Summary The degree of phosphorus saturation (DPS) represents the ratio of sorbed phosphorus (P) to the P sorption capacity (PSC) of soils. In some countries, DPS is used to evaluate the risk of P loss and surface water eutrophication. This study investigated DPS measurement and prediction in neutral loamy soils from Wallonia, Belgium. A total of 57 agricultural topsoil samples subject to diverse P management were evaluated. No satisfactory relationship could be found between PSC determined by a one‐point short‐term isotherm in the laboratory and the sum of aluminium and iron extracted by oxalate (Alox + Feox). The equation PSC = a Alox + b pHw appeared to be more appropriate for estimating PSC in the soils studied. These soils had a near‐neutral pH, and P fixation processes linked to the presence of calcium ions or carbonates were important. Comparisons of DPS with soil‐test P and water‐extracted P suggested that DPS could be a useful agronomic and/or environmental indicator. Our results also showed that DPS values between 20 and 30% corresponded to the agronomic optimum of soil P content. Consequently, DPS may be used as an indicator of P status in neutral soils, provided that the PSC assessment is adapted to the local soil characteristics. Résumé Le degré de saturation en phosphore dans les sols agricoles limoneux avec un pH proche de la neutralité. Le degré de saturation en phosphore (DPS) est un indicateur des risques de perte en phosphore (P) vers l'environnement utilisé dans de nombreux pays qui exprime la part de la capacité de sorption du P (PSC) réellement occupée par cet élément. L'objectif de cette étude est d'évaluer la faisabilité de l'utilisation du DPS pour les sols limoneux wallons présentant un pH proche de la neutralité. L'étude de 57 échantillons de surface de sols agricoles soumis à des modes de gestion contrastés n'a pas permis d'établir une bonne relation entre la PSC mesurée en laboratoire et la somme des teneurs en Al et Fe extraites à l'oxalate (Alox et Feox). L'utilisation d'une équation du type PSC = a Alox + b pHw semble plus appropriée dans les sols dont le pH est proche de la neutralité et où les carbonates de calcium interviennent dans la fixation du P. De plus, une comparaison du DPS avec les indicateurs agronomiques et environnementaux, P disponible et P extrait à l'eau, montre que DPS est compatible au système d'évaluation de la fertilité utilisé actuellement. Un DPS entre 20 et 30% correspond à une bonne fertilité du sol. Ainsi, le DPS peut être utilisé comme indicateur dans les sols neutres, à condition de préalablement adapter son calcul aux caractéristiques du sol.
Application of phosphorus (P) fertilizers to agricultural soils should be based on sound knowledge about soil P reserves, availability and lability to optimize conditions for plant uptake while limiting P transport to surface waters. This study investigated relationships between parent material (PM), land use and P fractions in agricultural soils at a regional scale. Surface and deep (100–120 cm) soil samples were collected from 120 agricultural sites (240 total samples) and analysed for three P forms (inorganic, organic and available) as well as for additional key properties. Land uses within the study area were continuous cropland, permanent pasture and temporary grassland. The area covered the 12 most common parent types in Wallonia. Mean total P contents were 904 and 401 mg/kg in surface and deep samples, respectively, with a 33% coefficient of variation for both horizons. The organic P fraction represented, on average, 30% of total P, while available P corresponded to 9% of total P. The influence of PM was apparent in both the surface and deep soil samples, while land use influenced only surface properties. The pattern in total P content relative to land use was pasture ≥ cropland ≥ temporary grassland. Correlation analyses highlighted significant relationships between edaphic parameters viz Alox, Feox, CEC, TOC, pH and P content, the main soil parameter being Alox which determines P sorption capacity. An enrichment factor was used to distinguish between geogenic sources and human activities in terms of topsoil P content.
Introduction. Phosphorus (P) is an essential element for plant growth. Therefore, it is essential to accurately evaluate its content in the soil. This requires reliable indicators of soil P status. Literature. This paper reviews literature regarding the indicators of P status in soils. Many indicators can be found, including single extractions (soluble, available, or total P), which are the most common indicators used worldwide. Over time, increasingly complex P indicators have been developed as sequential extractions which characterize the various forms of P, degree of P saturation, diffusive gradients in thin films, biological extractions, isotopic methods, or more complex models. To make a choice among them, different criteria should be applied, including relevance, cost and time, ease of interpretation, and, most importantly, the objective of the analysis. It is also necessary to analyze the appropriateness to soil and climate. Firstly, this paper describes the various types of indicators present in the literature, and proposes a classification system. Secondly, all cited indicators are evaluated and compared. Finally, the P indicators met in Wallonia, southern Belgium are discussed. Conclusions. Each P indicator presents advantages and disadvantages. This review highlights the importance of careful consideration of indicator choice, and the establishment of interpretation thresholds.
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