Investigating the use of silage effluent to improve available phosphorus from Gafsa phosphate rock

Plant phosphorus (P) requirements are largely met through the application of mineral fertilizers with an abundant supply of available P, but matching this supply from organic or alternative P fertilizer sources remains a challenge. An investigation was undertaken to determine if addition of phosphate rock (PR) to silage effluent could be used as a mechanism to increase available P. Addition of PR to effluent reduced recovery of available P, regardless of silage effluent batch, amount of PR added, particle size, and length of PR exposure. This reduction was attributed to a low dissolution of apatite, the presence of calcium (Ca) ions, and the formation of stable iron-phosphate precipitates. Additionally, the anion exchange membrane (AEM) method for available P analysis was found to be affected by organic acids concentrations. The implications are that dilution of silage effluent, rather than addition of PR, may provide improved short-term P availability from tank-recovered silage liquor.