Research Station Agroscope Reckenholz-Tänikon ART, CH-8046 Zurich

The soil P extraction with ammonium acetate EDTA (AAE10)

A prerequisite for the application of phosphorous values resulting from soil analysis methods is the comprehension of the chemical processes in the soil extraction. These processes were examined for the ammonium acetate EDTA extraction (AAE10-P) and compared with the extractions with CO2-saturated water (CO2-P) and pure water (H2O10-P). The average P-concentrations in the AAE10 extract are usually larger than those with CO2-P, whereby this factor varies between 1 an 11 for 95 percent of the samples. Exclusion of the EDTA reduces the extracted P by approximately two thirds. The AAE10 extracts of calcareous soils are saturated with respect to P, like those with CO2 and H2O10. Therefore, AAE10-P represents an «intensity», i.e. a P-concentration in mg/L, and not a P content in mg/kg or kg/ha. With lime-free and calcium-poor soils, mineral phosphates may be dissolved, whereby their proportion compared with the easily soluble P varies. While this method is often reported to determine the P-reserve of the soils, this pertains only for this portion in the AAE10-P content of lime-free soils. Addition of Lime or Ca2+ reduces the extracting power of AAE10-P, however in a differing manner compared with CO2-P or H2O10-P. The AAE10-P values may be similar to much larger than the CO2-P values depending upon the origin of the soil. Due to this divergence, each extraction method will need a principally different interpretation scheme for fertilizer calculation.

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