Different Biosolids, Different Phosphorus Availabilities.

Biosolids can be beneficially reused through land application to crop fields.  However, the processes used to clean wastewater at different treatment facilities impact the availability of nutrients in the resulting biosolids.  In order to protect water quality after land application of biosolids and yet provide adequate nutrients for crops, phosphorus availability from biosolids needs to be quantified.  One goal is to include biosolids in the Wisconsin Phosphorus Index to give farmers a tool to better manage nutrients.  This research is a compilation of data from three soil incubations using six biosolids collected from wastewater treatment plants in southeastern Wisconsin.  Each incubation applied the six biosolids and a fertilizer at the same total P rate (110 kg P/ha) and included a control (no P addition), but used different soils (Amnicon Cuttre complex, Kewaunee silt loam, Plainfield sand).  Soil and biosolids mixtures (replicated in quadruplicate) were incubated at 25^oC for 11 weeks at approximately 60% water filled pore space to simulate a Wisconsin summer.  At the end of the incubations, dried soil mixtures were analyzed for Bray P1, Mehlich 3, phosphorus saturation, water extractable P, and total P.  Statistical analysis was performed in R including ANOVA analysis with Tukey HSD post hoc separation if the overall F test was significant (p<0.05).  The results indicate that biosolids do not have the same phosphorus availability as measured by the various soil test phosphorus methods.  The class B biosolids with biological phosphorus removal (Fort Atkinson) had consistently the highest P availability across all three soils, even higher than the fertilizer treatment (applied at the same total P rate).  The biosolids coming from wastewater treatment plants using alum or ferrous chloride for phosphorus removal (Brookfield, Whitewater, East Troy, and Mukwonago) tended to be similar to each other as measured by soil test P.  The class A biosolids with no chemical treatment (Delafield) consistently had the lowest P availability, often not different than the control treatment (no P addition).  This data can inform the Wisconsin Phosphorus Index, and the goal is to include biosolids in the index in the future.