P's.

“P”s come in many forms.  Considering soil analysis and plant nutrition the abbreviation P stands for Phosphorous or should that be Phosphate?

Soil phosphorus (P) is the essential chemical element, vital for plant life, while phosphate (PO⁴) refers to the specific ionic compound (e.g. orthophosphate) that plants absorb from soil solution for growth, often used interchangeably in fertiliser talk but technically distinct.

Phosphorous exists in the soil in various forms, including unavailable organic and as the mineral.

Phosphate.  Usually only a small, active fraction is available as phosphate.

In essence, you add phosphorus (as a fertiliser) to increase the soil's total phosphorous content, hoping to maintain a sufficient pool of readily available phosphate for your crops.

Total Soil Phosphorous.

Soil concentrations can range from 2 to 9630 mg/kg, with an average global topsoil (0 - 300 mm) concentration of around 529 mg/kg.  In acidic soils it is locked to soil particles in combination with Iron and Aluminium and for alkali soils, Calcium.

Available Soil Phosphate. 

The value obtained by laboratory testing. 

The amount of phosphate available to plants (measured using the Olsen method).  This value is much lower than the Total Soil Phosphorous content.

Vertical Distribution.

Phosphorus tends to accumulate in the top 150 mm of soil, with amounts generally decreasing with depth.

Optimal Levels.

For agricultural production, an Olsen P index of 2+ (21–26 mg/lt) is typically considered optimal.  At this level, to achieve acceptable crop yields means that very small amounts of water-soluble phosphate are lost from soil in drainage water.

Phosphate in water bodies.  Where is it coming from?

Within a catchment area, from discrete source areas rather than from all parts of the catchment.

The transport of phosphorus from soil to water.

Transport from agricultural soils is often described as a “diffuse” loss and used to contrast with inputs to rivers from “point sources” like sewage treatment works.

Diffuse, meaning spread out, not concentrated, and its use in this context is incorrect.

Individual fields, (those with soils of high phosphorus content) or actions such as the inappropriate application of animal slurry under the right conditions of topography and climate can become the major contributors to the phosphate load in rivers and lakes.

To lessen phosphate load in water every effort should be made to identify fields likely to contribute most and manage them accordingly.