At pH 4.0 the predominant ionic form of phosphorus present is

Phosphorus is most abundant in the soil at pH 6 to 7, but at pH 4, i.e. in acidic soils, the dominant ionic form of phosphorus present is orthophosphates, i.e., $${{\text{H}}_2}{\text{PO}}_4^ - .$$   Availability of phosphorus decreased with an increase in pH. But to maintain the availability of phosphorus at a high pH, the percentage of organic matter in the soil is important.