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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 leaf compost	60-70% construction sand 15-25% top soil*15-25% organic leaf compost	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 performanceand long drawdown times
B	50-70% construction sand *30-50% organic leaf compost	70-85% construction sand *15-30% organic leaf compost	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 P content between 12 and 36 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 oflow P content	Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess ofsodium 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*P content between 12 and 36 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 sodium 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 can be tested, if sodium adsorption ratio becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the sodium and allow it to be leached from the soil (Pitt et al in press).