Phosphorus fractions in solid and liquid separates of Swine slurry separated using different technologies

Solid-liquid separation is a manure management option whereby P-rich solid is separated from N-rich liquid, allowing the separated liquid to be used as a fertilizer without oversupplying P. Little information is available on how the different P fractions in manures are partitioned to solid and liquid during separation. We examined the distribution of various P fractions in liquid and solid separates of swine manure, separated using different techniques, to gain information useful for making choices regarding the optimum use of manure separates. Samples of raw manure (RM) and their separated solid (SS) and liquid (SL) were obtained using three different separation techniques: (i) centrifugation without flocculant (CNF), (ii) centrifugation with a flocculant (CFL), and (iii) rotary press with a flocculant (RFL). These were subsequently analyzed for P using a modified Hedley fractionation scheme. Only a small proportion of RM, ranging from 5 to 12%, was recovered in SS, an advantage if SS is to be transported off-site. Concentrations of molybdate-reactive P and total P in all P fractions were less in SL than in the corresponding RM on a fresh-weight basis. The separation index (percentage partitioned to SS) for total labile P (water-extractable + NaHCO-extractable P) was 63, 81, and 75% for CNF, CFL, and RFL, respectively. The proportion of total P in labile form was significantly lower in SL than in RM. Therefore, using SL as a fertilizer instead of RM may help to avoid excessive buildup of soil test P with manure applications.