pH in water
Soil or media pH
Soil pH is a measure of the hydrogen ion concentration of a mixture of soil (or filter media or green roof media) and water. Neutral soil has a pH value of 7.0. An acidic soil has a pH less than 7 and a basic soil has a pH greater than 7.
Soil/media pH is an important parameter that affects fertility through the availability of nutrients needed to sustain plants and micro-organisms. It also affects the solubility of some elements which, in extreme scenarios, can reach levels toxic to plants and soil micro-organisms and increases the mobility and the potential for leaching of pollutants such as metals into the groundwater system.
In humid temperate regions, the optimum soil pH range for most plants is between 6.0 and 7.5 . More acidic soils inhibit the solubility of potassium, sulfur, calcium, magnesium and molybdenum, while increasing the solubility of iron, manganese, boron, copper and zinc.The solubility of phosphorus and nitrogen are reduced in both acidic and basic soils.
Design specifications for the media component of LID BMPs pertaining to pH are intended to ensure fertility and suitability for healthy vegetation cover. Where pH deviates from design specification, vegetation cover may be spotty or uneven, growth may be stunted, or in extreme cases, plantings may not survive and vegetation cover becomes dominated by weeds. To ensure the media will support the growth of plantings, which contributes to the drainage and water treatment performance of the BMP and adds aesthetic value, testing of pH should be done as part of construction, assumption and verification inspections.
Soil pH can be determined in the field using inexpensive soil pH testing kits where a small sample of surface soil is mixed with water and reagents which change colour according to the acidity/alkalinity. The soil pH value is determined by comparing the colour and shade to calibrated scales. Soil pH can also be determined using a portable pH meter which involves inserting a rod into a soil-water slurry mixture. Such soil pH tests should be conducted by creating a shallow (5 to 10 centimetre deep) hole in the soil, filling it up with distilled water, stirring to create a slurry mixture, inserting the pH meter rod into the slurry mixture and recording the value displayed on the meter. Alternatively, surface samples can be submitted to a soil testing laboratory accredited by the province of Ontario for testing by saturated paste method.. An acceptable procedure for testing soil pH is provided in ASTM D4972 - 13 Standard Test Method for pH of Soils.
When media test results fall outside of the specification range or acceptance criteria range, corrective actions are only needed if problems with vegetation cover, condition or composition (i.e., dominance by weeds) are also detected through visual inspection.
- Where vegetation cover is poor, unhealthy or dominated by weeds and soil pH is 'lower than the design/product specification or Acceptance Criteria ranges, corrective action involves amending the soil with ground limestone to raise the pH back to neutrality.
- Where soil pH is 'higher than the design/product specification or Acceptance Criteria ranges, corrective action involves amending the soil with sulphur or compost to lower the pH back to neutrality. Amendments to green roof media should be prescribed by the designer, product vendor or media supplier.
- Craul, P.J. 1999. Urban Soils: Applications and Practices. John Wiley & Sons, New York.
- 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.