Urban environments often don't have the best soil in which to plant trees. The soil can be compacted by construction and it may also contain too few nutrients due to an increased (building) sand content. Depending on the soil at the location and the function of the area, there are various ways to improve the soil.
In some cases, the soil around a planting hole may be compacted, which means that the soil has been pushed down by pressure. A mixture of different soil types can increase compacting. The roots of a tree cannot penetrate compacted soil as there is no oxygen present. This creates a 'flower-pot effect', the roots can only find oxygen and nutrients within the planting hole itself. This can continue to cause disappointing results years after planting and even result in the death of the tree. If the planting location and surrounding area are found to be highly compacted during planting, the soil will need to be loosened by being turned over using a crane digger.
Preparation of the planting location with tree sand or tree gravel
This method for preparation of the planting location is often used for trees planted in paving, as tree sand also forms a good foundation for streets. Tree sand is made up of sand mixed with organic material. It's not an optimum growing medium but rather a compromise mixture, which combines the requirements a tree makes of the soil with civil-engineering requirements in connection with subsidence. Tree sand can be used under cycle paths and footpaths. If further compacting occurs, under parking areas and streets with low driving speeds, for example, tree gravel is a better option. This material consists of volcanic lava, has sufficient nutritional value for the tree and can bear the load of cars. When tree sand and tree gravel are used, the volume should be at least one and a half times the space calculated for root development in topsoil.
Improvement with fungal-dominated humus compost
Forests are an optimum location for trees. The forest soil contains a high humus content and a complex variety of soil life, including many micro-organisms, such as fungi. Soil improvement using fungi-dominated humus compost recreates the optimal location for trees as closely as possible and enriches the biological soil activity, which eventually ensures that nutrients are made available for the tree. A maximum of ten percent humus compost can be mixed with poor soil to create a good growing medium.