**2. Phosphorus in soil**

Phosphorus is an essential element for plant growth and survival. However, the amount of phosphorus available in the Earth's crust is relatively low compared to other essential plant nutrients, ranging from 1100 to 1200 mg/kg. The average concentration of phosphorus in surface soils is estimated to be around 1100–50 mg/kg. Plants can only absorb phosphorus in the form of orthophosphates, with the highest concentration being in the form of H2PO4 at low pH levels. This form of orthophosphate is more readily absorbed by plants compared to HPO4 <sup>2</sup>, which increases in concentration with increasing pH levels. The average concentration of phosphorus in the soil solution is approximately 0.05 mg/L, although this value can vary depending on the soil type. Typically, the concentration of phosphorus in the soil solution that can satisfy plant needs ranges from 0.003 to 0.3 mg/L, as reported by [2, 3]. These findings suggest that while phosphorus is essential for plant growth and survival, it is often limited in availability in the soil. Therefore, it is crucial to carefully manage phosphorus fertilization practices to ensure that plants have access to the necessary amounts of this nutrient for optimal growth and development.

Phosphorus in soil exists in both organic and mineral forms. Typically, 20–80% of total soil phosphorus is in the organic form, while the remainder is in mineral form [4, 5]. However, these forms are not strictly distinct from one another and can transform into each other (**Figure 1**). The specific types of organic phosphorus are not well understood, but mineral phosphorus is typically associated with iron, aluminum, and calcium. These minerals each have varying solubilities in the soil. The mobility and speciation of mineral phosphorus in soil and aquatic ecosystems are regulated by reactions that occur on the surfaces of colloids [6–8]. Additionally, Lindsay noted that other reactions, such as simultaneous precipitation by calcium, iron, and aluminum, can also influence the phosphorus cycle in soil [9, 10].

The mechanisms responsible for converting soluble phosphorus into less soluble states are related to phosphorus storage and stabilization reactions. Nearly all soils contain iron and aluminum oxides and hydroxides, which can exist as separate mineral particles or as coatings on other particles, such as clay. Amorphous aluminum hydroxides may also be present between the expandable layers of aluminosilicates. Many H2PO4 adsorbents are present in the soil solution, and most of these adsorbents, such as iron or aluminum, use the hydroxyl (OH) exchange mechanism to absorb H2PO4 . In alkaline conditions, minerals like calcium carbonate can absorb H2PO4 /HPO4 <sup>2</sup> and cause precipitation by exchanging water, bicarbonate, and hydroxyl ions on their surfaces. Notably, each of these reactions occurs in response to the soil's pH level. At pH levels lower than 5, hydroxyl metal phosphates (Al (OH)2H2PO4) can form due to the presence of aluminum, iron, or active manganese. *Phosphorus Dynamics in Soil-Water-Sediment Environment DOI: http://dx.doi.org/10.5772/intechopen.113225*

**Figure 1.** *Phosphoric and mineral cycle in soil.*

In alkaline conditions, dicalcium phosphate deposits without water molecules can form due to the presence of active calcium [6, 11, 12].

As previously mentioned, the natural concentration of phosphorus in the soil is very low, and this small amount of phosphorus is not easily available to the plant. The plant has to compete with the reactions of surface absorption and phosphorus deposition in the soil to obtain the phosphorus it needs. Therefore, to achieve acceptable performance, phosphorus fertilizers should be added to the soil to aid the growth and nutrition of the plant. The most common phosphorus fertilizers are calcium orthophosphate fertilizers, including triple superphosphate and simple superphosphate. Triple superphosphate, with the formula Ca(H2PO4)2, has 44–53% phosphorus oxide [2, 13, 14].

### **2.1 Sources of phosphorous input**

### *2.1.1 Phosphorous fertilizers*

The growth and health of plants depend on several factors, including nutrient supply, soil quality, and exposure to sunlight. Among these, phosphorus is a crucial element that plays a significant role in promoting plant growth and health. Despite its importance in supporting respiration, photosynthesis, cell division and enlargement, and energy storage and transfer, the amount of phosphorus in soil is typically much lower than that of other essential elements like potassium, nitrogen, and calcium. As a result, various types of phosphorus-based fertilizers are produced today to supplement the soil's phosphorus content and help improve plant growth.

In the nineteenth century, researchers began conducting experiments to examine the effects of phosphorus fertilizers on plant yield. Britain (1843) and Germany (1878) were among the pioneering countries in this field, with subsequent experiments conducted at the Illinois Agricultural Research Station in 1888. Further experiments were carried out in Ontario, Canada, in 1916 [15]. After World War II, the use of animal manure to maintain soil fertility and provide food for the growing population began to expand rapidly [16].

#### *2.1.2 Sources of phosphorus fertilizers*

#### *2.1.2.1 Phosphate rocks*

These are a valuable source of phosphorus for plants, with the most reactive rocks containing Francolite, an Apatite mineral containing iron and carbonate. When crushed, phosphate rock can provide sufficient plant-available phosphorus in low pH soils. It is commonly used to cultivate plants such as Kathira, oil palm, and coffee in very acidic soils, particularly in warm weather conditions, moist soils, and long growing seasons.

#### *2.1.2.2 Phosphoric acid*

Phosphoric acid (H3PO4) is another important source of phosphorus for agriculture, containing 17–24% phosphorus (55–39% P2O5). It is produced through the reaction of phosphate rock with H2SO4 and can be either green acid or acid obtained from a wet process. Phosphoric acid is used to acidify phosphate rock and create calcium and ammonium phosphates, which are important fertilizers. It can also be injected into the soil or irrigation water, particularly in alkaline and calcareous soils.

#### *2.1.2.3 Calcium phosphates*

In the past, calcium phosphate fertilizers such as simple superphosphate, triple superphosphate, and enriched superphosphate were among the primary sources of phosphorus. However, unlike phosphoric acid and ammonium phosphates, superphosphates do not have a significant impact on soil pH.

## *2.1.2.4 Simple superphosphate*

Also known as single superphosphate, it contains 7–9–5% phosphorus (P2O5 16– 22%). It is a valuable source of both sulfur and phosphorus.

#### *2.1.2.5 Triple superphosphate*

This contains 17–23% phosphorus (P2O5 44–52%). Triple Super Phosphate is an excellent source of phosphorus, and its high concentration of phosphorus is advantageous because transportation, storage, and handling constitute a significant part of the total cost of fertilizer consumption.

#### *2.1.2.6 Ammonium phosphate*

Ammonium phosphate is produced by reacting phosphoric acid with NH3. Monoammonium phosphate (MAP) contains 11–13% nitrogen and 21–24% phosphorus (P2O5 48–55%), while diammonium phosphate (DAP) contains 18–21% nitrogen and 20–23% phosphorus (P2O5 53–46%). Both MAP and DAP are granular and watersoluble, making them suitable as starter fertilizers. Their high solubility allows for efficient uptake by plants, resulting in improved growth and yield.

#### *2.1.2.7 Ammonium polyphosphate*

Ammonium polyphosphate is produced by reacting pyrophosphoric acid with ammonia. Liquid APP is a cost-effective source of phosphorus that can be used alone or in combination with other liquid fertilizers. Typically, UAN and APP fertilizers are combined and applied in bands below the soil surface.

#### *2.1.2.8 Potassium phosphate*

Potassium phosphate products, such as KH2PO4 and K2HPO4, are highly soluble in water and are commonly used in the horticulture industry. Their high levels of both potassium and phosphorus make them a popular choice for crops such as potatoes, tomatoes, and many leafy vegetables that are sensitive to high concentrations of chloride found in KCl. Additionally, their low salt index makes them well-suited for application near seeds with minimal damage to emerging sprouts.
