Comparison of pros and cons of bioretention soil mixes

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

Mix	Composition in original Manual	Proposed updated composition	Pros	Cons
A	55-65% construction sand 10-20% top soil 25-35% organic matter2	60-70% construction sand 15-25% top soil 15-25% organic matter2 to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg (ppm) per Mehlich III (or equivalent) 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 (or equivalent) 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 in original MN Stormwater Manual	85-88 percent by volume sand and 8 to 12 percent fines by volume, 3 to 5 percent organic matter by volume recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) 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 imbalances1
D	Not in original 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 (Calculated separately from sand/silt/ clay total) 2 to 5 % organic matter recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) 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
E	Not in original manual	60-80% sand meeting gradation requirements of MnDOT 3126, ―Fine Aggregate for Portland Cement Concrete 20-40% MnDOT 3890 Grade 2 Compost 30% organic leaf compost	High infiltration rates, relatively inexpensive	As compost breaks down, nutrients available for plants decreases
F	Not in original manual	75% loamy sand by volume: Upper Limit: 85-90% sand with %Silt + 1.5 times %Clay > 15%. Lower Limit: 70-85% sand with %Silt + 2 times %Clay < 30%. Maximum particle size < 1-inch 25% MnDOT 3890 Grade 2 Compost	Finer particles in loamy sand holds moisture for better plant growth	Lower infiltration rates, requires careful soil placement to avoid compaction, requires custom mixing

¹This 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).
²MnDOT Grade 2 compost is recommended.