INFLUENCE OF AIR-DRYING AND SOIL GAS-PHASE CARBON-DIOXIDE ON DTPA-EXTRACTABLE IRON IN CALCAREOUS SOILS

The extraction of a field-moist soil with DTPA will result in a level of extractable iron (Fe) lower than that of the air-dried soil. Soil g as-phase carbon dioxide (CO2) levels may be considerably higher than a mbient atmospheric levels, especially in wet soils in the field. This study was undertaken to determine whether gas-phase CO2 level influenc es the quantity of Fe extracted by DTPA. Three moist calcareous soils were incubated for 21 days, each at three different partial pressures of CO2, after which the moist soils were extracted with DTPA. A sample of each soil was also air dried, and was subsequently extracted with DTPA. In each case, DTPA-extractable Fe from the moist sample was lowe r than that from the air-dried sample; however, DTPA-extractable Fe in creased with increasing CO2 partial pressure of in the moist soils. DT PA-extractable Fe concentration for a given soil following air drying was not significantly influenced by the CO2 partial pressure during in cubation of the originally field-moist soil. DTPA-extract pH of the mo ist soils followed the same trend as soil-solution pH (i.e., as CO2 co ncentration of the soil gas-phase increased, soil solution pH and DTPA extract pH both decreased); however, the slope of the pH versus log P CO2 curve was less pronounced in the DTPA extract due to the buffering capacity of the triethanolamine. From this study, it is concluded tha t elevated soil gas-phase CO2 partial pressure does not contribute to the lower level of DTPA-extractable Fe observed when the extraction is performed on a field-moist versus an air-dried soil; increased C02 pa rtial pressure actually resulted in a slight increase in concentration of DTPA-extractable Fe obtained from a field-moist soil.