Relationships of soil test phosphorus with soil properties and phosphorus forms

A comprehensive approach to soil P management in agricultural systems requires a balance between nutritional enhancement of the soil and a reduction in adverse consequences on the environment. Evaluation of forms in which P is present in the soil, dynamics of P transformation across these forms, determinants of spatial and temporal distribution of P pools in the soil, and impacts of agricultural practices on the rate and direction of these reactions were the key objectives of the present investigation.

Phosphorus extracted by the four widely used soil extractants, i.e., Bray 1, Mehlich 1, Mehlich 3, and Olsen, represented less than 25 percent of the total P content in 43 Virginia soils from the Coastal Plain, Piedmont, and Ridge and Valley regions. Correlation between Bray 1 (1 minute and 5-minute shaking), Mehlich 1, and Mehlich 3 extractable P was high: correlation of these extractants was poor with the Olsen procedure which extracted the lowest soil P contents. The Mehlich 3 procedure was more reliable in the extraction of labile P than the Mehlich 1 procedure in soils with a wide range of pH values. The Mehlich 3 procedure was less prone to extraction of plant unavailable, structurally adsorbed phosphorus, particularly in dithionite-citrate-bicarbonate extractable Al₂O₃ rich soils, than the other procedures. The process of P extraction by different extractants under varying physical and chemical conditions was satisfactorily explained by an acid-catalyzed nucleophilic substitution reaction mechanism. This mechanism satisfactorily links the solubilization of P from Al and Fe surfaces to the strength of nucleophilic agents such as OH⁻, CH₃COO⁻, CI⁻, F⁻, and H₂O, and to the acidity of the supernatant solution.

Inorganic P fractionation was determined by the modified Chang and Jackson (1957) procedure of Peterson and Corey (1966) with several modifications, and the organic P analyzed by the Bowman (1989) method. The total P content of the 43 soils in this study ranged from 0.02% to 0.4%. Twenty six percent of the total P was in the labile to moderately labile pool, 49% in the non-labile pool, and 25% in the organic fraction. The distribution pattern of Fe-P, reductant soluble P, occluded P, residual P, and organic P fractions was directly related to soil Fe₂O₃ and clay contents. The Al-P content was not markedly influenced by the clay content or pH. It was concluded that Fe-P, a dominant labile P form in agricultural soils was transformed over time to reductant soluble P, the dominant P form in the soils. The reductant soluble P fraction, which increases with P fertilization and has higher solubility under reduced conditions, through erosion poses a major potential eutrophication threat to surface waters. The eutrophication potential is high in the Piedmont and high clay soils.

Phosphorus adsorption in the Davidson and Tatum soils from the Piedmont physiographic region was well explained by the two-site-Langmuir adsorption model. The adsorption maxima was slightly (5%) decreased by long-term P fertilization over three decades. All soil inorganic and organic P fractions were increased by fertilizer application with the reductant soluble fraction increasing by 61 and 57 percent in the Davidson and Tatum soils, respectively. The residual effects of long-term application of P fertilizer increased labile P contents extracted by soil test methods, and in the Davidson soil the uptake from the residual fertilizer P was sufficient to maintain the tissue P concentrations of young corn seedlings at the sufficiency range.