Phosphorus adsorption by these Fe and Al oxides declines with increasing pH, and P availability in most soils is at a maximum near pH 6.5.

At low pH, P fixation/adsorption is largely from reaction with Fe/Al oxides and precipitation as AlPO₄ and FePO₄. As pH increases, solution Fe and Al decreases, which reduces the P adsorption/precipitation and increases P concentration in the soil solution.

Consequently, the concentration of total soil P has little or no relationship to P availability to plants. Maintaining optimal soil pH is critical for maintaining plant available soil P to plant root systems.

The most critical function of phosphorus in plants is in the storage and transfer of energy. Both adenosine diphosphate (ADP) adenosine triphosphate (ATP) act as energy currency within plants.

While P is immobile in the soil (does not freely move in the soil solution) it is considered somewhat mobile in the plant. Therefore, like nitrogen, deficiency symptoms first appear on older (lower) leaves.

Plant – Plants will be stunted and appear to be a dark green to bluish green.

Leaves – Symptoms appear first on older (lower) leaves. Leaf tips and margins will be reddish-purple.

Plant and Leaf Tissue Deficiency Levels

Early growth – recently mature leaf P of < 0.30%¹.

Tassel – recently mature ear-leaf P of < 0.25%¹.

Plant – Stunted growth, lack of canopy, and upright leaves.

Leaves – Dark green to bluish leaves with small, interveinal lesions. Symptoms first appear on older (lower) leaves.

Plant and Leaf Tissue Deficiency Levels

At-flowering – recently mature leaf tissue P (whole trifoliate) of < 0.30%¹.