Relationship between Phosphorus Levels in Three Ultisols and Phosphorus Concentrations in Runoff

TLDR

Soils with high phosphorus can release dissolved reactive phosphorus into runoff, accelerating eutrophication, and prior work on Captina soil showed that distilled water and NH4‑oxalate extraction methods best correlated with runoff DRP. The study aimed to determine the most consistent soil test phosphorus method for predicting runoff DRP across three Ultisols and to assess how site hydrology influences STP–DRP correlations. Surface soils (0–2 cm) from pasture plots were analyzed using Mehlich III, Olsen, Morgan, Bray‑Kurtz P1, NH4‑oxalate, and distilled water methods, with P saturation measured by three approaches, and 75 mm h⁻¹ simulated rainfall generated 30‑minute runoff from each plot. All STP–runoff DRP correlations were significant (P < 0.01), with most methods achieving r > 0.90; moreover, when DRP was normalized by runoff volume, H2O‑extractable STP correlated with the same regression line across soils, highlighting hydrology’s role and enabling a unified predictive relationship.

Abstract

Abstract Soils that contain high P levels can become a primary source of dissolved reactive P (DRP) in runoff, and thus contribute to accelerated eutrophication of surface waters. In a previous study on Captina soil, several soil test P (STP) methods gave results that were significantly correlated to DRP levels in runoff, but distilled H 2 O and NH 4 ‐oxalate methods gave the best correlations. Because results might differ on other soils, runoff studies were conducted on three additional Ultisols to identify the most consistent STP method for predicting runoff DRP levels, and determine effects of site hydrology on correlations between STP and runoff DRP concentrations. Surface soil (0–2 cm depth) of pasture plots was analyzed by Mehlich III, Olsen, Morgan, Bray‐Kurtz P1, NH 4 ‐oxalate, and distilled H 2 0 methods. Also, P saturation of each soil was determined by three different methods. Simulated rain (75 mm h −1 ) produced 30 min of runoff from each plot. All correlations of STP to runoff DRP were significant ( P &lt; 0.01) regardless of soil series or STP method, with most STP methods giving high correlations ( r &gt; 0.90) on all three soils. For given level of H 2 O‐extractable STP, low runoff volumes coincided with low DRP concentrations. Therefore, when each DRP concentration was divided by volume of plot runoff, correlations to H 2 O‐extractable STP had the same ( P &lt; 0.05) regression line for every soil. This suggests the importance of site hydrology in determining P loss in runoff, and may provide a means of developing a single relationship for a range of soil series.