Phosphorus solubility of agricultural soils: A surface charge and phosphorus-31 NMR speciation study

We investigated 10 soils from six states in the United States to determine the relationship between potentiometric titration derived soil surface charge and the concentration of water-extractable P (WEP). Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy was used to determine the chemical speciation of soil P. The surface charge value at the native soil pH was correlated to the WEP concentration, indicating that electrostatic interactions are involved in determining soil phosphate solubility. The titration curves were fit to a two-site Langmuir model and analysis showed that the native pH surface charge was accounted for by the low pH Type 1 (S-OH₂ ⁺) site, attributed to positively charged metal (oxy)hydroxides. The 31P NMR data indicated that 98% of the inorganic form of P was composed of orthophosphate species and 95% of the organic P was composed of the P monoester class compounds. The inorganic orthophosphate form of P was directly related to the total soil P content, suggesting that external fertilizer inputs control the level of this form of soil P. In contrast, P monoester class compound content was not related to total soil P content, suggesting that organic soil P is controlled by P cycling independent of external P inputs. The ³¹P NMR speciation data indicated that the inorganic orthophosphate, pyrophosphate, and DNA P concentrations in the soils were significantly associated wiThoxalate-extractable Al and Fe concentrations, which further demonstrates that metal (oxy)hydroxides are important surfaces where P species are interacting with soils.