Four basic organic landscape mixes were prepared using traditional and alternative organic materials: 1) 75% pine bark + 25% sphagnum peat moss, 2) 50% pine bark + 50% wastewater biosolids, 3) 100% municipal yard waste compost, and 4) 65% pine bark + 35% cottonseed hulls. Expanded shale was blended with each of these mixtures at rates of 0, 15, 30, and 60% (v/v). Three sets of pots were planted with Vinca (Catharanthus roseus), Verbena (Verbena hybrida), and Shantung Maples (Acer truncatum).

Plant growth was monitored for several months followed by harvest of Vinca and Verbena plants to determine total biomass yield. Vinca plant tissue was chemically analyzed to assess how the landscape mixes affected nutrient and heavy metal uptake. Vinca, grew best in the compost mixture and its performance was improved by blending up to 30% expanded shale into the mixture.

Both the perennial flower, Verbena, and the woody ornamental, Shantung Maple, grew best in the 50% pine bark + 50% biosolids mixture, followed closely by the 75% pine bark + 25% sphagnum peat moss mix and the 100% compost mix. For all three species evaluated, plant growth in the 100% compost mix was improved by the addition of 15 and 30% expanded shale, but plant growth in the 75% pine bark + 25% sphagnum peat moss mix and the 50% pine bark + 50% biosolids mix was diminished by including expanded shale in the mix.

Expanded shale increased the uptake Ca and Mg by Vinca, but decreased the uptake of phosphorus. Decreased P and trace metal uptake was probably due adsorption by the high pH expanded shale. Expanded shale can improve the performance of landscape mixes that primarily contain decomposed or composted organic materials. The increased performance is probably due to an improved porosity in the mixtures.

Learn more about this project by watching an interview with John Sloan.

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ASHS Members can access the rest of the interview by logging in on the ASHS home page. Learn more about this project: What were the results? What have you learned? Who is likely to benefit from this? What's being planned for the future? Is there renewed interest in composting from homeowners?