I measured total Iron and Aluminum in water extracts of soil samples (1:10) by an IPC-OES. For our soil quality analysis/index method we like to use, these elements turn out to be incorporated for distinguishing differences between land uses.
The soils have a pH of 7-8, a CaCO3 content of ~30-50% and are located in a semi-arid climate, so I wouldn't expect to find any toxic or harmful concentrations.
Now we measured 0.2-1.2 mg/kg (or 10-140 ug/L) of both elements, but it's hard to find any references on what are "good" or "minimum" values as most research focuses on toxicity (so more acidic soils) or on the solid soil solution.
Can anyone comment on these values or recommend literature on this which can help me establish good/bad/minimum boundary values?