Correlation of DTPA extractable Fe with indigenous properties of selected calcareous soils

Abstract

During the past few years the DTPA soil test (Lindsay and Norvell, 1978) has been widely accepted for the routine estimation of plant available Fe. The strength of the DTPA test lies in the ability of DTPA to extract labile forms of Fe from the soil, as the plant is able to extract these same labile Fe forms. The objective of this study was to determine the most probable sources of labile Fe which are being removed by the DTPA extractant.

Twenty‐four calcareous soils of Texas were extensively analyzed with respect to Fe oxide, organic matter, clay and solution phases. DTPA extractable Fe was highly correlated with ammonium oxalate extractable Fe (amorphous or poorly crystalline Fe oxide), but not with citrate‐dithionite extractable Fe (total free Fe oxide, including well‐crystalline forms), indicating that the amorphous phase was more readily solubilized by DTPA.

Clay content and organic carbon parameters were also correlated (though somewhat less) with DTPA‐extractable Fe. It is likely that the influence of clay and organic matter may have been indirect, since both of these phases would play a role in stabilizing and increasing the quantity of poorly crystalline Fe oxide in the soil.

Six consecutive DTPA extractions were made on each sample. The latter extractions correlated equally as well as the initial extractions with either the Fe oxide or the organic carbon parameters. These results indicated that similar labile Fe was extracted during each extraction of the series. There was little difference between correlation coefficients using 2 and 24 hour shaking times, although the absolute amount of Fe extracted was greater with the longer shaking period. These results indicate that the poorly crystalline Fe oxide is likely the dominant source of labile Fe extracted by DTPA.

Key words:

• DTPA
• Ferrihydrite
• Iron oxide
• Organic carbon
• Clay content
• CaCO₃