|April 1, 2004:
If it were necessary to choose one single test
to assess soil quality for crop growth, soil organic matter
(SOM) would probably be it. However, SOM has many different
forms and therefore is difficult to measure. Up until
now, most tests of organic matter analyze the total amount
of SOM. This is reported as something generally between
0.5% and 7% in soil. Soil organic carbon (SOC), which
makes up the bulk of SOM is also a common test and is
generally very close to percent SOM.
The problem with
percent total SOM and SOC is that they are not very
sensitive to management practices. They include highly
recalcitrant forms of organic matter which remain unchanged
for decades, regardless of management practices.
The active fraction of soil carbon, also known as the
labile fraction, is the component of organic matter
that feeds the soil food web. It is closely associated
with nutrient cycling and other important biological
functions in the soil. The active carbon fraction is
much more sensitive to soil management practices than
total organic matter.
A new procedure for analysis of soil active carbon
that is relatively accurate and also practical, has
been developed by Professor Ray Weil and associates
at the University of Maryland, College Park. Ray has
been a contributor to The
New Farm in the past.
The new test uses a dilute solution of potassium permanganate
(KMnO4). Potassium permanganate oxidizes organic carbon
and turns different shades of purple according to the
amount of organic carbon. Different strengths of potassium
permanganate oxidize different components of SOC; weaker
solutions oxidize the active forms, leaving the recalcitrant
forms of carbon, such as humus, unoxidized. In his soil
test it is this strategy that Weil takes to measure
the active carbon in soil.
There are several additional advantages to using a
weak solution of potassium permanganate: it is safe
– it is used in medicine as an antiseptic; the
oxidizing reaction, which shows up as different shades
of purple, is clearly visible to the naked eye; and
the solution is stable for many months.
Weil and his team have put together a field kit consisting
- a 0.02 molar solution of potassium permanganate,
- a palm-sized spectrometer for gauging the change
in color of the potassium permanganate,
- screw top tubes for shaking the soil suspension,
- measurement pipettes,
- a scoop for measuring soil.
The active carbon soil test procedures:
- sun- or air-dry soil for 15-30 minutes,
- in one of the tubes mix five cubic cm of crumbled
soil with potassium permanganate solution,
- shake for exactly two minutes.
- use the spectrometer to gauge the color of the
potassium permanganate solution,
- calculate active carbon.
In one research project in Honduras, active soil carbon
using Weil’s method correlated well with corn
grain yields, while total SOC, which is analyzed in
a laboratory, had no correlation. Other research has
shown the value of the test in distinguishing between
soil management regimes on U.S. farms.
Information about the active carbon test and test kit
can be obtained from Ray Weil at Rw17@umail.umd.edu.
Weil, R. et al. 2003. "Estimating active carbon
for soil quality assessment: a simplified method for
laboratory and field use."American Journal
of Alternative Agriculture. Vol.18, No.1: 3-17
Don Lotter has a Ph.D. in agroecology and has worked
in sustainable agricultural development in North America,
Latin America, and Africa over the past 25 years. He
can be contacted via his website www.donlotter.com