Introduction

0.1 General

The displacement of trace elements in soil cannot be described solely in terms of their solubility in water, since solid and dissolved compounds interact with one another in a large number of processes. Processes that proceed in one direction (e.g. weathering, mineralization) are accompanied by equilibrium reactions between solid and dissolved forms (e.g. adsorption/desorption, precipitation/dissolution).

Factors that determine whether substances dissolve include the pH value, the redox potential, the concentration of ions in a soil solution and its complexing agent content, all of which result in the soil solution varying with location and time.

To be able to compare increases in easily soluble trace element concentrations in soil by a standardized method, soil extraction with ammonium nitrate is used, which is applicable to all those compounds of elements in a soil that may contribute to the concentration of dissolved elements under the prevailing site conditions (water-soluble compounds, exchangeably bound ions, readily soluble metal complexes).

Extraction with ammonium nitrate solution fulfils the criteria to be satisfied by an extractant for routine analyses.

a) It can be applied to a wide spectrum of soils and elements in a wide concentration range.
b) Disposal of the spent ammonium nitrate solution is environmentally friendly.
c) The determination of the elements in the solution has low susceptibility to interference.

The concentrations of elements in extracts obtained with other unbuffered salt solutions (such as calcium chloride or sodium nitrate solution) often correlate well with those in ammonium nitrate extracts, but the absolute concentrations are different and, at some sites, there may be concentrations deviating substantially from average values. In addition, critical concentrations may be in the region of the limit of determination in the case of some extraction methods.

In conjunction with extraction methods for determining the reserves of elements in soil (e.g. extraction with aqua regia), the proportions by mass determined by extraction with ammonium nitrate solution can serve as a basis for site investigations and for making recommendations with regard to use and restoration of soil containing critical concentrations of elements.

0.2 Prediction of impact

0.2.1 General

The concentrations determined by extraction with ammonium nitrate solution only provide an indication of the effects of trace elements, since it is necessary also to consider numerous site factors and sometimes the amounts of elements present in the soil in order to predict impact. Thus, designations such as “biologically available”, “available to plants” or “components capable of migration” are not appropriate.

In soil that is sometimes poorly aerated (e.g. water-logged soil), variations in the redox potential need to be considered when assessing the potential of arsenic and manganese to migrate.

0.2.2 Migration of elements

Extraction with ammonium nitrate solution has been found to be a suitable method of assessing easily soluble trace element fractions of a soil, whose element concentrations should be determined from samples of undisturbed soil taken from adequately distant sites and at different times.

Major factors governing the transport of elements via seepage water are:

— their chemical and physical properties;
— the thickness of the unsaturated zone;
— the distribution of easily soluble elements over the depth of the soil;
— and the actual speed with which the seepage water travels.

The easily soluble trace element fractions in soil vary with time and location. Increased variability can generally be expected with varying soil acidity (e.g. in soil covered by forest vegetation) or hydromorphic modification (e.g. in warp soil), and can also be expected where the amounts of elements in the soil vary (e.g. due to mineralization). This should be considered when designing a sampling programme.

BS ISO 19730:2008

Soil quality. Extraction of trace elements from soil using ammonium nitrate solution

Current

Published:

31 Mar 2009