Pig slurries (PS) are widely applied to soil as fertilisers. Compositional variability, as shown in this study, is the main constraint on their efficient use. Slurry samples, collected from 126 commercial pig farms were analysed and organic nitrogen (ON), ammoniacal nitrogen (AN), total nitrogen (TN), total potassium (TK), total phosphorus (TP) content, pH, EC (electrical conductivity), DM (dry matter) and OM (organic matter) quantified. Relationships between major nutrient contents of PS and its physical and chemical properties were analysed. AN and TK were linearly related to EC (p < 0.0001) for values lower than 5.6 kg [AN] m−3 and 5 kg [TK] m−3 with coefficients of determination (R2) of 0.86 and 0.58 respectively, in agreement with previous linear models. At higher AN values (which means EC > 40 dS m−1) linearity cannot be maintained, which has important consequences for direct slurry EC measurements. ON and TP in slurries were closely related (p < 0.0001) to DM with R2 of 0.90 and 0.68 respectively.