Using a Soil Hydrometer to measure the Nitrogen and Phosphorus Contents in Pig Slurries

The hydrometer method to indirectly measure the nutrient contents, i.e. total nitrogen Nt, and total phosphorus Pt of pig slurries was examined against both single and multiple slurry sources. The data indicated that the estimation accuracy of the hydrometer method could be greatly improved if separate linear regressions were developed for slurry sources from pigs at different growing stages such as gestation, nursery, and finishing. In doing so, highly significant correlation coefficients for Nt and Pt with slurry total solids contents St were observed (0·9881 and 0·9935 for gestation; 0·9844 and 0·9873 for nursery; 0·9695 and 0·9628 for finishing). The slurry specific gravity Gs was also found to be well correlated with the Nt and Pt contents with correlation coefficients of 0·9909 and 0·9873 for gestation, 0·9836 and 0·9843 for nursery, and 0·9634 and 0·9745 for finishing, respectively. All these coefficients are much better than those reported by past researchers. The concept of using separate linear equations for different pig species was further supported by examining the results from the lumped data in this study. In this case, the correlation coefficients for Nt and Pt with total solids were 0·8878 and 0·6793, respectively, while those for Nt and Pt with slurry Gs were 0·8176 and 0·7022. In addition, the error analysis revealed that use of separate equations to estimate slurry nutrient contents had lower minimum error limits (±10% for Nt and 11% for Pt — gestation; ±13% for both Nt and Pt — nursery; ±17% for Nt and ±12% for Pt — finishing) than using combined equations (±40% for Nt and ±64% for Pt).