Phosphorus management, changes in soil P status over time and stream P loss in an intensive dairy catchment

Phosphorus (P) inputs are vital to maintaining agronomically optimal levels of production in intensive, grazed, grass-based dairy production systems. However, P is a costly input and is also a finite mineral resource and mismanagement of P inputs has been associated with P losses to water and impacts on water quality. This paper presents results from the Agricultural Catchments Programme; an integrated advisory/research programme working with stakeholders to assess the efficacy of Ireland’s Good Agricultural Practice (GAP) measures in meeting the targets of the EU Nitrates and Water Framework Directives. Results are presented for field P sources, management and losses in the stream for a 7.6 km2 catchment dominated by intensive, grazed, grass-based dairy production on well drained soils with permeable geology. Phosphorus management and source pressures were characterised in terms of field-scale P inputs and balances, recorded on-farm, and surface soil P status, assessed by sampling at a resolution of <2 ha across the catchment. Changes in soil P status over time were assessed by re-sampling the same sample areas after three years. Phosphorus loss was characterised in terms of P concentration and loads monitored continuously with high-resolution bank-side analysers at the catchment outlet. Mean fertilizer and manure P field inputs in 2011 were 26.5 kg ha-1 (SD, 27.4). Most P (83 %) applied to grassland was in organic forms (slurry and farmyard manure). Peak P application was in February to May (63 %) with no P applied from late October to mid January. Initially, 30 % of soil samples had excessive P, but this decreased to 25 % over three years. Total stream P loss in 2010-2011 amounted to 0.54 kg ha-1 yr -1, with 62 % of this as reactive P. Results suggest that the GAP measures related to rates and timings of field P application are largely being followed, that soil P status would appear to be responding as intended, and that P losses in stream water are small relative to the quantity applied and, on balance, are likely to decrease over time in response to implementation of the GAP measures. This paper considers further implications for effectiveness of GAP measures, agronomically and environmentally, in intensive, grazed, grass-based dairy production systems, including appropriate scales for implementation and monitoring of GAP measures.