Clay subsoil and missing or paltry topsoil (which will be richer in organic matter) is very common in developed and urban/suburban landscapes. Many of our native plants will be fine in "poor" and unamended soil as long as they are well-matched to the site conditions. (Such as how much direct summer sun the area gets, how well the soil drains or if it skews dry, etc.) For example, Butterflyweed (Asclepias tuberosa) does fine in clay, and many ornamental grasses (Panicgrass - Panicum, Bluestem - Schizachyrium, etc.) flop in soil that's too rich but stay sturdy and upright in soil that's not nutrient-rich or heavily amended with topsoil.

Organic matter, generally from compost, is the amendment of choice when improvements need to be made. It will help clay resist compaction and will support good populations of beneficial soil microbes which gradually give the soil texture (which also resists compaction). In natural settings, this organic-heavy layer is the topmost soil layer, and forms from the decay of leaf litter or woody debris like fallen trees or branches. In managed landscapes and more traditional gardens, this can come from biodegradable mulch (wood chips, bark, pine needles, etc.), leaf litter left in place and not removed in autumn, or a top-dressing of compost.

When planting, you could also mix-in compost with the existing soil, but in that case, mix it in thoroughly so the soil texture is more even throughout the zone the root ball will start growing into. Otherwise, water absorption and drainage will differ in soil layers of a different composition, which could negatively affect root health and establishment. In undisturbed areas (amending soil in areas already planted, for example), using a surface-applied organic matter (mulch or compost) is fine, as soil life will gradually do the natural equivalent of rototilling the material into deeper layers for you without disturbing roots or damaging soil structure that has already formed. Although there are situations where it's the most practical option, rototilling is usually discouraged since it can damage valuable soil structure from years of beneficial microbe activity that will take a long time to re-form.

Compost used can be from a vegetative source (leaf and grass clipping type of compost), manure-derived, or derived from mushroom farming. Avoid going to heavy on any of them, since the organic matter proportion of soil composition doesn't need to be very high to be effective at improving soil texture. For plants sensitive to herbicide residues (generally germinating seed, but sometimes older plants), be aware that manure-based compost could potentially contain un-broken-down herbicide residue, and mushroom compost is sometimes quite salty until rain leaches it a few times. (Salt in this context isn't necessarily sodium; it's any high level of mineral residue that can "burn" roots.) Good-quality topsoil will also have organic matter in it, though it's not a regulated material in Maryland, so check out the quality of a potential source before buying, or ask for a copy of its soil test results.

If useful, you can learn more about soil texture, composition, and similar subjects in our Soils, Compost, and Fertilizer compilation of web pages (see the linked page).

Miri