Comparison of soil phosphorus index systems for grassland in the cross-border region of Ireland^#

Background: The use of soil phosphorus (P) tests and index systems provides a guide for agronomic nutrient requirements and is frequently also used to estimate risk of P losses to watercourses. Use of soil testing and management based on the results thereof is mandated in some regions. Several P extraction methods are available which evaluate different P pools and are designed for particular soil types. Further to this, index systems categorising specific ranges of plant-available P, differ. Hence, translation between different tests and index systems is not straightforward. In cross-border regions, where hydrologic basins encompass more than one political jurisdiction, different tests and rules are implemented in adjacent lands. This can create disparities in land management, confusion as to what legislation applies and obscures the impacts of best management practices at catchment scale. Aims: The aim of this research was to compare the Morgan's and Olsen soil tests used to quantify plant-available P and the respective index systems, in a border region of the Republic of Ireland (ROI) – Northern Ireland (NI). Methods: Olsen, Morgans and water extractable P (WEP) were evaluated (n = 1,038). Statistical analysis was conducted to derive conversion equations to translate between the statutory test methods and comparison of the respective index categories was performed. Results: The conversion equations compared favourably with previous attempts. A stronger relationship was observed between Morgan P and WEP (R² = 0.60) than between Olsen P and WEP (R² = 0.45) (including pH and site as interaction factors). The ROI index system was found to indicate lower levels of plant available P in the soil compared to the NI system, for the same soils. Conclusions: The differences in categorisation of P availability using either index system creates differences in fertiliser recommendations and also perceived aquatic risks even within small cross-border catchments. This study points to a wider implication for international cross-border catchments, suggesting that evaluation of the relationships between adjacent national soil index systems is required to achieve harmonised management of shared waterbodies. Neither index system is preferred, but a combination of soil P tests incorporating both agronomic and environmentally oriented analyses may have utility in future decision support tools.