Sequential leaching to investigate speciation of phosphorus and other elements in organically farmed soils

For reasons of simplification and economy, the availability of nutrients and trace elements has been tried to approach via one-step extractions, which ideally rely on field experiments utilizing conventional fertilization, crops and varieties. However, particularly in the case of the main nutrient phosphorus, the commonly used extraction methods show significant weaknesses in reflecting actual plant availability. Zehetner et al. (2018) correlated 14 P-extraction methods to 50 different soils and obtained a maximum Pearson correlation coefficient of 0,365 between extracted P and crop yields.

A special sequential leaching sequence has been designed initially for sediments to assign the phosphate anion to exchangeable, Fe, Ca, Al, and humic-bound fractions, because they reflect different mobilization pathways. Fractionation is carried out in the following steps: 1. exchangeable (NH4Cl/NaOH pH 7), 2. Fe-bound (Na2S2O4/NaHCO3 pH 7), 3. Humics + Al-bound P (1M-NaOH), 4. Ca/Mg-bound P (0,5M HCl) and 5. residual P (boiling 1M-NaOH). Humics and its element loads are determined by alkaline extraction, which also contains exchangeables versus OH- and soluble hydroxo-complexes, and residuals from living cells. If the step with dithionite is omitted, the Fe-bound P moves to the subsequent fractions. At the same time, the NaHCO3-extract indicates the proportion of exchangeable P relative to HCO3- ;comparable to Olsen-P.

8 manure fertilized and 8 plant-residue fertilized organically farmed soils were fractionated using sequential extraction to quantify the different forms of phosphorus. The availability of other nutrients was also determined in the respective fractions. For comparison, the phosphorus content in the CAL extract was also analysed. In NaOH, precipitation of hydroxides and hardly soluble salts competes with soluble humic and hydroxo-complexes. Hardly alkali mobile cations might underestimate their part bound to humics, like for Mn, Mg, Ca, Sr and Ba.

As a result, the humics fraction turns out to act as a main carrier of phosphate and trace elements like Cu, and is underestimated by CAL or Olsen (NaHCO3)-extraction.

With respect to P released by CAL, dithionite released about double for samples classified as deficient (< 25 mg/kg), whereas it indicated slightly less amounts than P-CAL for 40-50 mg/kg.

For other nutrients was observed: K-CAL showed two separate linear relationships for manure and for plant residue fertilized soils versus NH4Cl exchangeable K. Mg-CAL correlated well with Mg-NH4Cl (ρ=0,851), unless dolomite was present. Exchangeable Li was higher with NH4Cl than with CAL, but well correlated (ρ=0,983).

 

M. J. Hedley,J. W. B. Stewart,B. S. Chauhan B.S.C.: Changes in Inorganic and Organic Soil Phosphorus Fractions Induced by Cultivation Practices and by Laboratory Incubations. Soil Science Society of America Journal 46(5), 970-976 (1982)

R. Psenner, R. Pucsko, M. Sager; (1984); Die Fraktionierung organischer und anorganischer Phosphorverbindungen von Sedimenten - Versuch einer Definition ökologisch wichtiger Fraktionen (Fractionation of organic and inorganic phosphorus compounds in lake sediments); Arch. Hydrobiol./Suppl. 70, 111-155

F. Zehetner, R. Wuenscher, R. Peticzka, H. Unterfrauner: Correlation of extractable soil phosphorus (P) with plant P uptake: 14 extraction methods applied to 50 agricultural soils from Central Europe. Plant Soil Environ. 64(4), 192-201 (2018)