Well-Grounded: Soil

The soil into which we attempt to thrust perennial plants often can't really be called soil, it is so compacted and contaminated and disturbed. The basic type of soil required for perennial plant thrival varies from almost pure sand (Penstemon haydenii, blowout penstemon), to rich alluvial silt (Asclepias incarnata, swamp milkweed), to heavy clay. Some perennials are remarkably easygoing, thriving happily in a wide range of conditions. Others sulk with the slightest variation in their preferred growing medium.

When we talk about soil, we are addressing its composition, or the sand, silt, and clay particles combined with pore space that make it what it is. Each of these three basic soil components has properties that make it more or less suitable to certain types of plants. The ability to hold water and allow it to move to the plants is critical to plant survival. The water-holding capacity of soil is linked to the slope of the site; flat sites with porous soils may be poorly drained. The depth of a particular soil type, and the interface between it and the next layer, can have a major impact on plant survival and growth. 

Asclepias incarnata

Many of the most disturbed construction sites have been stripped of their topsoil, graded and regarded with heavy equipment in less-than-ideal conditions, compacted and contaminated with everything from acid to the dregs from the cement truck.The preparation of this construction soil for landscape installation may be minimal. Topsoil may be dumped and spread directly over the concrete-like surface, which in effect acts like an insoluble pot into which roots cannot penetrate and water cannot drain. Soils may contain many or few of the minerals and elements necessary to provide plant nutrition over an extended period of time. Depending on the circumstances, the nutrients may be present but inaccessible to the plants. A common example of this is iron chlorosis. Soil amendments ranging from organic composts to commercial fertilizers and trace elements are often used as a means of improving the growing environment for herbaceous plants.

What can we control or manipulate in the soil that supports our herbaceous plants? If we are willing and financially able, soils can be completely removed and new soil brought in. Amendments can be added and pH adjusted. Drainage can be corrected. Does this effectively duplicate the native soil that would have been found on the site? Can we recreate the strata and complex organic world of original prairie soils, or any virgin landscape?