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The degree of phosphorus saturation (DPS) has been used in evaluating the risk of P loss from soil to runoff. While techniques are available for calculating DPS for acid soils, no widely used technique exists for neutral to calcareous soils that are typical of the Northern Great Plains, including Manitoba (Canada) soils. This study aimed to develop techniques of calculating the DPS of neutral to alkaline soils. Four measures of soil labile P and ten indices of P sorption capacity were used to calculate the DPS of 115 Manitoba soils. The various DPS calculated were evaluated using water-extractable ((H2O)) P as an index of P susceptibility to runoff loss. The DPS obtained using Olsen-extractable ((Ols)) P and the Langmuir adsorption maximum (ES(max)) ranged from 0.5 to 31.9% while those obtained from P(Ols) and the single-point adsorption index (P(150)) ranged from 0.9 to 73.9%. Of all the DPS evaluated, those that included P(Ols) and Mehlich 3-extractable ((M3)) P as the numerator with either P(150) or ES(max) as the denominator were fairly well correlated with P(H2O) (r values ranged between 0.45 and 0.63). Along with ES(max) and P(150), a new method of calculating DPS was formulated as the ratio of P(Ols) or P(M3) to Ca(M3) or (Ca + Mg)(M3). We found that the ratio of ammonium oxalate-extractable ((ox)) P to (Al + Fe)(ox), which has been widely used to calculate DPS in acid soils, was not suitable for neutral to alkaline soils of Manitoba. In these neutral to alkaline soils, Ca(M3) or (Ca + Mg)(M3) were better indices of P sorption capacity while P(Ols) and P(M3) provided better estimates of labile soil P. The DPS calculated using Ca(M3) or (Ca + Mg)(M3) were well correlated with P(H2O); however, they were numerically smaller than those obtained from the Langmuir adsorption maximum. As such, a saturation coefficient (alpha) with a value of 0.2 was generated to improve the numerical values of the newly estimated DPS. This new approach can be used to estimate the DPS in neutral and calcareous soils without the need to generate a P adsorption maximum.

Ige, D. V., Akinremi, O. O., Flaten, D. M., Ajiboye, B. and Kashem, M. A. 2005. Phosphorus sorption capacity of alkaline Manitoba soils and its relationship to soil properties. Can. J. Soil Sci. 85: [417][418][419][420][421][422][423][424][425][426]. The establishment of the P retention capacity of soil in Manitoba is essential for effective management of P in the region. However, the methods for determining the P retention capacity for neutral to calcareous soils in the Eastern Prairies are not well developed. The objectives of this study were to determine the P retention capacity of Manitoba soils and to generate equations that relate these capacities to other soil properties. One hundred and fifteen archived surface soils were selected and their physico-chemical properties were measured. These soils were used to generate a single-point P adsorption index by equilibrating 2 g of soil in 20 mL of 0.01 M KCl solution containing either 150 (P 150 ) or 400 (P 400 ) mg P L -1 . A subset of 26 of these soils was used for multipoint isotherms with P concentrations in the range of 0-1000 mg P L -1 . The data obtained were fitted to the Langmuir isotherm and the adsorption indices were correlated with the various soil properties that were then used to developed predictive equations of the P retention capacity of the soil. The values of the adsorption index, P 150, obtained from the single point adsorption study using 150 mg P L -1 , ranged between 88 and 891 mg P kg -1 , while that of P 400 ranged between 100 and 1250 mg P kg -1 . A better correlation was obtained between P 150 and soil properties compared with P 400 . For the 26 soil subset, the adsorption indices, S max 1 to S max 6, obtained from the Langmuir isotherm, ranged from 300 to 1330 mg kg -1 . A good correlation was obtained between the single point index and the multipoint isotherm (r = 0.93). Hence, S max for the 115 soils was estimated from the relationship between P 150 and S max 3 of the 26 soils. The best relationships between the adsorption parameters, P 150 and S max , and the soil properties were obtained with the sum of Mehlich-3 extractable Ca and Mg (R 2 = 0.66) and the sum of exchangeable Ca and Mg (R 2 = 0.64). Mehlich-3-Ca and -Mg each explained 56% of the variation, while clay content explained 40% of the variation in the P retention capacity of these soils. Unlike the widely reported influence of Al and Fe in acid soils, our study showed that the retention of P in Manitoba soils was influenced more by Ca and Mg and soil texture. Pour bien gérer l'utilisation du P au Manitoba, il est essentiel de connaître la capacité de rétention des sols. Malheureusement, les méthodes permettant de déterminer cette dernière dans les sols neutres ou calcaires de l'est des Prairies ne sont pas très développées. L'étude devait établir la capacité de rétention du P de ces sols au Manitoba et produire des équations reliant cette capacité à d'autres propriétés du sol. Les auteurs ont sélectionné 115 sols de surface en archive et mesuré leurs propriétés physicoc...

Phosphorus retention ability of soil has been predicted from different combinations of soil properties as a result of significant correlations between these variables. However, a significant correlation between P retention capacity and a soil property does not necessarily imply a significant direct effect of the soil property on P retention. The objective of this study was, therefore, to evaluate the direct and indirect influence of soil properties on P retention capacity of neutral to calcareous soils of Manitoba (Canada). One hundred fifteen archived surface soil samples representing major soils of Manitoba were used for the study. The P retention index (PRI) of these soils was determined by the single‐point adsorption method. The relationships between P retention and soil properties were evaluated by correlation analyses while the direct and indirect influences of these soil properties on P retention were evaluated using the path analysis procedures. Significant correlations (p = 0.05) were observed between PRI and pH; CO32−; organic C; sand content; silt content; clay content; exchangeable Ca (CaEx); exchangeable Mg (MgEx); cation exchange capaciaty (CEC); Mehlich‐3 extractable Ca (CaM3); Mehlich‐3 extractable Mg (MgM3); and oxalate extractable Al (AlOx). However, path analysis showed that only CaM3 D = 0.62), MgM3 (D = 0.26), and AlOx (D = 0.21) had significant direct effects (p = 0.05) on PRI. The significant effect of sand (soil texture) was due to the indirect influence of CaM3 Our results show that the PRI for the neutral to calcareous soils of Manitoba is best predicted from CaM3, MgM3, and AlOx Similar analysis conducted to relate the classical Langmuir adsorption maximum with the properties of a subset of soils also showed CaM3, MgM3, and AlM3 as the soil variables that had significant direct effects on soil's P retention.

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