The Role of Soil pH in Phosphorus Dynamics

Soil pH dictates various aspects of soil fertility. For phosphorus, soil pH impacts the chemical form present in the soil. As soil pH increases, the concentration of hydrogen ions decreases; likewise, the number of hydrogens associated with phosphate decreases.

Iron Fixation in Highly Acidic Soils

At soil pH levels below 3.0–4.0, the predominant form of phosphate in the soil is H₃PO₄. This form is not plant-available and has a high chemical reactivity with iron. This low pH also greatly increases the water-soluble of iron, which creates ideal conditions for the two to bond.

·        Oxidized Iron Soils (Red Clay): If the iron is in an oxidized state, the bond is very strong. This creates an insoluble mineral that has extremely low solubility and plant availability. Soil pH adjustments have little to no effect on releasing phosphorus bonded to oxidized iron.

·        Reduced Iron Soils (Grey Clay): If the iron is in a reduced state, the bond is also very strong but maintains a low level of water-solubility.

Aluminum Interactions and H₂PO₄^- Availability

As soil pH increases, the predominance of H₂PO₄^- increases, which is plant-available. The concentration of H₂PO₄^-starts at a soil pH of 3.5–4.0, peaks at a soil pH of 5.5–6.0, and ends at pH levels above 6.5–7.0. This form has limited reactivity with iron but is reactive with aluminum. Aluminum water-soluble occurs at soil pH levels below 5.0–5.5. The peaks of availability/ water-soluble between H₂PO₄^- and aluminum do not quite align. Additionally, as aluminum is included with iron, or replaces iron in the reaction, the water-soluble and plant availability of the resulting mineral increases slightly.

The "Sweet Spot" for Phosphorus Availability

Between pH 6.0–7.5, phosphorus exists as either H₂PO₄^- or HPO₄^2- _, both of which are plant-available, and the reactive partners of iron and aluminum are not water-soluble. This results in the optimal pH for plant-available phosphorus, with the lowest rate and severity of fixation due to the formation of insoluble minerals.

Alkaline Soils and Calcium Fixation

It is not until soil pH levels rise above 7.5 that HPO₄^2- is the prevalent form and excessive calcium drives the reaction of phosphorus with calcium. High soil pH is caused by the over-application of lime or naturally high calcium carbonate content in the soil. Because high soil pH and high calcium levels are strongly correlated, calcium is often confused with soil pH. The mineral formed in this environment between calcium and phosphorus closely resembles the rock phosphate mined for fertilizer production, which has low water-solubility. The amount of phosphorus participating in this reaction continues to increase as soil pH increases.  However, calcium phosphate minerals are more water-soluble than the minerals formed at a low soil pH. Like the mined mineral, the bond between calcium and phosphate can be broken by acidifying the material, leading to a significant increase in water-solubility. Acidic root exudates are effective at breaking this bond, leading to increased plant availability.

Comparison of Phosphorus Fixation by Soil Environment