What's the Right Mix?

What specific recipe or mix of soil ingredients is best depends on several factors. Important considerations include:

How will the soil be used?

If the soil will be used in a residential rain garden, likely topsoil can be used with subtle amendments of sand and compost to provide adequate infiltration rates and organic matter to support plants. If the mix will be used in a bioswale meant to capture and treat larger volumes of water, an engineered soil mix is more appropriate and often required by local standards.

What products are available locally?

When deciding what type of sand or compost to use it’s important not only to consider the qualities previously described, but what is available locally to reduce the environmental and economic impacts of transportation.

What are local requirements for soils?

Some states and local jurisdictions have established specific standards (see table below) for meeting water infiltration goals and to address concerns about nutrients loss from green infrastructure soils, which can contribute to eutrophication (the overloading of nutrients into water bodies). The ability of a soil blend to infiltrate water, remove contaminants, and support plants will depend on the ratio of sand to compost and other amendments. A general rule of thumb is that a healthy soil contains about 5% organic matter by weight. In a 60:40 mix of sand and compost that goal is just being met.

Table of soil mix standards

Several institutions and government agencies have also participated in research to determine what makes an ideal bioswale soil mix. Many of these studies have focused on reducing nutrient loss from bioswales by adjusting the amount of organic matter and restricting compost feedstocks to yard debris and foodwaste.

Phosphorus is typically the nutrient that limits the growth of algae and plants in fresh water—meaning that an excess of phosphorus may cause “blooms” of these organisms. So, a possibly more effective approach is to use a simple method called the phosphorus saturation index (PSI), a tool used to assess the mobility of phosphorus in agricultural soils. Researchers at the University of Washington are currently testing this method for application to bioswale soils. If the PSI does prove useful, it would provide builders of green stormwater infrastructure certainty that phosphorus leaching is not a problem, and allow for more customized soils mixes to be developed.


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