Phosphorus fixation in soil

Phosphorus fixation is the conversion of soil solution phosphorus to insoluble compounds by the soil components, causing a reduction in the amount that plants can absorb. Phosphorus availability is usually greatest in slightly acidic to neutral soil (pH 6-7). The amount and manner in which soluble phosphorus is fixed is influenced by the pH of the soil solution. Between pH 4.0 and 8.0, the main phosphate ions in soil are H₂PO₄^- and HPO₄^2-. PO₄^3-predominates above pH 9.0, but H₂PO₄^- is still present.

The extent of problems of phosphorus fixation in India

The efficiency of phosphatic fertilizers in India is around 15 to 20 percent because of their fixation in acidic and alkaline soils, and unfortunately, both soil types lead in India, accounting for more than 34% acidity affected and more than 7 million hectares of productive and fertile land salinity/alkalinity affected. Phosphorus fixation is greater in black, red, laterite, mixed red and black, red and yellow, and coastal alluvial soils than in alluvial, grey-brown, desert, and other soils. The maximum phosphorus fixation in Bihar and Jharkhand soils has been reported to range between 25 and 90 percent. Soil phosphorus fixing capabilities are listed in the following order: Red soil > Medium black soil > Heavy black soil > Grey brown soil > Alluvial soils.

According to another study, aluminum phosphate accounts for more than half of fertilizer phosphorus conversion, followed by iron phosphate. Similarly, it has been discovered that 60-70 percent of applied phosphorus remains fixed in the form of aluminum, iron and calcium phosphate following rice crop harvest.

Mechanisms for phosphorus fixation

Biological fixation

Biological fixation of phosphorus happens in all soils during the decay of organic residues low in phosphorus.

Chemical fixation

1. Sorption by iron and aluminum oxides and clay minerals: Phosphorus fixation by this is highest in acid soils but to some extent in all groups of soils.
2. Precipitation

• Formation of very insoluble carbonated hydroxyapatite in calcareous and alkaline 

• Acid soils cause the formation of iron and aluminum phosphates (insoluble).

• Calcium phosphate formation in slightly acidic to neutral soils.

Types of reactions in phosphorus fixation

(1) Adsorption: Adsorption of phosphorus happens on surfaces through constant charges, such as crystalline clay minerals, which interact with phosphorus primarily through the cations held tightly to their plate-like surfaces, and on surfaces with variable charges, such as ferric (Fe³⁺) and Al-oxides and organic matter, for which H⁺ and OH^- ions regulate the surface charge, and calcite (CaCO₃), for which Ca²⁺ and CO₃^2- are responsible for the development of charge.

(2) Isomorphism replacement: Phosphorus is fixed by the hydroxyl (OH^-) and silicate ions through isomorphous substitution. Hydroxyl (OH^-) ions are attached to Si and Al, and are liable to either dissociate as: ―(Si, Al)―OH  →  ―(Si, Al)⁺ + OH^- or, accept a proton (H⁺) ions as follows: ―(Si, Al)―OH  →  ―(Si, Al)―OH₂⁺ giving rise to positively charged clays, which take part in anion exchange.

(3) Double decomposition: A variety of reactions may be considered significant in the fixing (precipitation) of soluble phosphorus based on the solubility product principles. However, the development of insoluble phosphatic compound precipitation is deeply influenced by the pH of the system. This reaction can be divided into two categories: (a) reactions involving Fe and Al, and (b) other Ca reactions.

Phosphorus fixation in different types of soil

(1)    Neutral to alkaline soils: Phosphates are absorbed on calcium carbonate in neutral to alkaline soils (pH 7 and higher), precipitated as tri-calcium phosphate, and progressively transformed into insoluble apatites.

In slightly alkaline soils (pH 8.0), alkali earth elements such as Ca, Mg, and Zn predominate. They can precipitate when combined with phosphate. As a result, phosphorous becomes scarce. Alkali metals, such as Na, K, and others, prevail in very alkaline soils (pH 10.0). They can combine with phosphate to generate soluble phosphates. As a result, phosphorus is available.

(2)    Acid soils: Fe and Al react with phosphate to generate very insoluble molecules (aluminum hydroxyl phosphate) in acidic soils (pH less than 7).

Variscite (AlPO₄.2H₂O) and strengite (FePO₄.2H₂O) are the end products in severely acidic soils with pH less than 4. Iron and aluminum phosphates are precipitated or adsorbed on the surface of hydrated oxides of Fe and Al in strongly acidic soils, but in mildly acidic soils, these phosphates are largely precipitated or adsorbed on clay-humus colloids. Phosphorus is more strongly bound in acid soils than in calcareous soils. Manganese and titanium, in addition to Fe and Al, produce insoluble compounds with phosphate in acid soil.

Factors affecting phosphorus fixation

(1) Soil composition and quantity

(a) Hydrous oxide of iron and aluminum: Because of their larger surface areas, soils with amorphous Fe and Al oxides have more phosphate-fixing capacity than crystalline soils.

(b) Clay type and quantity: Soils with high clay content and quality retain more phosphorus than those with low clay content. The phosphate-fixing capacity of 1:1 clay minerals is greater than that of 2:1 clay minerals.

(c) Soluble silica: Increased soil weathering increases phosphorus sorption capacity (decreases Si solubility). Rice's phosphorus consumption efficiency has been observed to improve with the addition of sodium silicate.

(d) Amorphous colloids: Amorphous aluminosilicates, such as allophane, have a significant negative charge that is partially or completely balanced by complex aluminum cations. Phosphate is adsorbed when it reacts with aluminum ions.

(e) Calcium carbonate: In calcareous soils, calcite serves as a phosphate adsorbent site. The reactivity of calcium carbonate affects phosphate sorption.

(f) Cations: As the valency of the exchangeable cations in the systems increases, more phosphate is sorbed. Acid soils hold more phosphate than alkaline soils, while neutral soils absorb more phosphorus than saline/sodic soils.

(g) Anions: Anions reduce phosphorus sorption capacity more effectively in coarse-textured soils than in fine-textured soils. The effect is stronger in noncalcareous soils than calcareous soils. Fluoride ions are more effective than sulfate ions among inorganic anions.

(h) Electrolyte: As the concentration of the supporting medium increases, phosphorus retention increases and decreases with monovalent cation in the electrolyte.

(2) pH: Phosphate is mostly fixed in very acidic soils by precipitation with iron and aluminum in solution and by reaction with iron and aluminum hydrous oxides. Phosphate is fixed in moderately acidic soils through aluminum and iron hydrous oxides, as well as anion exchange on silicate clays. Phosphate is fixed as insoluble calcium phosphates in slightly acidic soils by hydrous oxides of aluminum and iron and silicate clays. In alkaline soils, soluble phosphate from fertilizer combines with calcium first to generate dicalcium phosphate, then tricalcium phosphate, and finally hydroxyapatite.

(3) Organic matter: Organic matter decreases phosphorus fixation by producing acids that solubilize phosphate during decomposition.

(4) Temperature: A rise in temperature improves phosphorus sorption in soils. Phosphorus is more abundant in warm soils than in cold ones.

(5) Overliming: It promotes phosphate fixation in soil by forming insoluble calcium phosphate complexes.

Conclusion

Phosphorus fixation is high in acid soils with high levels of iron and aluminum hydrous oxides, neutral or alkaline soils with high levels of CaCO₃, alkaline soils with high levels of soluble calcium (Ca2+), fine-textured clay soils with high clay content (clay soils), and soils with more 1:1 silicate clays (kaolinite) than 2:1 silicate clays (montmorillonite).