Organic matter

Organic matter (OM) is material 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 carbon containing 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:

1. living biomass of micro-organisms,
2. fresh and partially decomposed residues (the active fraction), and
3. the well decomposed and highly stable humus.

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 pollutant loading to receiving waters rather than improving Water quality. So an important part of construction, assumption and verification inspections involves sampling and testing the media 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:

• OM < 7.5 % by dry weight, the Walkley-Black method using a routine colourimetric determination procedure is acceptable .
• OM ≥ 7.5 % testing must be done by a loss on ignition (LOI) method .

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 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, 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 . 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 and United States Department of Agriculture.

Evaluation

Construction inspections

If laboratory testing indicates soil organic matter content is not within the design or product specification range, notify the media or topsoil supplier, issue a “do not install” order to the construction site supervisor and contact the design professionals and property owner or project manager to determine corrective actions.

Assumption and verification inspections

If laboratory testing indicates soil organic matter content is not within the design or product specification range, or the acceptance criteria range, schedule investigative work to do further sampling and testing to determine the affected area and depth and decide on corrective actions. Corrective action where organic matter is lower than the design/product specification or acceptance criteria involves amending the soil with compost.

Detailed guidance on implementing compost soil amendments can be found in Preserving and Restoring Healthy Soil: Best Practices for Urban Construction (TRCA 2012). Amendments to green roof media to address organic matter content deficiency should be prescribed by the designer, product vendor or media supplier.

Where organic matter is higher than the design/product specification or acceptance criteria, natural or simulated storm event testing should be undertaken that includes sampling and testing of nutrient concentrations (i.e., Phosphorus, Nitrogen, Soluble Salts) in sub-drain or surface flows from the BMP to evaluate if the exceedance is negatively impacting effluent quality.

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)

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

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.

Toronto and Region Conservation Authority