Particulate Organic Matter
Particulate organic matter (POM) fraction referred to in this document comprises all soil organic matter (SOM) particles less than 2 mm and greater than 0.053 mm in size (Cambardella and Elliot, 1992). POM is biologically and chemically active and is part of the labile (easily decomposable) pool of soil organic matter (SOM). Figure 1 shows tiny debris of POM (0.25 mm < POM size < 0.5 mm) at different stages of decomposition isolated from soil under no-till management. Studies have shown that POM accounts for few to large amounts of soil C (20% and more) in some soils of Eastern Canada and the USA depending upon agroecosystems and management practices (Table1). Relationship to Soil Function
As perhaps the most easily decomposable fraction of nonliving SOM after microbial biomass, POM fulfills many soil functions mediated by OM. It is a source of food/energy for microorganisms and soil animals as well as nutrients for plant growth. Particulate organic matter enhances aggregate stability, water infiltration and soil aeration; it increases cation exchange capacity and buffering pH. It also binds environmental pollutants such as heavy metals and pesticides. Particulate organic matter may play an important role in the suppression of soil borne diseases (e.g. damping off of cucumber) by compost. This may be explained by the fact that POM is an important source of food/energy in the compost for microorganisms responsible of disease suppression.
POM and Poor Soil Function
In poorly managed soils, the transport by erosion of sediments rich in POM into rivers and other water bodies can result in alteration of water quality and aquatic life. Build up and mineralization of those organic materials lead to the eutrophication of lakes and rivers. Incomplete mineralization of POM C in very poorly drained soils can lead to the formation of methane, which escapes into the atmosphere and contributes to ozone depletion.
Improving POM Levels
Management that affects SOM accumulation also affects POM content in soil (figs 2 and 3). More POM in the soil means that carbon and other nutrients are being stored in the intermediately available pool and are not subjected to losses (e.g., leaching) yet are available when needed.
The following practices enhance POM levels:
- Tillage management (no-till, strip till, and ridge till)
- Crop rotation, cover crops, and cropping frequency (reduction in fallow frequency)
- Application of manure/compost and organic byproducts
- Pasture and hay land management (e.g., rotational grazing and haying)
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