Evaluating Morgan’s Phosphorus Test as an Environmental Indicator

Abstract

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 upwards 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 might predict areas vulnerable to P loss. Soil test P is a typical parameter often used in risk assessment and catchment modeling 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 phosphorus (P) testing in Ireland uses Morgan's reagent from samples taken to 10cm depth for agronomic recommendations, however, it's sustainability as an environmental indicator has been questioned in terms of sample depth and extraction solution. In this study, seven grassland sites were sampled to 2, 5 and 10cm depths and extracted for Morgan's P, the standard agronomic test, as well as, iron-oxide impregnated paper strip P (FeO-P), calcium chloride extractable P (CaCl2-P)and water soluble P (WSP), all proposed as environmental soil tests. Extractable soil P increased with decreasing sample depth, as did variances in each test, such that, 2 cm samples had highest concentrations and variances. The current standard sample depth (10cm) was linearly correlated with corresponding data from samples taken to 2 and 5cm, indicating that surface soil P can be consistently estimated from the current standard depth. When soil tests were compared with Dissolved reactive P (DRP) in overland flow collected from field studies, certain soil tests were better indicators of P loss than others. The relative difference in Morgan's P values at the standard sample depth (10cm) was reflected in the relative difference in P loss between the two sites. Values of DRP collected from two sites ranged from 0.032-0.67 mg/l at the low P site and 0.261-0.620 at the high P site. Average DRP values from the high P site and maximum DRP values from the low P site were simulated using water-soluble P extracts. In this study, Morgan's P to 10cm gave a good indication of the relative difference in DRP loss between the two sites.