Farm-gate phosphorus balances and soil phosphorus concentrations on intensive dairy farms in the south-west of Ireland

Abstract

Phosphorus (P) loss to water is a significant threat to water quality in Ireland. Agriculture is an important source of this P. There is concern about balancing agronomic requirements and environmental protection in regulations prescribing P management on farms. This study examined farm-gate (P) balances and soil test P (STP) concentrations on 21 dairy farms in the south west of Ireland over four years, from 2003 to 2006 inclusive. Stocking density on the farms averaged 2.4 (s.d. = 0.4) livestock units (LU) per ha. Annual mean import of P onto farms was 21.6 (1.9) kg P/ha. Fertilizer P accounted for 47% (0.041), concentrates 35% (0.060) and organic manures 18% (0.034) of imported P. The mean annual P balance per farm was 9.4 (1.2) kg/ha, ranging from –3 to 47 kg/ha and mean P use efficiency was 0.71 (0.05) ranging from 0.24 to 1.37. The mean STP per farm following extraction using Morgan’s solution was 8.15 (2.9) mg/L of soil and ranged from 4.4 (2.2) to 14.7 (6.4) mg/L. There was a positive relationship (R2 = 0.34; P < 0.01) between STP and P balance; farms with a deficit of P tended to have agronomically sub-optimal STP and vice versa. The high between- and withinfarm variation in STP indicates that farmers were either unaware or were not making efficient use of STP results, and consequently there was agronomically sub-optimal soil P status in some fields and potentially environmentally damaging excesses on others (often within one farm). There was considerable potential to improve P management practices on these farms with clear agronomic and environmental benefits.