Traditional soil P test methods estimate plant available inorganic P but ignore the less available inorganic and organic P pools. In low‐input systems where fertilizer P additions are very low to nil, these less available P pools may be a better measure of potential plant available P, particularly in highly weathered soils. The objectives of this study were to determine the size and changes in soil P pools in the nonfertilized and fertilized treatments of a long‐term continuous cultivation experiment established on a Typic Paleudult in Yurimaguas, Peru. A modified version of the Hedley et al. procedure was used to sequentially fractionate soil P into increasingly recalcitrant organic and inorganic pools. The use of path analysis highlights the interactions among P pools and the different roles of the pools in P cycling between the nonfertilized and fertilized system. For the fertilized system, the NaOH‐extractable inorganic P pool acts as a sink for fertilizer P but desorption is rapid enough to maintain high levels of plant available P. For this system, inorganic P pools explain 96% of the variation in the level of available P. Organic P is the primary source of plant‐available P in the nonfertilized system and explains 44% of the variation in available P. Available P, measured by anion exchange resin, is dependent on crop residue (b values ± standard error = 4.98 ± 3.57), whereas yield depends most strongly on the available P (0.16 ± 0.11) and on NaOH‐extractable organic P (−0.17 ± 0.11). The lack of stability in organic P levels in the first 10 yr illustrates the need for long‐term experiments. The presented results support the notion that traditional soil P tests are inadequate for low‐ to no‐input systems.

Appalachian USA surface coal mines face public and regulatory pressure to reduce total dissolved solids (TDS) in discharge waters, primarily due to effects on sensitive macroinvertebrates. Specific conductance (SC) is an accurate surrogate for TDS and relatively low levels of SC (300-500 μS cm(-1)) have been proposed as regulatory benchmarks for instream water quality. Discharge levels of TDS from regional coal mines are frequently >1000 μS cm(-1). The primary objectives of this study were to (a) determine the effect of rock type and weathering status on SC leaching potentials for a wide range of regional mine spoils; (b) to relate leachate SC from laboratory columns to actual measured discharge SC from field sites; and (c) determine effective rapid lab analyses for SC prediction of overburden materials. We correlated laboratory unsaturated column leaching results for 39 overburden materials with a range of static lab parameters such as total-S, saturated paste SC, and neutralization potential. We also compared column data with available field leaching and valley fill discharge SC data. Leachate SC is strongly related to rock type and pre-disturbance weathering. Fine-textured and non-weathered strata generally produced higher SC and pose greater TDS risk. High-S black shales produced the highest leachate SC. Lab columns generated similar range and overall SC decay response to field observations within 5-10 leaching cycles, while actual reduction in SC in the field occurs over years to decades. Initial peak SC can be reliably predicted (R(2) > 0.850; p < 0.001) by simple lab saturated paste or 1:2 spoil:water SC procedures, but predictions of longer-term SC levels are less reliable and deserve further study. Overall TDS release risk can be accurately predicted by a combination of rock type + S content, weathering extent, and simple rapid SC lab measurements.

Methods for environmental risk assessment of P loss potential from soils lack uniformity and are generally difficult for routine analysis. Mehlich‐1 extractable P (M1‐P), an approach that is widely used to assess soil P status for plant growth, was used as a soil test P (STP) estimator of the degree of P saturation (DPS) of a soil. The concept of DPS integrates the dominant properties controlling the P sorption‐desorption status of soils. Soil samples from three physiographic regions of Virginia were analyzed for M1‐P and a wide range of other extractable P forms and selected chemical and physical soil properties. The DPS determined by ammonium oxalate (NH4–Ox) extractable P (Pox), Al (Alox), and Fe (Feox), ranged from 2 to 155%. Mehlich‐1 P, with a range of 1 to 1100 mg kg−1 was the most suitable single variable for estimating DPS. However, soil type and properties from the three physiographic regions were sufficiently different that regression models to estimate DPS based on M1‐P were significantly (P < 0.001) different between regions. Addition of other chemical or physical soil properties yielded insufficient improvements to the regression models over the strong relationships (r2 = 0.93, 0.98, and 0.75 for the Ridge & Valley, Piedmont, and Coastal Plain regions, respectively) between M1‐P and DPS. Interpretations/comparisons between studies are often limited by the numerous methods that are used to calculate DPS. We recommend DPS be determined as mmol kg−1 of NH4–Ox extractable P, Al and Fe and calculated as 100 (Pox) (Alox + Feox)−1

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Summary Andisols can absorb large amounts of phosphorus rapidly, and then release it slowly, yet the mechanisms by which they retain P and release it for plant growth are poorly understood. Ligand exchange of organic compounds from Al–humic complexes by P and/or Si release – due to breakdown of allophanic microstructure to provide sorption sites – might account for the retention of P, but its extent is not known. We applied a soil column flow‐through technique to quantify the release of anions and organic carbon (C) associated with P sorption by two andic soils, and we related the anion release to possible mechanisms for the retention of P. Phosphate (H2PO4–, HPO42–) sorption and concurrent anion desorption were obtained by passing a 1‐g P 1–1 (32 mmol KH2PO4 in 1 mm CaCl2) solution through the soil columns (25 cm3). Total dissolved P, Fe, Al, S, Ca, Mg, K, Mn, organic C and pH were determined in the eluent. Changes in eluent pH and the patterns of the retention of P and corresponding concentrations of Al, Si and organic C in the eluent were similar for the two Andisols. The general pattern and changes in pH of the eluent coincided with changes in the patterns of release of organic C and Si and the rate of P retention. Release of silica accounted for < 6% of the P sorbed and had only a minor role in P retention in these two Andisols. Release of organic C, however, accounted on a molar basis for 40% and 83%, respectively, of the P sorbed. Direct measurements of the pH of the eluent and release of anions and organic C concurrent to P retention contribute to rapid assessment of the controlling mechanisms of P retention. The results indirectly confirm the hypothesis of ligand exchange of solution P with organic complexes held on allophanic surfaces. The organic C release, however, is not specifically related to either the fast or the slow P retention phase. The shift in the controlling P retention reaction associated with a change from the fast to the slow P retention phase is clearly indicated by an abrupt change of the pH of the eluent. This shift, in previous studies identified graphically by a change in slope of the P sorption isotherm, can be identified directly by measuring the pH of the matrix.