Tuesday, November 14, 2006 - 9:30 AM

Agronomic Soil Tests are Adequate Indicators of Risk of P Loss to Water.

Suzanne Mccormick1, J. T. Sims2, Jim Stevens3, and Phil Jordan1. (1) Univ of Ulster, School of Environmental Sciences, Cromore Road, Coleraine, BT52 1SA, United Kingdom, (2) Univ of Delaware, Dept of Plant and Soil Sciences, 531 S College Ave, Newark, DE 19717-1303, (3) Agri-Food and Biosciences Institute, Newforge Lane, Belfast, BT9 5PX, United Kingdom

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.