Interest in urban agriculture is steadily increasing in the Mid-Atlantic region. The conversion of extensive green roofs to food production is particularly appealing due to space availability. The modification of a relatively unfertile shale-based substrate for increased water and nutrient availability was investigated, adding mushroom and yard-waste composts, but potentially contributing to nutrient runoff from rainfall and irrigation events.

Alumina and biochar were therefore tested as substrate amendments to determine their effect nutrient availability and retention. Fifteen substrate mixes were screened by column leaching tests, and four were further studied over nine-months, with crop and leachate studies. Basil, lettuce and peppers were grown and harvested in succession in replicated 50-liter tubs, with leachate collection systems. Biochar did not reduce nitrogen or phosphorus leaching and did not have an effect on plant growth. Alumina significantly reduced the amount of phosphorus leached from substrates with little to no effect on plant growth.