Soil types

As can be gathered from the section ‘Types of herbaceous plant communities: habitat stereotypes’, there is potentially an attractive naturalistic herbaceous vegetation for every site, no matter how wet or dry, fertile or infertile, providing the community is thoughtfully matched to site soils. In general, the most difficult soils to deal with are those traditionally prized by landscape architects and horticulturists—the moist fertile loam. On these soils, competitive exclusion as a result of the rapid growth of weeds and the most vigorous sown species is often a problem, and the use of uniformly vigorous sown species is the most sensible strategy. These soils are, however, very suitable for many annual forbs that are able to compete by growing vigorously. In some cases, such soils may be better planted with conventional herbaceous or woody plants and managed by mulching to suppress the weed seed bank. Extreme soil conditions, for example, dry infertile soils and, to a lesser degree, wet soils offer designers an opportunity to produce highly interesting vegetation that really does respond to the site context. Plant growth is, however, often slow on marginal soils and it is important to prepare clients for this, so they do not misinterpret this as failure. Designers need to establish the range and nature of the soils present on a site prior to earthworking and before overall master-planning commences so that various substrates present can positively inform the design concept.

In semi-natural habitats, soil pH is often an important factor in determining which species are present plus typical species diversity. Typically, more diverse plant communities are often associated with limestone-derived soils. Some of the species associated with such soils (calcicoles) perform better under alkaline conditions, others grow satisfactorily on neutral and acid soils. In many cases, high plant diversity is often related to the fact that limestonederived soils are not very fertile and, as a result, highly vigorous dominant species are absent or checked, allowing more species to co-exist. In our research on slightly acid soils in Sheffield, we have found many species that are often associated with limestone-derived soils, for example Origanum vulgare, to establish and persist satisfactorily. Overall, potential soil productivity appears to be more important for many species than soil pH per se.

Soil cultivation

In some cases, deep soil cultivation is essential, for example on crushed rubble soils to incorporate, for example, composted green waste, to improve root penetration and moisture retention. The cultivation of heavily compacted clay soils is also needed to improve root penetration and soil oxygenation. On less extreme soils, where established weeds have been controlled by herbicides, deep cultivation is often unnecessary and sometimes seriously detrimental. After standard rotavation to 200 mm, many soils can take more than six months to return to their pre-sowing density. This is often exacerbated by the destruction of soil structure due to too many passes with the rotavator. As a result, they tend to crack very deeply during dry periods and the surface layer containing the germinating seeds dry out more quickly, leading to lower seedling emergence and higher mortality. On most soils, cultivation for sowings of perennial species should be restricted to approximately a 25 mm deep surface layer to create a fine tilth for sowing.

Sowing practice

The goal of sowing is to obtain an adequate density of seedlings, relatively evenly distributed across the sown area with few bare patches.