Particulate organic matter

This can be used as a demonstration to show what soil organic matter is, in a very visual way. The particulate organic matter (POM ) can also be weighed as a quantitative measure that relates to recent organic matter inputs and their decomposition.

 

https://www.youtube.com/watch?v=3sVKH8W-Ixc

You will need
● 2mm (can also be 1mm window screen if a demonstration) and 0.25 mm (250 micron) sieves (at least 6” (150 mm) diameter are good, to avoid clogging with too much material during wet sieving). 0.25 mm sieves can be made from large diameter plastic pipe and 60-mesh silk screen fabric used in the screen printing industry.
● Water – reasonably clean tap water is fine.
● Small basins for the sieves to fit into, which also allow movement of the sieve into and out of water held in the basins (see photos below)
● Balance to weigh soils and aggregates.
● Squirt bottles with nozzles that allow rinsing of sieves to move and capture soil and aggregates.
● 250 mL beaker or similar container or jar that will allow decanting off floating organic matter from a water suspension.
● Funnel and squares of fabric (bedsheet-type) or paper filters for catching, visualizing, weighing the POM. Fabric or filters can be pre-weighed dry and weight written on them to simplify weighing of the aggregate sample.

More on the method

1. Weigh 70g air dried soil or 100 g moist soil
2. Place soil on the 1 mm sieve (made from window screen) or a standard 2 mm sieve in a small basin as for aggregate stability (above) and wet-sieve the soil as follows: Start by soaking the soil in the sieve in a basin with enough water to cover the soil, keeping it on top of the sieve. Then, you can begin to lift the sieve in and out of the water and gently break down the aggregates (note this is DIFFERENT from the aggregate stability test shown on this same webpage) where we wait exactly 5 minutes for the soil to soak, and do NOT manually break down aggregates).  Take care to not break aggregates so aggressively as to break down pieces of organic matter such as root fragments. You can either discard the material on the 2mm sieve, which will be a mix of stones and organic matter, (or save it in a beaker for decanting and visualizing the macro-organic matter, as below for the 250 micron fraction).

3. Next pour the water and mixture of soil/OM from the basin below the 2mm sieve (what passed through the sieve) through the 250 micron sieve into another basin, taking care to rinse out the first basin through the sieve. Repeatedly dip the sieve into the basin to provide extra rinses of the 250 micron sieve, and gently crush aggregates to liberate associated OM of size 0.25 to 2 mm. If you have access to running water some gentle stream of water can also help with this.

4. Rinse the contents of the sieve (fine to large sand plus corresponding sized organic matter) into a medium size beaker (250 mL or so). Fill the beaker ¾ full of water and repeatedly swirl and decant the water and suspended organic matter through the 250 micron sieve. You are trying to pour out any particles that are light enough (of low enough density) to float in the water column, and leave the denser sand at the bottom of the beaker. Continue until the water above the washed sand in the beaker is clear, and most of the organic matter is on the sieve. As you finish decanting each round, you will see a small part of the particles that look organic (darker) but hang back with the sand (ie their density may not be as high as sand but is quite a bit higher than the water). These are usually are organic/clay complexes and other mixed forms of organic matter, perhaps with a little black carbon or charcoal, and you should do your best to capture them on the 250 um sieves.

5. Transfer the 250 micron sieve contents to a filter paper or cloth. You can weigh the cloth previous to the demonstration, to allow the dry weight to be quantified if you want to dry the cloth and the organic matter on it. Otherwise, a light colored cloth will allow merely visualizing the organic matter, showing it to participants, and taking a photo for comparison to other soils.

4. You can follow a similar procedure with the macroorganic matter (>2mm) sieved out above in step 1. Both the > 2mm and 0.25-2mm fractions are interesting as sources of food for soil microbes and substrates that eventually are incorporated into long-term pools of organic carbon. However the smaller-size 0.25-2mm fraction will likely be more abundant and more comparable among soils as an indicator of soil OM status, so many times we just discard the >2mm fraction.
5. Additional note: in soils without a lot of residue inputs or where breakdown is very quick such as in warm, humid locations, it could be that more soil needs to be weighed into the test initially, to yield enough organic matter so that differences in soil organic matter are more apparent.

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