How much organic matter is in your soil?

Of all the factors that can be measured in soils, organic matter (OM) is probably the most useful and critical. This test measures the fresh or labile organic matter in soil. This organic matter supplies nutrients to plants because it is the food for microorganisms which in turn release nutrients like phosphorus and nitrogen.

Some tests like high temperature degradation or acid / dichromate digestion measure total or resistant organic matter. This is important for nutrient holding capacity, water holding capacity and soil structure. These tests are slightly difficult and can be slightly dangerous – been there, done that! Peroxide digestion tends to measure labile and perhaps more resistant SOM.

Potassium permanganate (KPM) is an oxidant that is safe to use. As the KPM oxidizes the OM it loses its purple / magenta colour. In the lab I use 0.2 M KPM stock solution with an added flocculant. The sample can be weighed or measured by volume (2.5 mls). The stock solution is diluted x 10 and then shaken with the soil by hand for a short time. So the method is simple and easy to carry out.

I tested 5 soils from around my property. A good compost, vegetable garden soil, soil from an old strawberry patch, soil from a paddock where very little fertilizer or compost had been added, and sub soil from an excavation. The soil in Gembrook is mostly a highly oxidized iron based soil that has a reddish colour. The results are below.

Permanganate digestion of labile organic matter in soils

Five soils from compost (left) to subsoil (right) were extracted using potassium permanganate. Soils with high labile organic matter remove most of the purple permanganate colour.The compost sample is on the left – very high OM, then from L – R vegetable garden, unfertilized paddock, old strawberry patch, subsoil. The big surprise was that the old strawberry patch soil had relatively high labile OM, slightly more than the vegetable garden. The subsoil (on the right) had almost no OM. The unfertilized paddock (centre) had relatively low OM.

The change in colour provides a simple way to compare the OM in samples. In the lab I used a photometer to read the absorbance of the solutions at 570 nm. This method needs a calibration curve but the end result is more accurate. The amount of carbon in the OM can be related back to the amount of KPM used up (based on a simple assumption). Therefore this method can give a reading for organic carbon in gm/kg. Then that can be converted to a value for OM (organic C is about 55% of OM). Its even possible to estimate the amount of nitrogen that could be potentially released from the OM because the C : N ratio is usually , based on soil type, between 10 : 1 and 33 : 1.

The amount of fresh OM in soils is related more directly to soil fertility factors like microbial respiration and biomass and to factors that can be linked to soil management like reduced tillage, green manures etc.

The inspiration for my exploration was Weil, Ray, et al 2003, Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use.

There are many good articles on soil organic matter available on the web including this one: Hoorman and Islam, 2010,  Understanding soil microbes and nutrient cycling. Ohio State University Agriculture and Natural Resources Fact Sheet SAG-16-10