Organic matter

Organic matter is matter that has come from a once-living organism (i.e., plants and animals), is capable of decay or the product of decay, or is composed of organic compounds. Once it has decayed to the point at which it is no longer recognizable it is called soil organic matter. When the organic matter has broken down into a stable substance that resists further decomposition it is called humus. Soil organic matter comprises all of the organic matter in the soil, exclusive of the material that has not yet decayed (i.e., surface litter). It can be divided into three general pools:

• living biomass of micro-organisms,
• fresh and partially decomposed residues (the active fraction), and
• the well decomposed and highly stable humus (USDA, 2015).

The structure, drainage and fertility characteristics of soil are all highly affected by organic matter content. In LID BMPs, if the soil does not contain enough organic matter it will lack porosity, water holding capacity and be difficult to maintain healthy vegetation cover without addition of chemical fertilizers. When organic matter content is too high, the soil may leach nutrients into the water that infiltrates through it, potentially contributing to nutrient loads to receiving waters rather than reducing them. So an important part of Construction, Assumption and Verification inspections involves sampling and testing the soil component of BMPs to ensure it meets the design specification for organic matter, or determine if it is still within an acceptable range.

Testing

To determine if the soil component of an LID BMP meets design specifications or is within an acceptable range for organic matter, representative samples must be collected and submitted to an accredited Ontario soil testing laboratory for soil organic matter analysis. The recommended test method depends on the organic matter content of the soil sample: - When organic matter is < 7.5 % by dry weight, the Walkley-Black method (Walkley, 1947) using a routine colourimetric determination procedure is acceptable. - When organic matter is ≥7.5% testing must be done by a loss on ignition (LOI) method (OMAFRA, 2006). Testing soil organic matter by LOI method involves drying a sample, typically at 105 to 120 ◦C for 2 hours, measuring the dry weight, igniting and ashing the dry sample, typically at between 360 to 425 ◦C for 10 to 16 hours (OMAFRA, 2006; McLachlin, 2016; Wright, 2016) in a muffle furnace and then reweighing the sample to determine the change in weight. The weight loss value (i.e., LOI value) is then used to calculate the organic matter content value based on the relationship between LOI and soil organic carbon established for the region through extensive testing of soil samples by the Walkley-Black method (McLachlin, 2016; Wright, 2016), with results reported as percent organic matter (%OM) by dry sample weight. Acceptable procedures for testing organic matter content of soils by both the Walkley-Black method and LOI method are provided by North Central Regional Research Publication No. 221 (Combs and Nathan,2012). Acceptable procedures for testing organic matter content of compost or highly organic soils is provided by ASTM D2974-14, Standard Test Methods for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils (ASTM International, 2014) and United States Department of Agriculture (USDA, 2002).

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.