of P increases [44]. P-availability in soil is dependent upon pH, organic matter, clay content and/ or type, moisture content, temperature, aeration, and other properties of the soil. In heavily P-fertilized soils of northern Iran, the reported higher P-adsorption was argued due to the high content of clay and Fe and Al oxides [43]. The capacity of soils to supply this nutrient to plants or the buffering capacity of soils is determined by quantity and intensity factors [63]. This is determined by sorption and desorption curves. Quantity is the solid phase that equilibrates with the solution and can also represent the buffering capacity of soils to meet the P requirement of plants [64]. Therefore, to improve the efficacy of P-fertilizer recommendations, buffering indices must be considered in our soil testing programs to know about P-uptake at different stages of plant growth [65]. The release of P occurs in simultaneous processes, viz., desorption at a higher rate and diffusion at a slower rate [66].

The availability of P increases with the addition of organic matter. As through mineralization of organic matter, the available form of phosphorus is released to soils. Retention of P reduces because phosphate absorbed into soil competes with organic molecules. In soils with high clay contents, the retention capacity of phosphorus will be high because the surface area per unit volume is very high of clay particles that absorb phosphorus very easily. On the other hand, the adsorption capacity of P increases when soils have a sufficient amount of minerals. The mineral composition of the soil influences the phosphorus adsorption capacity. The availability of P will be higher in soils that have 6–7 pH. At low soil pH, the Fe and Al make strong bonds with phosphorus, and phosphate tends to precipitate with calcium at high soil pH. Another factor also affects the availability of phosphorus to plants; like in cool weather, the organic matter takes a long time to decompose compared with warm and hot weather.
