b'4The influence of lime and phosphorus on soil P pools and soil pH in a controlled incubation experiment across a range of heavy soilsCorbett D1, Wall DP2, Lynch B3 and Tuohy P1,1Teagasc, Animal & Grassland Research and Innovation Centre, Moorepark, Fermoy, Cork, Ireland2Teagasc, Crops, Environmental and Land-use Programme, Johnstown Castle, Wexford, Ireland3School of Agriculture and Food Science, University College Dublin, IrelandSoil fertility is a major limiting factor on the output potential of grassland farms, particularly soils with low pH and soil test phosphorus (STP) concentrations. Optimum soil pH increases soil microbiological activity, promotes the release of nutrients and increases the efficiency of organic and chemical fertilisers. Soil pH is a fundamental factor that must be optimised when improving soil fertility as it regulates the availability of macro- and micro-nutrients. Phosphorus (P) isimportant for the growth, establishment and persistence of productive grassland species,however it can pose an environmental threat with potential impacts on water quality. Over time the soil pH and STP status of the soil changes depending on the intensity of production, fertiliser management, soil physical and chemical characteristics and climate. Limited knowledge exists on the quantity of P required to build-up soil P supply for grass production on high clay textured soils and organic soils and the P loss risks associated with such P fertiliser inputs.To elucidate the responses of soil P pools (STP and water soluble P (WSP)) to lime and P fertiliser inputs on soils with high levels of clay and, or, high OM levels a controlled soil incubation study was established using 25 soils from farms participating in the heavy soils research programme. Six treatments were imposed on the soil; namely 0, 50 and 150 kg ha-1 equivalent of P both with and without the addition of 5 tonne ha-1 equivalent of ground limestone. Soils were incubated at constant temperature, humidity, water filled pore space and bulk density for a total of 140 days with sampling at 35 day intervals. The initial soils were analysed for OM% and total P and metals. The change in soil chemistry (pH & P pools) was assessed over the trial period where at each sampling time the soils were analysed for STP and bioavailable metals, WEP and soil pH. There is a significant effect of soil type (p<0.0001) and treatment (p<0.0001) on the concentration of WEP. Increasing P fertiliser rate significantly increased WEP across all soils with a mean value of 0.545 mg l-1 on mineral soils and 5.349 mg l-1 on organic soils for the 150 kg ha-1 P fertiliser rate. Overall results indicate that there were higher risk of P loss on organic soils (>20%OM) compared to mineral soils, and that careful P fertiliser management is required on all soils to minimise such losses.48'