Knowledge of soil organic matter (SOM) content is important in herbicide
applications, pH maintenance, and general soil quality and productivity
assessments. Information about broad ranges of SOM content is needed for
determination of proper application rates of certain residual herbicides, for
determination of potential N contributions to crops, and for general knowledge of
changes in soil quality resulting from management practices that either favor soil
erosion and accelerated SOM decomposition or increase cover and crop residues.
Any field method requires basic steps that are easy to follow by operators from a
field office. Thus, some accuracy is normally sacrificed for simplicity and ease of
operation. Typical field tests rely on visual observations or portable battery-
operated field instruments, which are not as accurate as laboratory instruments
and equipment.
This method is based on the principle that basic EDTA releases SOM, and this
release is directly proportional to the color intensity of the extract. Thus, the
darker the extract, the higher the SOM content. This field method can be used to
estimate SOM for the revised wind erosion equation (RWEQ) so the soil erodible
fraction (EF) can be calculated. For example, if the soil contains 1.7% SOM (1%
organic carbon), the actual percentage of EF would be reduced by 4.66 percentage
points. The formula, therefore, is important to estimate soil loss from erosion and
how this loss is attenuated by SOM.
This method requires field standards of soils
from the general area with varying but known
SOM content. If these are not available,
previously extracted standard samples in clear
vials could be used. This method is based on
visual color comparison of the sample to color
obtained from a series of standards (usually
three: high, medium, and low). Thus,
estimation of SOM requires interpolation
(between 1.0 and 1.5% SOM) or indications
of less than (e.g., < 0.5% SOM) or greater
than (e.g., > 2.0% SOM). In the case of the
basic EDTA, the sensitivity of t