Interlaboratory Comparison of a Standardized Phosphorus Adsorption Procedure

Abstract

A standard P adsorption procedure was proposed and the ability of four laboratories to produce consistent results over a wide range of soils was determined. For this procedure, 0.5 to 1.0 g of soil were shaken in 0.01 mot L CaCI at a soil/solution ratio of 1:25 in containers allowing a 50% head space for 24 h at 24 to 26°C on an end-over-end shaker. Initial dissolved inorganic P concentrations of 0 to 323 smol P L (as KHPO4 or NaH1`04) were used and microbial activity inhibited by 20 g L chloroform. Excellent agreement between the four laboratories was obtained for the amount of P adsorbed by the 12 soils studied, with a mean coefficient of variation (CV) over all P levels and soils of 0.91 01o. The laboratories also exhibited a high degree of replication of individual treatments with no laboratory showing a strong consistent bias across all soils and P levels in terms of P adsorption. Langmuir, Freundlich, and Tempkin adsorption models were highly correlated with the adsorption data. Respective mean correlations for the 12 soils were 0.98, 0.97, and 0.95. The proposed method, therefore, has the potential to produce consistent results that can be used to predict partitioning of dissolved inorganic P between solid and solution phases in the environment.

Interlaboratory comparison of soil phosphorus extracted by various soil test methods

“…Similarly, Sharpley et al (6) found a close agreement between iron-oxide strip (Fe-strip) P measured by three different laboratories on a variety of soils (pH 5.5 to 8.0). Elsewhere, Nair et al (7) examined variability related to P adsorption procedures, reporting strong agreement in isotherm parameters as estimated by four laboratories for a wide range of soils (pH 5.6 -7.9).…”

Chemical Distribution of Phosphorus in Soils used during the Development of Sorption Isotherms

“…Although less than 25% of the sorbed P was found in the non-labile pools, the Naoh+hCl fraction contributed 2 to 61% of the overall sorption strength estimated for the soils studied. The results of this study suggest that sorption strength calculated from sorption studies are heavily skewed toward the nonlabile fraction, which was found to constitute a very small portion of the binding sites occupied during sorption studies.Abbreviations: b, the sorption maximum; ICP-OES, inductively coupled plasma optical emission spectroscopy; b f , sorption maximum determined using the sequential fractionation data; b i , sorption maximum estimated using the method of Nair et al (1984); k, a coefficient unique to each soil that has been used as a measure of the sorption strength between soil particles and P; k f , the sorption strength parameter determined using the sequential fractionation data; k i , the sorption strength parameter estimated using the method of Nair et al (1984); OM, organic matter; P i , inorganic P; P o , organic P; W kft , cumulative weighted sorption strength. W hen P is added to soils in the form of fertilizer, a series of reactions takes place, ranging from diffusion of P from the fertilizer granules into the soil solution, sorption of P into soil particles, and, with time, P precipitation (Hedley and McLaughlin, 2005).…”

“…To study the sorption capacity of soils, many researchers have used sorption studies, where a given amount of soil is mixed with solutions made up with various P concentrations (Nair et al, 1984). The amount of P adsorbed onto the soil is estimated as the difference between the initial and final solution P concentrations (Olsen and Watanabe 1957;Nair et al, 1984).…”

“…The amount of P adsorbed onto the soil is estimated as the difference between the initial and final solution P concentrations (Olsen and Watanabe 1957;Nair et al, 1984). Therefore, one can describe P adsorption in soils as:…”

“…For example, alkaline soils with a high amount of Ca will have higher accumulation of P in the HCl-extractable pool than in other pools (Bair et al, 2014); highly weathered soils will accumulate most of the added P in the NaOH-extractable fraction (Negassa and Leinweber 2009). Although several researchers have constructed sorption isotherms using the method of Nair et al (1984) or a modification of it, there is no clear indication of the partitioning of P into the various soil P pools after the sorption reactions are completed. Most often, the soil samples are discarded after the sorption study is concluded.…”