Effect of kinetic control, soil:solution ratio, electrolyte cation, and others, on equilibrium phosphorus concentration

Equilibrium phosphorus concentration (EPC) is the aqueous phosphorus (P) concentration at which no net adsorption or desorption occurs when a soil is in contact with water; and are typically determined experimentally by plotting the measured amount of sorption versus initial concentration of P. In this study we evaluated the effect experimental parameters such as equilibration time, soil:solution ratio (SSR), and solution electrolyte may have on EPC values for two distinctly different soil types, (i) organic muck soil, and (ii) loamy mineral soil. Results indicate that there is a significant difference in emerging EPC values with changing equilibrium time, SSR, and electrolyte. For the muck soil, we observed up to 97% decrease in EPC while increasing equilibration time from 1 to 8 h using 1:10 SSR. For the same soil the most profound effect of SSR was observed for 1 h equilibration where EPC value decreased from 0.19 mg L− 1 using 1:2 SSR to 0.03 mg L− 1 using 1:20 SSR, corresponding to an 84% change. The EPC of the loamy soil decreased by 94% while increasing equilibration time from 24 to 72 h using 1:20 SSR, whereas the same soil change in SSR was highly variable. Overall there was a 38% increase in EPC values using 0.01 M CaCl2 compared to KCl as the solution electrolyte in the experiment. These results indicate that experimental conditions can significantly affect EPC values (besides other factors) and therefore need to be standardized especially when used as a tool to assess P loading in natural systems.