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Minnesota Stormwater Manual

Comparison of pros and cons of bioretention soil mixes
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Comparison of pros and cons of bioretention soil mixes
Link to this table.

Mix Current composition Proposed updated composition1 Pros Cons
A
  • 55-65% construction sand
  • 10-20% top soil
  • 25-35% organic matter3
  • 60-70% construction sand
  • 15-25% top soil
  • 15-25% organic matter3
  • to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg per Mehlich III test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.
 Likely to sorb more dissolved P and metals than mix B because it contains some fines; best for growth of most plants Likely to leach P; if topsoil exceeds maximum allowed clay content, higher fines content could result in poor hydraulic performance and long drawdown times
B
  • 50-70% construction sand
  • 30-50% organic matter
  • 70-85% construction sand
  • 15-30% organic matter
  • to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg per Mehlich III test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.
 Easy to mix; least likely to clog Likely to leach P, lack of fines in mix results in less dissolved pollutant removal; harder on most plants than mix A because it dries out very quickly
C Not currently in MN Stormwater Manual
  • 85-88 percent by volume sand and
  • 8 to 12 percent fines by volume,
  • 3 to 5 percent organic matter by dry weight
  • recommended P content between 12 and 30 mg/kg per Mehlich III test
 Likely to sorb more dissolved P and metals than mix B because it contains some fines; less likely to leach P than mix B because of low P content Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances2
D Not currently in MN Stormwater Manual
  • All components below by dry weight:
  • 60-75% sand
  • Min. 55% total coarse and medium sand as a % of total sand
  • Less than 12% fine gravel less than 5 mm (Calculatedseparately from sand/silt/ clay total)
  • 2 to 5 % organic matter
  • recommended P content between 12 and 30 mg/kg per Mehlich III test
 Best for pollutant removal, moisture retention, and growth of most plants; less likely to leach P than mix B because of low P content Harder to find. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances

1See soil guidelines for important specifics about soil components and other important parameters
2This problem can be avoided by minimizing salt use. Sodium absorption ratio (SAR) can be tested; if the SAR becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the Na and allow it to be leached from the soil (Pitt et al in press).
3MnDOT Grade 2 compost is recommended.

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