The soil on a derelict site may lack topsoil, be compacted, be infertile, and/or contain pollutants such as lead, oil, pesticides, and so on. A sustainable landscape relies extensively on healthy soil to support all life in and above the ground, so it must be rejuvenated before other restoration strategies are applied. The soil should first be tested to determine its physical composition, pH, and availability of various soil nutrients. Then, the soil can be amended with what is deficient, but only in the necessary site locations and appropriate quantities. In some instances, clay or sand can give the soil more desirable structure. Calcium, gypsum, phosphorus, nitrogen, or other minerals might be added to affect fertility and pH.
The soil’s organic content and overall health can likewise be dramatically improved by adding compost. Some soil scientists recommend that compost be tilled
into the soil to achieve a 2:1 ratio between existing soil and compost by loose volume. This can be translated to adding 2"—4" of compost for every 6"—8" depth of soil (Figure 3—9). The compost not only adds organic content, but also provides a medium for microorganisms that in turn build soil by aggregating soil particles to create soil structure and pore spaces within the soil. The microorganisms can also break down organic pollutants and heavy metals in the soil. And compost-amended soil allows surface water to more effectively percolate down through the soil, thereby increasing soil moisture while reducing the quantity of runoff from a site (also see “Reduce Runoff” in this chapter). Adding compost is one of the most effective means of reviving poor and compacted soil.
Compost from on-site is best, though compost from a local source is acceptable if transportation distance is relatively short (also see “Reuse and Recycle” in this chapter). One caution in using community compost is to determine what materials were used to produce it. Unregulated compost may contain weed seeds, heavy metals, plastic remnants, or other pollutants. The compost should also be “mature” so that it doesn’t deplete nitrogen from the soil to which it is added.
The traditional practice of importing new topsoil to a site as a growing medium should be minimized if not avoided altogether. Trucks and heavy equipment integral to such an operation require a proportionally large amount of fuel. In addition, this convention requires topsoil to be removed from another site, thus contributing to its degradation. And topsoil is often not necessary where native plants are used.