It's more than dirt. The mixture of gravel and nutrient-rich organic material being spread at the construction site of the 9/11 Memorial is the thoroughly-planned foundation for more than 400 trees.
The so-called structural soil has two functions. The gravel helps create space for oxygen and water to reach tree roots, while providing enough stability to handle the plaza it will support. The organic material provides the nutrients to help the trees grow and remain healthy. Worm skin and excrement is part of the organic material combined with the gravel.
Currently, the soil is being spread on a nearly 13,000-square-foot area on the plaza, enough space to support up to 18 of the memorial trees, according to Gareth Eckmann, director of construction for the 9/11 Memorial.
The process of preparing the memorial plaza for the trees was the subject of a DNAinfo.com report written by Julie Shapiro. The report was accompanied by a photo slideshow, where you can find more photos like the one used for this MEMO blog post. The Port Authority of New York and New Jersey also posted pictures of the work on its website.
The plaza will eventually hold 18,000 cubic yards of this soil, according to the report.