Agronomic Effectiveness of Calcium Phosphate Recovered from Liquid Swine Manure

A new manure treatment technology developed as an alternative to anaerobic lagoons on swine (Sus scrofa domesticus) farms includes solid-liquid separation and subsequent recovery of soluble P as calcium phosphate from the wastewater. The objective was to determine the agronomic effectiveness of this calcium phosphate material. A greenhouse study was conducted with annual ryegrass (Lolium multiflorum Lam.). Three fertilizer materials were evaluated: the recovered P in two particle sizes (0.5–1.0 and 2.0–4.0 mm), and commercial triple superphosphate (TSP). Fertilizer rates were 0, 22, 44, 88, and 176 mg P kg soil. Three harvests of the ryegrass were made at 2-wk intervals. Total P uptake increased linearly with application rate for all three fertilizer materials. At the highest application rate, total P uptake was 37.8 mg pot for TSP, 26.2 mg pot for the recovered P with small particle size, and 9.0 mg pot for the recovered P with large particle size. Chemical analysis of the recovered P material revealed that over 99% of the P2O5 in the recovered calcium phosphate was plant available P, mostly as citrate-soluble. Mehlich-3 extractable soil P at the end of the experiment indicated that little of the large particle size of recovered P material dissolved and became available during this short 10-wk study. The recovered calcium phosphate appears to have potential as a fertilizer source. REPEATED MANURE APPLICATIONS to land on or around confined animal production facilities has, in some cases, resulted in high soil test P. Overapplication of P can be detrimental to surrounding ecosystems through contamination of surface and groundwaters (Ribaudo et al., 2003). Animal waste treatments that include P recovery from manure are a management option that could resolve problems of excess manure P on agricultural lands (Greaves et al., 1999; Smil, 2000). Recovery of P from animal liquid manures is possible by precipitating it in a variety of chemical forms. These include magnesium ammonium phosphates known as struvites (Webb and Ho, 1992) and calcium phosphates (House, 1999).Much of the literature on recovered phosphates, particularly struvites, from animal liquid manure has been concerned with their production and potential reuse in agriculture (Bowers and Westerman, 2005; Burns et al., 2001; Nelson et al., 2003; Suzuki et al., 2005), but there are very few reports on agronomic effectiveness of recovered phosphates (Johnston and Richards, 2003; Romer, 2006). A new manure treatment process has been developed for swine farms where the soluble P in wastewater is precipitated into a material containing a relative high concentration of calcium phosphate (Vanotti et al., 2003, 2005). The treatment process applied to systems without anaerobic lagoons consists of three steps which (i) mechanically separates solids from the liquid, (ii) biologically removes N from the liquid, and (iii) precipitates soluble P from the liquid with reduced amounts of hydrated lime (Vanotti et al., 2006). The calcium phosphate-rich precipitate can be dewatered to provide a material that is .90% solids (Szogi et al., 2006b). The resulting dry material contains sufficient P concentration (P2O5 . 20%) to be economically transported for fertilizer application or further processing into P containing products. The high phosphate concentration of this recovered P material suggests it likely has utility as a fertilizer source. Our objective was to evaluate agronomic effectiveness of the calcium phosphate recovered from swine wastewater. The study was conducted under greenhouse conditions with annual ryegrass using two particle sizes of the recovered P material compared with commercial TSP. MATERIALS AND METHODS