What is amended soil, and why is it so important?

January 23, 2019 Jennifer Young

Did you know that amended soil is a VIP for LID? Here’s a guide to amended soil and its key components


In previous blogs, we’ve discussed Low Impact Development (LID) and Green Infrastructure (GI) in municipal, development, and transportation settings. This time let’s dive into a very important part of many LID facilities: amended soil.

What is amended soil? It’s soil that has selected components added to meet specific needs of LID facilities. In this case, amended soil is a special soil mix that is used in vegetated (planted) LID practices like bioretention (rain gardens) and bioswales to soak up and filter stormwater. Many people think that the plants are the most important part of these systems, but the term “bioretention” refers more to the soil that forms the base of the practice. In fact, the soils in these systems are responsible for treating and cleaning the water, including for nutrients like phosphorus and nitrogen. The plants assist with this function but are not always the stars of the show.


The soils in bioswales are responsible for treating and cleaning stormwater, including for nutrients like phosphorus and nitrogen.


The typical amended soil currently used for stormwater LID features in Ontario is frequently called a filter media. This soil includes a high sand content, and it can be difficult to produce in a way that meets all the criteria listed below. It can also be a difficult place to grow vegetation. This is a design area where we still have a lot to learn, as far as what amended soil mixes are best for both plants and stormwater management over time. Amended soil mixes are evolving as LID facilities are installed and monitored.

Let’s have a closer look at the typical amended soil used in Ontario. What are we looking for and why is each component important?


1. pH: Small changes make a big impact

pH is a measurement of the acidity or basicity of a substance, with a neutral pH being set at 7. Rainwater tends to be slightly acidic with a pH of 5.6, unless affected by industrial activity, which can lower the pH to 3 or lower (acid rain). The pH of the soil is important because even small changes in pH can have big effects on plant growth and health. Low pH can limit phosphate absorption in plants. This means that bacteria growth slows, which can reduce available nutrients for absorption in plants. High pH results in phosphorous, iron, and manganese becoming less available to plants, with plants possibly showing signs of nutrient deficiency. The pH for amended soils should range from 6.0 to 7.5 (slightly acidic to neutral).

_q_tweetable:It is critical that soils be created that can provide for the needs of your LID feature._q_

2. Phosphorus index: Protecting from eutrophication

The phosphorus index—or P-index—of a soil is a measurement of how much phosphorus is in the soil. The P-index of the soil is very important because amended soils with a P-index over 30 ppm (parts per million) will leach phosphorus into the stormwater instead of removing it. This can cause increased phosphorus loading in downstream creeks and rivers, and that can lead to eutrophication—aka an excessive richness of nutrients—and algae blooms. The P-index for amended soils should be between 10 to 30 ppm, which is enough to support plant growth without exporting phosphorus from the soil.

3. Cation exchange capacity: Key for nutrient retention

The cation exchange capacity (CEC) is a measure of how many cations—aka positively charged ions—can be retained on a soil particle surface. The CEC is important because it is what allows the soil to retain nutrients like calcium or potassium, as well as retaining pollutant cations such as phosphorus and heavy metals, removing them from the stormwater runoff. The CEC for amended soils should exceed 10 meq (milliequivalent)/100g to best allow for removal of pollutants.

Amended soil is a special soil mix that is used in vegetated Low Impact Development practices like bioretention (rain gardens) to soak up and filter stormwater.


4. Infiltration rate: The fix for surface ponding

The infiltration rate of soils describes how quickly water drains into them. Soils with high infiltration rates drain water at a much quicker rate, allowing for water to be filtered and soaked into the ground faster. This is important because bioretention facilities often have surface ponding during rain events that needs to be quickly drained within 24 hours—a regulation in Ontario and British Columbia that may vary by location—to be ready for the next rainfall. Soils with lower infiltration rates may clog over time, increasing the time it takes to drain down the water within the facility. Infiltration rates for amended soils should exceed 25 mm/hour. Increased infiltration rates can result in drier growth conditions. Plants will need to be selected that can tolerate the soil condition.

5. Soil mix: It's about sand, fines, and organics

Amended soils are generally made of three components: sand, fines, and organics. Amended soils often have high sand content to offset the large amounts of fines that will be removed from the stormwater during treatment, but too much sand is not good for plant growth or nutrient removal. Fines are very small soil particles (silts and clays) and they contribute the bulk of the CEC in soil and improve pollutant removals. Organics are required to create a good environment for plants and soil animals like earthworms and bacteria. There must be a balance between sand, fines, and organics in amended soils for the best long-term performance. Currently, amended soils are recommended to have 85-88% sand, 8-12% fines, and 3-5% organics, although mixes with less sand can still meet all the other criteria.


In our experience, it is critical that soils be created that can provide for the needs of your LID feature. We have tailored soils for increased infiltration as well as specific mixes with increased organics and lower sands to promote more vegetation growth. It is important to understand what you need your soil to do within the LID, and for the vegetation within each feature. 

About the Author

Jennifer Young

Jennifer Young is a water resources engineer specializing in low impact development (LID), green infrastructure (GI), and integrated stormwater management planning.

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