Tuesday, November 14, 2006 - 9:30 AM
213-5
Agronomic Soil Tests are Adequate Indicators of Risk of P Loss to Water.
Excessive soil phosphorus (P) levels, above what is required for maximum economic production, are a problem in many areas and may make a significant contribution to diffuse losses of P into waterways. Identifying areas of high soil P will help target appropriate mitigation measures to tackle losses. Environmental soil tests such as Degree of Phosphorus Saturation (DPS), Mehlich-3 Phosphorus Saturation Ratio (M3PSR) and Water Soluble Phosphorus (WSP) have been suggested to be more suitable for predicting losses of P to the environment, particularly by subsurface pathways, than standard agronomic soil tests that measure plant available P. The ability of Olsen-P, which is the standard agronomic soil test used in Northern Ireland, and Morgan-P, the standard agronomic test used in the Republic of Ireland, to provide an indication of P loss to waterways was examined. The P content of soil within three 5-km2 catchments, two in Northern Ireland and one in Republic of Ireland, was measured. The recommended levels of Olsen- and Morgan-P required for maximum economic production were compared to critical limits of DPS (0.2), M3PSR (0.1-0.15) and WSP (8.6 mg kg-1) above which there has been shown to be a rapid increase in P losses by subsurface pathways. Strong relationships were found between Olsen- or Morgan-P and DPS, M3PSR and WSP (r = 0.7-0.91). A DPS of 0.2 was equivalent to either an Olsen- or Morgan-P index of 2, which is the recommended P level required for maximum economic production. A DPS above 0.2, which has been shown to equate with increased losses of P into waterways, is therefore in agreement with Olsen- and Morgan-P values above the agronomic optimum. The risk of P loss to water in these catchments can therefore be adequately predicted from Olsen and Morgan agronomic P tests, without the need for additional environmental soil tests.
Recorded presentation