Phosphorus fraction

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The term phosphorus fraction comes from soil science and hydrology . There he describes the different phosphorus groups and their occurrence in the soil or in the water. There are several ways to differentiate between phosphorus fractions, and different names are often used for phosphorus fractions in the literature. In principle, the fractions can be differentiated on the basis of theoretical considerations or on the basis of measurable fractions.

The following classification is one of the measurable fractionations :

  • Total phosphorus , the sum of all phosphorus fractions (also: total phosphorus, TP)
  • Dissolved phosphorus , the sum of all dissolved phosphorus fractions (also: dissolved phosphorus, DP; total soluble phosphorus, TSP)
  • Dissolved organic phosphorus , the organic part of the dissolved phosphorus (also: dissolved organic phosphorus, DOP; soluble organic phosphorus, SOP)
  • Dissolved inorganic phosphorus , the inorganic part of the dissolved phosphorus (also: dissolved inorganic phosphorus, DIP; soluble inorganic phosphorus, SIP)
  • Particulate phosphorus , phosphorus bound to particles (e.g. sediment ) (also: particulate phosphorus, PP)

Each of these fractions can be determined using various chemical analyzes. In addition, the following mathematical relationships exist between the fractions:

  • TP = TSP + PP
  • TSP = SOP + SIP

However, the simple mathematical calculation can lead to certain under- or overestimations of individual phosphorus fractions. Nevertheless, this mathematical approximation can at least offer a first clue.

Among the theoretical fractionations one counts the following classification:

  • Inorganic phosphate is the fraction that is present in primary phosphorus minerals (e.g. apatite), in secondary phosphorus minerals (Fe, Al or Ca minerals) or adsorbed on Fe and Al hydroxides.
  • Dissolved phosphate is the proportion that is present in dissolved form in the soil solution as H 2 PO 4 - or as HPO 4 2− . Dissolved phosphate is directly available to plants.
  • Organic phosphate is a collective term for all other organic compounds with phosphorus content in the soil. This includes phosphorus in humus, phosphorus adsorbed on organic molecules, microbial phosphorus and phosphorus in plant debris.

Plant-available phosphorus

Due to the different chemical and biological properties of each fraction, it is possible to infer certain properties of the soil by knowing the occurrence of these fractions. Above all, this includes the productivity of the soil examined. The fraction of phosphorus available to plants is of particular importance . All other phosphorus fractions are in dynamic equilibrium with one another and are therefore interchangeable, but since this exchange takes place extremely slowly and an overall analysis of the soil phosphorus is very time-consuming, in most cases only the phosphorus available to plants is determined.

In principle, dissolved phosphate, which is in the form of H 2 PO 4 - or as HPO 4 2−, is directly available to plants. However, plants can contribute directly to the mobilization of phosphorus from other fractions (especially from labile phosphorus) via complex mechanisms, which means that a simple water extraction would not be meaningful. In addition, the presence of other soil constituents can affect the extractions. In order to take these circumstances into account, various extraction methods for determining the phosphorus available to plants have been developed. The calcium acetate lactate extract (CAL method) according to Schüller is traditionally used in Austria , Switzerland and Germany . In England , where some of the longest continuously surveyed test fields are, and in the USA, Mehlich-3 and Bray extraction is predominantly used. The Olsen test (extraction with sodium hydrogen carbonate ) is also used. The comparability of these groups with one another is problematic. Depending on the extractants used, different components are dissolved and thus added to the proportion available to plants. Depending on the question, the measured soils or the plants that are being considered for cultivation, the individual analysis methods can allow meaningful conclusions.

The total phosphorus can be divided into solubility fractions via entire cascades of extractions that dissolve different proportions. The division into soluble phosphate, labile phosphate and stably bound phosphate is common here.

literature

  • AN Sharpley and AD Halvarson: The Management of Soil Phosphorus Availability and its Impact on Surface Water Quality . In: R. Lal, BA Stewart (Ed.): Soil Processes and Water Quality. Lewis Publishers, 1994, ISBN 0-87371-980-8 , pp. 7-90
  • A. Kreuzeder: Modeling Phosphorus Flows in Soils : Optimus Verlag 2011, ISBN 9783941274853

Individual evidence

  1. ↑ D. Dieter, H. Elsenbeer, BL Turner: Phosphorus fractionation in lowland tropical rainforest soils in central Panama . In: Catena. Volume 82, 2010, pp. 118-125
  2. ↑ BL Turner: Inositol phosphates in soil: amounts, forms and significance of the phosphorylated inositol stereoisomers . In: Turner, BL Frossard, E. Baldwin, DS (Eds.): Inositol Phosphates: Linking Agriculture and the Environment . CAB International, Wallingford, UK, 2007, pp. 186-207
  3. ↑ Pierzynski, GM, Sims, JT, Vance GF, Soils and environmental quality Boca Raton 2005, Taylor & Francis
  4. ↑ Schüller, H. (1969). The CAL method, a new method for determining the plant-available phosphate in soil. Journal of Plant Nutrition and Soil Science, 123, P. 48–63.
  5. ↑ http://www.agronext.iastate.edu/soilfertility/presentations/mbotest.pdf
  6. ^ Van Laak, M., Klingenberg, U., Peiter, E., Reitz, T., Zimmer, D., Buczko, U. (2018). The equivalence of the Calcium ‐ Acetate ‐ Lactate and Double ‐ Lactate extraction methods to assess soil phosphorus fertility. Journal of Plant Nutrition and Soil Science. doi: 10.1002 / jpln.201700366 .
  7. ↑ Neyroud, J.-A., Lischer P. (2003). Do different methods used to estimate soil phosphorus availability across Europe give comparable results? Journal of Plant Nutrition and Soil Science, 166, pp. 422-431.