Soil Health Assessment

potato field showing soil

Soil Particulate Organic Matter

This particulate organic matter (POM) method can be used as a demonstration to show what soil organic matter is, in a very visual way. POM washed from the soil in the method is also an indicator of recent organic matter inputs and their decomposition. Below is an instructional video illustrating the method.

Instructional Video:

Materials :

  • 2mm and 0.25mm (250 micron) sieves. There are different options for this, such as official laboratory sieves, 250 micron mesh bags, mesh fixed in a cylinder, modified containers with holes, etc. Here, two options are considered for 250 micron sieving, a 0.25 mm mesh bag or a handmade 0.25 mm sieve (Fig. 1), but it is easy to adapt the process to different types of sieves, as long as the size fraction between 0.25 and 2mm is isolated through wet sieving.
  • Rinse bottle. This can be made from a 500 mL water or soda bottle, poking about 20 to 30 holes in the lid with a safety pin, needle, or thumbtack.
1a. Bottle and mesh bag combination.
1b. Bottle and mesh bag assembled.
1c. An alternative to the mesh bag is a 0.25 mm sieve that can be made from local materials.
  • Water – reasonably clean tap water is fine.
  • Small to medium tubs or buckets (2 to 5 liters). These are for shaking the bottle/bag combination vigorously to accomplish the wet-sieving of soil inside.
  • Measuring cup (250 to 500 mL) or similar with a spout, to decant the particulate organic matter.
  • Open cylinder made of a plastic or metal container or bottle, and an elastic band to fix a cloth on it to capture the sample, e.g. a metal can cut open at both ends.
  • Fabric squares of approximately 15 x 15 cm to capture organic matter. It must be of a type that water can easily pass through, for example, T-shirt fabric.
  • (Optional) Small V-shape channel made of cardboard or rigid plastic sheet, about 20 cm long, to qualify the amount of POM (see fig. 8)
  • For research work, a precision balance (0.01 g or 0.001 g precision) can be used to more precisely quantify the amount of POM, in addition to visual or qualitative evaluation among samples from different fields. However the amount of POM can also be quantified by area on the cloth or volume in a small plastic or cardboard trough (see the procedure below).

Procedure:

  1. Obtain a well-sampled and homogenized soil sample, about 0.5 to 1 kg or more.
  2. From this larger amount, measure 100 g of soil. If the soil has not been sieved, remove the stones manually so that the 100 g does not contain stones larger than 5 mm in diameter. If the soil has already been sieved to 2mm, only the mesh bag or 0.25mm sieve below may be necessary.
  3. Assemble two sieves made of simple materials: a bottle with 2 mm diameter holes, inside a mesh bag with 0.25 mm holes. The bag can also be replaced with a sieve made of a plastic container and a 0.25 mm mesh (Fig. 1c), or a standard laboratory sieve of size 250 microns. The bag is a little easier to use in clay soils because it allows you to break up small soil aggregates by hand inside the bag (Fig. 3b), while both work well in loamy and sandy soils.
  1. Prepare tubs or small buckets of water, each with 1.5 l inside. If water is scarce, the second bucket may need only 1 liter of water. Also fill the wash bottle or bottles with water. Tap water or any clean water source is suitable.
2a. How the sieving bottle/mesh bag works
2b. Wash and rinse buckets prepared
2c. Washing using the mesh bag, top, versus with a small 0.25 mm sieve, bottom.
  1. Place the 100g of soil into the sieving bottle inside the mesh bag or 0.25mm sieve. Lower the bottle/bag into the first bucket with 1.5 L of water and shake well for a few minutes, so that all particles smaller than 0.25 mm come out of the bag into the bucket, while medium particles (0.25 mm to 2 mm) remain in the bag or on the 0.25 mm sieve (Fig. 2a, 2c).
  2. In clay soils, be patient as you gently break up any large aggregate inside the bottle to loosen its particles, using a stick or other utensil (Fig. 3).
  3. Once you have broken up all the aggregates inside the bottle, do a final rinse of particles into the bag from the bottle, making sure to capture those that have stuck to the inside and outside of the bottle.  Then begin washing and working the bag by hand, breaking up the small aggregates to release any organic particles that small clay aggregates may contain (Fig. 4). This process is important in clay soils, where smaller, more stable aggregates tend to form. As you do this you will notice of the contents of the mesh bag start to feel more like gritty sand than like smooth clay.  Be careful not to work so aggressively that you are breaking the POM inside the bag, and causing it to escape from the bag.
3. Use a stick or utensil to break up any clumps of soil inside the bottle.
4.  Break up the small aggregates or ‘crumbs’ of clay inside the bag by hand.
  1. When you see that what is left in the bag or mesh is only sand with organic particles, switch to the second tub or bucket to rinse the bag. When you finish rinsing, there should not be much cloudy clay or ‘dirt’ coming out of the bag or mesh.
  2. Transfer the mixture of sand and POM to the measuring cup or similar container for decanting (Fig. 5a). Use the wash bottle and water to move all material into the container.  Leaving a few grains of material in the mesh bag or sieve is acceptable.
  3. Add water to the measuring cup or other decanting container, and stir so that the organic matter in the water is suspended, wait a few seconds then pour or decant these grains of organic matter into the tube with the cloth (Fig. 5b), making sure that the fine mineral sand grains with the organic matter do not come out with the organic matter (Fig. 6).
5a. Transfer contents of the mesh bag (or alternatively the 0.25 mm sieve) to a measuring cup or other container for decanting.
5b. Prepare a plastic or metal cylinder like a jar with the end cut off, adding the square of cloth with a rubber band to catch the POM.
6. Decanting the POM floating in water from the sand which will sink to the bottom of the measuring cup.
  1. Repeat this cycle: add water, stir, decant, until there is no more organic matter floating in the cup. All the POM will then be on the fabric in the cylinder. In the cup you can see dark grains, floating just above the sand layer, these can be organo-mineral complexes, and sometimes carbon, and you should try to capture them as part of the POM. Don’t worry if some POM grains remain in the cup.
  1. Next, remove the fabric with the POM from the cylinder. Take care to rinse with a little water so that all the POM remains in the fabric and not in or around the edges of the cylinder.
7. Measuring the diameter of POM on the cloth square after spreading to a layer of about one or two particles thick.
  1. POM can be evaluated directly in wet form, based on its volume, or dried and weighed on a precision balance later. To evaluate the POM directly, distribute it on the cloth so that it forms a disk with a thickness of one or two particles only (Fig 7). If there is a small amount of POM, the particles must be brought together, if there is a lot of POM, they must be spread out..
  2. Measure the diameter of the circle of POM (Fig. 7)
  3. Another way to measure the amount of POM (Fig. 7b): you can make a right-angled channel out of thick cardboard or rigid foam or cardboard sheets like those used in architectural models. Then, in this channel, the POM is spread (wet or dry, but preferably dry) to be the width of a pencil at the bottom of the channel (Fig. 8). The length of POM in the channel is then compared to a guide.
  4. It is recommended to save the POM to dry and weigh it, and then store it to compare it with the quantity in a future measurement of the same field.
  5. Consult the guide to see the qualitative level of POM (Fig. 8) and management criteria, depending on the diameter or length of the POM channel that has been observed.
8. Rating the level of POM by measuring it in a small cardboard or plastic channel, with POM spread to the thickness of 1 pencil width across the bottom of the channel.
Table showing interpretation of the different levels of POM results