DR. Don Research Update
New organic matter test is a superior measure of soil quality, and can be done on-farm.

By Don Lotter

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 of:

  • 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

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