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:
- living biomass of micro-organisms,
- fresh and partially decomposed residues (the active fraction), and
- 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.
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.
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.
- Walkley, A. 1947. A critical examination of a rapid method for determining organic carbon in soils — effect of variations in digestion conditions and of inorganic soil constituents. Soil Science. 63:251-264.
- Ontario Ministry of Agriculture F and RA. Soil Fertility Handbook. (Reid K, ed.). OMAFRA; 2006. http://www.omafra.gov.on.ca/english/crops/pub611/pub611.pdf. Accessed October 17, 2017.
- McLachlin, I. 2016. Personal communication with Young,D. June 29, 2016.
- Wright, T. 2016, Personal communication with Young, D. June 28, 2016.
- Combs, S.M. and Nathan,M.V. 2012. “Soil Organic Matter.” In Recommended Chemical Soil Test Procedures for the North Central Region. North Central Regional Research Publication No. 221. Missouri Agricultural Experimental Station.
- United States Department of Agriculture (USDA). 2002. 05.07A Test Methods for the Examination of Composts and Composting. Holbrook, NY: Composting Council Research and Education Foundation