The Phosphorus Cycle

Overview

The phosphorus cycle describes the movement of phosphorus between minerals, organic matter, soil solution, plants, and microorganisms.

Unlike nitrogen: - Phosphorus has no gaseous phase - Movement is slow - Availability is strongly controlled by chemistry and pH

Major phosphorus pools

Mineral phosphorus

• Bound in primary minerals and secondary phosphates
• Slowly released through weathering

Organic phosphorus

• Found in organic matter and residues
• Must be mineralised before uptake

Soil solution phosphorus

• Present at very low concentrations
• Continuously replenished from solid phases

Key processes

Mineralisation

Microbial conversion of organic phosphorus into plant-available phosphate.

Fixation

Chemical binding of phosphate to: - Iron and aluminium at low pH - Calcium at high pH

Fixation reduces immediate availability but does not remove phosphorus from the system.

Desorption

Release of phosphate back into soil solution as plants remove P.

Plant uptake forms

• H₂PO₄⁻
• HPO₄²⁻

The dominant form depends on pH.

Phosphorus loss pathways

• Soil erosion and particulate loss
• Runoff of dissolved phosphate
• Very limited leaching compared to nitrate

Practical implications

• Phosphorus availability is highest in moderately acidic to neutral pH
• Excess P can induce micronutrient deficiencies (Zn, Fe, Cu)
• Building soil P is slow and persistent
• Over-application has long-term environmental consequences

Agronomic note

Phosphorus problems are usually availability problems, not absolute shortage.