Parks, commons, and areas of ancient habitat such as semi-natural woodlands may have soils that are entirely natural and have received very little human disturbance. Other areas, particularly abandoned industrial sites, sites of demolished buildings, and road and railway embankments have soils and substrates which have been greatly altered. Many sites have no topsoil, but comprise building rubble, concrete, Tarmac, and tips of industrial waste and mining waste, some of which cover huge areas. Site factors may include absence or shallow depth of workable topsoil, poor aeration, stoniness, extremes of pH, electrical conductivity, heavy metal content, iron pyrite content or organic contaminants. These characteristics are not necessarily problems, and many such sites support an interesting and even spectacular assemblage of plants. Generally, sites with ‘poor ’ soils and low fertility produce, in the initial stages of succession, a more varied and interesting flora and fauna than sites of higher fertility.
When considering sites in urban areas, it is important to find out the characteristics of the substrates, and generally to work with the opportunities that they present, rather than trying to impose changes which may well not be viable. In some cases, allowing any succession to continue naturally may be the best option, whilst intervening only to maintain paths or open areas, or to control undesirable invasive species.
Plants, including trees, that naturally appear on a site are nearly always more successful than introduced individuals, and on disturbed and variable soils this is even more likely to be the case. However, on some sites the need to maintain site diversity, improve appearance or site safety, or to encourage community action and use of the site, may be more important than natural succession of the habitat.
Before working on any abandoned industrial sites information should be obtained from landowners or agents about any possible hazards from polluted or hazardous substrates. The Environmental Health Department of the local authority should also be able to give advice. Methane leakage may be a problem on sites which have been used for tipping rubbish. This can cause a particular problem with pond creation, if methane bubbles form under pond liners.
Many sites have been disturbed due to the installation of underground services, which also need taking into account when planning site works.
Recently farmed land, grassland or abandoned allotments tend to have deep soils and high fertility. These soils, which are good for cultivated crops, are not easy to convert to attractive semi-natural vegetation, because the fertility encourages the growth of aggressive species, which may suppress more desirable plant species that are introduced to the site. Left to themselves, these sites tend to grow over with coarse grasses, nettle, bramble, elder and other species, which although supporting a range of invertebrates, birds and mammals, may not be the best vegetation for sites intended for public access. Cropping, without the addition of fertilisers, reduces phosphate levels in the soil, but nitrogen is replenished from the air through rainfall, and in urban areas, from car exhausts, and it may be difficult to reduce soil nitrogen levels. On some sites, where the desired aim is a wildflower dominated grassland, topsoil has been removed to reduce fertility, the resulting substrate being sown with a wildflower mix (Chapter 7 – Grasslands). Success has been variable.
The creation of woodland on fertile soils is also more difficult than on poorer soils, because of the rampant weed growth which will suppress the young trees in the early years. Weeding is vital for at least five years after planting. The formation of a woodland flora is likely to be a long and slow process, and in the initial decades the developing woodland will probably be dominated by couch, false oat grass, nettle, ivy, bracken and bramble (Chapter 6 – Trees and woodlands).
