Particulate organic matter

For the entire detailed protocol, see the full downloadable manual

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. Below are videos in three languages illustrating the method, which correspond to the streamlined method shown in the manual that can be downloaded above.

Instructional Video, English:
Instructional Video, Spanish:

Materials you will need:

  • Plastic bottle of size 200 to 400 mL approximately, with holes of size 2mm drilled (with a 2mm drill) or melted (with heated nails of size slightly less than 2mm). About 200 holes should be drilled or melted into the bottle side near the bottom.
  • A bag sewn out of 250 micron mesh (or #60 mesh), large enough for the bottle with holes drilled to enter it. A 13x20cm bag is generally large enough to accommodate most bottles.
  • Water – reasonably clean tap water is fine.
  • Buckets of size 4 to 8 litres (larger is alright too). These are for shaking the bottle/bag combination vigorously to accomplish the wet-sieving of soil inside.
  • Balance to weigh soils and particulate organic matter.
  • Rinse bottles with nozzles that allow rinsing of sieves to move and capture soil and aggregates.  These can also be made by punching narrow holes in the lid of a 500 mL or similar plastic water or beverage bottle.
  • 250 mL beaker or similar container, such as a measuring cup that will allow decanting off floating organic matter from a water suspension.
  • Plastic jar cut at the bottom to create a tube of diameter 8 to 12
  • Squares of fabric (bedsheet-type) or paper filters for catching, visualizing, and optionally weighing the POM. Fabric or filters can be pre-weighed dry and weight written on them to simplify weighing of the POM sample.

Method, step by step:

  1. Place the bottle with the 2mm holes in the 250 micron mesh bag to form two “layers” of screen material through which the soil has to pass
  2. Working above the bucket of water, or another place where soil can be spilled without issue, weigh out and add 100g of soil to the bottle with holes. If the soil was not sieved previously and there are large stones (> 5mm) it is preferable to remove them so that they do not present a bias in the starting weight of the soil. While pouring the soil into the bottle, it could leak a little through the holes and the mesh, this is not a problem, since we do not care about the soil fraction of size <250 micron.
  3. Holding the bottle inside the mesh, immerse and shake the bottle / mesh / soil vigorously in the bucket of water (Fig. R3). Silt and clay will start to come out of mesh bag. Take care that no soil leaves through the mouth of the bottle. The soil is washed in this way for 3 to 5 minutes.
  1. After approximately 3 minutes, examine the material inside the bottle. This should contain only large roots and / or stones> 2mm to continue with the next step. If there are still large aggregates or lumps, it is necessary to break them gently by breaking up clods by inserting a stick into the bottle, or simply continue shaking the bottle in the bucket. Because the soil weight has already been taken, at this point gravel-sized stones could also be inserted to break up the clods when shaking, but usually this is not necessary.
  1.  Once there are only small stones, sticks, and large roots in the bottle, remove it from the bag and rinse the bottle with the bottle or other water, into the mesh bag held open at the top, so that any material between 250 microns and 2 mm is kept in the bag and not lost.
  2. Rinse the mesh bag and bottle combination in a new bucket with clean water. Check whether the mesh bag contains only sand and organic matter or whether there are small clay aggregates, which will create cloudiness in the water. If there are small aggregates you may have to massage the mesh bag slightly to break them up while rinsing.
  3. Wash the contents of the mesh bag into a beaker, measuring cup, or other container for decanting off the organic matter or POM (cup of size 300-500 mL, see picture above).
  4. Once all the material in the mesh bag has been rinsed into the beaker or measuring cup, fill the cup, fill the cup or beaker ¾ full of water to suspend the organic matter and separate it from the heavier sand. Then prepare the cloth filter to catch the organic matter by fastening it to the end of the plastic cylinder with a rubber band (see pictures above in materials).
  5. Next, repeatedly swirl and decant the water and suspended organic matter through the cloth filter on the end of the tube, refilling the cup after each decanting. 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. It is very helpful to look at the video for this step.
  1. Continue until the water above the washed sand in the beaker is clear, and almost all of the organic matter is on the cloth filter. 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 cloth.
  1. To generate a particulate organic matter (POM) data, one strategy is simply to score the amount of POM visually, using a visual guide based on experience and comparison with other soils, or the scoring guide shown below which is based on our experience with this test.
  1. In addition to this visual scoring, you can store the small samples of POM particles in the cloths or filters and then dry them in the air or in an oven (40° to 60° C). These small amounts of dried POM can be weighed on a precision scale (1 mg or 0.001 g) which will produce a more precise quantitative result.
  2. If this POM weight is obtained (it will be very small, between 0.05 and 0.5 grams, say) it can divided out of the initial weight of soil that was washed to get a quantitative result as a percentage, to compare among agricultural plots or practices in an experiment. See the manual for more details on the calculations.