Phosphorus loss from soil to water is a major component of surface-water pollution in Ireland and water quality reports from the EPA have shown an increasing upward trend in slight and moderate pollution often attributed to agricultural phosphorus (P).

There is a need, therefore, for indicators of P loss from agriculture that can be related to water quality, such as models and risk assessment tools that might predict areas vulnerable to P loss.

Soil test P is a typical parameter often used in risk assessment and catchment modelling and although originally designed as an agronomic indicator of P requirements for crop growth, it is often used to indicate risk of P loss from agricultural areas.

Soil P testing in Ireland uses the Morgan’s P test to extract plant-available P as an indicator of crop requirements for fertiliser advice. This test uses an acidic extract on soils sampled to 10 cm and has been used in the context of potential losses of P from soil to water.

In Europe and the United States, there have been a number of alternative or environmental soil-test procedures developed as indicators of soil P loss that focus on simulating desorption and solubilisation of P from soil to solution. These procedures generally require a mild extractant, such as water or calcium chloride, in an attempt to simulate overland flow concentrations and often require shallow sampling depths and wider solution-to-soil ratios than agronomic P tests.

In addition, the use of P saturation as an environmental indicator has also been used to assess potential P loss from soils since it is a measure of P concentration in soil relative to amounts of Al and Fe, which symbolise P saturation sites in soil.

This project attempted to place soil P testing in an environmental context by questioning conventional sample depth, sample pre-treatment, choice of extractant and extract ratios against a background of dissolved reactive P (DRP) and suspended solids (SS) concentrations monitored from the same sample sites. The work examined laboratory-scale soil P extraction techniques and field-scale P loss using soils sampled from sites at Johnstown Castle Wexford.

Methods

Soil samples were collected from seven grassland field sites at Johnstown Castle covering a range of soil P levels. These sites were chosen since they are subjected to continuous monitoring of overland flow during rainfall events and could provide already installed instrumentation for gathering information on edge-of-field P losses.

Seven field sites provided soil samples for analysis and two of these sites were used to collect and analyse DRP and SS in overland flow. 

Full executive summary in report.