© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Note that the term fixation as applied to phosphorus has the same general meaning as the chemical fixation of potassium or ammonium ions. The chemical being fixed is bound, entrapped, or otherwise held tightly by soil solids in a form that is relatively unavailable to plants.

However, the fixation of gaseous nitrogen refers to the biological conversion of N₂ gas to reactive forms NH₄⁺ or NO₃¯ that plants can use.

Phosphorus Role As A Plant Nutrient

  1. ATP (adenosine triphosphate)
  2. DNA (deoxyribonucleic acid)
  3. RNA (ribonucleic acid) protein and enzyme synthesis
  4. Cellular membrane (phospholipids)

Low soil P a three-fold issue

  1. The total P content of most soils is rather low compared to the long term needs of growing plants
  2. Most of the P found in soils is in forms quite unavailable for plant uptake
  3. Soluble P added to soils is rapidly transformed into unavailable or fixed forms

High soil P

  1. high-P soils become non-point sources of nutrient pollution
    • particulate-P and dissolved-P enters waterways when water runs off the land, results in eutrophication

Global environmental problems

  1. Degradation of land
    • Too little available phosphorus
    • The result is erosion from poorly protected land due to sparse vegetive growth
  2. Degradation of water
    • With accelerated eutrophication caused by too much phosphorus

The Phosphorus Cycle

  1. The soil solution is much more dilute (typically 0.001 to 1 mg/L) with respect to P than with respect to other macronutrients.
  2. The main form of P species in solution are HPO₄²⁻ (in alkaline soils) and H₂PO₄⁻ (in acid soils).
  3. Often the process limiting the uptake of P by plants is the movement of P to the surface of the plant root.
    • Movement to the root surfaces
      • By diffusion of phosphate ions
      • By Mass flow of the soil solution containing phosphate ions
    • Soil solution phosphate ion concentration constrained by the strong fixation of the P by iron, aluminum, and calcium compounds lining the soil pores in which the soluble P must flow.
  4. Fine root hairs growing in the rhizosphere assist in P uptake by increasing the soil volume exploited and by decreasing the distance the P must travel to reach the root.
  5. Many plants, especially in natural ecosystems, the survival in low-P soils depends on the transport of soluble P to plant roots through the hyphae of symbiotic mycorrhizal fungi.

Soil Phosphorus exists either in organic forms or inorganic forms (mostly bound to Ca or to Fe or Al)

  • In neutral to alkaline soils calcium-bound P is dominant (extremely low solubility)
  • In acid soils, most P is bound to iron or aluminum oxides (extremely low solubility)
  • Very little P is lost by leaching because of the strong affinity for P by soil iron and aluminum.

Phosphorus Losses

  1. Most P losses occur as P in removed plant and animal biomass
  2. P adsorbed to eroded sediment
  3. P dissolved in runoff water

Phosphorus Additions

  1. Atmospheric deposition (principally on dust particles) generally contributes only enough to balance the very moderate leaching losses.

Organic Phosphorus in Soils

  • Half the total phosphorus is in organic forms
  • Availability of inorganic phosphorus is by (microbial) mineralization of the organic P
  • Less than C:P ration of 200:1 and mineralization can occur, but once the anion is in solution is then subject to fixation by calcium, iron, or aluminum

Inorganic Phosphorus in Soils

  • Soluble phosphorus ions in strongly acid soils (pH 4–5.5) are dominated by the monovalent anion H₂PO₄⁻
  • Soluble phosphorus ions in alkaline solutions are dominated the divalent anion HPO₄²⁻
  • Both anions are important in near-neutral soils
  • Of the two anions, H₂PO₄⁻, is thought to be slightly more easily taken up by plants, but effects of pH on phosphorus reactions (fixation and retention) with other soil constituents are more important than the particular phosphorus anion present.

Table 14.5 Inorganic Phosphorus-Containing Compounds Important for Soils . Reproduced after Brady and Weil (2017) The Nature and Properties of Soils.

In each group, the compounds are listed in order of increasing solubility.

Compound Formula
Iron and aluminum compounds
 Strengite FePO₄ ⋅ 2 H₂O
 Variscite AlPO₄ ⋅ 2 H₂O
Calcium compounds
 Fluorapatite [3Ca₃(PO₄)₂] · CaF₂
 Carbonate apatite [3Ca₃(PO₄)₂] · CaCO₃
 Hydroxy apatite [3Ca₃(PO₄)₂] · Ca(OH)₂
 Oxyapatite [3Ca₃(PO₄)₂] · CaO
 Tricalcium phosphate Ca₃(PO₄)₂
 Octacalcium phosphate Ca₈H₂(PO₄)₆. · 5H₂O
 Dicalcium phosphate CaHPO₄ · 2H₂O
 Monocalcium phosphate Ca(H₂PO₄)₂ · H₂O
 Magnesium compound Struvite NH₄MgPO₄·6H₂O

Very low solubility phosphorus compounds at low pH

(FePO₄ ⋅ 2H₂O)
(AlPO₄ ⋅ 2H₂O)

Very low solubility phosphorus compounds at higher pH

Ca(H₂PO₄)₂ moncalcium phosphater is slightly soluble
Ca₂HPO₄ dicalcium phosphate is slightly soluble
Ca₃(PO₄)₂ tricalcium phosphate has very low solubility

[Ca(H₂PO₄)₂ ⋅ H₂O] + 2 H₂O → 2 (CaHPO₄ ⋅ 2 H₂O) + CO₂↑ → Ca₃(PO₄)₂ + CO₂↑ + 5 H₂O

With decreasing solubility to the right.

Plant strategies to obtain enough phosphorus

Plants have evolved six basic strategies that plant roots may employ to enhance their uptake of various forms of phosphorus from soils.

  1. Increased root absorptive surface area.
  2. Chelate iron or aluminum to release P.
  3. Dissolve Ca–P compounds with acid exudates
  4. Exude phosphatase enzymes to release P from organic compounds.
  5. Exude substances to stimulate P-solubilizing rhizobacteria.
  6. Encourage colonization by mycorrhizal fungi that help plants take up P.

Practical Control of Phosphorus (P) Availability

Several means of optimizing P availability while using minimal amount of soluble P include:

  1. Enhance Mycorrhizal Symbiosis.
  2. Use or Breed P-Efficient Plants.
  3. Adjust P Application to Soil Status.
  4. Localized Placement. Phosphorus fertilizer concentrated in a localized zone of soil is less likely to undergo fixation reactions than fertilizer mixed into the bulk soil.
  5. Combine Ammonium with Phosphorus
  6. Control of Soil pH as phosphorus availability is usually best at a pH level between 6 and 7
  7. Cycling of Organic Matter. During the microbial breakdown of organic materials, phosphorus is released and P fixation is reduced.
  8. Use of P-Efficient Cover Crops or Rotation/Agro-forestry plants. Highly P-efficient plants grown as cover crops solubilize and take up phosphorus from the soil profile and release it into the surface soil when their residues decay.

Management Strategies for Controlling Over-enrichment of Soils and Water Pollution by P

  1. Avoid excess accumulation.
  2. Minimize loss in runoff and eroded sediment.
  3. Capture p from runoff.
  4. Capture p from drainage water.
  5. Tie up p with inorganic iron-, aluminum-, or calcium-containing materials.